Pilots Not Told About 737 MAX Auto Trim System (Updated)

  • E-Mail this Article
  • View Printable Article
  • Text size:

    • A
    • A
    • A

Boeing kept airlines and pilots in the dark about an automated background trim system on the 737 MAX that may be implicated in the first crash of the new model in Indonesia last month. The trim system, which is meant to improve pitch characteristics and stall protection, wasn’t even described in any of the documentation provided to pilots transitioning to the new aircraft.

Lion Air JT610, a new MAX8 with only about 800 hours on the airframe, plunged into the Java Sea off Jakarta on Oct. 29, killing all 189 people aboard. Prior to the crash, the aircraft flew through repeated pitch, airspeed and vertical speed excursions before diving almost directly into the water about 12 minutes after takeoff.

According to sources at two airlines operating the MAX series, the system in question is called Maneuvering Characteristic Augmentation (MCAS) and is intended to improve pitch response at high angles of attack. It was added to the MAX models partly because the aircraft has heavier engines than the previous 737 NG models and the airplane's center of gravity is biased more forward.

MCAS is activated without pilot input and would typically come alive in steep turns with high load factors, but only when the airplane is being flown manually. According to a minimal description provided to AVweb, MCAS operates only in flaps-up flight and is inhibited in any other configuration. MCAS intervenes at a threshold angle of attack and automatically trims nose down at a rate of 0.27 degrees per second to a maximum of 2.5 degrees. Stabilizer input is lower at high Mach numbers, but more aggressive at low Mach. The Wall Street Journal said Boeing didn't disclose MCAS details to cockpit crews because it was worried about overwhelming them with more technical detail than needed or could digest.  Boeing also said pilots were unlikely to encounter MCAS intervention during their normal flying.

Pilots trained on the MAX weren’t given even minimal briefings on MCAS, according to an interview with Jon Weaks, president of the Southwest Airlines Pilots Association published in the Seattle Times early Tuesday. “We do not like the fact that a new system was put on the aircraft and wasn’t disclosed to anyone or put in the manuals,” Weaks told the Times. And both Boeing and the FAA have warned that the system may not be performing as it's supposed to.

American Airlines, which also operates the MAX8, also provided its pilots with new documentation on MCAS hurriedly provided by Boeing after the Lion Air crash. Because MCAS relies on angle of attack data from the aircraft’s vane-type sensor, one focus of the Lion Air investigation is on the AoA sensor itself, which appears to have been replaced as faulty prior to the accident. The aircraft also reportedly had a history of unreliable airspeed indications. It's unclear if the two are related to the accident or how they affect MCAS operation.   

On the Pilots of America online forum, an American Airlines pilot posted an informational bulletin from a pilot’s association and added this: “We had NO idea that this MCAS even existed. It was not mentioned in our manuals anywhere (until today). Everyone on the 737 had to go through differences training for the MAX and it was never mentioned there either.” 

Boeing said Monday that it’s working closely with investigators and taking every measure “to fully understand all aspects of this incident.” Meanwhile, Indonesian investigators say they expect to release an initial report by the end of November.

Comments (13)

How is it possible for a major aircraft manufacturer to install a flight control system in the aircraft and NOT inform the pilots? If this kind of mistake can make it past FAA scrutiny then what purpose does aircraft "certification" serve?

Posted by: Mark Sletten | November 13, 2018 7:48 AM    Report this comment

I could not agree more Mark. This seems to be a breakdown by both Boeing and the FAA. In my opinion, both are equally responsible and must bear the burden of the souls recently lost due to both Boeing and the FAA negligences in what appears to be at this time the reason for those fatalities on Lion Air.

Posted by: Allen Churchwell | November 13, 2018 8:01 AM    Report this comment

You may well have asked "the $64,000 Question," Mark.

