Concerned about a number of loss of control (LOC) accidents, this past August, the issued an Advisory Circular recommending changes to stall and stick-pusher training.
Advisory Circular 120-109 states, “Evidence exists that some pilots are failing to avoid conditions that may lead to a stall, or failing to recognize the insidious onset of an approach-to-stall during routine operations in both manual and automatic flight. A growing causal factor in LOC accidents is the pilot's inappropriate reaction to the first indication of a stall or stick-pusher event.”
's Aviation Safety Reporting System (ASRS) database gives substance to the FAA's concern. Among the 464 stick-shaker reports submitted was one from March 2012 (#998415) involving an approach into a busy airport with closely spaced parallel runways. The pilots selected flaps 15 to maintain adequate separation from a preceding “heavy” while peering into the setting sun. Suddenly the stick shaker activated and the startled flight crew broke off for a go-around. The pilots later discovered that the trailing-edge flaps had not deployed and the angle of attack (AOA) warning system had prevented a further degradation in the aircraft's condition.
While none of the stick-shaker events in the ASRS database ended with a mishap, in the aftermath of thecrash in Buffalo on Feb. 12, 2009, and the crash in the Atlantic on June 1 of that same year, regulators faced public pressure over those crews' failures to respond properly to a stall warning. The FAA's Advisory Circular is the latest in the international aviation regulatory community bringing attention to stall training. As it happens, in 2005 issued Aviation Advisory Circular 0247, “Training and Checking Practices for Stall Recovery,” which called for modifications to stall warning and stick-pusher training. FAA and industry officials hope that training providers will modify their programs to instill in pilots the knowledge and skills to avoid undesired aircraft states that increase the risk of encountering a stall, and to respond correctly should a stall occur.
Certification flight testing of transport airplanes attempts to ensure that there is warning with sufficient margin to prevent an inadvertent stall. For many FAR Part 25 aircraft, a stick-shaker system tied to configuration sensors does the job. The one caveat to this preemptive warning occurs in ice conditions. We'll get to that.
Inherent in the certification require–ments is the assumption that the pilot will take the correct action to prevent the stall. Certification also requires that the stall characteristics be satisfactory, and not result in a sudden wing drop, pitch-up or some other rapid flight alteration. When the natural aerodynamic stall characteristics fail to meet the certification requirements, additional systems must be installed to prevent or tame them. Hence, some airplanes are equipped with stick pushers set to activate at an AOA lower than the angle at which the natural stall occurs, quickly and automatically lowering the nose to preserve controlled flight.
Most of pilots receive initial fixed-wing training in a basic aircraft such as a152, and the sensations experienced during stall demonstrations leave an imprint as to how an aircraft will provide stall warning. However, those sensations do not carry over to a business jet. Rather, the initial indication of a stall can be aural, tactile or visual and these can be either naturally or synthetically induced.
Transport Canada's Advisory Circular warns, “Airflow separation is often indicated by airframe buffeting and a reduction in controllability of the aircraft. However, for some aircraft, the loss in lift is sudden and without any preceding buffet.” Moreover, an aircraft can produce varying indications of an impending stall depending altitude, icing or other factors.
A natural or synthetic stall indication may include one or more of the following indications:
(1) Aerodynamic buffeting;
(2) Reduced roll stability and aileron effectiveness;
(3) Visual or aural cues and warnings;
(4) Reduced elevator (pitch) authority;
(5) Inability to maintain altitude or arrest rate of descent;
(6) Stick-shaker activation.
Failure to promptly reduce the wing AOA at stall warning can result in an increasing AOA and a stall. A high AOA can also cause engine surge and compressor stalls because of inlet flow interruptions. And the asymmetric lift associated with wing airflow separation or loss of thrust can result in a lateral-directional upset — that is a rapid wing drop-off.
Manufacturers' flight test crews and non-routine flight operations pilots who do actual stalls in their aircraft during post-maintenance flight checks report that stalls can be wicked. Colleagues who have done post-maintenance stall checks on Hawker 800s after replacement of the TKS panels have described the event as a sudden, rapid roll, and this in an aircraft otherwise known for fine flying qualities.
While stall training is conducted in full flight simulators (FFS), as noted in “Upset Recovery in Sims” (BCA, April 2012, page 34), even the most advanced of those systems have some inherent limitations involving the fidelity of their aerodynamic model near the edges of the flight envelope, the absence of g-loads, which could affect stall recovery and incomplete motion cues at the first indication of stall. Instructors need to be aware of a simulator's limitations to mitigate negative training.
