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.

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