I suspect they’d need to teach those pilots to add a lot of trim. Putting those big engines well ahead of the CoG makes them throw its nose up; it’s basic Newtonian levers.
Not that any plane flies perfectly level at all speeds, which is why they need trim in the first place. But presumably there’s a “sweet spot” which the original 737 was designed to nail in normal flight, which subsequent revisions have moved further and further away from.
At some point you really need an adult in the room to stand up and declare “no more”. But by accounts McDonnell-Douglas already showed all the grown-ups the door. That’s the nice thing about responsibility: just disseminate it enough, and no-one’s to blame.
A fix is being flight-tested.[1] The authority of the auto trim adjustment system is being limited. All planes that don't have it already will get the "AoA Disagree" detection, and a disagree will lock out automatic trim adjustment. Pilots will get more training on handling the aircraft with that system inoperative.
How bad is the handling with that system inoperative? The need for this came from adding bigger engines to a small airframe, which required putting them too far forward so they'd clear the ground.
> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem.
It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wheel.
This is extremely (EDIT: No longer) well documented by Boeing going back decades, and it presents solutions of either: 1) reengaging the electronic stabilizer trim and trimming electronically back to a point where the forces are manageable, then cutting electronic trim again, or 2) relaxing the stick, letting the nose go the way the stabilizers want it to, which slackens the force, and makes cranking the wheel manually doable. Great if you're stabilizers are trimmed severely nose down at a height of 40,000 feet; less great at a height of 1,000 feet. (EDITED TO ADD: Apparently this information & procedure was removed from Boeing's manuals after the 1980s, and pilots no longer train on it, as the issues that led to its necessity were thought solved. The MCAS apparently has reintroduced the need for them.).
This is compounded by a new change in the 737-MAX: in prior 737 series aircraft, there are two stab trim cutout switches: one to cut the autopilot's ability to command the stabilizer trim, the other to cut electronic power to the stabilizer trim adjustment entirely, so you could stop the autopilot from changing the trim but still retain electronic control and not need to worry about the manual crank forces. The two switches are MAIN ELEC and AUTO PILOT.
In the MAX, this changed, and it's now an all or nothing setup: you have to kill electronic power to kill autopilot commands to the stabilizer. The two switches are now PRI and B/U. Both must be cut out to stop any stabilizer runaway, including MCAS runaway.
This all seems to add up to Boeing's narrative of "The MCAS is safe, pilots just need to know to hit the stab trim cutout" being grossly insufficient. They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control.
Ok let's look at it another way. Do you think it's even remotely likely that Boeing haven't had one of their 737 Max aircraft try this out at a safe altitude? They staked the future of the aircraft by saying they believe they are safe to fly.
I've never suggested having a trim runaway close to the ground is trivial. It's certainly going to result in brown trousers when it happens, but it should be within the abilities of a well trained crew to handle.
Following the outcry over Lion air I just don't believe a pilot on type wouldn't have known the procedures for getting the aircraft to a safe altitude and disabling the stab trim.
As a pilot your hands are on the controls. You notice pretty instantly that your having to pull the nose up. You instinctively reach for the trim control on the yoke. This disables the MCAS system for 5 seconds. You get the pitch forces under control. A few seconds later MCAS decides you've still got the nose too high because of a faulty sensor and tries to trim it back down once more.
How many times before you remember that fatal bug from that crash a few months ago. Maybe twice? Three times tops.
Why don't you explain why previous 737 versions have a Mach trim to prevent Mach tuck, or why it had a speed trim system to control the pitch force response to speed changes.
I think the issue is the difference between the force needed to trim the aircraft and the acceptable control gain between the yoke and the elevator. I think with light aircraft trim is what you think of it. Slightly centers the yoke so the aircraft will fly level without control input.
I think what's going on with the 737-MAX is the needed trim is way beyond what's needed to fly the plane.
I'm now wondering is the MACS is needed because the tendency to pitch up also exceeds the pilots control inputs which is why it had to be on the elevator and have that much authority. Meaning once the aircraft pitches up the pilot doesn't have enough authority to push the nose back down.
> I don't find 1 particularly compelling, it would have been noted by now if it was impossible to trim the aircraft using the manual trim wheels.
It has been noted:
737 Flight Crew Training Manual, chapter Non-Normal Operations/Flight Controls, sub heading Manual Stabilizer trim:
"Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually."
In this case the excessive air load would be caused by the yoke control pulling the elevator up while the trim controlled stabilizer is doing the opposite.
Like you say, it's hard to know whether this actually happened.