According to the FAA's Advisory Circular, the first step in recovering from a stall is to ensure the autopilot and autothrottle are disconnected. Specifically, it states, “While maintaining the attitude of the airplane, disconnect the autopilot and autothrottle. Ensure the pitch attitude does not increase when disconnecting the autopilot. This may be important in out-of-trim situations. Manual control is essential to recovery in all situations. Leaving the autopilot or autothrottle connected may result in inadvertent changes or adjustments that may not be easily recognized or appropriate, especially during high workload situations.”
The second step in recovery is to pitch the nose down to immediately decrease the AOA, and implement nose-down pitch trim.
Step number three is wings level, and step number four is to add thrust as needed. Transport Canada points out that turbojet engines normally require 8 sec. to achieve go-around thrust from idle thrust at low altitudes, and even more time at higher altitudes.
Step five is to ensure the speed brakes/spoilers are retracted, and step six is return to the desired flight path.
For some time there has been concern among training, regulatory and investigatory officials as to whether flight crews have been adequately trained for the proper response to a stick pusher. Transport Canada's Advisory Circular 0247 states, “From observations, most instructors state that, regardless of previous academic training, pilots [on their first encounter with a stick pusher] usually resist the stick pusher and immediately pull back on the control wheel rather than releasing pressure as they have been taught. Therefore, pilots should receive practical stick-pusher training in an FFS in order to develop the proper response [allowing the pusher to reduce AOA] when confronted with a stick-pusher activation. Stick-pusher training should be completed as a demonstration practice exercise, including repetitions, until the pilot's reaction is to permit the reduction in AOA even at low altitudes.”
Meanwhile, the FAA's Advisory Circular encourages incorporation of stick-pusher training into flight training scenarios. Similarly, the Joint Aviation Authorities Joint Operational Evaluation Board Report regardingChallenger 300 training recommends that flight crews be exposed to operation of the Stall Identification System (stick pusher) since unfamiliar pilots could be misled into taking inappropriate action.
Common factors that have led to stalls include misinterpretations of AOA versus pitch angle, along with decaying airspeed, weight, g loading, bank angle, center of gravity, thrust and lift vectors, thrust settings and application of thrust, autothrottle protection, wind shear, configuration, altitude, Mach effects, uncoordinated flight, misuse of automation, situational awareness and wing contamination.
To help identify the relative frequency of these stall factors in business jet accidents, I've drawn upon research undertaken for a Loss of Control (LOC) presentation to the International Society of Air Safety Investigators in September 2011, involving a review of such mishaps from 1991 through 2010.
Of a total 71 business jet LOC accidents during that period, 18 involved an unintentional stall during approach and landing and in most of these the flight crews “failed to maintain adequate airspeed.”
Board Member Robert Sumwalt, a former airline captain, has stated, “A flight crewmember must carefully monitor the aircraft's flight path and systems, as well as actively cross-check the other pilot's actions, or safety can be compromised.” Many of the ASRS reports reviewed for this article showed how a distraction could compromise a flight crew's cross-checking and monitoring.
Twelve stall accidents occurred during circling approaches, while the aircraft was in a bank. In other words, the aircraft encountered an accelerated stall. The “threats” in a circling approach are significant: Attention is focused outside the aircraft, usually requiring a lot of bending of the head, which can induce sensory and perceptual illusions; there's no vertical guidance information; maneuvering occurs in relatively confined airspace very close to the ground, which severely decreases the pilot-flying's scan of airspeed, pitch, bank and sink rate. And when such an approach is executed in limited visibility and/or mountainous terrain — consider Aspen, Colo., and Truckee, Calif. — the lack of a distinct horizon induces further visual illusions.
Proximity to adverse terrain is not the only environmental factor contributing to the deterioration in aircraft control in these approaches. The significant density altitudes at these common destinations compound the problem. Higher density altitudes translate into higher true airspeeds, and a 10% increase in true airspeed increases the required turning radius by approximately 21%, further limiting aircraft maneuvering margins in canyon or mountain bowl terrain. The increase in the turn radius can quickly put an aircraft into a situation where any continuation of the turn places the flight path into the mountain. Even the inclusion of a relatively benign and undetected 10-kt. tailwind can greatly increase an aircraft's turn radius beyond safe margins in the confined maneuvering space.
