Breaking news: Ethiopian Airlines flight has crashed on way to Nairobi

[Evan]

The video shows how the slightest miss-trim at Cut-Out creates so high forces on the Stabilator trim jackscrew that a manual turning of the screw via the cables from the trim wheels in the cockpit is no longer possible.
If that's true, that's huge.

That would imply a mechanical change from the NG. MCAS is supposedly a software addition.

[3WE]

Just Heard on Natinoal News...

"Ethiopian pilots were following procedures when they crashed"

No doubt a media over-simplification, but...

...again reminded of Evan's general disdain when we mention the use of broad fundamental aerodynamic and airmanship rules which may not identically align with type-specific/incident-specific procedure and which may also be lacking in acronyms...

[Gabriel]

The first step in the runaway trim procedure is to turn off the autopilot, which in the case of an MCAS problem would not solve the problem.

I would just rephrase that saying that the AP was not engaged in the first place. The MCAS only acts in manual flight with flaps retracted.

The first item is actually hold the control column firmly. The second is Autopilot, if engaged, disengage.

Not that it changes in any way the rest of what you said, but just for accuracy.

[Gabriel]

As I understand it, the suggestion is that they couldn't correct the elevator trim with manual adjustments after disabling MCAS, so in desperation they re-engaged powered trim to help. Worrying if true.

That's very serious, BUT, that said, it would enable you to use manual electric trim which temporarily stops the MCAS and let's you trim as you please. You would need to click the thumb switch every time the MCAS starts trimming (which is 5 seconds after you stopped trimming with the thumb switch). That is exactly what the Lion Air crew was successfully doing for a few minutes, until they stopped doing it and crashed.

[Gabriel]

This is the Mentour video. I have not watched it yet.

https://www.youtube.com/watch?v=EzgBft-79U8&feature=youtu.be

[brianw999]

...........
Fact #2 is, if something like MCAS is planned for other Boeing families like 767, 757 or, God save us, the 747,
then I'll take a 24 month break from aviation with the thought, is aviation still something which I'm able to support worldwide.

Thank You.

Please Boeing, let it be so !!

[flashcrash]

if something like MCAS is planned for other Boeing families like 767, 757 or, God save us, the 747,
then I'll take a 24 month break from aviation with the thought, is aviation still something which I'm able to support worldwide.

Hmmm ... how about if I start a go-fund-me page to support the issue of an EASA directive mandating implementation of MCAS on all Lufthanza's remaining 747s? Any supporters?

[vaztr]

Gabe - the video is 'no longer available'???

[Black Ram]

Gabe - the video is 'no longer available'???

Mentour's employer (the one with the ugly yellow-blue interiors) wanted it removed

[Evan]

I would just rephrase that saying that the AP was not engaged in the first place. The MCAS only acts in manual flight with flaps retracted.

At this point, I don't even feel confident about that.

[Black Ram]

Posted on one of the other forums. Apparently the thumb switches do not always have full trim authority. Page 15:

https://www.easa.europa.eu/sites/def...20ISS%2010.pdf

[Schwartz]

More details from Leeham: https://leehamnews.com/2019/04/03/et...-to-stop-mcas/

Apparently they will upload a simulation by Mentour.

Great link. Here is an excerpt:

In subsequent discussions with MentourPilot, a YouTube channel with millions of viewers, I was informed this would probably not be true for higher speeds. MentourPilot’s experience when examining hundreds of pilots as Type Rating Examiner for the 737 was the elevator got exceedingly heavy at higher speeds and manual trimming at the slightest miss-trim of the Stabilator from neutral Yoke forces was very difficult.

He also pointed out the high speeds observed in ET302’s FlightRadar24 trace (Figure 1) were logical. It’s a consequence of following the Emergency checklist for “IAS disagree” (IAS is Indicated Airspeed, i.e. the dynamic air pressure experienced by the aircraft) after takeoff.

The combination of the preceding checklist followed by an MCAS Trim Runaway checklist could create a situation where manual trimming after a Trim Cut-Out would be difficult to impossible and would require non-checklist actions.

Combined with the speed which follows from an “IAS disagree” Emergency checklist procedure the Pilot Monitoring (PM) could have problems to move the trim while Pilot Flying (PF) would fight to hold the Yoke against the elevator forces. At a larger miss-trim, the situation is unattainable.

The excessive manual trim forces have been confirmed by an airline pilot which has done 737 test flights after elevator maintenance, where manual trimming needs to be checked. At a miss-trimmed Stabilator, you either have to re-engage Electric trim or off-load the Stabilator jackscrew by stick forward, creating a nose-down bunt maneuver, followed by trim.

