The Effect of Negative Pitch Changes on Landing Performance by Shem Malmquist

By Captain Shem Malmquist

One area of flight performance that is misunderstood by some pilots is the affect of pitch changes just prior to landing.  Specifically, some pilots will aggressively lower the nose just prior to touchdown.  While this technique will result in getting the aircraft on the ground in a hurry it also has some negative consequences.  Before discussing these issues, let us first dispel some of the misconceptions surrounding this issue.

The first misconception is that lowering the nose of the airplane will “raise the landing gear” thus resulting in a smoother landing.  While it is true that the geometry of the airplane does result in the main gear effectively moving up relative to the center of rotation, the effect is not significant.  Obviously the longer the airplane the more pronounced this effect would be, so we can use a large wideoby as an example as a “worst case”.  The main gear on the large widebody  is about 8 feet aft of the center of the CG range.  Using basic trigonometry we find that even an 8º pitch change only results in a net 1 foot movement of the main landing gear.

The second aspect that is sometimes raised is that forward pressure on the control column reduces the total load on the wing as less tail down force is required, resulting in more lift to slow the rate of descent.  While this does have some merit over very small pitch reductions (less than 1º or so of pitch change), it  requires precise timing and the possible negative affects of larger pitch reductions can be catastrophic.  To understand the reasons for this some background understanding is required.

For practical purposes, in stabilized flight all of the forces acting on the airplane are equal.  As long as the aircraft is not accelerating its rate of climb or descent and the wings are level (assuming no turbulence, etc.) the g meter in the airplane will be reading 1.0 g.  This is true whether the airplane is maintaining an altitude or in a constant rate climb or descent.  If the rate of climb or descent is changing then the g meter will be registering a number either above or below 1.0.  If it is in a descent, a reduction in the rate of descent will occur with an increase in g to some value over 1.0, and if the rate of descent in increased the g meter will show some value less than 1.0 as the aircraft temporarily accelerates to its new state of equilibrium.

Aircraft landing gear is designed to absorb load in two parts, which we can call “Phase 1” and “Phase 2”.  Phase 1 would be the force due to the vertical motion of the airplane.  If you do a “carrier landing” the landing gear is effectively stopping your vertical speed.  The wings are supporting all of the weight of the airplane until the nose is lowered, spoilers deploy and the airspeed is reduced.  Instead of this technique, most pilots try to smooth out the landing by increasing the lift a bit just prior to landing, commonly referred to as the “flare”.  If you were to be watching the g meter during the flare you would see it register something above 1.0 as the wings accelerated the airplane upwards (note that an acceleration upwards in this case means actually a reduction in the downward vertical speed and not actually motion away from the ground).   Any remaining vertical speed will be absorbed by the landing gear as part of  Phase 1.

Phase 2 is the weight on the landing gear that is present when the airplane is sitting on the ramp.  The basic force of gravity we all feel.  During landing this portion is not ordinarily a factor until after the aircraft has been derotated and the lift from the wings removed.  The design criteria for the landing gear assumes that Phase 1 and Phase 2 do not happen concurrently – that Phase 2 only occurs after any vertical speed is arrested.  If the nose is lowered prior to touchdown it can result in the wings not supporting the weight of the airplane prior to touchdown, thus adding some portion of the Phase 2 force to the initial touchdown.

Another way to describe this issue is by referencing the g meter.  Recall that the g meter uses 1.0 as the “baseline”.

The total amount of force absorbed by the landing gear can roughly be described as the difference or ∆ between what the g meter is reading just prior to touchdown and what it reads as it touches down.  If you are reading 1.1 just prior to touchdown and you touchdown with a net zero rate of descent the g meter not only will not increase, but will actually move down towards 1.0 as the acceleration stops.  This feels very smooth.  If you read 1.1 and touch down with some remaining vertical speed, the g meter may “spike” upwards a bit to stop the motion.  In a firm landing the g meter might go (for example) from a reading of 1.2 to 1.7, and the net of + 0.5 would be the force you and the landing gear “feels”.

If instead you pitch the aircraft over just prior to touchdown the force the landing gear “feels” is measured from the ∆ (change) of what ever point you started with to what you ended with .  For example, say the “pitch over” results in the g meter registering 0.8 g just prior to touchdown and 1.1 at touchdown.  For practical purposes the landing gear will now absorb 1.4 – 0.7 = 0.7 g.  Although 1.4 g in this case is a lower value than the 1.7 g in the above case, the net g of 0.7 g is higher than the net of 0.5, so the latter landing would feel harder and would be harder on the landing gear.

Weather Avoidance – Transport Aircraft Operations by Shem Malmquist

By Captain Shem Malmquist

While thunderstorms can occur during any part of the year, we all think of Spring as “thunderstorm season”.  With the warming of the weather comes an increase in convective weather. This is an excellent time to review the windshear and radar section of your Company Flight Manuals as well as the company Flight Operations Manual guidance.   More than 80% of the world’s severe storms occur in North America, but there are still plenty of storms in Asia and Europe.  Weather avoidance begins in the preflight planning stages. There are a number of tools available to aid us in our decision making.  What is surprising is how under-utilized some of these tools are.   This article is intended to provide some practical considerations on the tools available for weather avoidance.  Company and government publications are still your primary resource.

In the U.S., if there is significant weather, ATC may implement a Severe Weather Avoidance Plan, or SWAP, which may eliminate some of the planning that we need to do.  Absent a SWAP, we need to look at the big picture and get some preliminary ideas on the best way to minimize the weather encounters we might have in flight.  Our primary assets at this point are satellite, radar and lightning maps.  We can support this further by current weather reports, forecasts and PIREPS. Internationally, the availability of this information varies. We also have our flight dispatcher.  Dispatchers have access to a full array of weather maps, in addition to programs such as Flight Explorer, which depict the aircraft, navaids and a variety of weather products which can be overlaid.  Your dispatcher can help provide a route that will minimize the time you spend enroute avoiding weather.

A good dispatcher will be a great asset to utilize throughout the flight.  Their “big picture” displays, coupled with easy access to meteorologists or other weather data provide them with a lot of information. They can often tell you where the weather is, where it is moving and help you plan a route that will take you around the bulk of the weather.  Additionally, as smart phones and connected devices, such as I-Pads are becoming more common, many weather products are available from the aircraft ramp. The use of these devices after push-back may be limited by your company opspecs.  Regardless of how you accomplish it, it is important to try to get a picture of the weather close in to the airport, as well as along the route of flight.

