737 MAX Update

I haven't written about the saga of the MAX lately because there hasn't been much change in the situation concerning the grounded airliner. Progress is being made in fixing the MCAS system implicated in the two 737 MAX crashes, and estimates for the ungrounding of the aircraft range into the July-August timeframe.

The FAA recently convened a multi-agency Technical Advisory Board to review Boeing's proposed software fix for the MCAS system. The results of that review will be needed prior to FAA approval of the design changes.

That said, there have been a number of stories brought to light as to how the MCAS system came to be designed, and some more disturbing revelations about Boeing failing to disclose an inoperative warning feature to its customer airlines.

MCAS: What It Is and What It Is Not

The Maneuvering Characteristics Augmentation System (MCAS) has been routinely described in popular media stories as a stall prevention or mitigation device. It is really neither, but rather is a system designed to make the MAX "handle" just like the older Boeing NG series aircraft it replaces.

During flight testing of the MAX, test pilots and engineers noticed that in a small corner of the flight envelope: lightweight, aft center of gravity (CG) and approaching a stall, the forces on the stick varied from the NG version of the aircraft. MCAS was introduced in order to counter this divergence in longitudinal stability between the two models to make them handle the same. The genesis for the difference in handling is due to the MAX having larger, heavier engines which are set further forward on the wing for ground clearance.

The need for identical handling between the two aircraft was to maintain a common "type rating" on both aircraft thereby allowing pilots qualified on earlier versions of the 737 to fly the new aircraft without extensive training. Airline pilots, unlike, say, flight attendants, cannot fly separate types of aircraft but are generally only qualified on one "type" of aircraft (at a time).

The word "type" has a very specific technical definition in that the FAA designates which aircraft fall under the specific "type rating". For instance, being "type rated" in the 737 allows pilots to operate all the various sub-models of that series (-200, -300, -400, etc) without an extensive course of study for each sub-model. The Boeing 757 and 767 were also given a shared type rating as those aircraft were considered similar enough that pilots could fly both of them under a single "type rating". The same is true for the Airbus A320 series of aircraft. These ratings are annotated on all pilots' licenses.

As we now know, the MCAS system was flawed in its design due to being able to be triggered by a single angle of attack (AOA) sensor, and also by the ability of the system to reset itself and re-engage multiple times without limit. The question of how and why this design flaw happened is the subject of multiple investigations into the certification process.

Inoperative Angle of Attack Warnings

The WSJ has done some excellent investigative reporting on the MAX story and revealed recently that not only did the MAX aircraft not have a specific AOA warning indication which had been included on the earlier NG models, but that Boeing engineers were unaware that the warning on the MAX was inoperative. Furthermore, Boeing delayed notifying their customer airlines of the situation for nearly a year.

Cockpit angle of attack indicators or displays are not "primary flight instruments" such as airspeed, altitude, and attitude indicators. Pilots use primary flight instruments to directly fly the aircraft. An angle of attack indicator, however, is not required to operate most aircraft and is usually not included in cockpit displays. 

An analogy might be to a tachometer in your car. Nice to have, but not needed to safely drive. Much outrage has been vented over Boeing's not including this cockpit indicator as standard equipment, but I don't see it that way. AOA cockpit indicators are simply not needed for safe flight.

Angle of attack sensors themselves, from which AOA information is derived, are small vanes on the exterior of the aircraft. They are used to send AOA information to an airliner's flight control computers to provide "stick shaker" stall warnings. There are two AOA sensors installed on the 737 and a cockpit warning "AOA Disagree" would display should the two indicators return different readings, indicating a malfunction in one or both.

It was this "AOA Disagree" warning which was inadvertently deactivated on the MAX aircraft. Had the AOA only served its previous function of activating stall warning, this would be no big deal. But because the MCAS system was triggered by a single AOA sensor, not having this warning quickly became a very big deal. 

A single malfunctioning AOA sensor has been implicated as a possible cause for the inadvertent activation of the MCAS system on both the Lion and Ethiopian crashes. Having this alert enabled might have helped the pilots of those aircraft figure out what was going wrong.

Pinto, Tylenol, MAX?

It is becoming apparent that the MAX will be back flying at some point, but the question now arises as to how well Boeing will weather the ongoing tsunami of negative PR. Even President Trump weighed in on this question recommending that Boeing rebrand the aircraft.

If you recall, the Ford Pinto never rose above safety concerns after several accident caused fires and the model was eventually discontinued. The response to the Tylenol poisonings, however, is now considered a textbook example of how to manage a public relations crisis. One thing learned is that transparency and being forthcoming in light of a tragedy is essential. Boeing has only made matters worse by their perceived lack of candor.

My guess is that once the MAX is back in the air, the crisis will be quickly forgotten. My reasoning is that the public has a notoriously short memory for these sorts of things. Other aircraft have had spotty beginnings and went on to become successful. The MAX, which is still a 737 at heart, has a long and enviable safety record. As the aphorism states, the dogs may bark, but the caravan moves on.

Captain Rob Graves is a veteran airline pilot and retired Air Force officer. He currently flies a Boeing 737 for a major American airline where he has over 25 years of experience. His Air Force career included instructing future USAF pilots in the T-37 primary jet trainer, aerial refueling in the KC-135 Stratotanker, and conducting worldwide logistics in the C-5 Galaxy cargo aircraft. He is the author of This is Your Captain Speaking, an aviation blog. It can be found at robertgraves.com. He also writes for Avgeekery.com.