How the 737 MAX Flew into Aviation Infamy

Four months after Lion Air Flight JT610 plunged into the sea, killing 189 people; ten days after Ethiopian airways Flight 302 dug a crater, atomising its 157 passengers and crew, there is still a mystery surrounding the Boeing 737 Max aircraft.  No – not why these planes crashed, but why “the why” has not been told. Here’s the real story.

Four years ago, the aviation press was agog with excitement about the race between the two great airline manufacturers, Boeing and Airbus, to produce the latest version of their popular narrow-bodied single-aisle airliners. The Boeing 737 had long been pitted against the Airbus 320 series. More than 7000 versions of the 737 are flying; various airlines operate more than 8000 Airbus 320s and 321s.

Airbus won the race. Its A320 Neo – largely unchanged except for new engines and “sharklet” winglets – went into service with Lufthansa in January, 2016, and quickly garnered 6500 orders. Boeing’s first 737 MAX – a re-design with similarly more powerful and economical engines — went to Indonesia’s Malindo (a subsidiary of Lion Air) in May, 2017. Loyal Boeing customers placed 5000 orders for the plane.

Boeing’s modifications placed the new, bigger engines further forward; to compensate for the change in trim, changes were made to the nose, the tail assembly and a new “split winglet” was designed. And a new piece of automation to help pilots cope with emergencies was added to the flight-control system. Enter MCAS – the manoeuvering characteristics augmentation system. It was, helpfully, to over-ride the controls to push the nose of the plane down, when an imminent stall was detected.

The MCAS reacted to inputs from instruments indicating airspeed and angle of attack – that is the angle of the wings to the airflow. A stall occurs when the angle is too great for that airspeed; the result is the lift provided by the wings is too low to keep the plane flying. Stall recovery involves lowering the nose, picking up airspeed and thus increasing the lift. That is what the MCAS was supposed to do, automatically, especially at the high speeds of modern jets, when a high-speed stall is highly unlikely, but particularly dangerous.

When Indonesia’s National Transportation Safety Committee (NTSC) analysed the information on the flight data recorder from JT610, the chain of events that caused the accident was immediately obvious:

  • The angle of attack sensor was faulty and wrongly indicated the plane was climbing too steeply.
  • The “stick shaker”, a Boeing device to warn the pilot of impending stall, began vibrating the control column as soon as the plane became airborne.
  • When the flaps were retracted at 3000 feet, the MCAS started trying to push the nose down.
  • Twenty-one times, the captain cancelled the action with the thumb trim switch on the control column, and raised the nose. Each time the system replied by raising the nose. He then handed command to his co-pilot.
  • The co-pilot quickly flicked his thumb switch twice. The plane went into a  steep dive.
  • Ten minutes after takeoff the plane had lost 3000 feet. Its last recorded height was 2500 feet.
  • The captain took back control and tried with all his might to pull back on the control column, but he could not stop the dive. The plane hit the sea at  500 kph.

It soon emerged that the flight manual for the 737 MAX made no reference to the MCAS. The system was supposed to operate in the background, coming into play only in an emergency. A month after the Lion Air crash, Boeing issued a bulletin explaining the characteristics of MCAS. In so many words, it clarified that:

  • Erroneous AOA data could be cancelled or reversed with the trim switches on the control column.
  • Even so, the nose-down operation would re-start itself after 10 seconds.
  • Brute force on the control column could not override the system.
  • The only way to stop what it called “uncommanded nose-down stabiliser”  was to turn off both Stabiliser Trim Cutout switches.

That is what the crew did when they struck the same problem on the plane’s previous flight, from Denpasar to Jakarta. The first release of information from the cockpit voice recorder (CVR) indicates that first one pilot, then the other, was frantically trying to find an explanation for the aircraft’s behaviour in the manual. They either didn’t know about, or think about, the trim cutout switches. The cockpit voice recorder may tell us why the JT610 crew didn’t do that, but the transcript won’t be released until the final report, between August and September.

Now, more information has emerged about the inherent design weaknesses of the MCAS. First, although the 737 MAX has two angle-of-attack sensors, MCAS is connected to only one. That breaks a cardinal rule of aircraft design – not to have a single point of failure. Second, each operation of the pitch trim system produces a 2.5 degree change in the tail stabiliser. The FAA certification documents are reported to have shown an effect of only 0.6 degrees. So the two flicks of his switch by the Lion co-pilot produced a 5 degree irreversible nose dive.

Crew of Ethiopian Flight 302 this month had even less time than the Indonesians to deal with their problem of “uncommanded nose-down stabiliser”. Their plane reached a height of only 3000 feet when controllers heard a scared voice asking for permission to return to Addis Abbaba. A minute later, the dot on their radar screens disappeared.

