Boeing 737 Max 8 Q: Inherently unstable?

Factual Questions
1

I heard an interview with Ralph Nader and he claimed, or at least seemed to imply, that the Boeing 737 Max 8 is inherently unstable or needs software to keep it from stalling. He said it was due to the larger engines they put on it in order to compete with some Airbus. These larger engines moved the center of gravity such that the Max 8 requires sensors and software in order to keep it stable. He further seemed to imply that this was unique among commercial airliners.

I don’t really go to Nader for my aeronautical information – I come here for that. So:

  1. Does the Boeing 737 Max 8 require software and sensors to keep it stable and flying?
  2. Is this unique among commercial airliners?
  3. Bonus question: Is it unique among all non-military airplanes (private jets, turboprops, other personal aircraft)?

I know that modern fighter jets are unstable and would fall out of the sky if their computers failed. Is that true for any non-military aircraft?

RS

2

The Boeing 737 Max 8 is not an inherently unstable aircraft. There are plenty of military aircraft that are inherently unstable and require fly-by-wire in order to keep them in the air. The F-117 is a perfect example of this. Even with its computers and fly-by-wire systems, it is still nicknamed the Wobbly Goblin. That instability gives them better performance. An inherently stable aircraft doesn’t want to turn as quickly as an inherently unstable plane with computer stabilization. Since passenger jets generally aren’t trying to outmaneuver other aircraft, there isn’t much need for this type of instability on them.

The Boeing 737 Max 8 has bigger engines than the previous 737s, and in order to make them fit, they had to move them forward and upward. While the 737 Max 8 is not inherently unstable, in a low-speed, nose-up attitude, and possibly a steep turn, the engines tend to want to rotate the nose further up, which could cause a stall. The MCAS system that you are hearing a lot about is designed to prevent a stall in this rare case, and only this rare case, of low-speed, nose-up, flaps-retracted condition.

If you aren’t in a low-speed, nose-up attitude or a steep turn, the MCAS system should not be engaging. It’s not necessary to keep the plane flying in most conditions.

Computers taking control of airplanes is nothing new in commercial aircraft. Airbus tends to have even more computer controls than Boeing. There is an old joke that planes are moving towards having only one pilot, a computer, and a dog. The computer flies the plane and the dog is there to bite the pilot if he attempts to touch anything in the cockpit. :slight_smile:

The problem in this case is that the MCAS system seems to be triggering when it shouldn’t, possibly due to a fault in a single sensor. The MCAS system rotates the entire rear wing of the plane to pitch the nose down, and yanking back on the control column only pushes the little flaps on the ends of those wings back up, meaning that the MCAS can push the nose down a lot harder than the pilots can pull it back up.

Boeing published a procedure to get out of this problem. Switch off the MCAS and trim the plane (rotate the entire back wing) back up so that the plane flies level. The problem is that the Ethiopian pilots apparently did just that and it didn’t work. If you switch the elevator (rear wing) controls completely off, you can still crank the rear wing’s pitch angle by hand, but it is apparently difficult to do, possibly next to impossible if you are in a high speed dive caused by the plane pitching down on you.

This video from Mentour Pilot explains why the 737 Max 8 has the MCAS system.

3

It’s probably best to think of it as being not quite stable enough to meet certification standards in a specific limited case.

  1. No. It can fly fine without the software.
  2. It is not unheard of for airliners to have little helper modules that make up for a lack of natural feedback to the pilots. Stick shakers exist because swept wing jets don’t have natural stall warning characteristics. Stick pushers exist because some T tailed aircraft don’t pitch down once stalled. Artificial feel systems make the controls feel heavy at high speeds and/or large control deflections. MCAS doesn’t seem unreasonable in that context.
  3. I think all certified civilian aircraft are inherently stable, but some may have flaws at the edge of the envelope.
4

I asked the more general question a few years ago about negative stability in passenger aircraft; the answer I got was that the design trend among commercial aircraft manufacturers has been to move the center of mass rearward somewhat to reduce the required downforce by the tailplane (thereby improving fuel economy), but not so much as to induce negative stability like a modern fighter jet. The MD-11 reportedly uses computers to help with pitch stability, but it’s not clear from that article whether the MD-11 truly employs negative stability, or just something close to neutral stability.

