Here is what the NTSB had to say about it in the Final Report about the Wellstone crash. Comments?
2.10 Low-Airspeed Alert Systems
Current Federal airworthiness standards require that airplanes be equipped to provide a clear and distinctive stall warning to the flight crew at a speed that is at least 5 knots higher than stall speed. However, 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.
The accident airplane was equipped with a stall warning system designed to sound a horn in the cockpit 5 to 8 knots before the actual stall speed of the airplane in any configuration. However, as discussed previously, because the airplane was not equipped with a CVR, because of the approximate nature of the airspeed calculations, and because abrupt airplane 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 accident flight crew failed to maintain airspeed during the approach. As discussed in section 2.3, radar data indicate that the accident flight was operated below Aviation Charter.s recommended approach speed for about the last 50 seconds of the flight.
The Safety Board has investigated numerous accidents and incidents involving commercial flight crews that inadvertently failed to maintain adequate airspeed. For example, the Board has investigated at least 11 events since 1982 involving Part 135 flights and at least 7 events involving Part 121 flights in which 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 addition, the Board has investigated other events in which the drag associated with airframe ice and pilot inattention led to a critical loss of airspeed. Failure to maintain airspeed during these flights resulted in catastrophic and other unsafe circumstances, such as loss of control, impact with terrain or water, hard landings, and tail strikes.
A 1996 FAA Human Factors Team86 Report titled, .The Interfaces Between Flight Crews and Modern Flight Deck Systems,. expressed concern about the history of accidents involving lack of low-airspeed awareness in the context of flight crews monitoring automated systems. This report states the following:
“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.”87
This accident history was also cited by the Flight Guidance System Harmonization Working Group of the Aviation Rulemaking Advisory Committee (ARAC
), when, in March 2002, it proposed revisions to FAR
25.1329 and AC
25.132988 to provide low-airspeed protection and alerting during autopilot operations for newly certified transport-category airplanes. The proposed regulatory revision would require, .[w]hen the flight guidance system [FGS] is in use, a means ‘to avoid excursions beyond an acceptable margin from the speed range of the normal flight envelope’. The proposed new AC
, which is intended to provide an acceptable means for showing compliance with this new requirement, states the following:
The requirement for speed protection is based on the premise that reliance on flight crew attentiveness to airspeed indications, alone, during FGS.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 FGS operation..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,  for the landing flap configuration has been found to be acceptable.
The proposed changes to FAR
25.1329 reflect the advanced avionics capabilitiescharacteristic of modern transport-category airplanes. However, the Safety Board notes that a low-airspeed alert system has been developed for Embraer EMB-120 airplanes; installation of the alert system was mandated by FAA Airworthiness Directive 2001-20-17.91 The system is designed to alert flight crews of low-airspeed conditions in certain airplane configurations and in icing conditions through the use of an amber-colored indicator light installed in the control panel and an audible alert. The Board is also aware that several avionics manufacturers offer low-airspeed alert devices associated with approach and maneuvering speeds for use in less sophisticated general aviation airplanes. This demonstrates that it may be feasible to develop low-airspeed alert systems for most airplane types.
2.10.2 Need for Improved Low-Airspeed Awareness
The Safety Board recognizes that the development and requirement of a low-airspeed alert system is a departure from the previously accepted premise that adequate low-airspeed awareness is provided by flight crew vigilance and existing stall warnings. However, the circumstances of this accident and the history of accidents involving flight crew lack of low-airspeed awareness suggest that flight crew vigilance and existing stall warnings are inadequate to reliably prevent hazardous low-airspeed situations and that this unsafe condition is not unique to autopilot operations or flight in icing conditions. If a low-airspeed alert system had been installed on the accident airplane, it might have directed the attention of the flight crew to the airplane.s decaying airspeed in time for them to initiate appropriate corrective action. For example, if a low-airspeed alert had activated when the airspeed dropped below 1.2 Vs (about 92 knots for the accident airplane), the flight crew would have received about 15 seconds advance notice before reaching the airplane.s estimated stall speed. In addition, if the flight crew had maintained an airspeed at or above the threshold set by such an early low-airspeed alert, the additional airspeed could have prevented an accelerated stall initiated by an abrupt last-second maneuver or provided an improved speed margin above a premature stall caused by ice accumulation on the wings.
This change in philosophy is evident in the ARAC
.s proposed changes to FAR
25.1329 and AC
25.1329, Embraer.s requirement for a low-airspeed alert system on the EMB-120, and the fact that several avionics manufacturers offer low-airspeed alert devices for general aviation airplanes. The Safety Board supports this change in philosophy. A low-airspeed alert associated with the minimum operationally acceptable speed for a particular phase of flight would likely help flight crews maintain airspeed awareness in much the same way that altitude alert systems help flight crews maintain altitude awareness. Enhanced airspeed awareness would also likely provide an additional safety margin against stall and loss of control events at low altitudes where recovery is difficult, as was the case in this accident.
The Safety Board 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.92 The Board encourages the FAA to consult with representatives from NASA and other aviation industry specialists in resolving and addressing these issues. 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. Therefore, the Safety Board believes that the FAA should convene a panel of aircraft design, aviation operations, and aviation human factors specialists, including representatives from NASA, to determine whether a requirement for the installation of low-airspeed alert systems in airplanes engaged in commercial operations under 14 CFR
Parts 121 and 135 would be feasible, and submit a report of the panel.s findings. The Safety Board further recommends that if the requested panel determines that a requirement for the installation of low-airspeed alert systems in airplanes engaged in commercial operations under 14 CFR
Parts 121 and 135 is feasible, the FAA should establish requirements for low-airspeed alert systems, based on the findings of this panel.
86 This team comprised FAA and industry representatives.
87 Federal Aviation Administration, Human Factors Team Report, The Interfaces Between Flightcrews and Modern Flight Deck Systems (Washington, DC: FAA, 1996).
88 Similar changes were also proposed to Joint Aviation Requirements (JAR) 25.1329 and Advisory Circular Joint 25.1329. The FAR
/JAR and ACs are currently titled, .Automatic Pilot System.; the proposed new titles would be .Flight Guidance System..
89 Vs is the stall speed or the minimum steady flight speed at which the airplane is controllable.
90 Vsr is the reference stall speed.
91 This low-airspeed alert system was developed as a result of the January 9, 1997, accident involving Comair flight 3272, an EMB-120RT that crashed near Monroe, Michigan, during a rapid descent after an uncommanded roll excursion in icing conditions and the March 19, 2001, accident involving Comair flight 5054, an EMB-120 that departed controlled cruise flight and descended 10,000 feet after it encountered icing conditions.
92 Some of the issues that should be addressed include the following: 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 (see, for example, A.R. Pritchett, .Reviewing the Role of Cockpit Alerting Systems,. Human Factors and Aerospace Safety Vol. No. 1 : 5-38), differentiating such an alert from other kinds of cockpit alerts and warnings, and developing flight crew procedures on and training for the use of such systems.