Yeah I don't disagree. I don't fly, but have sort of been following this out of curiosity. One comment I've seen from some pilots is basically "ehh, runaway trim isn't a new thing, we train for it in the sim, and there's a standard way to deal with it (disengage the automatic system and trim manually)."
So perhaps Boeing felt that this didn't really change anything in that regard. They seem to have been proven wrong.
The point, which I think you missed, is that you don't need to manually trim. Look at the image I linked to. It's MCAS down 2.5 degrees. Pilots up 2.5 degrees. MCAS down 2.5 degrees. Pilots up 2.5 degrees. It's a repeated cycle of the pilots getting trim to where they want it and the MCAS kicking in to nose them down
Yeah that's what the Lion Air crew did, and it worked until it didn't (lost control in a turn?). But counter the trim with the push buttons and then hit the cutout is not in any Boeing QRH (yet?). As a non-pilot this seems like a reasonable option, but there may be something glaringly obvious as to why it's a bad idea. Even if this is the ultimate solution it's quite different to how you'd react in an NG and will come with extra training. Basically a bad situation all around.
If I've parsed everything appropriately the Ethiopian crew did try to turn the stab trim motors back on presumably to regain control over the stabilizer and we saw how that worked out. There's just not a lot of room for diagnostic work at that altitude.
Their response would have worked to return to level flight in non-MAX variants of the plane, though. Regardless of what the manual or checklist says, they had years of experience flying 737s that behaved in a specific way, they developed an unconscious/intuitive mental model of the plane based on those behaviors (that is faster and quicker to react than consulting checklists), and then a significant and deadly change was made to those behaviors and not communicated to them for cost-cutting reasons.
That’s a great explanation, even if it is oversimplifying.
We don’t build planes with training wheels anymore because the performance cost was too high. Planes are still the safest way to travel even without the training wheels.
I don’t think 737 MAX 8 pushes the envelope too far. I think they screwed up on re-training the disengage, and they may have screwed up on redundancy by only using a single AoA sensor, but I also am guessing the latest crash has absolutely nothing to do with trim.
Yes, in particular the 737 is a cables and levers and pullys design. It can fly totally manually. If the pilots had realized they had a trim problem, and switched off the electric trim, and trimmed manually (literally turning a crank in the cockpit) they would not have crashed.
“When Boeing built the MAX, in order to increase fuel efficiency, they went with a different engine, explained Fred Tecce, a commercial aviation expert. “Because the 737 sits pretty low on its landing gear, [Boeing] had to move the engines up a little bit and move them forward a little bit” on the MAX versions. “In order to compensate, they extended the nose gear by eight to 12 inches" and repositioned the engines which "affected the airplane’s pitch characteristics and center of gravity.”
Tecce concurred that control inputs and the resulting pitch changes were challenges that had to be overcome in the latest version of the world’s best-selling aircraft.
“In order to compensate for what the engineers perceived to be an issue with respect to pitch, they added this MCAS system that operates when the autopilot is off and the angle of attack exceeds certain limitations and when the airplane is banked pretty steeply.” He said the technology runs the stabilizer pitch down for several seconds and it “reassesses and will start again until it believes the airplane has reached a safe angle of attack, and it operates without the pilots knowing [about it].”
Tecce noted that in the case of the Lion Air Boeing 737 MAX crash, “now the airplane is pitching down and actually moving the control wheel will not stop that system. If the pilot uses the trim system on the yoke, the [MCAS] system will stop" but "if the airplane isn’t in the proper attitude it will reactivate,” Tecce said, further forcing the aircraft downward if pilots fail to recognize the situation and take proper corrective action.
A pilot familiar with the system pointed out that recognizing this scenario was crucial to determine if there was a problem that warranted activating the trim cutoff switches. Additionally, if the autopilot is engaged, activating a yoke trim switch disconnects the autopilot and gives full control back to the pilot immediately.
Tecce did not fault the FAA for taking a wait-and-see approach. “A lot of people throwing a lot of rocks at the FAA. Since 2010 we’ve had one aviation fatality” in the United States. “Our safety record is astonishing,” he said. “I don’t think there’s anything wrong with the airplane. If you talk to the pilots who fly them, they’ll tell you it’s not the airplane so much as whether or not the manual properly describes what’s going on.”
Boeing issued an unactionable memo to shift responsibility onto the operators.
73n vertical trim design and procedures are suboptimal. There should be an automation trim cutout switch to maintain powered trim control except under the most unusual circumstances/malfunctions. It's like turning off a car's brake boosters when ABS is at fault. They can't be retrimmed in full deflection nose up or down by 2 mere Schwarzenegger mortals in time to recover at either low altitude or airspeed because of surface loading.
