Stall By Surprise: Aircraft’s AOA May Not Be Or Behave As You Believe
Angle of attack (AOA) is an important parameter to an aircraft’s performance, stability and control. A wing has a limited range of AOA in which to function efficiently, and going beyond those limits has negative consequences.
Ground school during the primary stages of pilot training imbeds the concept that a wing always stalls at the same angle regardless of the airplane’s speed and pitch attitude. In truth, a wing’s critical AOA is affected by a number of other factors, sometimes surprising even professional flight crews when their aircraft stalls at an angle far below normal.
On June 28, 2002, a Saab 340B departed Sydney on a 35-min. flight to Bathurst, Australia. At the controls were a pilot in command (PIC) with nearly 10,000 total flight hours, with 1,939 hr. in type, and a second in command (SIC) with 6,620 total flight hours of which 1,451 hr. were in the twin commuter. They were flying their sixth leg of the day.
The area forecast indicated the freezing level would be near 4,000 ft. and that moderate icing conditions could be expected in clouds above that. The forecast for Bathurst included snow showers, a surface temperature of 2C, a broken ceiling of 800 ft. and southwesterly winds gusting to 28 kt., necessitating a circling approach that night.
The pilot was flying the aircraft on autopilot and during descent from 12,000 ft., entered clouds several times before reaching the initial approach altitude of 5,700 ft., then continued down to 3,810 ft., the minimum descent altitude for the GPS approach. The pilots observed ice accumulating on the windshield wipers but not on the wings’ leading edges. However, due to conflicting information among the SOPs, airplane flight manual (AFM) and the aircraft operating manual (AOM), the crew activated the engine anti-ice system but not the prop deice or deice boots on the wing and tail leading edges.
A circling approach at night and in icing conditions comprised a triple threat. To make matters worse, the GPS approach was quite steep, so even with the flaps extended to 20 deg., landing gear extended and the torque reduced to flight idle, the aircraft arrived at MDA traveling significantly faster than the normal 130-kt. circling speed. The PIC left the power levers at idle to begin slowing to the appropriate speed while beginning a right-hand turn to track downwind for Runway 17. The SIC pointed out that the airspeed had begun to decay below target speed, and as the PIC added power and began to roll out of the right turn, the aircraft suddenly rolled left past the vertical and pitched nose-down without warning.
The FDR recorded the AOA at 9.5 deg. at the moment, far below the normal critical angle for the wing. Normally the stick shaker wouldn’t activate until 13.1 deg. AOA to warn the pilots of an impending stall. Why the sudden and abrupt stall? The wing leading edges were contaminated with a relatively small but aerodynamically significant 0.5-in. layer of ice and that is what caused the left wing to stall so prematurely.
The PIC overpowered the autopilot, aided no doubt by the adrenaline surging through his veins, and rolled the aircraft back from 109-deg. left bank to approximately 35 deg. At that point the right wing stalled, rolling the aircraft to about 56-deg. right-wing down. According to the Australian Transport Safety Bureau’s Air Safety Investigation Report 200203074, as the aircraft passed through 688 ft. AGL, its pitch attitude was 19-deg. nose down! The PIC then rolled the aircraft to a wings-level attitude, increased power to 100% torque, applied nose-up pitch inputs and the crew recovered the aircraft from what might have been disaster.
During its investigation of the incident, Australian authorities quickly found five other Saab 340 incidents involving trace to light amounts of icing leading to premature stalls with little or no warning to flight crews.
One of the effects of airfoil contamination is a reduction in the critical AOA. If a flight crew unintentionally allows the AOA to reach this contamination-induced critical angle, the first obvious sign to the flight crew can be an abrupt uncommanded roll, buffet or other aerodynamic cues without stick-shaker activation. Data collected from a British Aerospace ATP that was involved in an icing upset in 1991 determined that the ice-induced stall occurred at about 140 kt., compared with a normal stall speed of about 110 kt.