Taxiing out, the portable electronic devices are generally required to be turned off.  While dispatch is still available, contacting them is a bit more challenging due to the workload.  While monitoring ATC can offer more clues as to what other aircraft are experiencing, we need to be careful to not read too much into that information. History has shown that crews following other aircraft have often been led into the teeth of a developing severe weather situation.  This is not to say that there is no value in evaluating what preceding flights are experiencing, just to emphasize that it needs to be one of many data points, rather than a primary decision factor.  In many International locations, the dispatcher may be your best support. ATC may not be able to provide any information due to lack of equipment and/or language differences, and operations in local languages can pretty much eliminate any information we might gain by monitoring other traffic.

Arguably, our best available tool for real time decision making is our onboard weather radar. Our onboard radar is limited by several factors.  In addition to only displaying weather directly in front of the aircraft, when on the ground, or low altitudes, the radar is effectively limited in its ability to “see” convective weather that is relatively close in.

Our radars are limited to 15 degrees tilt.  One degree of tilt moves the beam approximately 1,000’ every 10 miles.  Using that formula, you can see that if you are 5 miles from the weather, the highest you can “see” above you is 7,500. At 10 miles, just 15,000 feet.  Rain at 7,500 is too low to be able to determine if the weather is convective or not.  It might be heavy rain, but only rain.  It is also possible for light or moderate returns to be displayed while heavy to extreme precipitation is up high, above the tilt range for your radar (more on this later).  Additionally, heavy rain will be attenuate, as the aircraft radar will not penetrate very far into heavy returns.  For this reason, prior to takeoff, if there is any doubt, utilize ATC, dispatchers, visual cues and any other resources you can think of, to the extent possible to provide a complete picture of what you are departing into.

While the capabilities of ATC outside the U.S. is a very mixed bag, ATC in the U.S. has several layers of weather display capabilities, depending on the type of facility.  On departure, the TRACON (Terminal Radar Control) utilizes what is known as Airport Surveillance Radar (ASR).  The displays are near real-time, and most locations are equipped with the more advanced ASR-9 or ASR-11, which can display six levels of precipitation intensities.  Unlike our aircraft radar, ATC radar is not limited by vertical tilt, nor will it fail to display weather due to attenuation.   This means that terminal ATC may have a better picture of the weather close in to your position than we can get with our onboard radar units.

The capability of TRACON weather depiction varies.  In addition to the basic displays described above, 36 locations have additional capability through a system called Weather System Processor (WSP), which provides enhanced weather displays which also can detect the doppler wind shifts present in low altitude windshear.  The ASR display is updated with each sweep of the antennae, so every 4.8 seconds.  Additionally, there are over 40 airports that have Terminal Doppler Weather Radar (TDWR), which provides far more capability in detecting weather. If the facility does have TDWR, that is part of a system called Integrated Terminal Weather System (ITWS).  This system, actually part of the Center Weather Service Unit (more on this below), provides windshear, microburst, gust front and tornado signatures from the TDWR antenna and precipitation information via NEXRAD.  It will display this information for the final approach course, to three miles, and to two miles on the departure course.  Gust front information is shown in a 10 and 20 minute prediction line, with direction and speed. You do not get both. Those terminal locations that have TDWR do not have WSP, and, unfortunately, many locations have neither.

While the TDWR is displayed so the Tower controllers can see it, the TRACON radar scope does not show TDWR displays.  The TDWR display terminal for the TRACON is located only at the supervisor’s station.  The controllers will get an alert for a potential windshear, but the weather displayed on their scope will be limited to the standard ASR-9.   Workload could limit the ability for the supervisor and controller to communicate the particulars of the weather timely enough to be of value to our decision making process.  The time that we may need the most weather information is also when the ATC workload is the highest.   Unfortunately, FAA funding has not been sufficient to allow for the installation of these displays at the controller stations.

Once we are airborne, the dynamics change a bit as onboard radar starts to find some advantages due to our altitude.  When we are above the “low altitude” stuff, we can start to differentiate between what is convective and what is not.  Airborne radar’s inability to “see through” heavy returns is still a disadvantage, however, with that, comes more definition of the outer edges of the cells.

The aircraft radar runs about ten times the frequency of ATC radar, and it is much less powerful (peak outputs in the 25KW range vs. just 150 watts!).  This combination is why our airborne radar attenuates easily.  On the plus side, though, our higher frequency allows the airborne system to “see” smaller water droplets and provide more definition than what ATC can get.  We also can focus it to get an idea of the vertical development of the weather.  ATC radar is a great complement to our limitations, as it can still power through the strongest precipitation without attenuation to see through the other side of the radar echo you’re seeing.  The best combination can be obtained by utilizing ATC’s ground-based radar to supplement our higher definition airborne radar.

We do not need to deviate around heavy rain that is just low altitude, but that weather that extends up into the flight levels is a different story.  Use the radar tilt to determine how high the wet rain is getting.  The higher the rain (wet precipitation) is getting, the stronger the storm.

Changing the tilt 1-degree will move the radar beam 1,000’ at a distance of 10 miles.  Utilizing this formula, we can adjust the radar beam to find the weather at the altitudes that matter. However, before we do that, we need to be sure where the bottom of the beam is.  All of our aircraft have a 30” dish, which results in a beam width of approximately 3.5 degrees.  That means that if we are set on “zero”, the bottom half (1.75˚) of the beam is going to be moving below the aircraft altitude at about 1,750’ for every 10 miles (although this is offset a bit by the curvature of the Earth).  As the calibration can be slightly off, depending on the aircraft and radar unit, in order to determine what we need to avoid, we first need to know exactly where the bottom of that beam really is.

Utilizing the same tilt formula, we can easily determine it by the following fact:  When the bottom of the beam is tilted down at a 10˚ angle, the beam will hit the ground at the same distance in miles as the aircraft height about the ground, divided by one thousand.   For example, if you are at FL350 over terrain that is about 1,000’ MSL, you are about 34,000’ above the ground.  Tilt the radar down until the ground return is at 34 miles, and that is the 10˚ down point for the bottom of the beam.  Very simple and, once you understand it, requires none of the math calculations which might be challenging when you’re getting bashed about in turbulence.  Now, just rotating it up 10˚ will align the bottom of the beam with the aircraft altitude until the curvature of the Earth starts to have significant impact.

We want to look for weather that is extending vertically into the flight levels.  Use the tilt formula, and find the altitude where the bottom of the beam is first hitting the weather.  If you are at FL200 and the bottom of the beam is hitting weather aligned with your altitude, you are probably looking at convective weather.  If you are at 10,000’, you will probably want to rotate it upwards.  Let’s assume the weather is 50 miles in front of you.  If you are hitting weather with the bottom of the beam 2˚ above level, you are seeing something that is extending up to 20,000’.  If you are 30,000’, you will want to tilt it down to ensure that you are capturing the weather.  The reason is that the precipitation at FL300 will likely be frozen, and may not be picked up by your radar.  Momentarily turning your gain off the CAL position might help with this, but frozen precip just does not reflect very well. It is, therefore, very important to remember that just because you are not showing precipitation at your altitude,  does not mean that you do not have severe weather at your altitude.  The rain shaft might stop at FL 250, but the severe weather may extend thousands of feet above that, and just not be reflecting due to it being frozen.

