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Problems with the 738s...

Started by cagarini, Fri, 9 Nov 2018 07:01



John H Watson

The information coming in is still very confusing. i.e. what happened on previous flights and what might have happened on the fatal flight. Maintenance records seem to be saying that the pitot-static system was flushed out and an AOA sensor was replaced prior to the last few flights.

Why would the pitchover be caused only during manual flight?

The trim system is controlled by the Flight Control Computers (FCCs). Elevator feel is controlled by the EFSM (Elevator Feel Shift Module).

QuoteDuring a stall, the FCCs command the stabilizer to trim the
airplane nose down.

The FCCs also send a signal to the EFSM to increase the force required to pull back on the stick during a stall (as sensed by AOA vanes).

QuoteThe EFSM and column cut-out switch modules operate to make sure the pilot cannot easily stop this automatic stabilizer movement with the elevator control column
nose up input.

I'm not sure what they mean by the column cutout switch modules preventing pilot action. On the 744, moving the control column in the opposite direction to trimming will stop the trimming (manual, speed or autotrim).

United744

The 737 just screams poor design.

John H Watson

 I think that "make it hard for the pilot" logic is a recent addition to the 737NG (i.e. it's found on the -800/-900/etc... ). The more stuff they add, the more it becomes like an Airbus  >:(

It also has an LE slat automatic extension system close to stall (i.e. if the LE slats are extended, it extends them further)

cagarini

John,

with my uncommensuarbly inferior knowledge about airliners and their technical details and operation, I have to say that I would put it more like the new ( NG ) versions probably being a rude copy of what Airbus offers.

My experience from using airbuses is that of a simmer, a desktop simmer, the 1 hr in a CAE A320 sim doing all sorts of stuff that came into my mind :-), and many many talks to fellow glider pilots, some of them who came from the old BVoeing guard... and are pretty much unanimous in saying they do like the Airbus stylle and actually many prefer it over Boeing...

As a simmer, I find it much simpler to opperate a good / detailed airbus in P3D than the Aerowinx PSX. If I am some time away from Aerowinx, I have to re-learn every step ... Airbus is already in my ROM :-)

United744

The 737 has its roots in the 707/727 days I think.

It's not good that there is an automatic trimmer that appears by design to not be able to be overridden by the crew. Don't the stab cut-out switches override it??? Is it not trained as a possible stab trim runaway???

Did the accident aircraft crew get any stall warnings? I would assume if the AoA senses a stall condition to the point of taking action, that they would get stall alerts (even if bogus) as well?

John H Watson

This is more complicated than I thought. The 737 MAX8 seems to have another system for augmenting control:

Quote737 MAX8 Emergency Airworthiness Directive
The recently released Emergency Airworthiness Directive directs pilots how to deal with a known issue, but it does nothing to address the systems issues with the AOA system, which may be the causal system in the Lion Air accident. This is significant. The positive takeaway is that we are advised, as pilots, that once we recognize the issue, we can stop the negative impacts by taking the trim system out of the loop.

At the heart of this investigation is the MCAS system (description from Boeing):

MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

United744

Yes - I think many missed that in the bulletin. There is a new, separate trimmer system, and it appears that it is hard to override, does NOT have cut-out capability, and failed spectacularly in the Lion Air case. :(

John H Watson

The bulletin seems to say that you can override it, but I'd say you would have to apply continuous manual trimming to get the stab back to a normal position, then hit the cutout switches. Identifying the problem, however, is the tricky part. If your (faulty) indicated AOA/stickshaker is telling you you're stalling, you would be hesitant to pull up.


torrence

 "MCAS is activated without pilot input and only operates in manual, flaps up flight."

It seems to me that this part of the airworthiness directive is effectively an oxymoron since it basically says that there is no truly manual flight (stick, rudder, throttles), with the automatics cut out.  MCAS is there to 'help' you. This flies in the face of much of what I've seen in training videos etc. concerning getting back to basics if the auto systems are not making sense.

Cheers,
Torrence
Cheers
Torrence


John H Watson

Looks like the Left AOA feeds the Left ADIRU which feeds the Left A/P. Similarly, the Right AOA feeds its onside units.

Unfortunately, both AOAs seem to feed the Stall Warning computers (which activate the stick shakers and other devices). The Stall Warning Computer/s may use the highest value of the L and R AOA vanes, rather than an average.


John H Watson

Looks like my last post was deleted....

I posted a link to a YouTube video by Mentour Pilot... 

https://www.youtube.com/watch?v=zfQW0upkVus&t=601s

I was told the link was the same as one above, but the video seems different  ;)

Hardy Heinlin

I saw even two posts. (I didn't delete them.)

Jeroen Hoppenbrouwers

My bad. Both posts referred to a link posted already above -- as far as I could see. And both posts said that in an edit. I may have been short of coffee...