When the flight data recorder was recovered, Ethiopian Airlines opted to send it not to the NTSC in the     US, but to BEA at Le Bourget airport, near Paris. The Bureau d’Enquêtes et d’Analyses pour la sécurité de l’aviation civile had determined the cause of the mid-Atlantic crash of Air France Flight 447 from their examination of its recorder. BEA quickly issued a public statement drawing attention to the significant similarities with the Lion Air crash. It said nothing else.

What is known, however, is that in both incidents the aircraft were travelling far too fast. Airspeed below 10,000 feet is normally restricted to 250 knots (460 kph), but the Lion Air plane was flying at 300 knots, and the Ethiopian plane at 400 knots. That suggests the pilots increased power to counter what they believed to be an impending stall.

The Lion Air Boeing had made three trips with the faulty AOA sensor before the fatal flight. At Bali, mechanics changed one sensor; perhaps it was the wrong one. The FDR data  showed that there was a 20 degree difference between the readings of the two sensors on the plane – even when it was on the ground!

A further complication is the revelation in American press reports – led by the Seattle Times in Boeing’s home town – that the certification of the MCAS was delegated to Boeing by the Federal Aviation Authority.

Aircraft accidents rarely have a single cause, so many investigations reveal a tragic convergence of errors, mistakes, misjudgements or mechanical faults, trivial in themselves but deadly in conjunction. So it seems with the 737 MAX.

An automated system designed to save pilots at a time when they are likely to be under most pressure, not properly executed or explained, flummoxed two flight crews when confronted with its malfunction. Whether from inexperience or sheer panic in the face of the unexpected, they failed to take the correct action to save their plane and passengers.

It is already obvious what needs to be done to make the 737 MAX a safer aeroplane, and it’s under way. The MCAS will take data from both AOA sensors, the movement of the tail stabiliser will be limited, and the system will be restricted to only one, instead of repeated actions.

What is not at all clear is where these tragic stories will end, and how much it will cost all those involved.

Geoffrey Luck, a veteran pilot, was an ABC journalist from 1950 until 1976. In January, 2014, he recalled how inexperience and power lines very nearly cost him his life

  • Doubting Thomas

    Very interesting, Geoffrey. An old RAAF air traffic controller friend found this on YouTube. Forgive me if you’ve already seen it. Confirms what you are saying.


  • Lacebug

    Heck, I love Geoffrey Luck’s writing.

  • Doubting Thomas

    Don’t we all?

  • chuckp61

    I’m sure Boeing will rue the day they decided to follow the path they followed with the MCAS system in terms of pilot education and PARTICULARLY the single point failure aspect of only the captains AOA/ASI feeding data to the MCAS – but having said that these crashes were pure crew incompetence.

    What ever the underlying cause the crew were presented with a Stab Trim Run Away for which there is a memory NNC (Non Normal Checklist). That is a procedure that is REQUIRED to be second nature because there is not time to consult reference material contained in the QRH (Quick Reference Handbook)

    There is also a memory NNC for unreliable airspeed.

    If on takeoff you suddenly get stick shaker just airborne you have either;

    A/. Ignored the ASI crosscheck at 80kts
    B/. Something else is amiss.

    What that something else might be is largely irrelevant and whether you understand what it is has no relevance either – your immediate actions are identical. You implement the Unreliable Airspeed memory items. These are prescribed thrust settings and pitch attitudes which ENSURE the aircraft CANNOT be stalling OR overspending. You can now get the aircraft away from the ground so you have the aircraft in a safe configuration/altitude and TIME to get the copilot to pull out the QRH – within which is all the information needed to get the aircraft back on the ground.

    You probably wouldn’t retract the flaps in this scenario – you’re not in a hurry to go anywhere so you’d just fly around with your takeoff flaps out until it was time to put the wheels down and add more flap to land. But just say you’ve taken off in really bad weather so you decide to clean up and track for your departure alternate airport where the weather is fine – perfectly good decision.

    Now when the flaps are retracted all of a sudden the aircraft wants to behave like a lawn dart. It doesn’t matter whether you understand why there is;

    A/. Reflex actions that all pilots would do
    B/. Visual and aural clues as to the problem
    C/. A memory NNC.

    Any pilot experiencing the nose going down will pull back on the stick and add some nose up stabiliser trim with the trim switch under his thumb on the control column. When he released the switch and the nose down trimming resumes would he really NOT NOTICE the large manual trim wheel clattering away by his right knee (another by the copilots left knee)?