In addition to the problems our aircraft radar has with “close in” weather, due to tilt limitation, we run into another set of problems with weather that is farther out.  The weather intensity the radar displays starts from how much of its beam is reflected back to the unit.  If the beam has widened to the point that a cell is only occupying a portion of the beam size, the energy being reflected back will be less.  Although our radar units have some compensation for this, there are limitations on that.[i]  Furthermore, as our radar is not really designed to penetrate the weather, attenuating fairly easily we only have limited ability to see “through” a storm to the other side, and, as is fairly well known, significant weather can be “hiding” behind the weather we see.  Again, we can gain additional insight into the weather by utilizing ATC, dispatchers or other ground-based sources to put together the “big picture”.

The weather displayed on enroute controllers ARTCC (Air Route Traffic Control Center) displays differs from what the terminal controllers have.  First, all U.S. ARTCC’s have a Center Weather Service Unit (CWSU). This is staffed with a NOAA NWS meteorologist, who can assist ATC with weather issues.  The CWSU provides regular briefings to the ARTCC traffic management unit (TMU) as well as the TRACONs (Terminal Radar Control) within their service area.  The CWSU has the ITWS system, which is monitored by the TMU and, as previously described, the Tower and TRACON.

A limitation, like the terminal controllers, is that during the times when the most deviations are occurring, the workload is very high on the controllers, so they may not have the time to request more detailed assistance.  Offsetting this limitation a bit, the Center weather depiction on the individual controllers scope is more sophisticated, utilizing a Weather and Radar Processor (WARP), which integrates weather data from one or more NEXRAD sites.  It does not display anything less than moderate precipitation.  Additionally, controllers can set their scopes to display weather in three altitude blocks, starting at the surface, starting at FL240 and starting FL 330.  In each case, the display will include all the weather above the floor selected.  By asking ATC what altitude range they are depicting, you can determine if what they are showing is low altitude stuff, or something at your altitude.  You can ask them to toggle to FL240 or 330 to get a better picture of the vertical development, thereby possibility differentiating real cells from just general areas of rain.  ATC radar seems to pick up dry precip better than airborne units also.

ARTCC can provide valuable “big picture” weather information.  The limitation is that the controller may not be able to provide much information beyond the area of their individual display, unlike many dispatcher’s ability to zoom out to your entire route of flight.  The CWSU has that capability, and ATC uses it for traffic management, but obtaining that information for your individual case would be dependent on ATC workload. Still, for that weather that is within the controller’s coverage area, ATC can be easier and faster to communicate with than a dispatcher or other ground-based person, and can provide immediate information on weather directly in front of you, which can be particularly valuable when the airborne radar is attenuated.

Generally, we see attenuation occurring in two scenarios.  One might be when there is heavy rain in front of us, and that is blocking the radar’s “view” of what is on the other side.   This sort of attenuation is important to be aware of as we might discover that the area behind the storm is not a gap like we might have thought.  The other scenario is when we are in the rain itself.  Heavy, low altitude rain over a wide area makes it a lot more challenging to identify any convective activity that might be hidden within it.  The flight manual may provide some guidance for this scenario, recommending a higher tilt, to hopefully, be able to “see” weather that is rising above the low altitude rain, or, to avoid the area that is bowing “inward” toward you, as that might indicate an area of even higher attenuation.  It can be deceptive, with the area with the higher attenuation potentially appearing to be a short cut out of the weather. Southern Airways flight 242 encountered such a trap in 1977, chasing what appeared to be the closest way out of the weather led them directly into the jaws of a level 5 thunderstorm.

At altitude, the airborne radar has an advantage that we can get a lot better definition and may see gaps we can fly through at our altitude – just be cautious to ensure that the gap is not just a dry pocket within a storm.   However, if we are flying along in the precip itself the entire display can attenuate.  In this scenario our radar is likely to just depict precipitation in a solid arc of weather across the display.  It can be just a little better than useless.

If you find yourself in this situation, flight planned to fly at 10-15,000 feet, stuck in the rain, one strategy might be to continue up to the lower flight levels, to a point that you are out of the rain and able to utilize the airborne weather radar.   Another choice, and an equally valid one, would be to request ATC’s assistance in this scenario.  ATC will not be faced with the attenuation issues, and the controller should be able to identify areas of more intense weather.  As previously stated, you might consider asking Center to toggle their display to only depict the weather above FL 240, as anything that was depicting at that altitude is very probably significant convective weather.

A different scenario is a longer flight, where you know that at some distance ahead there is some weather.  For this situation, dispatchers can give you a good idea of how you might plan the route by providing fixes or even lat/lon’s that will take you around the weather with minimal deviations.

When selecting a heading to avoid weather, be cognizant of the wind drift.  A nice advantage for those aircraft that can display the aircraft track is the ability, in track mode, to move the track cursor so the aircraft will actually track around the weather, reducing the amount of deviation corrections that otherwise might be required.

Approaching the terminal area brings about a new set of issues.  Recall the limitations on radar tilt discussed earlier.  On August 2, 1985, Delta 191 approached the DFW area[ii].  The ATIS described the weather as benign, scattered clouds at 6,000’, 10 miles visibility and calm wind.  There were scattered thunderstorms in the area, and the flight made a few diversions inbound.  At 1756 CDT, ATC transmitted that “…there’s a little rainshower just north of the airport…”.   The Delta 191 crew told ATC they were at 5,000 feet at 1800. At 1802 they were 6 miles from the outer marker.  At 1804:18 the first officer stated that there was lightning coming out of the cloud in front of them.  They reached 1,000 AGL at 1805:05, and the aircraft crashed at 1805:58.   During this same time period, NWS radar showed a level 3 cell off the end of the runway at 1756, which had intensified to a level 4 cell by 1804.  But what did the crew see on their radar?

This was a storm that was developing significant weather at higher altitudes, which, once reaching critical mass, would essentially dump the rain downward, along with the significant wind, which would doom this flight.  1800 CDT, and 5,000’, the flight was about 20 miles from the airport.  If, like many crews, they had their radar tilt at around 5˚ nose up, they would be viewing weather at around the 15,000 foot range.  It is probable that this was not high enough to detect the severity of the storm at that time.  Tilting the radar higher would have yielded the storm’s deadly secret of having a lot of moisture up high, and little down below.  This is a recipe for a microburst.  USAir 1016 had a similar set of circumstances, with just a “thin veil of rain” between them and the airport before the downburst of rain began. There are only a few scenarios that would lead to the precipitation being up high and little or nothing lower, among these are developing thunderstorm (microburst “setup”), virga or hail.  None of them are things we want to be flying under!

