Gusty Winds
Today we canceled flight
operations because of the winds. Crosswinds and gusty conditions
contributed to one third of the reported landing accidents in the
2010 Nall report. The good news is that most of these accidents were
not fatal. Every year the Nall report reveals trends in General
Aviation and gives pilots a chance to focus our attention on common
errors. Unfortunately, most of these errors remain consistent year
after year. As a flight instructor we try to teach pilots to avoid
becoming a statistic. Don't run out of gas, don't stall/spin, don't
fly into a thunderstorm, don't exceed the capabilities of the
aircraft or, more importantly your competency.
The nature of the soup in
which we fly is fluid, fluid dynamics. The airplane flies in the
relative wind. The relative wind is the direction of movement of the
atmosphere relative to the airfoil. Aerodynamics is a complex
subject and I do not claim to be an expert. I have a working
knowledge and this is greatly simplified, so take this with a grain
of salt.
When the wind is calm our
relative wind is controlled by the movement of the airplane. When
the aircraft is flying in calm air at 90 knots it will travel across
the ground at 90 knots. If the aircraft is flying directly into a 20
knot headwind at 90 knots it will travel across the ground at 70
knots. If the aircraft is flying with a direct 20 knot tailwind at
90 knots it will travel across the ground at 110 knots. This is easy
to understand. This makes sense. When we start to explore the
effects of winds at other angles the explanation becomes more
complex.
Since a picture is worth
a thousand words here is a few thousand.
FAA Pilots handbook of
aeronautical knowledge.
If we fly east and the
atmosphere is moving south we will end up south of where we pointed
the aircraft.
FAA Pilots handbook of
aeronautical knowledge
We can use math to figure
out how far north (left) we need to point the aircraft to track the
direction we want to go. The heading (where the aircraft is pointed)
and the track (the path along the ground) are different, and we can
use math to determine the exact wind correction angle to fly. We
would be in coordinated flight, wings level, ball centered nose
pointed one way and ground track in a crab. Landing in a crab would
cause problems since the landing gear would be pointed in a different
direction than the runway.
Engineers have developed
solutions for this problem. The B-52 could pivot the landing gear.
I do not fly B-52s. There are techniques to insure the longitudinal
axis of the aircraft is aligned with the runway at touchdown.
The “crab and kick” and the
“wing low” are the two methods to deal with crosswind approachs
and landings. The crab and kick method involves flying the final
approach segment in a crab and right before touchdown aligning the
aircraft with the runway. This method requires very accurate timing.
FAA airplane flying
handbook
FAA airplane flying handbook
The wing low method or slip uses
ailerons to control lateral drift and rudder to align the
longitudinal axis. This is uncoordinated flight and increases the
sink rate. The slip would not be appropriate for aircraft with long
wingspans since the wingtips could contact the ground before the
landing gear contacted the runway. This is one of the reasons
airliners do not slip to land. One of the reasons general aviation
aircraft favor this type method is to maintain directional control
after landing. They say “fly the airplane all the way through the
landing.” Properly executed the upwind wheel will touchdown, then sometime later the downwind wheel, then finally the nosewheel (or tailwheel). A sudden gust of wind may cause the aircraft to lift off
again and directional control is essential. Once we have landed we
want to keep the aircraft on the ground and in control. Large
aircraft are less susceptible to being blown around and often have
spoilers that spoil the lift and make the wings stop flying.
The airplanes have limits on the
amount of crosswind they can safely handle. Many airplanes have a
published “maximum demonstrated crosswind component” others have
a “maximum crosswind limit.” The first is not considered
limiting but is the maximum that was demonstrated in testing. The
second is the manufacturer stating “that would be really stupid.”
Both allow the manufacturer to avoid liability if you have problems.
The airplane is limited by rudder authority, wingtips dragging in
the dirt, landing gear collapse and other realities we can not
change. Understanding these limits is vital so that a pilot can make
an informed risk management decision.
As a flight instructor I help pilots
understand these limits. Their ability to safely land in a crosswind
is more often restricted by their technique rather than the aircraft.
Consistent practice in challenging conditions will help expand
personal capabilities. Confidence should be tied to competence. It
is an interesting risk management decision for an instructor. Is it
a conducive learning environment? The aircraft, the environment, the
client and my interaction with each factor into the decision.
Today was interesting. I was
scheduled to fly with a low time student and two rated pilots. The
winds were strong. The crosswind component was within my personal
limits and at a level that I would feel comfortable letting the
client explore. However the forecast called for stronger winds and a
shift in direction that took the crosswind component outside my
personal limits. It was not supposed to get that bad until much
later. However the observed conditions were already higher than
predicted, so much so that the forecasters amended their predictions.
The difference between peaks and lulls was the deciding factor. If
we are on final approach and the relative wind drops fifteen knots my
reactions must be nearly instant and flawless. Being out of
airspeed, altitude and ideas all at the same time is not a conducive
learning environment. So we canceled.
Have fun, be safe.