> the aircraft will tend past a certain angle of attack to start nosing into a stall, rather than away from one.
Virtually all aircraft will do that, including the original 737. Different aircraft do this at different points; provided the pilot is trained for the aircraft, it's not a fundamental problem.
The problem for the MAX is they wanted pilots to be able to treat it like an airplane it wasn't.
> The problem is that the issue may cause the plane to be unflyable even by a pilot aware of the issue.
That’s partly true, but it’s not the whole story. MCAS and its use to make the MAX 8-9 fly “more like the 737” is part of a longer trend of manufacturers coddling pilots instead of expecting them to behave as competent professionals and treating them as such. They’re effectively opting for a more familiar normal aviating experience, with the tradeoff being longer emergency/exigency checklists and so much more to manage/remember/process when the system malfunctions. With the end-result being that even more experience is required to safely and reliably pilot modern aircraft.
Side note: Disabling trim should only be done as part of a problem remedy, not as part of normal flight.
(Disclaimer: I’m not a pilot. I’ve read for many hours on these issues.)
Trim controlling stabilizer, yoke controlling elevator, and elevator being unable to counteract the more extreme ends of trim settings, is part of the original 737 design. They all behave that way, and 737 pilots are aware of this, and know they must neutralize trim to have ample elevator control.
However, only the 737 MAX has MCAS, and MCAS doesn't take airspeed into account. An appropriate corrective action for a low airspeed stall, will be way too aggressive for a high airspeed stall (a rare event, suggests overly aggressive dive recovery leading to a stall). And designers + regulators assumed that pilots would treat such behavior the same as an ordinary runaway trim incident, but does it seem ordinary or is it a shockingly bad overcorrection?
We've seen many times the scenario where pilots become so surprised at a particular situation that their rational ability to work through the problem can be compromised. Yes, transport pilots are substantially trained to avoid such a compromise, but problems at low altitude combined with the lack of a consistently positive rate of climb is an urgent situation. Pilots are hyper aware that recovery from either a dive or stall consumes a huge amount of altitude, which you simply don't have at low altitude. That itself could delay proper decision making: i.e. automation has betrayed us for unknown reasons, disable it now.
With MCAS disabled, what's the 737 MAX stall behavior like? Is it meaningfully easier to unwittingly get into a stall?
> I have not read anything to suggest that a normally trimmed, MCAS-disabled Max would pitch itself into a stall from thrust application. That would be bad, but is also almost surely far from the actual situation.
?? Most aircraft will do this. Even docile trainers.
There's been a couple 747 crashes due to this exact scenario. Or regarding the 737 in particular, to quote "Mike734" in 2007:
> In the B-737 too much nose up trim can make a go-around very exciting. The under wing engines create a very large pitch up when adding full power for a go-around. The pilot has to really push hard to stop the jet from pitching too far nose up.
and "Cac737":
> Now in the Boeings for example, B737 and bigger with underslung engines, this situation is aggravated even more because when you push the power levers forward for go-around thrust, due to where they are and their thrust lines, that act alone will cause the nose to rise very noticeably, and if you are light you can actually find yourself pushing forward on the control column on a go around. now trim "back" as you say and forgetting you did so, can find yourself in a very nosehigh attitude if not careful.
That's why it is so essential to recognize and stop the runaway trim (for whatever reason) ASAP or you could get into a situation where it would be physically impossible for the crew to do so.
> Pilots are trained for how to recover from these conditions and regain trim authority.
I agree with all of your corrections except this one. I've read that this yo-yo or rollercoaster maneuver was described in old 737 manuals decades ago, but then removed. It's not in the current manuals and I haven't heard anyone else claim that it was ever trained for by airline pilots, either back then or now.
I get the impression that Boeing decided some decades ago that needing to retrim without the motor and at extreme airspeeds and aerodynamic load was so unlikely that they didn't need to prepare pilots for it.
This would probably be true if they hadn't introduced a system that makes the stabilizer try to kill you, and then tied the off switch for that system to the off switch for the trim assist motor.
(A reason to be pedantic on this point is that it's important to know whether the lack of having performed this maneuver can be reasonably described as a pilot error.. although in the MAX cases there probably wasn't enough altitude to perform it anyway at the speeds involved.)
Not that any plane flies perfectly level at all speeds, which is why they need trim in the first place. But presumably there’s a “sweet spot” which the original 737 was designed to nail in normal flight, which subsequent revisions have moved further and further away from.
At some point you really need an adult in the room to stand up and declare “no more”. But by accounts McDonnell-Douglas already showed all the grown-ups the door. That’s the nice thing about responsibility: just disseminate it enough, and no-one’s to blame.
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