This scenario is a difficult one, but one that can be somewhat mitigated through proper tilt techniques.  In the approach environment, the closer we get to the weather, the less our ability to “see it” due to the limitations on tilt.   Start scanning the area and look for convective weather before you get into the terminal area.   As you get in closer, supplement the onboard radar with additional queries to ATC.  If you are approaching an area where you are unable to scan above the low altitude environment due to the limitations of tilt and distance, and only depicting light to moderate rain, yet the controller in the terminal environment is showing intense rain, you might be witnessing a Delta 191 scenario.

When asked, most pilots understand what rain aloft but not lower down might indicate, but few have put together that knowledge with how they might utilize the weather radar and ATC to handle the situation.  According to a study conducted by the FAA Lincoln Laboratory, many pilots penetrated thunderstorm cells in the vicinity of airports. The main difference between those flights that had no incidents and those that ended up in accidents appears to just have been chance.  “Until at least one pilot deviates, nearly all pilots stay on course even when strong storms move into the approach path[iii]

ATC weather depiction ability in the U.S. has improved greatly since Delta 191, but one thing that has not changed is the ATC primary function of separating known traffic.   Telling us about the weather is not ATC’s primary responsibility, but controllers do want to help where they can.  Many controllers are not aware of the limitations of our radar displays, and they are also limited by their equipment and FAA legal has placed additional limitations on what they are, and are not, allowed to tell us.  With this in mind, we need to have a solid understanding of what they can depict, and directly request the information that we need.

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[i] For more information, see your aircraft flight manual.

[ii] http://www.airdisaster.com/reports/ntsb/AAR86-05.pdf

[iii] An Assessment of Thunderstorm Penetrations and Deviations by Commercial Aircraft in the Terminal Area http://www.ll.mit.edu/mission/aviation/publications/publication-files/nasa-reports/Rhoda_1999_NASA-A2_WW-10087.pdf

