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Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

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Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#131

Post by dingaling »

petschge wrote: post #97
fooqux wrote: post #93

Earlier quoted context omitted.

You realize that many modern aircraft are all fly-by-wire, right?

Fly-by-wire does not necessarily imply that there is software interfering. You can have a very linear system that just translates yoke input into control surface deflection, but uses software and cables instead of hydraulic lines.

That's an obtuse definition of FBW that has no implementation that I know.

There's no benefit in developing and certifying software that just passes raw pilot commands to actuators.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#132

Post by Rooster61 »

VBprogrammer wrote: post #128
Rooster61 wrote: post #123

Earlier quoted context omitted.

"The undated EASA certification document, available online, was issued in February 2016, an agency spokesman said. It specifically noted that at speeds greater than 230 knots (265mph, 425kph) with flaps retracted, pilots might have to use the wheel in the cockpit’s center console rather than an electric thumb switch on the control yoke. " So, yes and no. It gives SOME control over the trim, but if the trim keeps gett…

This appears to be a reference to https://www.easa.europa.eu/sites/default/files/dfu/IM.A.120%... issue 10. They are saying there that near Vmo (maximum operating speed) with an aft centre of gravity (the aircraft loaded in a specific way) there might not be enough nose down trim authority available within the limits of the electric trim to completely trim the aircraft (allow the pilot to relax the forward pressure o…

Just to clarify, it's B-05/MAX under Issue 10. Took me a minute to find it.

> there might not be enough nose down trim

After reading it, I'm not sure how you are gathering that it's only nose down trim. It only mentions an inability to completely set the trim longitudinally. Longitudinal doesn't mean in one direction. It only indicates which plane we are talking about, which in this case is pitch.

> Vmo (maximum operating speed) with an aft centre of gravity (the aircraft loaded in a specific way)

"The aisle stand trim switches can be used to trim the airplane throughout the flight envelope and fully complies with the reference regulation Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts"

The issue mentions that there are certain corners, and lists what you posted after e.g. That isn't the only corner case in which the yoke trim control becomes insufficient.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#133

Post by crooked-v »

amanaplanacanal wrote: post #119
briandear wrote: post #114

Earlier quoted context omitted.

They could have disabled the whole thing if they simply followed the standard runaway trim procedure. A system malfunction should appear to a pilot the same way a runaway trim wheel appears. The result is that there is runaway trim checklist—and a procedure to work around it. You turn off the electric trim and go to a manual reversion. It’s something every 737 pilot trains for. Here’s some discussion about this from…

> A system malfunction should appear to a pilot the same way a runaway trim wheel appears. I thought the problem here was that it doesn't appear the same.

From what I understand, runaway trim looks like a continuous change, where runaway MCAS is intermittent changes every 5 seconds with longer delays if a pilot just touched the throttle trim controls.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#134

Post by VBprogrammer »

Rooster61 wrote: post #132
VBprogrammer wrote: post #128

Earlier quoted context omitted.

This appears to be a reference to https://www.easa.europa.eu/sites/default/files/dfu/IM.A.120%... issue 10. They are saying there that near Vmo (maximum operating speed) with an aft centre of gravity (the aircraft loaded in a specific way) there might not be enough nose down trim authority available within the limits of the electric trim to completely trim the aircraft (allow the pilot to relax the forward pressure o…

Just to clarify, it's B-05/MAX under Issue 10. Took me a minute to find it. > there might not be enough nose down trim After reading it, I'm not sure how you are gathering that it's only nose down trim. It only mentions an inability to completely set the trim longitudinally. Longitudinal doesn't mean in one direction. It only indicates which plane we are talking about, which in this case is pitch. > Vmo (maximum oper…

I was reading between the lines on the fact that the scenarios the listed involved high speeds (which generally need nose down trim) and an aft centre of gravity which also needs nose down trim.

Further, the section on EUSA's position clarifies that they wanted to improve the margin of safety on an out of trim dive, this is where 3 seconds of nose down trim (incidentally, that's how long they expect it to take for a mediocre pilot to identify a trim runaway) are applied without pilot intervention and the pilot has to demonstrate controllability.

But that point is pretty moot because the electronic trim cannot drive the jackscrew to the extremes that the manual trim wheel can, but it can return from the extremes to the centre. MCAS is driving trim to the nose-down extreme and the pilot is trying to return it to neutral.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#135

Post by DuskStar »

armitron wrote: post #65

I'll never fly 737 Max. No matter what changes they do, barring a complete redesign of the plane. Boeing deserves to go out of business for this.

And I'll say that I'll happily fly the 737 Max with a first-rate airline, and avoid flying any plane on an airline with a worse safety record than Aeroflot. Both Lion Air and Ethiopian Airlines have similar safety records to this century's Aeroflot, whether by incidents/hour, incidents/rotation or incidents/plane. No comparison to the American/Delta/United/BA/KLM/Air France/Lufthansa/etc of the world. (Seriously, if you've had more 'incidents resulting in fatalities' this century than Aeroflot while flying half as many planes... Let's just say that Ethiopian does not meet my standards for a safe airline. And that's only counting 2000-2018)

Similarly, I'd like any plane I'm flying in to have a fully qualified pilot AND copilot, and I don't consider 200 hours of total flight experience to be 'fully qualified'.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#136

Post by Rooster61 »

VBprogrammer wrote: post #134
Rooster61 wrote: post #132

Earlier quoted context omitted.