The Max 8 has a different design issue in that the newer engines are mounted in a different location from that of the original 737 design, and this reportedly causes a pitch-up tendency. I must admit I don’t fully understand this: moving the engines forward should move the center of mass forward and cause a greater pitch-down tendency, and moving them upward should put the thrust line more in line with the center of mass, reducing any previously existing pitch-up tendency. Supposedly the pitch-up tendency of the Max 8 happens under conditions of low speed and high thrust, e.g. immediately after lifting off from the runway and during initial ascent. Under these conditions there’s not enough elevator control authority to effectively manage the increased pitch-up tendency, so the MCAS system changes the angle of the entire horizontal stabilizer to reduce overall downforce from the tailplane, allowing the pilot to maintain something closer to a neutral elevator input.

The problem seems to be that this same MCAS system is also used to avert a stall condition, rather than the conventional anti-stall stick-pusher system. Whereas the classic stick-pusher could be overridden by a pilot with a good strong pull on the yoke, overriding the MCAS system (in the event of an erroneous anti-stall pitch-down event) required an unusual procedure that supposedly was not well-communicated to its pilots and/or was more difficult to implement during an in-flight control crisis. Moreover, the MCAS system was vulnerable to failure in just one of the angle-of-attack sensors.

ninja’d by e_c_g…

5

The videos I’ve seen explain the pitch-up tendency being due to engine thrust pushing against a longer lever (the engine mounting pylon) compared to previous models, especially when you’re increasing thrust.

The 737 was designed to be very low to the ground, making it easier to load large cargo. The max 8’s larger engines wouldn’t have enough clearance if they were simply slung under the wings like previous models, so they had to be mounted a bit forward and higher up.

6

When it comes to measuring pitch-up moment on the airframe due to thrust, the lever-arm is measured as the vertical distance from the line of thrust to a horizontal plane containing the aircraft’s center of mass. This means that by raising the engines higher above the ground, the designers have reduced the length of the lever-arm used for calculating the pitch-up moment caused by engine thrust. ISTM that for a given thrust level, this should reduce the pitch-up tendency as compared to previous versions of the 737.

Moving the engines forward would not change the length of the lever-arm associated with pitch-up tendency.

So…what am I missing?

7

In the airplanes I’ve flown, there’s a little wheel where you can adjust the trim. They’re pretty foolproof. It seems to me that a 737 could simply be trimmed for thrust, rather than relying on a sensor that may fail.

8

@Machine Elf

It is the added lift from the nacelles of the larger engines.

9

This is how I’ve heard it explained. Past a certain angle of attack, airflow hitting the undersides of the nacelles creates substantial “new” lift forward of the wing, thus increasing the tendency for the nose to go up.

10

Ah, this makes perfect sense. Moving the engines up and forward (and making the nacelles larger) certainly would increase this effect. It would appear then that this has no relationship with the thrust output of the engines, and is entirely due to angle of attack and airspeed.

11

Yeah it’s the nacelles creating lift. And its effect is just to change the handling characteristics slightly at high angles of attack (not necessarily low speed) so the pull force required on the control column reduces. The MCAS makes it so the required pull force increases as angle of attack increases. It’s to do with sufficient feedback to the pilot rather than control authority. The MCAS is only active in manual flight as the autopilot doesn’t care what the pitch forces feel like.

12

See? This is why I go to the SDMB rather than Ralph Nader for answers to these questions. I understood the first few, but then it got into details that only a pilot or aeronautical engineer would understand, but that’s OK! I got my answer and it looks like some others learned something as well.

Thanks, everyone!

13

Here’s a video done by a B-777 co-pilot that goes a pretty thorough explanation of the Ethiopian Preliminary Accident Report:

https://www.youtube.com/watch?v=HBqDcUqJ5_Q

14

But isn’t the 737 Max 8 fly-by-wire? Shouldn’t this mean that the feedback force on the stick is entirely artificial? Why wouldn’t they just modify the feedback force behavior in the software?

15

It is not fly-by-wire. The feedback is still artificial, you don’t get the required feedback from hydraulic actuators so there are systems built into the control circuit to provide that feel. That is the case with all large aircraft, FBW or non-FBW. I’m not sure why they couldn’t adjust the elevator feel system in that way, maybe it wouldn’t be enough or the problem is more nuanced than what the feel system can fix. As I understand it there is increased feel put in by the feel system as well as the MCAS doing its thing.

Edit: If they’d made it fly-by-wire it would have required more training and they’d marketed it to the airlines as not requiring special training.

16

It looks like the best (aerodynamic) solution would have been to put longer landing gear legs on the 737 to provide increased ground clearance for the larger engines to be mounted where they wouldn’t cause problems. As this would have required major structural redesign of the wing to move the main gear outwards and provide the required retraction space, I’m not surprised that Boeing didn’t want to do this. It may be time to retire the 737 and start over with a new design.