Aircraft Pilot Coupling (APC) PIO Transport Aircraft by Shem Malmquist

Written by Shem Malmquist, Captain MD-11

Most pilots have heard the acronym “PIO” (pilot-induced-oscillation) at some point in their careers, and most assume it is related to poor piloting skills. Recognition and recovery from PIO is not even taught in the airline world. This is coupled with a lack of understanding of the topic by many accident investigators. In fact, outside of the aircraft certification community, there is a general lack of appreciation for these issues.Recognizing this problem, the National Research Council established a committee to study the topic. The committee included participants from industry and government. Members included all the major manufacturers of transport aircraft, flight test pilots from the military, FAA and manufacturers, engineers and ALPA representatives. The final product was a publication titled “Aviation Safety and Pilot Control – Understanding and Preventing Unfavorable Pilot-Vehicle Interactions”. The lack of attention to the topic was noted by the National Research Council (NRC): The committee was disturbed by the lack of awareness of severe APC (PIO) events among pilots, engineers, regulatory authorities and accident investigators.i Traditionally, the letters P.I.O. have been taken to mean “Pilot-Induced-Oscillation”. Today there has been some movement towards redefining PIO to move away from the traditional “blame the pilot” mindset. In that vein, PIO came to be defined as Pilot-Involved-Oscillation. The USAF Fight Test Center prefers the term as “pilot-in-the-loop oscillation”, and many in the field have replaced the letters PIO completely by a new acronym, “APC”, or Aircraft-Pilot-Coupling, although APC can refer to either an open or closed loop event.1 The NRC states: “Aircraft-pilot-coupling (APC) events” are inadvertent, unwanted aircraft attitude and flight path motions that originate in an anomalous interaction between the aircraft and the pilot.ii Pilots need to understand what APC is, what factors can lead to it, and how to recover from it. The importance of reporting APC events cannot be overstated. Current data recorders lack the sampling rates to adequately ascertain that a APC has occurred and accident investigators are often not familiar with the dynamics. As a result, some recent APC events were erroneously blamed on the pilot. On July 31, 1997, an MD-11 crashed on landing at the Newark International Airport. The NTSB stated that the probable cause was the “captain’s overcontrol of the airplane during the landing and his failure to execute a go-around from a destabilized flare.” In the analysis section of the accident report was the following statement: The captain’s large and rapid elevator control reversals, which resulted in an increasing divergence above and below the target pitch attitude, were consistent with a “classic” pilot-induced oscillation (PIO). Essentially, the captain made each increasingly larger elevator input in an attempt to compensate for the input he had made in the opposite direction about 1 second earlier. PIO in the pitch axis can occur when pilots make large, rapid control inputs in an attempt to quickly achieve desired pitch attitude changes. The airplane reacts to each large pitch control input, but by the time the pilot recognizes this and removes the input, it is too late to avoid an overshoot of the pilot’s pitch target. This, in turn, signals the pilot to reverse and enlarge the control input, and a PIO with increasing divergence may result.iii Similarly, the Japan Accident Investigation Board (AIB) faulted the Captain of JAL 706 for another MD-11 accident in which PIO occurred – this one descending through approximately 17,000 feet. The AIB faulted the Captain despite its own analysis that clearly demonstrated problems in the aircraft handling qualities. These two events highlight the need to educate accident investigators about APC. While the NTSB and AIB analysis did cite PIO as a factor in the above accidents, they both concluded with a probable cause which was directly contrary to studies on the topic by NRC researchers: It is often possible, after the fact, to carefully analyze an event and identify a sequence of actions that the pilot could have taken to overcome the aircraft design deficiencies. However, it is typically not feasible for the pilot to identify and execute the required actions in real time.iv This sentiment is also echoed in a paper written by USAF Test Pilot School flight test engineers: Pilots must be in the loop for a PIO to occur, but pilots do not induce these unwanted oscillations. If anything, it is the airplane that induces them. This is easily shown by noting that the same pilot, flying two different airplanes in the same manner may experience many PIOs in one but never experience PIO in the other.v  Factors related to APC Aircraft are designed to be inherently stable, however, there are limits to how stable (or unstable) an aircraft can be and still be controllable. If an aircraft were too stable it would not be controllable due to lack of response (a problem in some pre-Wright brother’s designs), and if too unstable, it would be difficult to control due to the extremely high workloads involved in maintaining the desired flight path. Between those two extremes, aircraft are fairly stable yet do have several dynamic modes, some of which are oscillatory. Oscillations in pitch can be simplified for analysis to two separate and independent motions, a short period and a long period (also known as a phugoid).vi The short period essentially assumes no change in the path2 of the aircraft, and is the pitch change due to the aircraft’s natural stability. The longer phugoid period involves change to the actual aircraft path, and, being a longer and relatively slower oscillation, is not a problem for the pilot to control. Not necessarily so with the short period. The short period is normally a fairly fast cycle, where the pilot will see the aircraft pitch changing to correct itself as soon as what ever caused the deviation is removed. If the cycle is a bit slow, the pilot is apt to intervene to correct the pitch to the desired attitude. A problem could occur if the pilot does this just as the natural stability is restoring the aircraft attitude on its own. The combination of the aircraft pitching on its own coupled with the pilots input can lead to a very fast “correction” and a resulting “overshoot” of the desired attitude, with a repeat of the process in the reverse direction. The result is an oscillation, with the pilot’s attempts to arrest the pitch changes actually contributing to them. A PIO occurs when a pilots inputs combine with the natural frequency of the aircraft’s motion such that the pilot inputs sustain and/or perhaps even increase the amplitude of the motion. A PIO can occur in any axis, although pilots are most familiar with oscillation in pitch and roll. A faster natural cycle is preferable as the pilot can predict the changes more easily and is thus less likely to become part of the oscillatory dynamic. The oscillation frequency itself is influenced by several factors, including (but not limited to): • The basic stability of the aircraft (more stable, faster rate). • The amount of damping in the design, (the larger the horizontal stabilizer and/or the more moment arm, the more damping, (think of a paddle at the end of a broomstick, the longer the handle or the larger the paddle the more it wants to align with the wind). • Moment of Inertia, (the amount of mass that is forward or aft of the wings (or, in the case of roll, way from the longitudinal axis), which means more inertia for the natural stability to counter once the oscillation starts – more moment of inertia nets a slower rate). • The altitude (high altitude thinner air damps less so slower rate). • Advanced flight control inputs, (Fly-By-Wire (FBW), Stability Augmentation Systems (SAS). Some of the above are fairly intuitive while others are not. Stretching an airplane has a well known (and accounted for) effect on the moment of inertia (even if the CG is the same). The same physics hold true for cargo load distribution on a freighter aircraft. Unfortunately, current regulatory and flight test requirements for cargo consider only the CG and structural limit weights of the cargo. Although the moment of inertia is far greater with a full cargo load distributed as it would be for line operations, flight testing is not required for this condition. The same CG does not necessarily mean that the short period oscillation rate is the same. More research should be conducted on the handing qualities of freighter aircraft and certification standards should be modified accordingly. FBW and SAS has allowed for aircraft designs where the natural stability of the aircraft is less of a factor. The systems can, and do, mask the natural dynamics of the aircraft such that in some designs the control of the aircraft can require a different thought process than in conventional aircraft. Finally, it is important to remember that high altitude tends exacerbate any unfavorable handling characteristics due to less natural aerodynamic damping. PIO Triggers There are many ways to manipulate the controls and still perform with the certification standards required to pass checkrides. Some pilots fly by making small inputs, predicting what is needed (known as “low-gain”), while at the other end of the spectrum we have pilots who tend to use relatively large control inputs to accomplish the same task (high-gain). While this is, to some extent, attributable to pilot experience and technique, there are events that can drive a “low gain pilot” towards the “high gain” side, and high-gain can precipitate APC. Pilots are incredibly adaptable to the way an aircraft handles. Pilot ability to compensate for unfavorable handling qualities is a large part of what keeps flying as safe as it is. Part of the key to that is pilot experience with the feel of a wide variety of aircraft. As pilots gain experience in a particular aircraft they are also able to adapt and “work around” problem areas that might be peculiar to a particular aircraft. In terms of handling qualities, this obviously requires the pilot to spend some time hand-flying the actual aircraft. While this is does not excuse the manufacturers from designing aircraft that handle well, it could make the difference should a pilot find his or herself in an unexpected situation requiring superior flying skills. Being proficient in handling the aircraft makes it less likely that a pilot will experience PIO to begin with. However, not all airline pilots spend a lot of time hand-flying. In fact, hand flying has been discouraged by many in the industry. Autopilots can fly the aircraft more precisely, leading to fuel saving and greater passenger comfort, plus the design of modern cockpits is such that hand-flying can lead to unacceptably high workloads. Airlines train pilots to fly using the autopilot with less emphasis on hand-flying. Consequently, the first time some line pilots spend more than a few moments hand flying their aircraft at altitude could be after a malfunction disengages the autopilot. The surprise of being presented with an aircraft that is not flying the way one expects also contributes to the pilot response. A “startled” pilot is more likely to fly with “high gain”. That adrenalin surge is not always helpful. The “startle factor” is actually a term used in the test flight community. It can be due to a system failure, ice accumulation, TCAS RA or an unexpected mode change in the flight control system. For example, a pilot is flying in cruise when the autopilot disconnects due to an out of trim condition or similar anomaly. The pilot now tries to bring the aircraft back to the assigned cruise altitude with a large control input. The sudden necessity for the pilot to intervene often results in a much bigger correction than needed, and if conditions are right, a PIO may result. Another factor is the task. Higher precision tasks, such as landing, tend to require higher gain. A sudden shift from a low precision task to a high precision task can lead to the pilot gain increasing suddenly. TCAS RA is an example of this as the TCAS may direct the pilot to fly within a relatively narrow vertical speed window. Response to a TCAS RA could force a pilot to make rapid pitch changes to hold the aircraft within a very narrow vertical speed. A TCAS RA therefore includes facets of the “startle-factor” as well as a shift in task precision. This in turn can lead to a sudden onset of PIO in aircraft that are susceptible. Similarly, turbulence at high altitude has also been linked to several severe PIO events in transport aircraft.vii Mixed-mode control, where an aircraft is flown using partial automation (such as hand-flying with autothrottles engaged) has been identified as a PIO trigger.viii Unlike older systems, where the vertical path is controlled entirely with pitch, and airspeed controlled separately by power, advanced flight control systems are designed such that the automatic system has control of the entire flight regime – which includes interrelating the pitch