On October 29th 2018, Lion Air Flight 610 plunged into the Java Sea after taking off from Jakarta with 189 souls on board. There were no survivors.
Since the crash there have been accusations that Boeing withheld vital information about the 737 Max. Information that may have been a contributing factor.
Time for a quick refresher.
An aircraft works like a teeter-totter. It balances at its center of gravity, usually just forward of the wing. The horizontal stabilizer, that little wing in the back, works opposite the big wing to keep the plane from nosing over. If you’re old enough to remember balsa-wood models, that’s what they usually did.
The horizontal stabilizer has two movable sections at the back called the elevators. On my 767 they’re about the size of garage doors. When I pull or push on the yoke, hydraulic actuators move the elevators up and down. Pull back, houses get smaller. Push forward, houses get bigger.
Pretty basic stuff I realize.
However, as the plane changes airspeed, the aerodynamic forces change and I would have to hold the yoke forward or back just to stay level. That’s why we can “trim” the entire stabilizer. That little wing back there pivots up or down by hydraulic motors. As we change speed, we trim the stabilizer so that the yoke stays neutral.
Now this is really important: the stabilizer has more “authority” than the elevators. In an aerodynamic tug-of-war, the stabilizer will win. Remember that for later.
Since that stabilizer is so important we want to make sure it moves when we want it to but stays put when we don’t. Several aircraft have been lost over the years due to “runaway trim”.
The other half of the equation is stall protection. Time for another quick refresher.
How much lift the wing produces is determined by angle of attack or AOA. Angle of attack is kind of like money (go with me here). Some is good, more is better, too much and now you’re writing million dollar checks to the RNC.
Too much angle of attack and the air over the wing gets all discombobulated and the wing stops working. Pull back, houses get smaller. Keep pulling back and the houses start getting bigger again.
We never want to stall an airliner. For one it’s hard on the airframe. Plus everybody in back would spill their drinks. And then there’s whole crashing and burning part.
Modern airliners have various mechanisms to prevent a stall and to warn us if we’re getting close to one. Airbus pretty much won’t let you do it if the controls are in “Normal Law”. The Jacques 9000 computer will say “I am sorry monsieur, I cannot let you do that”. Airbus is big on what they call “envelope protection”. In Normal Law their fly-by-wire system limits tries to keep the plane out of trouble.
Boeing, at least the ones I’ve flown, take a different approach. Boeing’s philosophy has always been “The pilot know’s what he’s doing”. The 757/767 will warn me if I’m about to stall but it will still let me do it.
If the flaps are extended (takeoff and landing) my attitude indicator will display a pitch limit. Looks like a pair of eyelashes. If I were to raise the nose beyond that we would get a stall warning. That’s when the “stick shaker” kicks in and starts vibrating the yoke. After that the “nudger” will try to push the yoke forward. In true Boeing fashion, however, I could override the nudger if I wanted to.
The stall warning is triggered by angle of attack sensors. These are just little weather-vanes mounted on the side of the airplane. Fairly simple devices but they can malfunction like any other component.
Finally we get to the 737 Max and Lion Air.
Now I haven’t flown the 737, but I’m told it’s not nearly the jet the 757 is. Pilots love the 757, corporate bean counters love the 737. Guess which one everybody bought.
The 737 Max is Boeing’s latest and greatest version of the plane they’ve been building since 1967. Better engines, nicer cockpit plus a few other tweaks, but still a 737 at the end of the day.
Apparently Boeing put one additional tweak in the 737 Max and didn’t bother to tell anybody. If the 737 Max senses a stall, it will automatically run the stabilizer trim nose down.
Remember how I said the stabilizer has more authority than the elevators? You can probably see where this is going.
Now we won’t know exactly what happened until the crash can be properly investigated. What it looks like is that a malfunctioning AOA sensor caused the plane to falsely sense a stall and activated the MCAS (Maneuvering Characteristics Augmentation System).
In the 757/767 I can stop the trim from running by moving the yoke in the opposite direction. This is a nice, intuitive system. If the nose starts moving up or down on its own, my initial reaction is going to be pulling back or pushing forward. Especially at 3:00 AM when I’m not exactly firing on all cylinders.
We also get a “UNSCHED STAB TRIM” light and an aural warning if the trim decides to move on its own.