As Paul opines in his earlier blog on this topic, pilots are having to learn ways AROUND automation. Now toss in a system they didn't even know about fed from an AoA system that -- itself -- may have been unreliable and you have a recipe for ... what happened.

And there are still people who think automation will make pilotless airliners possible!! No way.

IF MCAS contributed to this crash ... Boeing has a big problem on its hands.

Posted by: Larry Stencel | November 13, 2018 8:30 AM    Report this comment

Aha ... a "second day" and "next edition follow on rewrite " two days later with more info. Great.

See my comments in your previous blog.

Posted by: Larry Stencel | November 13, 2018 9:28 AM    Report this comment

This is not the first time a manufacturer forgot to inform the pilots. In december 1991 SAS flight SK751 with a pretty new MD-81 had an emergency landing near Stockholm because of ice sucked into the engines. The pilots weren't able to throttle down the engines due to new software that prevented throttling down during flight at altitude. The plane crash landed on a snow-covered field, but all survived. Some organisations are seemingly too big to handle even essential safety-related information.

Posted by: Peter Olsen | November 13, 2018 9:40 AM    Report this comment

Boeing had better get the checkbook ready.

Posted by: John McNamee | November 13, 2018 10:13 AM    Report this comment

No argument that the FAA should have done better oversight of Boeing during certification. On the other hand I'm sure that any FAA oversight doesn't affect Lion Air and Indonesian certification except indirectly.

Posted by: jvo fnr | November 13, 2018 11:15 AM    Report this comment

There are many systems that pilots don't know about. Also, the 737 has had a still pusher forever--the MCAS is stick pusher.

Yes, maybe there should have been more communication on MCAS, but it is not like they put in an afterburner...

Posted by: SV MASSIMINI | November 14, 2018 8:40 AM    Report this comment

This will not go well. As a new system, it should have been pointed out as a 'new and novel' approach in the certification documentation. OEM documentation should have had this system in them. And the FAA should have (did not have to be but SHOULD have) been on board for flight testing to see this system. This was a failure on the OEM for sure (or so it seems). But the FAA should have been tipped off as well during the certification process. Unless it was missing from that as well.

Posted by: Rob McDowell | November 14, 2018 11:34 AM    Report this comment

MCAS is not a "stick pusher" per se. Its more tied into the auto-trim system.

Pushing or pulling the stick adjusts the control surfaces on the horizontal stabilizer. Trim on the other hand pivots the entire tailplane.

Posted by: Jan-Willem Korver | November 15, 2018 6:29 AM    Report this comment

Auto Trim......but we didn't tell the pilots about it in manuals or training material because we didn't want to overload them!!! Pilots have be trained on run away trim since autopilots began. Automation is fine, lowers the workload of the crew, but you can NEVER delete the pilots ability to turn it off.

Posted by: John Fenoglio | November 15, 2018 9:39 AM    Report this comment

Recently I was reviewing digital/computer control avionics with my father who was a fighter/test pilot for three decades, his take was the new options were amazing except for one thing, he said he would want a big red override button for all the computer systems to return full flight control to pilot should computer systems start to fail.

Posted by: Christopher Lawrence | November 15, 2018 10:17 AM    Report this comment

Although I think Boeing should provide all info, including what happens 'behind the panels', there are procedures in place for pilots to counter both unreliable air speeds as well as stab trim runaways. However, these two occurrences happening simultaneously while a stick shaker is also activated very easily creates an overload for any crew. I wonder ... if the crew had known about the system they still would have to use very effective CRM and flying skills to come to the correct conclusion and take appropriate actions.
I know of flight crews which have encountered air speed problems with a stick shaker all the way to touchdown...on top of that having to disable stab trim would have 'loaded' them up that much more.
I am very curious as to the end result of this LionAir accident investigation.

Posted by: Mauro Hernandez | November 16, 2018 2:00 PM    Report this comment

Add your comments

Log In

You must be logged in to comment

Forgot password?

Register

Enter your information below to begin your FREE registration