and power inputs for the vertical mode much similar to what a pilot does when hand-flying. When flying the aircraft entirely manually, the pilot will naturally add power immediately as pitch is increased and vice versa. When both are engaged, the autopilot and autothrottles on the newer aircraft work together in a similar manner, as the flight control system “knows” what each component is doing. Clearly this is not possible if using only part of the system. When hand-flying with authothrottles on, the autothrottles will lag behind manual changes in pitch, attempting to hold the selected airspeed. In order to maintain the desired vertical path the pilot needs to augment the pitch inputs to make up for the lag of the throttle response. The net result is the pilot is essentially flying with higher gain, with associated higher risk of APC. The more complex flight control systems in modern transports can create a mismatch between what the pilot is doing with the controls and how they are actually responding, particularly when the stability augmentation or flight control systems inputs combine with pilot inputs to the same control surfaces. The NRC stated: The pilot, unaware that the systems are operating at their limits, may call for a greater response from the control surface than is allowed by the system’s rate or position limits for that (control surface).ix Accident investigators should be aware that flight control surfaces are not always moving the way the pilot is commanding, and in many modern aircraft the pilot has no feedback to that effect. If there is a bit of a delay in the control response (rate limiting), this may not be apparent when the pilot is making small, smooth (low gain) inputs, but may become very evident during abrupt (high gain) inputs. This could result in a handling qualities “cliff”, where the pilot suddenly encounters PIO. In certain cases it is very difficult to reduce the control inputs (gain) once the PIO is encountered. For example, if PIO is encountered in landing, the pilots natural reaction is to work harder to get the situation under control, possibly exacerbating the situation. As aircraft flight control design increases in complexity we could see more PIOs during line operations. Flying qualities requirements for transport aircraft are not well defined in the FARs or JARs. More comprehensive military specifications are available, but they are more fighter oriented and the problem has been further complicated by newer flight control systems (FBW, SAS) which have made existing handling qualities criteria obsolete.x It is interesting to note that every FBW aircraft has, at one time or another, experienced PIO. System failures are another trigger for PIO. On November 25, 2000, an MD-11 experienced a PIO event while climbing out of Newark, New Jersey. The first officer was hand flying and at FL260 the aircraft began to pitch up and down at a fast rate. The PIO continued until the autopilot was engaged. Post event analysis found a fault in one of the electronically controlled hydraulic actuators that essentially gave the stability augmentation system (SAS) more authority than it was designed to have, leading to the oscillation. The autopilot had no problem flying the aircraft as the flight control computer had full control of all the inputs. The discussion of failures listed as PIO triggers by the National Research Council provide further insight: System failures can alter the effective aircraft dynamics either by changing the aircraft’s response to control inputs or by changing the feedback with the pilot. Control system failures, such as failures in the hydraulic system, actuator failures, or uncontrolled changes in aircraft trim, may significantly compromise the controllability of the aircraft. Intermittent control system failures can result in highly nonlinear or discontinuous control responses that act as potential triggering events.  Sensor and display failures that alter the feedback dynamics to the pilot or control system are also potential triggers. Even a simple mechanical failures, such as a loose pilot’s seat, can alter the acceleration feedback the pilot receives and has been observed to trigger an APC event.xi Complexity in the automation is another area of concern. If the automation is extremely complex a pilot may not be able to fully understand the nuances of the system, and may develop an ad hoc understanding based on normal operations. This can lead to incorrect response in an unusual or emergency situation.xii Flight Test The design engineers do attempt to eliminate any unfavorable handling qualities during the design phase without sacrificing other design requirements, such as maneuverability. After the prototype is built, much of the task of rooting out any handling problems is transferred to the flight test pilots and flight test engineers. Often times, problems in aircraft handling qualities do not manifest themselves until very late in the certification process, as noted by one researcher: On most cases of PIO experienced in the past, the problems were discovered in a relatively late stage of development, or even, during routine operation.xiii There have been several instances in flight test where the first several test pilots did not experience any problems, but a subsequent test pilot did experience PIO. This illustrates the need to have as many pilots as possible involved in the test process. The situation is somewhat analogous to new software on a home PC (and, indeed, in newer aircraft the handling qualities are literally new software!). The test program could be considered the “beta test” phase, where a relatively small group of users “test” a new program to work out any “bugs”. Even with a large pool of beta testers (a much larger group than test fly new airplanes), new software problems are virtually always found once the software is released to the public. The larger numbers of users leads to more variation thus exposing “bugs”. Similarly, when an aircraft is flown by a large number of line pilots, each with different handling techniques and skill levels, any residual problems in handling are likely to manifest themselves. Another potential problem area occurs when flight computer software code is modified to address a specific issue. In many cases, the aircraft is only tested for that flight specific task or flight regime without regard to the possibility that the particular modified computer code might affect other flight regimes as well. Ideally, full flight regime testing should be used after any change to control law, and operators should ensure that the line pilots receive adequate training in the changes to the control laws. Finally, sometimes problems have been dismissed with statements such as “pilots wouldn’t fly like that”. This is a convenient “out” for schedule-driven engineering managers, but one that clearly not conducive to ensuring good handling qualities.xiv Exiting PIO First, it should be recognized that by definition, APC/PIO cannot happen unless the pilot is making inputs that are sustaining the maneuver, i.e., the pilot is in the loop. Consequently, the first step is to get out of the loop. This presents three primary possibilities: • The pilot freezes the controls. • The pilot releases the controls. • The pilot significantly reduces the aggressiveness of control.xv Because a pilot may be highly focused on a task when PIO develops, it is important that the pilot-not-flying assist the pilot-flying in recognizing the situation. It may take forceful intervention to get the pilot to reduce his gain, freeze the controls or, in particular, release the controls altogether. If near the ground, this means a go-around is likely the only option, as was used by a NWA A-320 crew when they experienced a lateral PIO in April, 1995.3 Reducing aggressiveness of control is a lot easier said than done. Reducing the size of the inputs is tough, even for experienced test pilots. A few can increase their gain when asked, but it is rare that a pilot, once in a “high-gain” situation can choose to reduce it.xvi It also may be possible to exit the PIO by engaging the autopilot, but this may not be a realistic option in many circumstances and would depend on the design of the particular flight control system. There are many that argue that an APC/PIO represents a loss of control, as the situation is out of control until the pilot takes action that is probably not conducive to completing the task when the PIO began. It would then be logical to conclude that if a pilot has to abandon the task (if only temporarily) to maintain control, that the aircraft was out of control previous to that abandonment. While in most cases this type of loss of control may not lead to a crash, incidents of APC should be categorized as loss of control events by safety and regulatory organizations. Reporting of Events An order of magnitude increase in the data rates utilized for accident investigations is required to establish that APC is, or is not, a primary causal factor.xvii Recognition of APC by accident investigators has been problematic not only due to lack of training but also a lack of data. Unfortunately, many current FDRs lack the parameters to spot a PIO, such as control column position, and even the new generation FDRs that do include those parameters do not have adequate sampling rates to ascertain whether a PIO has occurred or not. Many parameters are currently only recorded once per second, or 1 hertz. Current data recording ability would easily accommodate at least 20 hertz, which is adequate to identify PIO. The current lack of data recording capability adequate to identify APC makes it all the more important that pilots report APC should it be encountered. Pilots should be trained to identify APC. The NRC notes:  Operational line pilots have little or no exposure to APC potential and are not trained to recognize the initial symptoms or to understand that APC does not imply poor airmanship. This may limit reporting of APC events.xviii If a PIO is ever experienced it is vital that it be reported and not dismissed as the fault of a new pilot. Pilots tend to blame themselves rather than realize that the PIO may be indicative of a fundamental problem in the aircraft itself. It is a recognized problem that APC events are under-reported, and without such reporting it is difficult for manufacturers and regulators to fix real problems. Systems such as ASRS and FOQA should be utilized to identify and report APC events. All too often, pilots only submit an ASRS if they are worried about a violation, however, pilots should utilize ASRS any time that they experience something that does not seem quite right, even if they do not feel that the flight was in jeopardy. 1 “Closed Loop” in this context refers to an event where the pilot’s actions act to continue the unwanted behavior, while “Open Loop” refers to an event that does not continue in a repetitive cycle. PIO is, therefore, a subset of APC that is a closed-loop, where pilot feedback into the system is an essential component. 2 The Center of Gravity continues along a straight path. 3 An interesting footnote to that incident was that the manufacturer had issued a temporary flight bulletin to the operating manual, but it was not implemented – nor was it required to be. i National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997) Page 3. ii National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997) Page 14. iii NTSB DCA97MA055 iv National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997) p.2. v Twisdale, Thomas R. and Nelson, Michael K. (1999). A method for the Flight Test Evaluation of PIO Susceptibility, (NASA Dryden PIO Workshop, 1999) p.2. vi Carpenter, Chris. Flightwise – Stability and Control. (Shrewsbury, England: Airlife Publishing Ltd. 1997) p.113. vii National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997), Page 50. viii National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997), p.53. ix National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997)Page 15. x Rossitto, Ken F. and Field, Edmund J., Boeing Phantom Works, Criteria for Category I PIOs of Transports Based on Equivalent Systems and Bandwidth. (NASA Dryden PIO Workshop, 1999). xi National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997), p.52. xii National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997), p.55. xiii Weerd, Rogier van der, Delft University of Technology/Aerospace Engineering, The Prediction and Suppression of PIO Susceptibility of Large Transport Aircraft. (NASA Dryden PIO Workshop, 1999). P.189. xiv National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997),, p.111. xv Twisdale, Thomas R. and Nelson, Michael K. (1999). Pilot Opinion Ratings and PIO, (NASA Dryden PIO Workshop, 1999) p.2. xvi Lee, B.P. Airplane Handling Qualities, Boeing Commercial Airplane Group. PIO Flight Test Experience at Boeing (Puget Sound) – and the need for more research. (NASA Dryden PIO Workshop, 1999). P.152. xvii A’Harrah, Ralph A. and Kaseote, George, A Case for Higher Data Rates, (1999, NASA hq, FAA hq). xviii National Research Council. Aviation Safety and Pilot Control (Washington, D.C.: National Academy Press, 1997), p.163.