According to George Bershinsky, pilot of the University of Wyoming’s King Air being used in icing research for the National Center for Atmospheric Research (NCAR), “less than 1/16th inch of icing can reduce a wing’s lift by 25%. This little is sometimes hard to see, but the stall speed increases by around 20%.”
It is important to realize that the wing’s critical angle of attack may change with no apparent visual, tactile or performance cues associated with a contaminated condition. In many of the ice-induced accidents and the inflight icing research conducted by agencies such as NASA-Glenn and the NCAR, the airplane provided no advanced warning to these experienced engineering test pilots. NCAR test pilots have noted airplane response and kinesthetic cues to an ice-related stall can be substantially different from those in simulator training scenarios. Given this information, it shouldn’t be surprising that there have been accidents in which the stall and upset occurred prior to stick-shaker activation in ice-contaminated airplanes.
When you think of airfoil contamination, frost and ice come immediately to mind. But what about rain? Might it affect a wing’s boundary layer enough to cause a premature stall? Dr. John Hansman, professor of aeronautics and astronautics at the Massachusetts Institute of Technology (MIT), is among the select group of researchers who have studied the performance of airfoils in a “heavy rain” environment. His research, along with that of several dozen other highly respected scientists in this specialized area of boundary layer aerodynamics, identifies two mechanisms, a “splash-back” effect and a roughened airfoil surface, that contribute to a degradation of the boundary layer. As raindrops strike an airfoil, they form an “ejecta fog” of splashed-back droplets. These rob momentum from the air particles in the boundary layer, or more simply, slow the airflow. (The rainfall rates to create this effect exist in severe downbursts.)
Dr. James Valentine, a fluid dynamicist whose work has been recognized by the National Research Council’s Transportation Research Board, found that a thin water film forms on the airfoil surface by the fraction of the raindrop that is not splashed back. The raindrops form small impact craters and surface waves in the water film, which roughens the airfoil surface.
So, as with ice and frost, this creates additional surface friction on the boundary layer, with the net effect of loss of lift, increase in drag and a premature separation of the boundary layer, all of which compromise an airfoil’s performance. A typical airfoil experiences a decrease in maximum lift of up to 18%, a drag increase of up to 40% and, perhaps most importantly, a decrease in stall AOA of up to 8 deg. An “average” airfoil typically stalls around 17 deg., thus one with a contaminated surface could stall at just 9 deg. AOA.
During a microburst/wind-shear recovery procedure, the goal for the pilot is to attain the maximum possible lift without proceeding into a stall. This is why many aircraft wind-shear recovery procedures recommend the pilot pitch the aircraft to a fairly significant attitude, while also “respecting the stick shaker.” However, this procedure is based on a dry and uncontaminated airfoil’s performance curve.
With that in mind, assume the airfoil on your wing typically stalls around 17 deg. AOA and during a wind-shear recovery procedure, ideally you are just shy of that — say, 16 deg. Now, if the airfoil is contaminated by a heavy rain encounter in a “wet” microburst and you apply the wind-shear recovery technique to arrest the alarming sink rate, that airfoil is deep into the stall at 16 deg. AOA. So, you’ve actually worsened the aircraft’s ability to “max perform” in that situation.
Meanwhile, stall warning systems do not provide accurate, preemptive stall warning indications when the wing is contaminated. This isn’t the fault of the manufacturers. There currently is no accurate method to detect the surface condition of a wing and accurately predict the behavior of the boundary layer under that exact condition. (See “Limited Measures” sidebar.)
A wing’s critical AOA is substantially changed during transonic flight with the presence of Mach waves, as discussed extensively in “Low-Speed Buffet” (BCA, September 2018). A flight test project conducted by the National Research Council of Canada and presented at the 24th International Congress of the Aeronautical Sciences used a highly swept-wing high-speed business jet to conduct low-speed buffet testing. At an altitude of approximately 13,000 ft. and an aircraft weight of 29,270 lb., the buffet onset AOA occurred at 16.84 deg. By contrast, when flying straight and level flight at FL 450 and 31,000 lb., the buffet onset AOA was 6.95 deg. — a difference of almost 10 deg.
