Weather Depiction
By Captain Shem Malmquist
“Flight 1210, I am currently
depicting an area of extreme precipitation from your 10 o’clock to 2 o’clock
approximately 40 miles in front of you”.
We were enroute from TPA to MEM at FL380. Our radar was set to about 2 degrees down,
and we were showing absolutely nothing
on the radar. Suspecting snow, I turned
the radar’s gain all the way to maximum, and at that setting, a few scattered
green (lowest level of intensity) very small dots (more like pin-points)
appeared in the general area where the controller said he was depicting
weather. Why such a difference? When pilots ask for more information, we are
often met with a response from ATC with words similar to “Your weather
depiction on your radar is better than mine”.
The statement highlights the following fact: Many pilots and many
controllers do not fully understand each other’s capabilities.
So, who does have the better
display? The answer is “it depends”.
First, a caveat. Aircraft have a wide variety of equipment. Many
non-airline aircraft have various uplinked weather products. Few (if any) U.S. airline aircraft have more
than airborne weather radar. As this
article concerns U.S. air carriers, we will not discuss those tools that may be
available to corporate and general aviation aircraft.
Airline aircraft use x-band
radar, which is very good for getting sharp definition of wet rain, but
attenuates very easily and is virtually unable to get a return on a dry
particle (snow or hail). It is also low
power. Unlike the radar that ATC uses,
which has a peak power output in the 25kw range, the modern airborne weather
radar units only have a maximum output
of 150 watts. Yes, you read that right.
They put out less energy than some of the light bulbs likely installed
in your house! They make up for that by
having very sensitive receiving antennae and good processing computers. Still, that small amount of power only can go
so far – is it any surprise that it cannot pass very far through a band of
weather?
This capability is even
worse when we are flying in the precipitation itself. The rain coats the radome and greatly reduces
the ability for the airborne weather radar to “see” what is happening. Couple that with the attenuation once the
beam leaves the aircraft and it is attempting to pass through a wall of
water. Flying in the precipitation,
sometimes all we see is an arc of red extending only a few miles in front of
the aircraft. During these times, what
is interesting, is that the most intense
storms will actually attenuate the radar even more, and depict as a thinner
spot, with a “bow” inward in the depicted arc.
If the pilot is not sharp, what it looks like is that the fastest way
out of the weather is to turn towards that bowed in area. On April 4, 1977, Southern Airways 242, a
DC-9, was sucked into such a trap.
Attempting to find a way through severe weather, they turned towards
that bow like a moth to a light. They flew into the jaws of a Level 5
thunderstorm, crashing a short time later after both engines flamed out in the
heavy rain and hail.
Contrast this against the
WARP (Weather Radar and Processing Unit) on the ARTCC displays. The higher power, multiple sites and
integrated NEXRAD entirely eliminate the attenuation issue for all practical
purposes. Although the display can be a few
minutes old due to the processing time, it is very valid information. Further, the display can be selected to
change the base altitude from the surface, FL240 or FL 330. This is important. Storms with vertical development can be
analyzed this way to get some idea of what is being looked at. Further, remember my story earlier about not
depicting anything while ATC was showing an area of intense precipitation? I asked the controller what altitudes he had
selected. He was showing weather FL 330
and above. Most precipitation at that
altitude will be frozen. So, why could I
not see it on the airborne unit? The
reason is that it was snow. The ARTCC
display shows dry precipitation.
Airborne radar does not. This
means that a controller can potentially know where the thunderstorms really
are. If there is a wide area of rain,
with some thunderstorms popping out of it, by displaying the higher altitudes,
the actual individual storms can be depicted.
As most of the precipitation at that altitude is likely snow, the
airborne system is very limited in what it will display.
So, when is the airborne
system better? Well, if we are out in
the clear, we can tilt to remove the lower altitude stuff and differentiate
that way, looking at the FL250 range to try to find the part of the storm that
is not just frozen precipitation. We can also adjust the beam tilt to show the
weather that is at our altitude, and not below it. The other advantage is much higher
definition, which, if we are clear of the storm, allows us to come much closer
to the edge of the weather or thread through and area of storms.
In the end, both ATC and
airborne systems have their advantages.
By letting airline crews know the altitudes and intensities you are
depicting the weather, the crew will be in a lot better position when making
safety critical decisions in the enroute environment.
A similar difference exists
in the terminal environment, but for different reasons. Not only is the airborne radar system subject
to attenuation, but it is also limited by tilt.
Airborne radar on airline aircraft send out a beam that is approximately
3.5 degrees wide. It sweeps to the left
and right, usually about 45 degrees to each side. The pilot can adjust the tilt of the beam from
15 degrees down to 15 degrees up. Using
trigonometry, for every 1 degree of tilt change, the beam is moving 1,000 feet
at a range of 10 miles from the aircraft.
This means that at 20 miles the beam is moved 2,000 feet, and at 5 miles
it is moving just 500 feet per degree.
Using the same formula, we find that 15
degrees nose up tilt will put the beam at 15,000 feet at the 10 mile range, and
7,500 feet at the 5 mile range. This
means that the ability for an aircraft to depict whether the rain is a thunderstorm
or just low altitude rain is very limited for close-in weather when the
aircraft is on the ground, or close to the airport.
Unlike the aircraft, ATC
radar is not tilt limited, so ATC has a much better depiction of weather that
is close in to the aircraft. Just
something to keep in mind. On August 2,
1985, Delta 191 approached the DFW area[i]. 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.
There is no doubt that a
good air traffic controller can bring a lot to the table in improving flight
safety, and, as been highlighted in this article, ATC has capabilities that
pilots do not have access to. There is
also an aspect to consider that is not related directly to the equipment. Pilots, being human, can get caught in
something called “plan continuation bias”, where the mindset of continuing is
strong enough that they discard any information that goes against the
plan. ATC can sometimes be in a better
“big picture” position, and by providing more accurate information regarding
the physical nature of the storms, may be able to break through that bias. Sometimes a query is all that is needed to
prevent a deadly dynamic.
