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.