Consider cruising your aircraft at FL 340 and Mach 0.55 and you roll into a turn. The additional AOA required to maintain altitude causes the airflow to accelerate markedly over the leading edge to the point where a strong Mach wave is created just behind the leading edge. The associated strong pressure gradient causes a separated boundary layer immediately behind the Mach wave, leading to buffet. The aircraft utilized in the Canadian research project in this maneuver demonstrated buffet onset at 8.5-deg. AOA.
The air is even thinner at FL 450, which means an aircraft needs to fly even faster and/or at a higher AOA to produce sufficient lift. In the Canadian flight test program, as the airplane slowed to Mach 0.65, the buffet occurred at 6.95 deg. This was an even lower buffet AOA than was experienced at FL 340. The buffet onset AOA varies inversely with the Mach number, meaning that the higher the Mach numbers, the onset of buffet occurs at lower AOAs. This also means that the airspeed for low-speed buffet increases with altitude.
Most business aircraft depend upon high-lift devices for takeoff and landing. The deployment of trailing-edge flaps, leading-edge devices and spoilers will change the airfoil’s critical AOA. As trailing-edge flaps are extended, the camber (and depending on flap design, the wing area) is increased. Although the amount of maximum lift is increased, the critical AOA is decreased because the airflow separates earlier. Even though trailing-edge flaps provide increases in the airfoil’s maximum lift, their use markedly decreases the critical AOA.
However, airflow separation is delayed by Krueger flaps and slats, allowing a substantial increase in the critical AOA, and thus provide a better margin over the stall when deployed.
Note, that if you fly an aircraft equipped with both trailing- and leading-edge high-lift devices and experience a hydraulic failure that prevents their normal deployment, the backup extension system often puts priority on extending the leading-edge devices because of the added AOA margin they provide.
The tragic loss of a Gulfstream G650 and its crew during flight testing at Roswell, New Mexico, on April 2, 2011, highlighted some of the negative effects on critical AOA when in ground effect. The experimental aircraft was conducting a planned one-engine-inoperative (OEI) takeoff when a stall on the right outboard wing produced a rolling moment that the highly experienced flight test crew was unable to control. The right wingtip contacted the runway, and the aircraft departed the right side of the runway and struck a concrete structure and airport weather station, destroying the aircraft and killing those within. The NTSB found that the airplane had stalled and its report highlighted some common misconceptions and misunderstandings about ground effect.
When an aircraft is close to the ground, negative changes occur to its aerodynamics, which especially affect swept-wing jets. This is particularly the case during the landing flare and takeoff rotation when the aircraft is at a precarious energy state with very little margin for error. As a swept-wing aircraft is rotated, the wingtips are momentarily closer to the runway, changing the airflow significantly, further increasing the negative effects of ground effect.
The NTSB’s John O’Callaghan, a national resource specialist in aircraft performance, noted that the stall of all types of aircraft occurs approximately 2-4 deg. AOA lower with its wheels on the ground. The flight test reports noted “post-stall roll-off is abrupt and will saturate lateral control power.” The catastrophic roll-off of the wing in the Roswell accident was due in part to no warning before stall in ground effect.
Be advised that the AOA gauge should not be used as a direct indication of the wing’s condition during takeoff. Ground effect and crosswinds will affect the AOA sensor reading. It will not provide valid information until the aircraft is airborne and at a sufficient altitude.
On a day in which falling precipitation necessitates the need for anti-icing fluids, an application is meant to keep the wing from suffering the substantial loss of lift from surface contamination. This does not mean that a deiced wing coated with a layer of anti-icing fluid is without performance degradation. According to a study by NASA Glenn Research Center and the National Research Council of Canada, the application of anti-icing fluids has a significant negative effect on aerodynamic performance. The study discovered the stall angle was reduced to 15 deg. compared to the clean value of 20 deg. (reference: Andy Broeren of NASA Glenn, Sam Lee of Vantage Partners, Catherine Clark of NRC Canada. “Aerodynamic Characterization of a Thin, High-Performance Airfoil for Use in Ground Fluids Testing.” Fifth AIAA Atmospheric and Space Environments Conference, Fluid Dynamics and Co-located Conferences, AIAA 2013-2933).
