Tag Archives: B737

B737 MAX MCAS: Maneuvering Characteristics or Stability Augmentation or Stall Recovery?

Aerodynamics for Naval Aviators was written by Harry Hurt in 1959 and published by the US Navy in 1960. It is still in print and is still in use as an authoritative handbook for pilots to learn the aerodynamics behind the safe operation of aircraft through all areas of the flight envelope. Whether going very fast, or very slow, whether going very high in the sky or flying low to land, the aerodynamic principles explained in this book have helped many aviators fly safely in all manner and variety of aircraft.

On stall recovery in Chapter One, Basic Aerodynamics, Hurt states, “Recovery from stall involves a very simple concept. Since stall is precipitated by an excessive angle of attack, the angle of attack must be decreased. This is a fundamental principle which is common to any airplane.” (See Aerodynamics for Naval Aviators, Ch 1, Effect of High Lift Devices, pg 29).

In my flying experience, there are two ways to reduce the angle of attack (AoA) and a third step in the stall recovery procedure. The first way to reduce AoA is to lower the nose of the aircraft with the elevator towards or even below the horizon. The second way is to add all the power you have and accelerate the aircraft. The third step is to roll wings level. While this does not directly influence the AoA, this step does re-direct the lift force vertical and opposite the weight of the aircraft.

In Chapter Four, Stability and Control, Hurt writes, “The initial tendency to continue in the displacement direction is evidence of static instability and increasing amplitude is proof of dynamic instability.” He writes further, “In most cases, the contribution of the fuselage and the nacelles is destabilizing.” Under the topic of Longitudinal Dynamic Stability, Hurt writes, “dynamic instability will exist when the amplitude of motion increases.”

(See Ch 4,, ibid, Dynamic Stability, pg 245, 256, 279).

According to public media reports of various B737 MAX mishap investigations, the Maneuvering Characteristics Augmentation System or MCAS moves the elevator trim relatively rapidly and to a large displacement angle, in much the same manner that a pilot, hand flying a stall recovery would move the elevator. (See: https://news.yahoo.com/ethiopian-airlines-crash-mcas-system-boeing-737-max-194126559.html). Interestingly enough however, the MCAS does not engage the auto throttle system to add full power. Not sure why?

So while the concept that the MCAS is improving the longitudinal stability of the B737 MAX, most likely due to engine nacelle longitudinal destabilization factors, this is the engineering argument offered for the MCAS system not being a pilot procedural controlled stability control, such as an elevator, in actuality the MCAS acts very similar to the control inputs of a pilot recovering from a high AoA induced stall, by moving the elevator due to input of a high AoA.

In that regard, it might be a very good idea to:

-first rely on several AoA probe inputs to validate the high AoA data input is in fact a high AoA

-second to display that reading in the cockpit for all to see

– and third let everyone in on what appears to be both a longitudinal stability augmentation system and stall recovery system, so that the owners and operators can maintain it well and operate it safely.

Is the B737MAX Angle of Attack Comparator Systems news story a Distraction from Two Critical Safety Issues?

Is the discussion about the optional equipment of B737MAX angle of attack AoA data comparator systems diverting the discussion from two of the most central safety issues, safety hazard reporting and rapid response, and the full extent of flight crew training?

Why ask that question?
First, the specific hazard of a malfunctioning AoA probe and/or data value input to the flight control system computers, ordering an erroneous massive nose down stab trim control input, was reportedly known to exist on the B737MAX acft before both the Lion Air mishap or the Ethiopian Air mishap.
Who knew and who reported it?
According to reports from pilot groups, 3-5 US based passenger carrier flight crews experienced a similar malfunction and reportedly entered same in the acft maintenance log book, and may have written an airline safety event report as well. Thus at least one or more US certificated airlines may have known of this hazard, meaning that their FAA (principal operating or operations inspector (POI) and their principal maintenance inspector (PMI) either knew or should have known of this hazard, and thus through their Boeing reps, the manufacturer knew or should have known of the hazard. This is to some degree speculation based on reports and the reported procedures for reporting and sharing flight safety hazards between flight crew members, the airlines, the regulators and the manufacturers.

This could mean that FAA and Boeing, if knowing of the hazard,  should have immediately and rapidly issued a safety of flight notice to all B737MAX certificated operators, informing of:
A. the flight hazard
B. the maintenance fix
C. the aircraft operators procedures, either standard, supplemental, non-normal or emergency, as appropriate to respond to this hazard if encountered in operations.

Did FAA or Boeing do this? It is not clear yet how far along this path the FAA and Boeing proceeded. News reports of the Boeing fix underway in January 2019 show that some action was underway prior to the Ethiopian mishap.

