Worrying: The Boeing 737 MAX Has Hit Another Design Flaw

Boeing’s beleaguered 737 MAX aircraft has received another blow today, as the FAA have discovered another major issue with the flight controls. Despite working on fixes for several months, it appears far more work is needed before the jet can return to service.

Boeing 737 MAX
There is yet another problem with the MAX. Photo: Boeing

The aviation industry has been abuzz today with the worrying news that Boeing have found another flaw in their 737 MAX jets. It appears that the procedure which Boeing have put in place to correct an inadvertent MCAS ‘nose down’ situation just doesn’t work.

Now, Boeing is faced with needing to develop another software fix for the plane, which will clearly set them for much longer than anticipated. Not only that, but with the world media reporting yet another Boeing mistake, public trust in the plane maker has reached its lowest ebb yet.

What’s the problem?

Initially, reports from the FAA were somewhat vague, with no particular reason given other than that a fault had been identified. However, over the course of the day, more information has come to light regarding the nature of the fault.

Flight Global reports that the issue is related to the ‘runaway stabilizer’ procedure, which is what Boeing is calling it when the plane noses down in response to erroneous activation of the MCAS. This procedure is intended to help pilots regain control of the aircraft and take it out of the nose down position.

Lion Air 737 MAX
The MCAS is considered responsible for the disasters at Lion Air and Ethiopian. Photo: Flickr user
Bathara Sakti

According to CBS, during flight tests in a simulator, FAA pilots found a problem with the trim functionality. In testing the simulator in a situation where the nose was being pushed down, pilots found that it took too long for the nose to be returned to the normal position, despite following all the correct procedures.

The BBC quoted an unnamed source as saying,

“During simulator testing last week at Boeing, FAA test pilots discovered an issue that affected their ability to quickly and easily follow the required recovery procedures for runaway stabilizer trim (i.e., to stop stabilizers on the aircraft’s tail moving uncontrollably). The issue was traced to how data is being processed by the flight computer.”

As such, Boeing is being asked by the FAA to format another software update in order to fix this latest identified defect.

What’s next for the MAX?

The FAA have demanded that this software issue is fixed before any airborne flight tests of the software updates are conducted.  While it seems to come down to a ‘data processing issue’, the consequence of such a software glitch could have been deadly in real life.

The FAA have released the following statement regarding the current situation.


According to their statement, they will only lift the ban when they deem it safe to do so, which could mean the MAX is grounded for much longer than anyone originally thought. As a result, Standard are reporting Southwest Airlines have extended their cancellation of the MAX through to October. We expect other operators will follow suit soon, although at this point, even October is looking somewhat optimistic.

Southwest Boeing 737 MAX 8 aircraft
Southwest have removed the MAX through to October. Photo: Southwest.

This comes as the IATA meet in Montreal with world leading aviation authorities to discuss how to return the plane to service. Clearly, they’re going to have a fair bit more time to mull this problem over.

What do Boeing have to say about this?

It seems Boeing are happy with the FAA’s decision, and agree that more work needs to be done. In a statement, the plane maker said,

“The safety of our airplanes is Boeing’s highest priority. During the FAA’s review of the 737 MAX software update and recent simulator sessions, the Federal Aviation Administration (FAA) identified an additional requirement that it has asked the company to address through the software changes that the company has been developing for the past eight months.

“The FAA review and process for returning the 737 MAX to passenger service are designed to result in a thorough and comprehensive assessment. Boeing agrees with the FAA’s decision and request and is working on the required software.

“Addressing this condition will reduce pilot workload by accounting for a potential source of uncommanded stabilizer motion. Boeing will not offer the 737 MAX for certification by the FAA until we have satisfied all requirements for certification of the MAX and its safe return to service.”

Boeing 737 MAX
Boeing will not offer the plane for certification until they have ‘satisfied all requirements’ Photo: Boeing

This latest development not only jeopardizes the 737 MAX’s timely return to service, but it looks like it will hit Boeing where it hurts too. Boeing stock was down a massive 6% following the break of the news, affecting their profitability and ability to keep paying those sizeable dividends.

