November 01, 2012
By Patrick Veillette firstname.lastname@example.org
Concerned about a number of loss of control (LOC) accidents, this past August, the FAA 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.”
NASA'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 the Colgan Air Q400 crash in Buffalo on Feb. 12, 2009, and the Air France A330 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 Transport Canada 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 a Cessna 152, 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.