Stick forward to trim was not an option for ET302, they were at 1,000ft above ground. According to The Wall Street Journal, the ET302 crew re-engaged electrical trim to save the situation, to get the nose up. It was their only chance. But too late. The aggressive MCAS kicked in and worsened the situation before they could counter it.

The way I read this, is it becomes physically impossible for the Pilot not flying to move the trim wheel under those speed and elevator conditions. Perhaps some of you can help me understand. Is the trim wheel mechanically linked to the elevator actuator?

[Gabriel]

Posted on one of the other forums. Apparently the thumb switches do not always have full trim authority. Page 15:

https://www.easa.europa.eu/sites/def...20ISS%2010.pdf

I don't think that this has anything to do with these accidents. Apparently and from what I understand, Boeing limited the travel of the trim when actuated with the thumb switch to meet the out-of-trim dive requirements (that would be a dive with the plane trimmed too nose-down, since a nose-up out of trim will facilitate not complicate the recovery). The "thumb switch" is in fact two switches, a nose-down one and a nose-up one. Micro switches in the stabilizer open the nose-up thumb switch circuit when the stabilizer reaches a certain nose-up limit position, thus disabling the nose-up thumb switch, and the same happens with the nose-down thumb switch and the stabilizer reaching a nose-down limit position. In other words, you if the stabilizer reaches the nose-down limit position for the thumb switch, the nose down thumb switch will not work anymore, but nothing will prevent you from trimming nose-up using the nose-up thumb switch.

Note that the bold parts are bold for a reason. I may have totally misunderstood the EASA document.

[Gabriel]

Great link. Here is an excerpt:

The way I read this, is it becomes physically impossible for the Pilot not flying to move the trim wheel under those speed and elevator conditions. Perhaps some of you can help me understand. Is the trim wheel mechanically linked to the elevator actuator?

No. I'll try to explain, but let's start forgetting about the elevator, as I understand it has nothing to do with what happens with the trim (rather the opposite).

The horizontal tail has 2 movable parts. The "fixed" (not so fixed) stabilizer in the front and the elevator in the back. The elevator is controlled with the control column. The elevator is hinged in the front, hinged to the back of the stabilizer, so when it moves the front part of the elevator stays in place and the back moves up and down. Under normal conditions they are hydraulically actuated, so aerodynamic forces cannot move it. But it is easy to visualize that if you deflect the elevator up (to push the tail down and the nose up), there will be aerodynamic forces that will tend to return it to the equilibrium position. Very much like a weather vane.

The "fixed" stabilizer is really not fixed. It is hinged in the back. So, if you could move it freely, the back part will stay in place and the front of the stabilizer moves up and down. It can be intuitively seen how, if you start from the equilibrium position and displace the front of the stabilizer a little bit say up, and let it go, it will not tend to return to the equilibrium but rather move more up. It's like a weather vane facing back (this is pretty much what happened with the Alaska MD80 accident).

The stabilizer is controlled by a jackscrew that goes through a bolt in the front of the stabilizer. When the jackscrew rotates in one direction or the other, it moves the nut up or down and the elevator with it. If the jackscrew doesn't turn, the stabilizer won't move, so you will not have the unstable behavior described above. I mean, the stabilizer will try to move farther away from the equilibrium, but the jack screw won't let it. How? Because the threads of the nut will make pressure against the thread of the jackscrew. The stronger the pressure, also the stronger the friction. And when is the pressure stronger? When the loft on the stabilizer is stronger, and that happens the farther away the trim is from the equilibrium position and the faster the airspeed (because the lift is proportional to the angle of attack, of the stabilizer in this case, and the speed squared).

The trim wheel doesn't control the elevator. It controls the stabilizer. They are mechanically connected to the jackscrew. Rotating the wheels rotates the jackscrew.
If the stabilizer is very out of trim (very far from the equilibrium position) and the airspeed is high enough, the friction between the jackscrew and the nut can be very high and it can be almost impossible to manually rotate the jackscrew using by manually rotating the trim wheel.

[Evan]

Great link. Here is an excerpt:

Jesus. So, another stealth factor in which a provisional recovery technique for manually-unattainable high-speed trim recovery in the NG is contradicted by MCAS in the Max... and no one thought of it?

The way I read this, is it becomes physically impossible for the Pilot not flying to move the trim wheel under those speed and elevator conditions. Perhaps some of you can help me understand. Is the trim wheel mechanically linked to the elevator actuator?

The mechanical trim wheels operate a cable spool that drives the stabilizer jackscrew. This is why the trim wheels move when the electric pitch trim is operating.