NTSB 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.