Phraseology in International Air Transport Operations by Shem Malmquist

Avoid the use of Slang

by Captain Shem Malmquist

There is a precaution in the MD-11 CFM that pilots should “avoid the use of slang” in international communications.  As the many pilots will fly some international trips during their career, it is worthwhile to explore what that statement really means.  The challenges of international flying include more complicated IFR procedures, country differences, lack of radar coverage, and generally less ATC “support” than domestic pilots are used to.  All of these issues are often complicated by the limited English abilities of controllers (and pilots) who are from countries where English is not the native tongue.  Even in those countries where English is spoken, their version of English is often dramatically different than what Americans are used to.

In all countries, Air Traffic Control English is a language unto itself, where precise meanings of words are defined within the ATC framework to avoid ambiguity and general miscommunication.  Therefore, safety dictates that pilots stick to correct phraseology only, without adding extraneous verbiage.  Additionally, using non-ICAO standard phraseology in the international arena does little for our professional image.

Pilots and controllers who speak only limited English are often only familiar with the English words as defined in PANS-OPS.  As they are limited to these words and meanings, it is very easy for us to confuse them by using other words that are not defined, or using terms they are familiar with in a different context.  An example from the world of computers are the words “bug” and “virus”.  All computer people know the definition of these words as they pertain to computers, but most non-English speakers have no idea that there are other definitions of these words.

The phraseology used by U.S. ATC is unique to the U.S.  The rest of the world is actually more or less standardized to the ICAO meanings and phrases.  For example, such terms as “level”, which is taken to mean the same as “maintaining an altitude” in the U.S., is, instead, used as a shortened version of Flight Level in most of the world outside the U.S.  A Chinese controller will likely only understand the word “level” in the context of “flight level”, and have no concept that the word also means that you are flying at a constant altitude.   A clear example of miscommunication was heard in an ATC response to a pilot saying “level 4000”.  The controller replied “roger, QNH 1010, transition level FL90”.  The controller assumed that the crew had not switched over to the local altimeter setting, so was actually reporting “maintaining flight level 40”, an incorrect altitude reference when below the transition level.

In another example, Air Traffic Controllers will know what the symbol “O” is, and that the phonetic of that symbol is “Oscar”, yet they may not have any idea what you mean if you say “Oh” when you mean to say “Zero”.  Similarly, they are not taught what “fifty-one” means, but they will certainly understand “five-one”.  The digits zero through nine are in the PANS-OPS – higher numbers are not.  Although some foreign controllers do know more than this basic English, using terms other than these is just inviting confusion.

Here are a few more examples:

“Point”.  The word is simply not defined in PANS-OPS.  Along with this, “two four point seven” is not a flight level or altitude.  There is a flight level “two four seven”.  The use of the word “point” has no place in international communication.

“Looking for lower”.  Should net an interesting response in some of the back-water places we fly to.  Correct term is “Request descent”.

“With you”.  This phrase has no place in international or domestic operations, as many controllers will tell you.

“Level”.  As discussed above this is a standard shortened version of the term “flight level”.  If you want to tell them you are at an altitude, use the word “maintaining”, or just omit it altogether.

Attempt to avoid extra verbiage in general.  It may be nice to say “good morning” or “any chance of” and the like, but in using those terms you are assuming that the controller (or ground maintenance on the headphones) understands more than basic ICAO English.  Be careful with the terms “to” and “for” in the context of altitudes.   While these terms are defined in the PANS-OPS, they contain obvious ambiguity.  Omitting them might be worthwhile if there is any doubt, i.e.  “4000 climbing 5000” or “FL145 descending FL80”.

While it would be nice if all non-U.S. controllers were truly fluent in English, the fact is that most are not.  It is not unreasonable for them to expect us to stick to the narrow scope of English that is in the official publications. Instead of being frustrated that most of the world’s controllers do not really know English, just be thankful that the French did not get their way on the choice of languages for aviation!

Those pilots who fly international should read and be familiar with the ATC phraseology section of the PANS-OPS, which is contained in our route manuals.  Those new to international can also learn by listening to the radio techniques of more experienced pilots, whether they be from our own company or airlines such as Cathay Pacific or British Air.  If you have trouble reverting from ICAO to domestic phraseology, keep in mind that a U.S. controller will always understand what you mean when you use an ICAO term, such as “decimal”, but the converse is not true.