17 comments
  1. OK, so let’s summarize here – the Boeing approach to introducing stellar new aircraft:
    – Shoddy, rushed design to start off with; plane is aerodynamically unstable (displaced center of pitch due to larger engines); poorly developed MCAS – severe shortcuts taken in fundamental software architecture / points of failure; don’t tell pilots anything about MCAS.
    – First plane crash, with all on board killed: blame the pilots, do nothing else.
    – Second plane crash, with all on board killed: blame the pilots, attempt to do nothing else, but eventually get forced into grounding the plane as a result of pressure from foreign aviation regulators.
    – Continue to blame the pilots.
    – Produce copious amounts of hot air about how easy the MCAS fix will be, and keep shifting the date of expected re-introduction.
    – Deny that any simulator training is necessary for pilots; maintain that a 1-hour tablet course is enough.
    – Fool IAG into signing an LOI for 200 new MAXs, and have IAG publicly state that the plane is safe.
    – Pay fat dividend to shareholders.
    – Submit shoddy, patched-up version of MCAS for review by FAA. Be amazed when FAA discovers further flaws during a simulator test of the new software.
    – Fool yourself into thinking that any airline, and/or the public, are ever going to trust the MAX again.
    What a stunning recipe for success!

    1. A great summary! You just forgot to discuss the important issue of rebranding! The difference between Airbus and Boeing is that Airbus can play with the idea of a pilotless plane, but Boeing desperately needs the pilots to have somebody to blame when their Maxes crash!

        1. I personally like the 737 LawnDart….I know it harkens back to another plane from the 60’s, but it works well.

    1. Agree this is fascinating and it’s all happening in 737 Max simulators at the moment. Boeing also admitted the Max simulator 2 months ago had bugs and didn’t reflect the flying characteristics fully of the aircraft!

      Fix it for the simulator – who is going to be in the aircraft as they “test” just how safe this plane is? Not me for at least 5 years!

  2. Also, did Boeing’s professional testing program somehow miss this latest ‘software’ problem or did they find it and hope that they could hide it from the FAA so they wouldn’t have to fix it?
    If that’s the case how many more ‘glitches’ are there waiting to be found?
    And was the FAA anything this thorough when they were testing the NG?

  3. Reports from Forbes are saying that the software is causing a hardware lockup and the FAA wants Boeing to replace some chips, but Boeing insists they can do it by rewriting software.
    Pretty soon I expect to see band aids, duct tape and bubble gum covering holes in the Max’s…

  4. It’s an incredulous state of affairs how Boeing are trying to down play the serveity of what is now surfacing.

  5. Boeing should do something to overcome the problems of 737. Perhaps reintroduce a new 707 or 727 that can replace the 737 max and trouble free like the 747. If not Airbus 320 or similar model will take the place of 737!

