It’s More Than Angle of Attack
By Captain Shem Malmquist
As many in the flight test community know, Pete Reynolds (Learjet, later Bombardier) was one exceptional test pilot. Pete possibly had more time in transport category jets (Learjet) beyond the actual stall angle of attack than any other test pilot in history as a result of the work to mitigate the stall characteristics in the Lear. He was prescient in many ways. Prior to his death he worked with one of his favorite engineers, Andy Skow, on a system to alert pilots and bring them rapidly “into the loop” when the energy state of the aircraft was at risk of becoming dangerously low. Note the term “energy state”. For the record, I have no connection with Andy aside from knowing about his product and discussing the system with him and others.
Most current low energy alerting systems set their alerts only on measured angle of attack. The alert thresholds are presented on the airspeed tape utilizing an algorithm that “synthetically” determines the airspeed appropriate to the measured angle of attack. To avoid nuisance alerts, system alerting thresholds created from this algorithm result in airspeeds lower than needed to mitigate the risk. Earlier alerting is better as long as false alarms are mitigated. This builds in more time for pilots to understand the problem and respond correctly.
A delay in crew alerting means the energy state is potentially too low to take corrective action. While current stall detection systems are based on angle of attack, there is another issue related to the problem of low energy. The challenge is not too high an angle of attack, nor is it lack of airspeed. Assuming high lift devices are not changed, the pilot has two controls to the lift, changing angle of attack or airspeed. Essentially, pitch and power. As Skow and Reynolds explain, the issue is not angle of attack nor lack of airspeed. It’s lack of lift. By measuring lift it is possible to add a significant amount of margin to detect a deteriorating energy state. By adding airspeed to angle of attack in a low energy alerting system it is possible to set the alerts much earlier while avoiding the potential for a false alarm. The sooner a pilot perceives an issue the sooner the pilot can mitigate the problem.
Skow and Reynold’s system, called Q-Alpha, normally sits quietly in the background not disturbing the normal flow of actions on the flight deck. It responds when sensing the impending low energy state. The design is such that it is instantly recognizable, clear and unambiguous, especially to a distracted pilot. This approach brings a pilot rapidly back into the frame of reference needed to fly an airplane. The pilot rapidly receives the information needed to eliminate the surprise factor.
This may not be the only solution, but concepts like these deserve a close look. Like any important innovation, the Q-alpha system still needs some work to achieve certification. The alerting system must demonstrate adequate performance under various failure conditions (e.g., the angle of attack system), gusts, icing, fight control malfunctions and more, all of which would be part of a detailed certification plan. As outlined, Q-Alpha would not only support low energy alerting but would also mitigate the risk of too much energy which can lead to runway excursion. This solution is quite inexpensive and can be retrofitted easily on existing aircraft. Q-Alpha would not affect existing aircraft systems. I wonder why the FAA or similar organizations have not dedicated some research funding or support to determine the effectiveness the Q-Alpha system as an accident mitigator? Are we going to continue to blame pilots for not paying attention when flying with automated systems that encourage them to not pay attention?
A system such as Q-Alpha might actually have prevented accidents as diverse as what happened to Asiana 214 in San Francisco or Air France 447 over the South Atlantic, stretching to mitigating the factors leading to the Southwest runway excursions at Midway and Burbank. The system would also alert for too much energy in an approach environment.
Another high risk area, particularly (but not exclusively) for general aviation, are stall/spin accidents. Lockheed test pilot Sammy Mason wrote:
“Many accidents that fall under the category of stall/spin do not involve a spin and are not full stalls. They are…the result of ‘mushing,’ that is, being almost or partially stalled. Because the airplane didn’t ‘feel’ stalled, the pilot felt secure. Then, too often, the first reaction is to further increase the angle of attack instead of applying power. The moment the pilot pulls back on the stick/wheel, the airplane stalls. Whether or not the airplane spins at this point depends on the characteristics of the airplane”[i]. A system such as Q-Alpha might capture this sort of event as well.
Reprinted from Curt Lewis’ Flight Safety Information. Subscribe to FSI at http://curt-lewis.com/