    Stab Trim Runaway!

    It doesn’t matter whether you understand what is causing it – doesn’t matter whether MCAS is an utter mystery to you or that Boeing never mentioned it – there is a memory NNC!!

    Hold your left thumb down on the trim switch to stop further stab movement and select, or get the other pilot to select, both Stab Trim Cutout switches to cut out. Re trim manually and continue on to a safe landing.

    Notice where these crashes happened – there is a reason for that. The next big step in improving air safety is politically impossible thus won’t happen – that’s why both Boeing and Airbus go all out to design idiot proof aircraft – but various airlines keep finding better idiots.

    Boeing B737-800NG Training Captain.


    An operator has to sack dumb pilots. I sacked three out of my 25 year operation. They went on to kill themselves flying for other operators. VH-EGT, VH-DIL and VH-AMF. [AMF was 25 years ago and still not on the ATSB website, but you have to be patient with government workers]. As chuck says, some airlines specialize in recruiting dumbos; they are cheaper for a while – the pilot of VH-DIL was flying for hours only, and mustering cattle all day. He used to sleep on the autopilot. Autopilots don’t have the survival instinct and he woke up in a 300 knot spiral dive. EGT attempted takeoff with the parking brake applied. AMF mishandled a engine fire on startup.

  • brandee

    Like Lacebug I love the writing of Geoffrey Luck and was struck by his pithy comment in the Oz today [next to the Johannes Leak cartoon]:

    “The NSW election result was bad news for the ABC. Its reporters face another four years in opposition”.

  • pgang

    T B Lynch, it’s tragic when we allow the wrong people to take control. In my own industry we are seeing ever more inexperienced and often incompetent people making important decisions because proven people are considered to be too expensive. It is a dreadfully false economy. If there is one thing that a business should never under-invest in it’s people.
    It looks like the crash at Essendon was due to similar circumstances. The pilot took off with full left rudder trim, killing himself and four passengers. How does that happen?
    Even in the early days of aviation, redundancy was understood. I’m building a flying scale model of a Sopwith Camel, and one of its interesting features was a propeller driven pump that was intended to pressurise the fuel system. In case of (inevitable) pump failure, the pilot also had a hand pump in the cockpit. Of course that didn’t stop the Camel from killing a notable percentage of its pilots…

  • pgang

    Chuck it was of significance, to my mind, in noting the airlines that had these crashes. Thanks for confirming those suspicions. The aircraft has faults that require sorting, but nothing that should have been fatal. Perhaps the A320 has a few too, who knows. I think it is becoming ever more imperative for travellers to choose their airlines wisely. There are many operators out there that are simply incapable of dealing with the management of complex passenger airliners.

  • chuckp61

    Boeing and Airbus are in business to sell aeroplanes. They know what is going on but they are too afraid to make a noise about pilot standards in the, shall we say, less developed parts of the world, so they work hard to make their products as idiot proof as possible. Unfortunately there is no end to human engenuity at the stupid end of the scale.

    I was having this discussion with a mate this morning and we both reckon it’s going to get so bad in the next few years the aircraft manufacturers will be forced to intervene.

    Look at the reputational and financial damage Boeing is facing right now – all down to pilots who should not have been in the seats they were occupying. I would bet next months pay the test pilots and Boeing management are pulling their hair out screaming (behind closed doors) “FFS it’s not brain surgery!!!”

    It’s been very clear to me, and my peers, for years that if the same standards were imposed on new airline pilots that were imposed on me and my peers 30 years ago Boeing and Airbus would have sold a shitload less aeroplanes.

  • pgang

    Interesting, but I hope you’re wrong about the accident rate. (Although I guess it’s not just accidents either – why would an airline fly over a combat zone?).

    It seems to be a real conundrum. If they’re not building and selling enough aircraft to make a profit, what then?

  • Roger Franklin

    Via email, Jim Eames writes:
    Geoffrey old son, just a few comments as a former journo and later Director of Public Affairs for QF:
    “Revealing for the first time -…” the story of the DH-86 is not quite right: Two of Hudson Fysh’s books refer to its problems, along with John Gunn’s volume of the Qantas history, not to mention numerous other articles over the years.

    As for the myth of Qantas never having lost a passenger: I attempted to put that to bed in my Flying Kangaroo book several years ago. The myth exists but for anyone who ever asked the correct QF response was: No we never lost a passenger since the jet age. We certainly lost a few before then as the appendices of the foregoing books illustrates. The problem is, while we made it clear as often as we could, not enough journos bothered to ask.
    Best wishes,
    Jim Eames

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