Boeing 737 MAX

[Delfin]

Also FWIW:


A fellow I've networked with for a couple decades who flies the 737NG for a US carrier which has grounded its MAXs says that all of the US operators have reviewed the retained digital Quality Assurance data and reported no instance of MCAS anomaly.

There are several ASRS reports, all of which were investigated and found to be other than MCAS events.

Here's a report by a Seattle TV station. Not particularly enlightening until the last 20 seconds or so when the American Airlines union rep says the same thing.

https://youtu.be/oYolKJSrBmg

My friend also confirms something I was wondering about. The Ethiopian crew was confronted by a stick-shaker at lift-off. Their response was to retract the flaps.

Interesting video, thanks. I've only GA experience, and it's been awhile since I sold my plane but from memory, flap retraction when you get a stall warning seems like the reverse of what you would want to do, since that is just going to pitch the nose up. Have I got that correct?

 

[Xsbank]

“Stall recovery, check thrust, flaps 20, positive rate, gear up.”

So yes, rote stall recovery would be to retract the flaps from take-off position to 20 degrees, which if the Ethiopian pilot did, that was how he was trained.

 

[AlaskaProf]

, since that is just going to pitch the nose up. Have I got that correct?

Differs from airplane to airplane, but probably true in this airplane.

More critical, it instantly reduces lift, aggravating the impending stall. ...and initiating MCAS?

This was a heavy airplane departing a high -altitude airport at a temp, which we would not consider exactly hot at 64f, but was well above the standard atmosphere and would require a performance adjustment. Not hard to develop a departure stall scenario.

 

[AlaskaProf]

“Stall recovery, check thrust, flaps 20, positive rate, gear up.”

So yes, rote stall recovery would be to retract the flaps from take-off position to 20 degrees, which if the Ethiopian pilot did, that was how he was trained.

The info I have says they selected "Flaps 0" .

 

[Delfin]

Differs from airplane to airplane, but probably true in this airplane.

More critical, it instantly reduces lift, aggravating the impending stall. ...and initiating MCAS?

Retraction also moves the center of lift forward, which causes the nose to pitch up.

 

[Ski in NC]

So they went to flaps 0?

 

[ssobol]

“Stall recovery, check thrust, flaps 20, positive rate, gear up.”

So yes, rote stall recovery would be to retract the flaps from take-off position to 20 degrees, which if the Ethiopian pilot did, that was how he was trained.

737 takeoff flap settings are 1, 5, and sometimes 15. There would be no retracting flaps to 20 after takeoff. Pilots are also trained not to change the aircraft configuration during a stall recovery until after the plane is under control and flying again.

In the case of the 737 crashes if the pilots retracted the flaps to zero then the MCAS could operate. Flaps>0 stops MCAS trim changes (provided that part of the system is operating correctly).

 

[Xsbank]

(I cheated- Challenger 605 stall recover from memory).

 

[Baker]

MCAS system description: 737 MAX - MCAS

The MCAS is sort of a "feel" system, but not exactly. There is already a feel system in the 737 which controls the forces on the control column using hydraulic pressure. Depending on which phase of flight you are in, the pressure is controlled by speed inputs to give the pilot the correct feel on the column to manually fly the plane. The MCAS system actually moves the stabilizer to change the performance of the airplane. This reduces the pilot force required to keep the nose down so it does affect the "feel", but not in the same way.

The control feel system does not move any aerodynamic surfaces to change the control feel. You can experience the column control feel system effects with the plane parked on the ground using a test set. MCAS will not affect the control feel if the plane is not moving.

I expect that there is more to the MCAS system moving the stab than just the pilot feel. If the only need for MCAS was for pilot feel, then Boeing could have accomplished the same thing with the elevator control feel system without actually moving the stabilizer.

We are getting caught up in semantics. Boeing's purpose of the MCAS system was to make it "feel" like and NG airplane. It was not a stall avoidance system...although it does do that as a byproduct. ssbol, your link and reference is a blog....I could have written it.

 

[Baker]

737 takeoff flap settings are 1, 5, and sometimes 15. There would be no retracting flaps to 20 after takeoff. Pilots are also trained not to change the aircraft configuration during a stall recovery until after the plane is under control and flying again.

In the case of the 737 crashes if the pilots retracted the flaps to zero then the MCAS could operate. Flaps>0 stops MCAS trim changes (provided that part of the system is operating correctly).