If you fly into a destination such as Colorado's Eagle County Regional Airport (EGE), take the opportunity during your next sim training to practice circling there. I flew a circle-to-land rather than the straight-in to EGE during a simulator session and realized how a pilot could get very target fixated operating into this Rocky Mountain airport and inadvertently permit the airspeed to decay. Just as important as a hands-on sim experience is an insightful debrief that helps flight crews discover a set of multi-tiered preventive measures to ensure their aircraft would never get close to an undesired state when operating in such an unforgiving environment.
Some ASRS reports invite attention as was the case with No. 720231 (December 2006) in which the submitter wrote, “Push the nose over!” The brief narrative described a Gulfstream captain deciding to take the jet up to FL 450 due to worries about having enough fuel to make the trip. The PIC shrugged off the SIC's assertion that they were a bit heavy for FL 450 and started to climb. With that, the airspeed started to decay, but when the SIC noted the speed drop, the PIC pointed at the AOA gauge and said, “See, we're fine.” Shortly thereafter the aircraft began to porpoise, followed by activation of the stick shakers and a stick pusher and then the onset of a full stall. The captain kept the pitch up and tried to hold attitude while squeezing in more power. The SIC insisted they needed to get the nose over, and then reached for the yoke and pushed forward. Afterward the PIC denied they'd stalled since there had been no aural alert.
The experience of airspeed decay while at high altitude was repeated in many other ASRS reports reviewed. Buffet tends to be the first stall identifier in that environment. Gust loads created by high-altitude turbulence can increase the local Mach speed over the wing, resulting in shock-induced buffet or even stall. While modern aircraft generally have much more generous “buffet margins” than those of earlier generations, turbulence gusts have the potential to create a high-altitude “coffin corner” in the flight envelope. Proper use of buffet boundary/maneuver capability charts is one of the tools pilots should use to determine the maximum altitude that can be flown safely. Pilots also need to know what speed to maintain for peak buffet resistance, and whether the engines can produce enough thrust to maintain airspeed. Where is an ideal “non-threat” environment to practice this scenario? In the classroom and the simulator.
Airspeeds slower than L/D max subject the airplane to increased drag, which will cause an even further decrease in airspeed. High-altitude flight at speeds slower than L/D max must be avoided.
Typical primary flight displays (PFD) indicate airspeed trending as well as the low- and high-speed limits. However, information on the PFD can be misinterpreted. It does not indicate that adequate thrust is available at that altitude to maintain the current airspeed.
At higher altitudes there is insufficient excess thrust to “power out” from a stall while attempting to minimize altitude loss. It is impossible to recover from a stalled condition without reducing the AOA. The only effective response is the deliberate and smooth reduction in the AOA, while keeping in mind that there's less pitch damping at altitude and thus the flight controls tend to be more sensitive, and trading altitude for airspeed — possibly thousands of feet of altitude. At all times inputs should be smooth, deliberate and positive.
Also keep in mind that an AOA gauge probably won't give a direct indication of the aircraft's true AOA. Most civil aircraft do not have full-flight, Mach-compensated AOA indicators and thus the displays need to be adjusted for Mach and density altitude to provide flight crews with accurate information about stall margins.
The FAA's Advisory Circular as well as the NTSB and other investigation authorities have recommended incorporating high-altitude operations into initial and recurrent training curriculums. The Airplane Upset Recovery Training Aid Team recommends line oriented flight training (LOFT) to familiarize crews with high-altitude slowdowns and approach to stall. Crews should always recover at the first indication of an impending stall.
However, as already noted, most simulators don't have the proper mathematical models to replicate the aircraft's high-altitude stall behavior. In addition, simulators need to be programmed with high-altitude atmospheric characteristics so that pilots can learn to recognize signs of a high-altitude upset and rehearse recovery techniques.
Icing has contributed to nine business jet stall accidents, five of which involved aircraft equipped with pneumatic deicing boots, and three of those had deployed the boots during approach but still had residual ice on them. Ice contamination can result in stall onset at a lower AOA and an increase in stall speeds without activation of the stall warning system.
In the wake of a non-fatalaccident at Lubbock Airport, Texas, in 2009, the NTSB noted that in icing condition the activation angle of the stick pusher should be reduced or the system's benefit in reducing AOA prior to stall and during recovery efforts is lost. The Board concluded that a lower stick-pusher activation AOA would enhance safety in icing conditions and provide stall protection before an uncommanded roll develops during stall. Accordingly, it is recommending an evaluation of transport-category airplanes equipped with stick pushers to ensure their activation at an AOA that will provide adequate stall protection in the presence of airframe ice. According to Daniel Meier Jr., aviation safety inspector, flight operations, FAA headquarters, “A stall caused by icing is extremely hazardous because you cannot conserve altitude by maintaining attitude. Adhering to the standard of minimum altitude loss ingrained in training has resulted in pilots failing to recover from ice-related stalls and upsets that have resulted in altitude losses in excess of 5,000 ft.”