  6. 737 MAX: A Software Fix Might Not Be the Complete Answer
    A very talented mechanic in the food processing industry once explained to me that there would always be those who knew how to do a thing (think highly skilled engineers in the case of the 737 MAX) and those who knew what to do; think generalists with less specialized knowledge, but across a wider range of disciplines, those whose talents and interests allow them to “think outside the box”. I think this element may be missing in the search for solutions in case of the 737 MAX.
    A few days ago I watched (on YouTube) Mentour Pilot’s co-pilot struggle to adjust the 737 Max’s pitch trim manually in the simulator.
    https://www.youtube.com/watch?v=aoNOVlxJmow
    The copilot needed his (Mentor pilot’s) assistance because of the forces required to accomplish this, even in the calm and low pressure environment of the simulator. In the real world the effort required by this mechanically flawed system would have caused both pilots to be limited in their ability to find other solutions to the immediate problem.
    In an earlier video, I had noted (with discomfort) the apparent difficulty of being able to grip the runaway trim wheel and stop it by force. He also stated that the manual system is a mechanical system based on cables, which I think is generally a good thing. Hindsight is easy but…
    From a human engineering and safety standpoint, the design of the horizontal tail surfaces and the pilot’s mechanical trim system on the 737 MAX seems poorly thought out and poorly implemented. The speed at which the trim wheel runs in the simulator video seems to indicate a poor choice of mechanical advantage and mechanical ratios in the manual trim system, producing large forces that the pilots must overcome manually in an emergency and also require too many revolutions of the trim wheel to accomplish the needed trim control adjustments. If this is in fact true, large forces would be required to control a runaway trim by force; or to simply use the manual trim in normal flight. In order to get a good grip on the runaway trim wheel one would need to be able to grasp the outer perimeter of the wheel using the full capabilities of one’s grip. This appears impossible in the video because of the design and placement of the trim wheel.
    If such a stabilizer, as opposed to an elevator trim system was disabled after the stabilizer had been run to an excessive nose down trim position before the MCAS system was disabled and; under a busy and pressured emergency environment; with an excessive number of turns of the (difficult to operate) trim wheel required to correct the stabilizer position; and with relatively limited elevator vs stabilizer control authority (read: area relationships), it is understandable that the pilots of the Ethiopia flight might have elected to re-engage the (faster) electric trim system, while unfortunately possibly simultaneously re-engaging the MCAS system.
    The Boeing engineers seem to have made an (unsuccessful) attempt to address some of these issues by providing a fold-out handle attached to the trim wheels, but it appears to be marginally effective. I also doubt that it (the handle) could be accessed while the trim wheel is running. There are numerous other ways that these difficulties could be addressed:
    • Increasing the mechanical advantage available to the pilots by changing the mechanical ratios involved;
    • The trim wheels could be a larger diameter, clearing the console and thus providing the ability to better grasp the wheel;
    • Changing the relative areas of the elevator and stabilizer in ordered to create a more balanced control authority between them;
    • Adding a completely independent (from the autopilot, MCAS, and other computer controlled systems) and redundant second trim motor and control system for the pilot’s emergency use;
    • Utilizing a (recirculating) ball screw mechanism in place of the conventional jackscrew in order to reduce forces and the number of turns of the trim wheel required to be effective. Since ball screw mechanisms require significantly less force to operate and can be “self-driven” their use opens up additional possibilities for redundancy in the system.
    • Another advantage of ball screw actuators is that they do not require lubrication; think of the Alaska Airlines Flight 261 accident. The probable cause was stated to be “a loss of airplane pitch control resulting from the in-flight failure of the horizontal stabilizer trim system jackscrew assembly’s acme nut threads. The thread failure was caused by excessive wear resulting from Alaska Airlines’ insufficient lubrication of the jackscrew assembly”, similar to the system we are discussing on the 737 MAX.
    • Self-actuating aerodynamic servo and/or anti-servo tabs on the elevator and/or the horizontal stabilizer (while un-conventional) might be a part of the solution.
    • Other aerodynamic, possibly self-actuating, solutions having nothing to do with the trim system may be possible in addressing the thrust vector caused issues that MCAS was designed to address.
    Is this same (737 MAX) trim system installed on all versions of the 737? Have these issues been addressed in earlier versions? If so, were they lost in later design iterations, perhaps not requiring a change to the type certificate?
    The industry, the FAA, and many others worldwide have created perhaps the safest transportation system the world has ever seen, but we need to maintain that system under constant review, surveillance, and improvement by competent parties to ensure decisions and rules are made, and compromises decided upon, by those best qualified to do so.
    Design always involves compromise and trade-offs. This requires good judgment, good management, and oversight by qualified people, but the teams can become too specialized and lose sight of the forest. When you bring in one or more “outside” team members into a discussion, their seemingly un-informed insights can be profound. For example, in another YouTube video
    https://www.youtube.com/watch?v=kERSSRJant0
    “Sully” explained the root cause of the Air France Flight 447 accident (poor human engineering, very similar to the 737 MAX issues we are discussing). I would guess that his wide range of separate areas of knowledge and experience (while seemingly unrelated to engineering) allowed him to reach this insight, and I expect that he probably has many others related to possible improvements of the characteristics of the control system of the A330 and other Airbus aircraft. Unfortunately I have seen no evidence of response to his accident prevention insight and lesson by Airbus or the industry in general.
    Sincerely,
    Phil Hertel
    The Practical CFI
    CFI ASMEL-I

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