Yep...although we have performance for every flap setting short of 30(except flaps 2....flaps 2 seems to be the bastard child of flap settings). So while it is uncommon to do a flaps 25 takeoff, I have done them.

 

[Baker]

Also something that was mentioned that I did not address....Single input to MCAS was the "standard" OEM package. There was an option box to check if you wanted dual inputs to the MCAS along with a "disagreement" light/warning. Southwest and American opted for the second input to MCAS. United did not. Obviously SWA and AA's purchasing department has some pilots or engineers in it. UAL could be accused of counting beans.

 

[FF]

Usually retracting flaps to 0 will retract the leading edge devices, which near a stall is not a great idea.

 

[LN-RTP]

Stall warning at lift-off and they retracted the flaps...?
Only way to fly them could be to pull back on the yoke and get an abnormal nose high, firewall the motors and hope to power out of it. Now MCAS goes to work and keep trimming down..
But they were not stalling to start with, instead damage to the AoA vane caused the fake stall warning , either from a bird strike or from a jet bridge or something.

They turn off the Stab Trim Cutout Switches but can’t Trim Manually for some reason, still accelerating at level flight and take-off power making the Stab more powerful as speed increases, in desperation they turn the Stab Cutout Switches back on and MCAS trims the nose further down, the elevator can not overpower the Stab and they go in at 383 knots..
Not sure the above is what really happened, but putting together bits and pieces, it could have...?

 

[Baker]

Stall warning at lift-off and they retracted the flaps...?
Only way to fly them could be to pull back on the yoke and get an abnormal nose high, firewall the motors and hope to power out of it. Now MCAS goes to work and keep trimming down..
But they were not stalling to start with, instead damage to the AoA vane caused the fake stall warning , either from a bird strike or from a jet bridge or something.

They turn off the Stab Trim Cutout Switches but can’t Trim Manually for some reason, still accelerating at level flight and take-off power making the Stab more powerful as speed increases, in desperation they turn the Stab Cutout Switches back on and MCAS trims the nose further down, the elevator can not overpower the Stab and they go in at 383 knots..
Not sure the above is what really happened, but putting together bits and pieces, it could have...?

Not an unreasonable sequence of events.

 

[Delfin]

Not an unreasonable sequence of events.

Baker, or others, do you know the spool time from firewalling to Max power on those new engines?

 

[LN-RTP]

Baker, or others, do you know the spool time from firewalling to Max power on those new engines?

Probably instantaneous if you are already @ take-off power.

Just read the preliminary report in the accident this morning, sobering reading..
Here it is..

http://www.ecaa.gov.et/documents/20...AVJ).pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e

 

[Baker]

Baker, or others, do you know the spool time from firewalling to Max power on those new engines?

Yeah, it is instantaneous for the most part. Also the computers keep the engines from over boosting so you're not gonna get much more out of them. We do use reduced thrust take offs so you may be going from 90% to 101%. This particular flight had a reduced thrust setting of 94%...just starting to read through it.

 

[AlaskaProf]

Is this not a description of the MCAS? (copied from the Boeing AFM in the prelim.)

 

[Baker]

Is this not a description of the MCAS? (copied from the Boeing AFM in the prelim.)

Yes but you have to be able to read through those symptoms. Ultimately, those are symptoms of bad information being fed to the MCAS system. Most of those symptoms would occur even if you did not have MCAS....the big one though is the nose being pushed over. So you "MAY" get some or all of those symptoms. My AFM is 2000 pages. I am not expected to read through all of those when we get a "new" airplane. They need to give me the "differences".

 

[Ski in NC]

Probably instantaneous if you are already @ take-off power.

Just read the preliminary report in the accident this morning, sobering reading..
Here it is..

http://www.ecaa.gov.et/documents/20...AVJ).pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e

I'm no pilot or aircraft guy, but looking at the FDR readout I did not see the runaway stabilizer trim that I expected. Seems the stab stayed steady and the AC still dove? Am I reading it right?

 

[Baker]

I'm no pilot or aircraft guy, but looking at the FDR readout I did not see the runaway stabilizer trim that I expected. Seems the stab stayed steady and the AC still dove? Am I reading it right?

No.... 2 units of trim is unheard of....especailly with the speeds they were doing. I'll leave it at that!!! Gotta go to bed!

 

[Ski in NC]

thanks..

 

[Baker]

thanks..