One of the important misconceptions that the new FAA Advisory Circular serves to dispel is that evaluation criteria for a recovery from a stall or approach-to-stall do not mandate a predetermined value for altitude loss.
The University of Illinois-Urbana's aeronautical engineering department conducted a research project to determine the effect of residual and inter-cycle ice accretions on airfoil performance. The study found that inter-cycle ice reduced the maximum lift coefficients about 60% from 1.8 (clean) to 0.7 (iced) and stall angles were reduced from 17 deg. (clean) to 9 deg. when iced. The effect of the small ridge-like features was local boundary layer separation on the airfoil's upper surface, particularly at higher AOA.
The U.S. NTSB has recommended “additional research to identify realistic ice accumulations, to include inter-cycle and residual ice accumulations . . . and to determine the effects of criticality of such ice accumulations; further, the information developed through such research should be incorporated into aircraft-certification requirements and pilot training programs at all levels . . .”
The NTSB has also recommended that manufacturers of turbine-powered airplanes be required “to provide minimum maneuvering airspeed information for all airplane configurations, phases and conditions of flight [icing and non-icing conditions]; minimum airspeeds also should take into consideration the effects of various types, amounts and locations of ice accumulations, including thin amounts of very rough ice.”
For the Advisory Circular to be effective, pilots need to be properly trained in the use of anti-icing and deicing devices, and know the proper speed and configuration to fly in potential icing conditions, especially when slowing to configure for landing, and through touchdown. Additionally, these speeds must be established with adequate stall margin, even when ice remains on critical portions of the aircraft, and there must be a proper recovery procedure to employ at the first indication of loss of control.
ASRS No. 964029 (July 2011) highlighted the conundrum that can result from deferred maintenance on a component, as permitted by an MEL. “During preflight, we found the stick pusher was not working properly,” the pilot reported. “During the stall protection test, the stick pusher would make a loud grinding sound. The sound could be heard as far back as the emergency exit row, as well as being felt in the floorboards till at least row three. It had been written up a few days prior, but the corrective action was “Ops check good.” Maintenance Control tried to get the contract mechanics to defer the stick shakers. We determined that the worst possible outcome would be the stick-pusher servo seizing, which would prevent the control column from being pulled aft, an essential part of controlled flight.”
Probably all flight crews, but especially post-maintenance flight crews, and maintenance controllers should be trained to know the warning signs of a malfunctioning stall warning and/or stick-pusher system, and know when the inoperative component should be repaired prior to the next flight rather than deferred. This information should be contained in ground training, and probably reinforced in the simulator.
Notably, in September 2009, Transport Canada Airworthiness Directive CF-2009-36 states: “There have been several stick-pusher capstan shaft failures causing severe degradation of the stick-pusher function. This directive is issued to revise the first flight of the day check of the stall protection system to detect degradation of the stick-pusher function. It also introduces a new repetitive maintenance task to limit exposure to dormant failure of the stick-pusher capstan shaft. Dormant loss or severe degradation of the stick-pusher function could result in reduced controllability of the airplane.”
Among the ASRS data were numerous reports in which the stick shaker and/or stick pusher activated when other cockpit indications revealed the aircraft was at a safe flight condition. When these false warnings and activations occurred during benign phases of flight, the crews were invariably startled and then perplexed, both of which were distractions. In some cases the pilots found little guidance in their Quick Reference Manuals to deal with the false indications. In one case, the AOA vane had actually fallen off the aircraft after it left the ramp. In all these false alarms, the flight crews performed the most important task — that is they kept flying their aircraft at a known pitch and power setting.
One report in which an aircraft was placed in severe jeopardy by an errant stick-pusher activation occurred in an MD-83 at 50 ft. AGL during the landing flare. The pilots instinctively yanked (probably with plenty of adrenaline to boost their power) as the stick pusher wrongly nosed the aircraft toward the ground.
The FAA's Advisory Circular encourages trainers to develop realistic scenarios such as the forgoing push-over that could be encountered in operational conditions. That helps, to be sure, but preventing stall accidents will require a multi-layered methodology that includes procedures that optimize cross-checking and monitoring, thorough flight testing of icing protection systems, MELs that eschew the easy deferral of vital components without full consideration of the effects on flight safety, more informed ground school instruction and improved simulator fidelity, particularly near the edges of the aircraft's performance envelope. BCA