The NTSB believes currently required stall-warning systems are not adequate to cover all critically low-airspeed conditions and has recommended that the FAA require the installation of so-called “low-airspeed alert” systems on all airplanes used in FAR Parts 121 and 135 commercial operations.
The Safety Board's recommendation stems from its investigation into several Part 121 and Part 135 accidents since 1982 (most recently the crash of a King Air that killed the two pilots and six passengers, including Sen. Paul Wellstone [D-Minn.]) in which “a stall or failure to maintain airspeed during the approach or landing phases was cited as a causal or contributing factor and in which icing was not cited as a factor.”
In the Wellstone accident, the NTSB determined that the airplane was being flown below the “recommended approach speed for about the last 50 seconds of the flight.”
Current systems provide a warning at a speed that is at least five knots higher than the stall speed. But the NTSB doesn’t believe this is adequate for all situations. “Stall warnings do not always provide flight crews with timely notification of developing hazardous low-airspeed conditions. For example, abrupt maneuvering can increase angle of attack so rapidly that a stall could occur nearly simultaneously with the stall warning, and ice accumulation, which raises the stall speed, could degrade the stall-warning margin to the point at which little or no stall-warning is provided.”
Because the King Air carrying Wellstone did not have a cockpit voice recorder, because of the approximate nature of the airspeed calculations and because abrupt maneuvering or even small amounts of ice accumulation can defeat the airplane’s stall-warning system, the Safety Board was not able to determine “when or if the stall-warning horn activated before the onset of the stall.” Regardless of when or whether the stall-warning horn activated, it is clear that the pilots “failed to maintain airspeed during the approach,” the NTSB said.
A 1996 FAA/industry report titled “The Interfaces Between Flight Crews and Modern Flight Deck Systems” expressed concern about the history of Part 121 and 135 accidents involving lack of low-airspeed awareness in the context of flight crews’ monitoring automated systems. This report states: “Flight crews may not be provided adequate awareness of airplane energy state, particularly when approaching or trending toward a low-energy state…Transport-category airplanes are required to have adequate warnings of an impending stall, but at this point the airplane may already be in a potentially hazardous low-energy state. Better awareness is needed of energy state trends such that flight crews are alerted prior to reaching a potentially hazardous low-energy state.”
Low-airspeed Warnings Proposed for New Airplanes
Regulatory action toward requiring a low-airspeed warning system when using autopilots is already under way as a result of a Jan. 9, 1997 accident involving a Comair Brasilia that crashed near Monroe, Mich., during a rapid descent after an uncommanded roll excursion in icing. Also considered was the March 19, 2001 incident involving another Comair Brasilia, whose crew lost control during cruise and descended 10,000 feet after the airplane encountered icing. A low-airspeed alert system was developed for the Brasilia, and installation is mandated by an AD.
An FAA/industry aviation rulemaking advisory committee has proposed a change to FAR 25.1329 (automatic pilot systems) that would require low-airspeed protection and alerting during autopilot operations for newly certified transport-category airplanes. But the ARAC recommendation has yet to be turned into an FAA notice of proposed rulemaking.
The requirement for speed protection is based on the premise that reliance on flight-crew attentiveness to airspeed indications alone during autopilot operation “is not adequate to avoid unacceptable speed excursions outside the speed range of the normal flight envelope…Standard stall warning and high-speed alerts are not always timely enough for the flight crew to intervene to prevent unacceptable speed excursions during [autopilot] operation,” the NTSB said.
For example, a low-speed alert and a transition to the speed-protection mode at approximately 1.2 Vs or an equivalent speed defined in terms of Vsr (reference stall speed) for the landing flap configuration has been found to be acceptable. If a low-airspeed alert activates when the airspeed drops below 1.2 Vs, pilots would receive several seconds advance notice before reaching the airplane’s estimated stall speed. In addition, if the pilots maintain an airspeed at or above the threshold set by such an early low-airspeed alert the additional airspeed could prevent an accelerated stall initiated by an abrupt last-second maneuver or provide an improved speed margin above a premature stall caused by ice accumulation on the wings.
The Board said it recognizes “that there are unresolved technical, operational and human-factors issues that will need to be carefully evaluated and addressed in connection with the design and implementation of a low-airspeed alert system.” Some of the issues that should be addressed include defining the target speed at which the alert system would activate; effectively integrating such a system with other aircraft systems; preventing nuisance alarms and flight-crew over-reliance on such a system; differentiating such an alert from other kinds of cockpit alert and warning; and developing flight-crew procedures and training for the use of such systems.
“Despite these unresolved issues, the Safety Board concludes that the development of and requirement for the installation of low-airspeed alert systems could substantially reduce the number of accidents and incidents involving flight-crew failure to maintain airspeed.”
At press time the FAA had not responded to the recommendation.
Two Other Recommendations
In addition to its recommendation for low-airspeed alert systems and the convening of an FAA/industry panel to determine the technical requirements for such a system, the NTSB issued two other recommendations as a result of investigating the October 2002 crash of an Aviation Charter King Air that killed all eight people aboard (including Sen. Paul Wellstone), as well as other on-demand charter accidents.
In the first of the two remaining recommendations, the Safety Board called on the FAA to require the “conduct of en route inspections” and to “observe ground training, flight training and proficiency checks” of Part 135 on-demand charter operations as is required for Part 121 and 135 airline operations. Under current rules, there is no requirement for FAA inspectors to conduct en route inspections of on-demand charter operators. According to the NTSB, such a requirement might have detected “discrepancies that existed at Aviation Charter.” The Safety Board also expressed concern that the FAA is “relying too much on company check airmen and not enough on its own staff.”
Crew resource management is the subject of the second of the two remaining recommendations. Specifically, the NTSB asked the FAA to require that on-demand charter companies that conduct two-pilot operations “establish and implement an FAA-approved CRM training program” in accordance with the same type of program mandated for major airlines–namely, Part 121 Subparts N and O.