MCAS didn’t bring the 737 MAX down, It played a major role, but something else did and it looks like it is going to jeopardize the 737-MAX-10.

On March 24th Dominic Gates published an article titled “FAA tells Boeing it doubts MAX 10 will meet safety standard deadline” Gates is one of my favorite aviation writers. This article holds the answer to what the FAA really thinks caused the accidents, probably Mark Forkner’s not guilty verdict of and the fate of the MAX-10 program.

MCAS was initially designed to help mimic 737NG flight characteristics on the 737MAX at high speed. It was later given bigger authority over the flight controls than initially intended. Mainly to override pilot commands at slower speeds to help protect the crew from stalling by applying forward trim.

Installing MCAS on the MAX, was also the first time Boeing gave the airplanes computers the authority to override the pilot. Without fully understanding the implications on pilot perception and Human machine interface and interaction.

On older 737 models you cannot trim the airplane in the opposite direction to the pilot’s yolk or trim commands. MCAS was the first time for this to be allowed – overriding pilot inputs.

The cause of both accidents; the Lion Air and the Ethiopian was a common one, faulty Angle Of Attack AOA sensor. The faulty sensor in both airplanes transmitted an extremely erroneous AOA information – which in turn caused the MCAS activation eventually leading to the loss of control.

But a faulty AOA transmitter causes two more phenomena, It triggered a “Stickshaker” i.e. stall warning, warning the pilots from an impending stall.

It also caused erroneous Airspeed indication – leading to an Unreliable Airspeed condition.

This requires some elaboration but simply put, Airspeed is calculated by subtracting the static pressure from the Total pressure. While the Total pressure tube is measuring away from the airplane’s exterior skin this is not the situation with the static sensor. It measures the pressure on the skin. This is making it exposed to friction and turbulent flow. AOA sensor is used to compensate for these influences. Now we can understand why a faulty AOA will cause an Unreliable Airspeed situation.

In a 737 the pilots get an obvious Stickshaker (stall) warning – The stick is vibrating and rattle noise in the cockpit.

A not-so-obvious Airspeed unreliable caution – namely an amber “IAS DISAGREE” flag in small fonts, on the PFD indicating the two indicators disagrees. This is the best-case scenario. if they are both wrong, suffering the same error – this flag will not display. There is no alert for a faulty stabilizer trim other than the very loud sound of the trim wheel turning.

If you were to induce the same three malfunctions on a 787 the indications would be: A red Warning for the STABILIZER together with a Siren, indicating it is the highest priority, an Amber “UNRELIABLE AIRSPEED” indicating an impending non-normal checklist waiting, and a Stickshaker.

Both accidents crews were faced with the same three malfunctions – Airspeed unreliable, Stall warning, and a runaway stabilizer, accompanied by all the bells and whistles, loud sounds and noises.

These might be the three most stressful warnings a pilot will ever see at any given day, let alone all three at the same time.


Now the crews had to choose what was the bigger threat to them, both crews chose Airspeed unreliable as the highest threat. Both crews lost control of the airplane due to the MCAS commanding a nose-down stabilizer trim based on a faulty sensor.

Let’s go back to 2010. In December that year the FAA finally issued an AC titled “FLIGHTCREW ALERTING” AC 25.1322-1 This AC specified the guidelines.

The philosophy set forth by the AC is simple – pilots need assistance prioritizing non-normal situations therefore the Airplane manufacturer should develop a system to support Alerting, Urgency prioritization and its presentation. (Section 5)

NASA, the NTSB and others came to a conclusion that such an aid is required to assist pilots during non-normal situations characterized by stress and high workload.

All modern airplanes have such a system., Gulfstream calls it CAS, Boeing, and Embraer, use the acronym EICAS and Airbus uses ECAM on their airplanes. All three systems employ the same concept, Prioritizing and displaying the various warnings cautions and advisories – using visual and aural cues, Separating to different levels of severity and urgency.

Some use Electronic Checklist ECL and some do not.

On airplanes I have flown Runaway Stabilizer is a red warning while airspeed malfunction is a lower-level caution.

Not so on 737. The first 737 was built in the late 60s. This technology was not available back then. The 737-warning system is not really a system. It consists of a warning light panel, and other lights somewhat easily missed, of sounds and lights scattered in the cockpit and in the case of the stabilizer – not even that. The only way to identify a runaway stabilizer is by feeling the pull on the yolk or seeing the trim wheel rotate.

This AC was released in 2010.

737 is the only commercial jet by Boeing that does not meet the AC requirements.

But in 2014 the financial pressure from Airbus’s A320 neo was mounting and Boeing announced the 737MAX program.

The aviation industry, Boeing’s airline clients, and Boeing’s Brass demanded minimum investments and minimum operational changes to the MAX compared to the NG.  One airline (most likely SWA) even negotiated a one million dollars fine for Boeing in a case where a simulator training for pilots will be required by the FAA FSB.

It is detailed in the M Forkner indictment

Boeing’s lobbyists on the hill were successful in getting a waiver for EICAS requirement. It was based on the excellent safety record of a single serious accident for every six million takeoffs. And a ten Billion dollars price tag and job losses to boot.

At the end of the day the FAA granted permission, a waiver was granted.

The reason these airplanes crashed is not just MCAS. It is the combination of multiple severe conditions immense workload on the crews, and the lack of an existing aid installed on the 737MAX. Runaway stabilizer is easily resolved by flipping a couple of red-guarded switches.

In wake of the two accidents the US congress decided to conduct its own investigation.

One of the major outcomes of the congressional investigation was the “Aircraft Certification, Safety, And Accountability Act” setting forth grounds for a robust supervision of the airplane type certification process.

Still, this act gave a specific waiver to the 737MAX-10 type certification until the end of 2022. As Boeing argued, solving the MCAS issue will reduce the likelihood or probability of accident in terms of Risk Management to an acceptable level.

At the time, it was expected that the MAX-10 certification process, will meet this deadline. It is quite clear now that it will not.

Cancelling the 737MAX-10 program will probably carry a much higher price tag than the 10 billion dollars price tag put on introducing EICAS on the 737MAX design.

Make no mistake, The 737 still has one of the best safety records of all flying airplanes.

The MAX today is a much better airplane than it was in 2014 when the first waiver was given.

But it is not just Boeing and the MCAS coverup it is many within the industry that pushed for this exemption.

Boeing’s lobbyists have their work cut out for them.

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