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confused pilots reacting without think- ing, inputting control forces that ulti- mately doomed the aircraft.
The failure to f ly deliberately has produced a history of inappropriate inputs in all three axes of flight, but the worst offender has been the rudder. It is one of the most confounding devices on an aircraft. The controls are buried beneath the pilot’s feet, so it can be difficult for an observer to determine how a pilot is manipulating the pedals. This reduces the ability for instructors (or more experienced copilots) to pass along corrections for ineffective rudder technique. Sometimes pilots are released into the wild sans a substantial idea of how
to use one of the biggest control surfaces on an aircraft.
Over time we develop some ideas about the rudder. Pilots of taildraggers cannot stop yapping about it (it is hard to forget the experience of a ground-loop). It is used in props to counteract p-factor and as an aerodynamic cudgel to eliminate excess energy on final. In aircraft from LSAs to A380s, it is the means to maintain directional control in crosswind conditions. The rudder is alter- nately the most demanding and the most dangerous device aboard an aircraft.
Va is the maximum speed at which full def lection of the f light controls can be commanded without undue risk of structural failure. It is an arbitrary number
that should be memorized. As important as it is, Va comes with a rather important caveat: Flight con- trols are only protected when inputs are initiated from the neutral position. Alternating def lections (stomping on the right rudder and then stomping on the left) can result in structural failure – even at speeds well below Va. Although this caveat technically applies to the elevator and ailerons as well, it is the rudder that is most at risk in the event of oscillatory inputs.
Enhancing the danger is the fact that the rudder is one of the least pre- dictable devices on an aircraft. Both the ailerons and elevator are fairly
linear – a given control force will result in a fairly predictable rate of roll or pitch. The rudder is quite a bit more finicky. If the aircraft is in a slip, rud- der towards the side of the slip will have to overcome the aerodynamic forces already acting against the ver- tical stabilizer, and a little bit of rud- der will have little effect. If rudder is opposite the direction of the slip, the aerodynamic force of the rudder and vertical stabilizer will sum into a large rate of yaw. Thus the same amount of pressure on a rudder pedal can
The scale of those forces is insidious due to the fact that pilots often have no kinesthetic notion that they are occurring (a rudder may experience thousands of pounds of force, while the rotational acceleration felt by the pilot is relatively minor).
FAA Technical Report
The issue of rudder usage was central to an FAA report released in October of 2010. The survey focused on transport category aircraft but is applicable to high-performance pas-
senger aircraft as well. The sur- vey focused on unusual attitudes, with 914 pilots relating various experiences encountered over the course of their career.
Nearly two-thirds of pilots re- ported unusual attitudes during low-level flight associated with takeoff, landing and initial climb. The vast majority were attrib- uted to wake vortex encounters (62 percent) and atmospheric disturbances (21 percent). It is important to note that nearly all respondents flew aircraft that weighed in excess of 100,000 pounds. Wake turbulence is not a problem for only small aircraft.
With wake turbulence making up the majority of disclosures, it was unsurprising that roll upsets were predominantly described. Interestingly, 56 percent of pi- lots reported using the rudder in response to these roll events. This almost exactly matches the number of pilots who indicated that they had received aerobatic
training at some point in their life. Aerobatic f light demands a substan- tial amount of rudder coordination, sometimes applied quite aggressively. The law of primacy in training is ap- parent here as pilots routinely transfer techniques learned in earlier f lying experiences to aircraft for which they may be inappropriate and sometimes dangerous.
In many cases, the respondents described using rudder in a manner inconsistent with industry and manu- facturer guidance. The final report also noted that pilots tend to consider an aircraft “upset” based on motion
The failure to fly deliberately has produced a history of inappropriate inputs in all three axes
of flight, but the worst offender has been the rudder. It is one of the most confounding devices on an aircraft.
produce significantly different results (a notable percentage of pilots report- ed experiencing “unexpected rudder characteristics” in an FAA survey).
This ambiguity greatly increases the probability that a pilot will fall into the trap of divergent oscillation, cycling from left to right rudder at an increasingly dangerous rate. The yaw generated from an engine failure on a multi-engine aircraft, for example, can create a startle response inappro- priate for the situation. Kicking the rudder in alternating directions can place massive forces on the vertical stabilizer risking structural failure.
February 2021 / TWIN & TURBINE • 7