Radio technique may seem a small thing, but experienced pilots use correct radio technique because it increases the safety of the operation as well as preventing a great deal of unnecessary frustration.

Landing Kinematics by Shem Malmquist

Pilots flying large aircraft need to be very cautious when making pitch changes near to the ground.  A review of flight data shows that pilots sometimes make forward (negative) pitch changes or rapid large aft (positive) pitch changes just prior to touchdown.  In both cases, the pilots are probably doing this in an effort to achieve a smoother landing or possibly “save” a landing when the descent rates appear to be too high.

Conventional wisdom suggests that because the the CG is ahead of the main gear, the main landing gear will move upwards as the nose moves down when pitching forward; essentially rotating around the CG. This theory does not hold up to analysis. An 8 degree pitch change will only move the main landing gear about 1 foot on the largest aircraft – hardly enough to affect the outcome of the landing.  When this fact is illustrated to pilots, many will state “but I have done it and it works for me!” A review of aircraft dynamics reveals the answer to why some say it works, and, more importantly, why it is a very risky technique.

At the other side of the spectrum is the scenario where the pilot attempts to “save” a landing at the last moment utilizing a pitch change (upwards) as the primary mechanism.  This also can create significant problems in the landing environment.

Pitch Transient

 Pushing the controls forward moves the elevator surface downward, increasing the lift at the horizontal stabilizer.  This increased lift at the tail, raises the tail and moves the nose down, in the direction of the control input.  The opposite is, of course, true as well.  Pulling back creates a downward aerodynamic force at the tail, raising the nose.

Both of these are effected by something formally known as a “pitch transient”.  Here is how it works:

When a pilot pushes forward, the elevator moves down and increases the lift force at the tail. It takes a moment for this force to overcome the aircraft’s inertia, and to actually pitch the aircraft downwards, reducing the angle of attack. The reduced angle of attack causes a change in  aircraft lift, and it takes yet another moment for the actual aircraft motion to change as a result of that change of aircraft lift. During this time, the tail lift force is still acting on the aircraft, and is adding to the total lift force the aircraft is producing.  As the weight does not change, for a very shorttime, the aircraft will have more lift than weight, changing the flight path accordingly. The aircraft will initially move up slightly (or reduce its descent rate).  With very precise timing it is possible to get a smooth landing this way.  Why is it so risky?

The pitch transient only lasts long enough for the pitch to actually change.  The pitch transient is the initial reaction to the change in effective weight that happens before the angle of attack has actually changed as a result of the pitch control input.

The problem is that it is difficult to judge the exact height above the ground in a large aircraft. If a pilot misjudges height even slightly, it can be dangerous.

The larger effect of pushing forward on the controls is that the aircraft will lower its pitch attitude, and along with it, the angle of attack of the wing will decrease. The loss of lift will result in the aircraft now moving in the direction of the control input.  Once the aircraft pitch actually starts to decrease, the loss of lift on the wings will contribute to an acceleration downward, as the wings are now producing less lift than the aircraft weighs.

Landing gear is certified based on the assumption that the wings are producing at least as much lift as the aircraft weighs. The engineers assume you will be flaring, or at the very least, holding the pitch constant.  Even a constant pitch attitude results in the angle of attack slightly increasing as the aircraft enters ground effect, softening the landing.  Flaring adds a bit more lift, further softening it.

Reducing angle of attack has the opposite effect. If the wings are producing less lift than weight the aircraft willaccelerate downwards.  Landing with a reducing angle of attack is  outside of the engineering assumptions, and it does not take a lot of downward acceleration before the ultimate design limits are exceeded. This can this bruise your ego; it can also break the airplane

The pitch transient effect works the other way too.  If a pilot pulls back on the controls, the elevator deflection creates a “down-force” which will pitch the aircraft upwards.  However, before the pitch change has occurred, the down-force from the elevator deflection is added to the total weight of the aircraft, and, for a short time, the aircraft will actually accelerate downwards before the increase in pitch results in more lift to start the acceleration upwards.

The magnitude of these pitch transients vary with several factors.  These include, but are not limited to, the design (plan form) of the aircraft, the amount of wing sweep, the wing loading and how fast you move the flight controls.

When flying more than a few feet off the ground, this effect is virtually undetectable from inside the aircraft.  Even a hard pull on the controls is unlikely to move the aircraft downward enough to see on the altimeter before the aircraft starts up or vice versa.  However, near the ground it’s a different story.  For example, an attempt to “save” a landing by pulling back just before touchdown can create more downward acceleration which in a relatively small aircraft could, under some circumstances, be visible due to the height change.  If the pilot is relatively close to the center of gravity, they will likely perceive this downward acceleration and react to it.  As the perception (correctly) is that the change of pitch did not accomplish the goal of arresting the descent, the likely response would be to add power.  The pilot does not need to understand the dynamics involved to make the correct response if they can readily perceive it through outside cues.

However, in large aircraft it is more complex. The motion of any aircraft occurs through, and rotates about, the center of gravity.  Because the pilots are seated at the far end of the aircraft, as the aircraft rotates around the center of gravity, the pilots can be moving relatively upwards even while the CG is moving down and vice versa.  This is true of any aircraft, but the larger the aircraft the more pronounced this effect will be.  It can lead to a significant mismatch between what the pilots are perceiving verses what the airplane’s motion is actually doing, particularly when making rapid pitch changes near to the ground.

Changes in the flight deck height are very easy to see when looking out the window.  If the flight deck is moving up while the aircraft CG (and landing gear) is moving down, and the flight deck is moving down while the CG/gear is moving up, it is not hard to see that a pilot could get in trouble really fast.

Consider this scenario: A pilot pulls back hard in a last moment attempt to “save” a landing.  This action accelerates the aircraft downwards at the CG. Simultaneously, the aircraft rotates to pitch up, so the pilot’s eye-height levels off, or at least slows in its rate of descent.  The pilot perceives that the landing is “saved”, when in reality it is not. The pilot is likely to be very startled when the subsequent landing is very firm, and that can lead to additional inappropriate control inputs.  If the pilot continues to use pitch to compensate for their perceptions of what the aircraft is doing, the pilot can get more and more disconnected with what the aircraft is doing, with catastrophic results.

Pilots can reduce these risks by establishing a safe landing attitude and then holding that attitude through touchdown. Then they should momentarily maintain the pitch attitude on landing, until positive they are on the ground. They should derotate the aircraft at a rate no greater than the rate used on rotation at takeoff.

So what should the pilot do if they encounter a last moment downdraft or some other factor leads to a rapid “sinker” just prior to landing?  If pitch is maintained at a constant, then the only other input that can control the vertical path is power.  Pilots should be ready to add power as required rather than attempt to make rapid pitch changes near the ground.  In severe cases, a go-around is the only viable option.