Upset Prevention, Part 3
This is the final part in a three-part series on upset prevention. If you missed them, here's the first and second parts.
Airbus sought to better understand the causes that would lead to longer-lasting and more-severe wake encounters. Most of the encounters were performed stick-free--in other words, allowing the aircraft’s own stability to dictate its resulting motion. The flight test program also conducted several hundred events with the pilot trying to minimize the bank angle. The results clearly showed that pilot action does not improve the situation.
Airbus’ flight testing demonstrated that if the pilot tries to counteract the first rolling motion, these control deflections will roll the aircraft into the vortex, placing it onto an intercept with the core of the vortex. Worse yet, when the aircraft enters the core of the vortex, the rolling motion will be amplified by the pilot’s initial inputs. The result will be a final bank angle greater than if the pilot would not have moved the controls.
At this point in our discussion, it is appropriate to recall that pilot perceptions on the rapid and unexpected motion of the aircraft have been wildly over-exaggerated due to the startle. They are inappropriately perceiving the aircraft’s initial motion and reacting with excessive control inputs that are deepening the upset.
Results from the Airbus flight tests showed that most of the time the autopilot will control the encounter and will keep the aircraft adequately and safely within its flight and maneuvering envelope.
If the autopilot is disconnected, Airbus urges pilots to wait for a reasonable stabilization of the aircraft before any reaction, then roll wings level and re-establish a stable flight trajectory. In the rare cases when pilot intervention was required during the Airbus flight test program, they emphasized a smooth recovery and minimizing actions on the flight controls.
Avoid Aggressive Reactive Inputs
The data clearly shows that “aggressive, reactive inputs may result in large excursions in airplane attitude and may lead to a loss of control.”
This reiterates the findings in the JFK accident, and correlates well with recommendations from the FAA’s Airplane Upset Recovery Training Aid, Revision 2, which reminds pilots that “Airplane upsets have occurred when the pilot has made incorrect adjustments…. If the pilot’s control inputs are reactionary, unplanned and excessive, the airplane reaction may be a complete surprise. A continued divergence from what is expected due to excessive control inputs can lead to upset…. Control deflections at one point in the flight envelope might not be appropriate in another part of the flight envelope….”
The authors of Airplane Upset Recovery Training Aid, Revision 2 noted, “Any loss-of-control recommended recovery techniques and procedures provided by a manufacturer for a particular aircraft take precedence over those in the training aid.”
It then specifically states, in bold, italicized letters, “For example, the handling characteristics of fighter-type airplanes cannot be assumed to be similar to those of a large, commercial, swept-wing airplane.”
A pilot may find that his own control inputs can cause unwanted motion that could lead to an upset or loss of control.
Known as pilot-induced oscillation (PIO), this condition occurs when a pilot’s commands become out of phase with the aircraft’s motion. When the aircraft doesn’t respond as quickly or as aggressively as the pilot desires, the pilot often responds with inputs that grow increasingly out of phase with the aircraft's response.
Remember that high-speed, high-altitude flight produces considerable changes to an aircraft’s stability and handling qualities. As air density decreases at higher altitudes, an aircraft’s aerodynamic damping decreases, thus it becomes more responsive to control inputs.
Over-controlling is a distinct threat at high altitude.
For the same control surface movement at constant airspeed, an airplane at 40,000 ft. experiences a higher pitch rate than an airplane at 5,000 ft. because there is less aerodynamic damping.
Therefore, the change in angle of attack is greater, creating more lift and a higher load factor. It takes less force to generate the same load factor as altitude increases. Erratic and large pitch inputs, possibly from a startle/surprise effect, can very rapidly bring the aircraft into an upset. It is imperative to not overreact with large and drastic inputs. Pilots should smoothly adjust pitch and/or power to keep the aircraft within the center of the maneuvering envelope.
In many of the incidents, the aircraft were near the upper limits of the flight envelope where its margin between low-speed and high-speed buffet is likely to be small.
Do not forget that the angle of attack for buffet onset is considerably less than the stall angle of attack at low altitudes. For example, a flight test project conducted by the National Research Council of Canada titled “Aerodynamic Low-Speed Buffet Boundary Characteristics of a High-Speed Business Jet” utilized an intermediate capacity highly swept-wing high-speed business jet to conduct low-speed buffet testing.
At an altitude of approximately 13,000 ft., the buffet onset angle of attack occurred at 16.84 deg. In contrast, in straight and level flight at FL450 the buffet onset AOA was 6.95 deg. In other words, be wary of the limited range of AOA due to Mach effects when at high altitudes.
The Airplane Upset Recovery Training Aid, Revision 2 provides important advice. “Transport pilots should be aware that certain prior experience or training in military, GA or other non-transport aircraft that emphasizes the use of rudder input as a means to maneuver in roll typically does not apply to transport aircraft or operations.
When normal means of roll control have been unsuccessful, careful rudder input in the direction of the desired roll should be considered to induce or augment a rolling maneuver or to provide the desired bank angle. A rudder input is never the preferred initial response for events such as a wake vortex encounter or wind-shear encounter, or to reduce the bank angle preceding an imminent stall recovery.”
Pilots should understand their airplane’s feel and response characteristics to flight control inputs. By becoming familiar with these characteristics, they will learn to apply the appropriate control input in response to various flight situations.
Training programs must make certain the recommended flight control responses to a potential upset be thoroughly “vetted” for the given aircraft’s specific dynamic response, flight control system, structural limitations and other flight envelope limitations consistent with the highest-quality evidence-based engineering.