Crosswinds can likewise create a stall at a lower AOA. During crosswind takeoffs and landings in a swept-wing jet the “upwind” wing experiences airflow that is more direct (i.e., perpendicular) to the wing’s leading edge, and this generally improves the wing’s performance. Conversely, the “downwind” wing experiences the airflow at a greater angle (essentially increasing the “sweep” of the wing), which decreases its lift, increases drag, promotes the span-wise flow of air, and thereby reduces its stall AOA.
For example, a crosswind from the right effectively increases the sweep of the left wing and reduces the sweep of the right wing. Clinton E. Tanner, Bombardier’s senior technical advisor in flight sciences, cites flight test results showing that sideslip reduces the stall AOA of the left wing by up to 3.5 deg. when it experiences a sideslip of 20 deg. Large rudder applications during a highly dynamic stall event will also generate high sideslip angles. Either of these conditions may result in asymmetric stall of the downwind wing.
Tanner is concerned about the combined effects of anti-icing fluids, ground effect and crosswinds on lowering the overall margin of safety during takeoffs. Given the very real possibility that the negative influence of ground effect, crosswinds and deicing fluids have an additive effect on the reduction in stall AOA, the margins over an actual aerodynamic stall during a takeoff decrease. The possibility of an aircraft encountering an actual aerodynamic stall is real, and particularly without aerodynamic warning.
Incidentally, during preflight inspections you should examine the condition of the aerodynamic seals on your wing, particularly on “hard wing” regional and business jets. Deteriorated aerodynamic seals, especially those near the leading edge of the wing, cause a significant loss in the maximum lift of a wing as well as decrease the stall AOA. Is your aircraft allowed to operate with deteriorated wing seals? It depends. The MEL or the CDL for your aircraft might contain relief for minor wing seal protrusion and deterioration.
If you fly an aircraft with a TKS system be aware that removal of the TKS panels for maintenance requires extreme care for replacement to make certain that the panels are aligned with laser-like precision on the proper location along the leading edge of the wing.
Secondly, the adhesive used to reseal the TKS panel to the wing must not protrude above the metal edges. That seemingly miniscule protrusion of adhesive can negatively affect the wing’s critical angle of attack. Good friends who were Non-Routine Flight Operations captains in TKS-equipped aircraft cite incidents in which the aircraft rolled to beyond 90 deg. of bank during post-maintenance test flights due to very slight misalignments of TKS panels after removal and reinstallation.
While on the topic of TKS panels, it is recommended that they be frequently operated even in the summer to keep the plastic supply tubes from cracking due to dryness as well as the micro-pores from clogging up. Astute colleagues who have paid close attention to the preflight of Hawker 800XP aircraft during aircraft acceptance checks have noted that TKS panels did not exude a proper distribution of TKS fluid during preflight checks. This would create a condition in which part of the wing would not be deiced or anti-iced in flight when the TKS system was applied, leading to a dangerous wing condition of an ice mass building up on a portion of the wing. At this point you have involuntarily become a test pilot with no certainty of the aerodynamic performance of your wing. (See “TKS Considerations” sidebar.)
If you learned to fly in light general aviation aircraft, you might have been exposed to the tactile indications of an aircraft nearing the critical angle of attack. However, many of the transport aircraft used by business flight departments do not provide tactile indications, a fact especially true for swept-wing aircraft or high-performance aircraft equipped with sharp leading-edge airfoils. Such aircraft are particularly prone to stalling abruptly without warning and are even more susceptible to aerodynamic performance degradation from airfoil frost, ice, anti-ice fluid, decaying aerodynamic seals and such near their leading edges.
A professional pilot must know an aircraft’s aerodynamic limitations in all phases of flight, with varying flap conditions, from changes caused by environmental conditions, and must also be familiar with conditions that cause instruments to display misleading information.