Ski...I guess that is the problem. 5 units is normal. 2 units is a handful at the very least and as speed increases, the more of a handful it becomes. MCAS was allowed the ability to trim 2.5 degrees at a time....that was a last minute addition. Originally it was only supposed to be able to trim .6 units at a time. Had they left that at .6, we likely would not have had these accidents.

 

[twistedtree]

So one of the past comments was that standard operating procedure in the event of un-commanded/disruptive trim action is to disable automatic trim via a switch between the two seats, and trim manually with the trim wheel.


Did the pilots do this, and if they had, should they have been able to bring the plane under control?


Please note that I'm not suggesting this might be the sole cause of this accident. By all indications there are lots of contributory "issues" and plenty of corrective action required in many areas. But this seems to be one of those contributory issues.

 

[LN-RTP]

So one of the past comments was that standard operating procedure in the event of un-commanded/disruptive trim action is to disable automatic trim via a switch between the two seats, and trim manually with the trim wheel.


Did the pilots do this, and if they had, should they have been able to bring the plane under control?


Please note that I'm not suggesting this might be the sole cause of this accident. By all indications there are lots of contributory "issues" and plenty of corrective action required in many areas. But this seems to be one of those contributory issues.

First of all, it is not “Automatic Trim”, but rather Electric Trim the switches will disable, although there is an Automatic Trim Function with the autopilot engaged, but with the autopilot on, the MCAS would also be disabled. (Clear as mud? )

Yes, the Ethiopian pilots disabled the electric trim using the Cutout Switches, but they forgot to dial the speed back, either manually or using the auto throttle command bug function, hence the speed kept building and the nose down trim became more “effective”. In a last desperate attempt they turned the electric trim back on, but then the MCAS system, getting erroneous signals from a sick sensor, trimmed further down and they were doomed.R.I.P.
They airplane could have been saved and was flyable, if the pilots had done a few things differently, but they didn’t.

 

[Baker]

So one of the past comments was that standard operating procedure in the event of un-commanded/disruptive trim action is to disable automatic trim via a switch between the two seats, and trim manually with the trim wheel.


Did the pilots do this, and if they had, should they have been able to bring the plane under control?


Please note that I'm not suggesting this might be the sole cause of this accident. By all indications there are lots of contributory "issues" and plenty of corrective action required in many areas. But this seems to be one of those contributory issues.

According to the play by play in the report, they tried to use the manual trim. The captain commanded it...the first officer attempted it. He said that it did not work. I have a few thoughts here. First one is did the FO understand what the Captain was asking and tried the electric trim switches on his side while the trim switches had already been "cutout". My other thought is that the FO did indeed understand the captain and that he tried and that the stabilizer was so "loaded up" that it was difficult or impossible to move. I am not sure this would happen....I always had hoped there is enough mechanical advantage in that trim wheel that aerodynamic forces would never overcome yout ability to move it. Lotsa questions still for sure. I go to the simulator in a month. I am gonna try things like this.

 

[LN-RTP]

. I always had hoped there is enough mechanical advantage in that trim wheel that aerodynamic forces would never overcome yout ability to move it.

Hmm, I thought the upper limit was 230 knots, but maybe Gorilla forces can make it work at higher speeds?

 

[Baker]

They airplane could have been saved and was flyable, if the pilots had done a few things differently, but they didn’t.

That is easy to say after the fact. We have the benefit of hindsight and now knowing all of the subtleties of MCAS. They were faced with a lot of data to process and problems to solve. The stick shaker...erroneous airpseed...numerous caution lights...etc. Then add MCAS into the mix.

There have been airliners that have crashed with erroneous airspeed alone. Google Birgenair 301. They just had erroneous airspeed without all of the other crap and they still managed to crash it.

These guys were dealt a very bad hand.

 

[Baker]

Hmm, I thought the upper limit was 230 knots, but maybe Gorilla forces can make it work at higher speeds?

Is that in the book??? I don't remember that little nugget of info...damn my memory!!

 

[Ski in NC]

Just another ignorant non-pilot question:

So the trim wheels are mechanically attached to the trim jackscrew mechanism? If the stab was in the nose down position per the crash AC, how many turns would it take of the wheels to get it to a normal position?

And it sounds like at their airspeed the aero loads may have prevented that motion? With elevators held nose up the handwheel would have to counter the loads from the elevators (since they are on the back of the stab)?