The 737 Max, however, is a bit different. If the MCAS activates, apparently there is no “trim brake”. As long as the system senses a stall, it will keep trying to run that trim nose-down even if you pull back on the yoke. There is a certain logic to this, because in a real stall, pulling back on the yoke is probably what got you there in the first place.
Since the crash, Boeing has stated that pulling the control column on a 737 Max will not arrest stabilizer movement if the dive was caused by faulty angle-of-attack data. "Control column force will not stop electric trimming" reads a Nov. 7 memo from American Airlines to its pilots.
Gee, that would have been useful information.
In this case, it would seem that there was no stall. The MCAS may have reacted to a faulty sensor and tried to fix a problem that wasn’t there. Causing the plane to nose down despite the crew’s efforts. This particular aircraft had previously had issues with an AOA sensor so that seems to be where everyone is focusing right now.
Once you get an airliner nose down, it wants to start to picking up speed very rapidly. Which would cause it to nose down further, which would cause to speed up even faster, and so on.
It looks like JT160 hit the water at an insane descent rate of 30,976 feet per minute! A normal descent is maybe 1,000-1,500 feet per minute and if we’re trying to get down in a hurry maybe 3,000. I would guess it hit the water in a near vertical dive.
Remember, this is all speculation until they do a proper crash investigation. So far we only have the Flight Data Recorder. The Cockpit Voice Recorder has not been found yet.
The reason everyone is pointing fingers at Boeing is because they didn’t bother to tell anyone about MCAS!
So here we have a system capable of overriding a pilot’s flight control inputs and they didn’t let anyone know about it.
Assuming this was a runaway trim scenario, the crew could have stopped it by shutting off power to the stabilizer trim. Most of the jets I’ve flown have a trim cutout switch/switches located somewhere on the center console.
I suspect that not all the blame lies with Boeing here. There are usually multiple factors that contribute to an aircraft accident.
Lion Air has been around since the year 2000 and has managed to destroy 8 aircraft and damage several others in that relatively short time. That’s a pretty appalling safety record. For a while, the US and European Union went so far as to prohibit them from their airspace.
January 14, 2002
737-200 - 0 Fatalities - Aircraft Written Off
Takeoff accident due to incorrect flap configuration
November 30, 2004
MD82 - 25 Fatalities - Aircraft Destroyed
Landing runway excursion. Aircraft hydroplaned.
March 4, 2006
MD-82 - 0 Fatalities - Aircraft Written Off
Landing runway excursion. Left reverser was listed as INOP. Reverse thrust used on landing causing the aircraft to veer.
December 24, 2006
737-400 - 0 Fatalities - Aircraft Written Off
Landing accident. Incorrect flap setting and the aircraft was not aligned with the runway.
March 9, 2009
MD90 - 0 Fatalities - Aircraft Written Off
Landing runway excursion. The pilots continued with a non-stabilized approach. Only one reverser opened, causing the aircraft to pivot around the nose off of the runway.
November 2, 2010
737-400 - 0 Fatalities - Aircraft Written Off
Landing runway excursion. Non-stable approach, speed brakes not used until 42 seconds after touchdown.
April 13, 2013
737-800 - 0 Fatalities - Aircraft Destroyed
Landed short due to unstable approach.
August 6, 2013
737-800 - 0 Fatalities - Aircraft Damaged and Repaired
Landing excursion. Aircraft hit cows on the runway and veered off as a result of trying to avoid them.
February 1, 2014
737-900 - 0 Fatalities - Aircraft Damaged and Repaired
Hard landing resulting in tire damage and a tail strike. Four bounces before the impact.
February 20, 2016
737-900 - 0 Fatalities - Aircraft Not Damaged
Runway excursion. Late touchdown, delayed spoiler deployment, thrust levers not at idle position, and late of brake application. Aircraft came to a stop with the nose gear one meter past the end of the threshold.
April 29, 2018
737-800 - 0 Fatalities - Aircraft Damaged and Repaired
Runway excursion. Heavy rain was reported as they touched down and veered off to the left of the runway.
October 29, 2018
737-MAX8 - 189 Fatalities (Not confirmed) - Aircraft Destroyed
Aircraft crashed into water shortly after takeoff.
Rarely is there a single cause for an aircraft accident. We always talk about the “chain of events”. In this case the chain may have been both an equipment failure as well as a human failure. In my mind this still doesn’t excuse Boeing for not telling us about MCAS.
As my union (ALPA) likes to say “The best safety feature is a well trained crew.”