In a recent question, one of the answers states:

MCAS has nothing whatsoever to do with stall recovery or stall behaviour.

It was implemented strictly to make the MAX's pitch stick force response in certain regimes match older 737s, to avoid an additional training requirement necessitating Sim time.

But after reading this article, I'm confused about what exactly MCAS was intended for, as I thought it was stall recovery.


This website has a very comprehensive description and analysis of MCAS: http://www.b737.org.uk/mcas.htm

I find it to be the most accurate of all the various sources often quoted.

Here is a small quote from that website:

MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.


Don't go by Wikipedia. Read the congressional report on the issue here. Stall comes into the issue in a peripheral way, but it's not what the system itself was addressing. It was addressing behaviour that could nudge the unwary pilot closer to the stall region you might say, but not stall recovery, or behaviour in the stall itself (maybe "nothing whatsoever" is a bit too strong, but it was a secondary issue at minimum).

The main issue was, while doing turns while flaps up and at fairly low speed, the force required to hold the nose attitude while in the turn would go down in a subtle way, and a pilot holding, say, 15lbs of back pressure to maintain the pitch attitude in the turn would notice the nose drifting up without increasing back pressure. Normally, someone paying attention while they were flying would notice this and react with a relaxation of back pressure instinctively, but it was an odd characteristic nonetheless, certainly different from older 737s. The other large issue was the pitch up with thrust was more pronounced than with with older 737s, a result of the same factors.

These sorts of software band aids are not that unusual. The 747-8 actually uses the FBW ailerons, which are normally inactive at high speed, to work as active oscillation dampers to stop the wing tips from shaking up and down at high speed. And whenever you see vortex generators stuck on a wing or tail somewhere, they are usually a physical band aid to fix something found during testing.

The scandal about the MAX is mostly in Boeing's implementation of the band aid.

  • $\begingroup$ “They might have been able to just train pilots to recognize this behaviour and probably certified it as is, but at minimum it would have forced airlines to implement a simulator session to make sure pilots recognized the condition and could respond to it.” I disagree. The FAA Regs would not allow it to be certified without MCAS. You can’t make exceptions and then train pilots to fly an aircraft that does not behave the way aircraft normally behave. It would not be safe. $\endgroup$ Oct 13 '20 at 14:13
  • $\begingroup$ I'll take that para out. My understanding was that the behaviour wasn't bad enough to be "uncertifiable" in absolute terms, just uncertifiable without adding a sim training requirement and MCAS was intended to replicate the older 737 more precisely to avoid that. But perhaps I had the wrong idea. $\endgroup$
    – John K
    Oct 13 '20 at 15:41
  • $\begingroup$ There has been lots of speculation, and lots of misunderstanding, by all kinds of “experts”. I have never seen any actual documentation to say MCAS was ONLY needed for fleet Type Rating commonality. To me, it would have to be one reason or the other, not a little bit of both. $\endgroup$ Oct 13 '20 at 17:50
  • $\begingroup$ I'm hearing now that the final go forward with the software/wiring changes to recertify the system will include special sim training anyway, so that ship has sailed for good and all that grief was for naught. I used to joke at my company that there needed to be a "False Economy Committee" of ppl with huge brains and special powers who can predict the ultimate cost of attempts to cut corners. Usually it was related to choosing supplier X who's junk parts are cheaper, and ultimately cost 3 times as much in the long run. This is on a whole new level. $\endgroup$
    – John K
    Oct 13 '20 at 19:50

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