Second, and more specifically, a Lion Air flight crew reportedly experienced this hazardous malfunction, on the mishap acft, during the flight previous to the mishap flight. How did Lion Air respond to the reported hazard? Was the hazard reported?  Why did Lion Air maint management and flight ops management assign that acft to fly again, knowing it had experienced the hazardous malfunction on the previous flight? Had that hazard had been resolved? Was the event entered in the aircraft maint discrepancy log book? What possible justification was used to clear the maint discrepancy? Was the airline’s Boeing maint rep asked what to do in this case? Why or why not?

If an additional crew member (ACM), [ACM because he was on the flight deck vs in a passenger seat in the cabin, where he would have been a dead heading crew member DH], a qualified and certificated airman was on the flight deck, assisted the operating crew members by either moving the horizontal stab motor cutoff switches or recommending the same to the operating crew, was that event recorded by Lion Air Flight Safety Manager, investigated and this information promulgated immediately by Lion Air Safety to all Lion Air flight crew members and to their CAA regulators and the manufacturer Boeing? Why or why not?

Why did one flight crew member, the ACM, know how to turn off power to the trim motors with the horizontal trim cutoff switches, and know to turn off the power at that time as a response to the malfunction, but other crew members, and the operating crew members did not?
Boeing B737 Runaway Horizontal Stabilizer Trim Emergency Procedures are published in the Boeing Aircraft Operating Manual or AOM, and Quick Reference Handbook or QRH, and are very similar to the procedures on most or all Boeing passenger acft, such as the B757 and B767. Why did some certified and trained Lion Air crew members know that and others seemed to not know that? Why did the Ethiopian Air flight crew apparently not know that? How is that possible?

Would have having an AoA comparator and disagreement warning system have made a difference if the system did not automatically disengage the horizontal stab automatic trim system known now reportedly as the MCAS Maneuvering Control Augmentation System? If the flight crew was not trained on MCAS, how would a comparator system have helped them? Even without a comparator, merely knowing to flip the horizontal stab trim cutoff switches at the first instance of uncommanded horizontal stab trim movement, should have resolved the runaway stab trim issue enough to allow the flight crew to land the aircraft, in our opinion.

So, in our opinion, these mishaps reveal two massively glaring safety of flight issues:
First, why are known and reported equipment hazards and procedural short falls not resolved quickly, rapidly and universally by the airline, their CAA, EASA or FAA regulators and the equipment manufacturers such as Boeing or Airbus?  Why does the hazard reporting system seem to work most of the time but not all of the time? Is there a regulator difference in hazard reporting system and resolution? If so, why?
Second, why are some flight crew members trained, qualified and certified to operate with only a bare minimum FAA type rating training at some airlines in the global commercial scheduled airline community, and then little or nothing else further, while flight crew members at the three major US based airlines are given four to five times more training subject-wise, twice as deep training hands-on sim-wise, trained to competence and then regularly receive repetitive in the cockpit line oriented safety audits, line checks with immediate feedback, Advanced Qualification Program or AQP reviews, plus take home systems and procedural tests, exposures to safety forums, monthly or weekly ASAP Event Review Team reports and FOQA training reports?
Why is there such an enormous difference in flight crew procedural training?
Do you think that flight crew training and flight safety are closely related and perhaps that is an unaddressed global or international commercial scheduled airline safety issue?

In the opinion of SafetyForecast, two issues may be being ignored here, two issues which are literally the foundation elements for mishap -free commercial scheduled airline flight operations: [Yes we believe that mishap free commercial scheduled flight operations can be achieved by constantly reporting and resolving flight safety hazards and by training flight crew members on all procedures, equipment , limitations available and repeating and refreshing that training regularly.]

First, where is the flight hazard reporting and resolution safety program in non-US based airlines? Does it exist and how robust is it? Does it seek to both report safety of flight hazards and rapidly resolve these hazards? “All safety is local.” Citation: Paul Miller, SafetyForecast.com Better Safe Inc.

Where is the hazard reporting and resolution interface joint safety program between the manufacturer, regulator, certificated commercial scheduled airline and the certified flight crwe member?

Remembering that “All Safety Is Joint”, (Citation Paul Miller, SafetyForecast.com Better Safe Inc.) meaning that safety is really a joint effort between the certificated airmen, the certificated airline, the CAA regulator and the equipment manufacturer.

Second, where is the highly developed, broad based and in depth flight crew procedural based training program, above and way beyond type training,  globally, in many non-US based airlines? Why are some operating flight crew certified in type and then given little if any additional training?
“If you seek one level of safety in commercial airline operations, should you seek one level of training?” (Citation: Paul Miller and David Williams, SafetyForecast, Better Safe Inc and Safety Net Inc.)

There is a much bigger flight safety story here, in our opinion and it involves both safety hazard and resolution programs and flight crew training programs.


Paul Miller, int’l B757/767 captain, retired, 43 years of line and instructional flying
David Williams, US FAA designated airline check airman and airline captain, retired