Just to clarify, it's B-05/MAX under Issue 10. Took me a minute to find it. > there might not be enough nose down trim After reading it, I'm not sure how you are gathering that it's only nose down trim. It only mentions an inability to completely set the trim longitudinally. Longitudinal doesn't mean in one direction. It only indicates which plane we are talking about, which in this case is pitch. > Vmo (maximum oper…

I was reading between the lines on the fact that the scenarios the listed involved high speeds (which generally need nose down trim) and an aft centre of gravity which also needs nose down trim. Further, the section on EUSA's position clarifies that they wanted to improve the margin of safety on an out of trim dive, this is where 3 seconds of nose down trim (incidentally, that's how long they expect it to take for a…

That makes sense. Thanks for the clarification.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#137

Post by cjbprime »

briandear wrote: post #114
wp381640 wrote: post #96

Earlier quoted context omitted.

> Why isn't that enough to disable MCAS From what I understand it did but it would reengage again. It's the reason why the Lion Air flight lasted longer in the air than Ethiopian, because the pilots in that case kept hitting the trim and buying themselves 40-60 seconds each time

They could have disabled the whole thing if they simply followed the standard runaway trim procedure. A system malfunction should appear to a pilot the same way a runaway trim wheel appears. The result is that there is runaway trim checklist—and a procedure to work around it. You turn off the electric trim and go to a manual reversion. It’s something every 737 pilot trains for. Here’s some discussion about this from…

> In the US, you can’t even fly Cessnas commercially with 200 hours and this first officer was copiloting a modern 737.

To be fair, it's not clear that the time someone would spend flying a Cessna between their 200-1500 hours in the US is actually helping them become a better 737 first officer..

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#138

Post by cjbprime »

zaxomi wrote: post #22
sergiosgc wrote: post #14

Earlier quoted context omitted.

Worse. Why don't they do it with three sensors and a voting system? By using two sensors, when they disagree, MCAS will have to be disabled. It seems the plane is unstable without MCAS, due to the forward positioning of the engines, requiring specific training and abilities from pilots.

No, it is not unstable without MCAS. MCAS is only designed to change the trim if the airplane is hand flown in flaps-retracted, low-speed, nose-up flight. This is something almost everyone is missing: A normal flight should not encounter this.

> low-speed

Not sure this is right. General aviation pilots often think of wing stalls as a consequence of low speed (their planes don't have AoA indicators!) but they can happen at any airspeed: they simply happen whenever the critical AoA is reached.

I think the idea is that the extra lift generated by the nacelles due to the engine position causes the plane to reach high AoA at e.g. full engine power, not just when flying slowly.

I think it's also the case that the plane does not actually become aerodynamically unstable, it just starts to handle differently (yoke pressure-wise) in a way that fails airworthiness requirements.

So it would only cause a stall if the pilots continued to pull back on the yoke into the stall while it's not fighting them as much as they're expecting it to.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#139

Post by cameldrv »

just_steve_h wrote: post #92

Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? Think of an analogy: when the autopilot commands a nose-down elevator input, and the pilot pulls back on the control column, that's enough to disengage the autopilot. Or think of cruise control in a car:…

This is incorrect. MCAS is required to meet the stability requirements of FAR 25 in addition to being required for the common type rating.

Re: Preliminary conclusion MCAS misfired in Ethiopian's 737 max crash

#140

Post by cameldrv »

losvedir wrote: post #115
just_steve_h wrote: post #92

Here's the design decision that puzzles me the most: MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? Think of an analogy: when the autopilot commands a nose-down elevator input, and the pilot pulls back on the control column, that's enough to disengage the autopilot. Or think of cruise control in a car:…

> MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? It does. It's just that MCAS tries again in 5 seconds. That's why the experienced pilots in both the Lion Air and Ethiopian crashes kept the MCAS in check - trimming to neutral column force is second nature to them - and it only crashed when the first of…

The problem is:

1. MCAS failure produces an intermittent stab trim movement.

2. Boeing claimed that the stab trim runaway checklist should have been run and would have let the crew recover.

3. Boeing's checklist only calls for action on "continuous" uncommanded stab trim movement https://www.satcom.guru/2019/03/taking-next-steps-while-awai... (second image)

4. Intermittent uncommanded stab trim movement is normal. The Speed Trim System and Elevator Feel Shift system regularly adjust trim, so merely seeing the wheels move by themselves is not cause for alarm.

5. MCAS is not documented at all in the flight manual, and the stab trim runaway checklist was not updated to address how MCAS might cause a failure necessitating its use, presumably to avoid the need for additional transition training.

6. MCAS is unreliable due to its reliance on a single AoA sensor.

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