MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
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Keep in mind that the MCAS software has already been changed to prevent the type of erroneous activation that occurred in the accident aircraft. The only time then that one would see MCAS activate during a TCAS event would be if the pilot mishandled the procedure and brought the aircraft close to a stall. In that case, you want MCAS to activate!
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I never said MCAS was the best solution to resolve the MAX's high-AOA handling characteristics. Heck, I'm not sure anyone would claim it was the "best" solution. It was, however, the solution Boeing went with. And yes, it was poorly designed in as much that it activated at a time that it wasn't needed and created a hazardous flight situation. However, I haven't seen anyone claim that it would not actually do what it was designed to do in an actual stall situation.
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yoko1 said in answer to my awkward query
It was my understanding that the installation/addition of MCAS software AND the wiring change shown in peter lemmes blog resulted in the disconnect of the control column cutout switch-. Thus the change from NG wherein at anytime the pilot pushed or pulled in opposition to the stabilizer trim, the electric motor stabilizer trim was shutoff leaving only elevator? and manual trim wheel and ? .
Apologies if I've totally botched this up ;((
When you refer to the "removal of the yoke/stick switch," I think you are talking about the control column cutout switches that prevent trimming in the opposite direction that the control column is displaced (that is, stops nose down trim when pulling back, stops nose up trim when pushing forward). This feature has not been removed and still functions as designed except that these switches are bypassed for MCAS activation (and only for MCAS activation).
Apologies if I've totally botched this up ;((
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C: The aeroplane must be shown by analysis, test, or both, to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems) within the normal flight envelope, without requiring exceptional piloting skill or strength.
C3: A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable. (Probability of 1 x 10-9 or less per flying hour )
C3: A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable. (Probability of 1 x 10-9 or less per flying hour )
I don’t understand why fixing the MCAS algorithm helps, when THS is controlled by a DAL C system that is expected to fail at less than 1 x 10-5 pr. Hour.
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If you fly an "RA" TCAS event, then the autopilot and Auto throttle should be disengaged. TCAS "TA" is to alert crew of other "traffic" but is not deemed to be a threat.TCAS "RA" maneuvers are generally flown with "smooth" control inputs for pitch up or down depending on the course of action. These events are not subject to high or low pitch angles in general, so MCAS probably wouldn't be a factor.
From what I have heard in that situation you may need to pull up until the stick shaker activates, and then keep around that AoA until you clear the terrain, with the stick shaker activating intermittently, as demonstrated in this simulator session (the relevant part of the video starts at 13:30):
Could MCAS interfere with the terrain escape maneuver? I think that could be the worst moment for MCAS to activate.
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Indeed. But what I meant was if MCAS could interfere with the terrain escape maneuver when the AoA vanes are working properly.
More precisely I think my question would be if MCAS activates before the stick shaker, simultaneously with the stick shaker, or after the stick shaker.
More precisely I think my question would be if MCAS activates before the stick shaker, simultaneously with the stick shaker, or after the stick shaker.
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I'm not a pilot, but what about: "Terrain! Terrain! Pull up!" ?
From what I have heard in that situation you may need to pull up until the stick shaker activates, and then keep around that AoA until you clear the terrain, with the stick shaker activating intermittently, as demonstrated in this simulator session (the relevant part of the video starts at 13:30):
https://youtu.be/4qsFI9l0bJk?t=13m30s
Could MCAS interfere with the terrain escape maneuver? I think that could be the worst moment for MCAS to activate.
From what I have heard in that situation you may need to pull up until the stick shaker activates, and then keep around that AoA until you clear the terrain, with the stick shaker activating intermittently, as demonstrated in this simulator session (the relevant part of the video starts at 13:30):
https://youtu.be/4qsFI9l0bJk?t=13m30s
Could MCAS interfere with the terrain escape maneuver? I think that could be the worst moment for MCAS to activate.
Last edited by yoko1; 25th Jun 2019 at 22:50.
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According to what is generally posted on this forum :
Depending of what the stickshaker and MCAS set point are, this potentially could happen.
With MCAS 1.0 it was unrecoverable.
With MCAS 2.0, you still have the authority for 1.2 g maneuver. Or so they say.
Actually, no one outside Boeing actually knows much about this MCAS operation.
Not sure even people at Boeing really knew what they were doing...
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Another question comes to mind.
What would have happened in a real terrain avoidance maneuver with the stickshaker active ? Wouldn't it have run for 9-10 seconds, adding 2.5 units of nose down trim ? And so overwhelming the elevator authority ?
What's the pilot to do in that case ? Counter trim, with a risk of negating the MCAS raison-d'etre ?
Flip the trim cutout switches on the pedestal ?
If you have to counter/cut off the MCAS at the very moment it is supposed to be useful, why not have the freedom to counter it just by pulling on the yoke ?
What would have happened in a real terrain avoidance maneuver with the stickshaker active ? Wouldn't it have run for 9-10 seconds, adding 2.5 units of nose down trim ? And so overwhelming the elevator authority ?
What's the pilot to do in that case ? Counter trim, with a risk of negating the MCAS raison-d'etre ?
Flip the trim cutout switches on the pedestal ?
If you have to counter/cut off the MCAS at the very moment it is supposed to be useful, why not have the freedom to counter it just by pulling on the yoke ?
Last edited by Fly Aiprt; 25th Jun 2019 at 23:09. Reason: Typo
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Actually, it would be more of a problem if MCAS did not activate. Keep in mind the purpose of MCAS - to augment the control feel at high AOA. If you have a high AOA for any reason - including windshear or GPWS maneuvers - then you are in the flight regime for which MCAS was designed. The original problem occurred when MCAS activated when it wasn’t needed.
So, from what I understand, MCAS activates when it detects an angle of attack "it doesn't like", and it keeps adjusting the stabilizer until the angle of attack returns to a value "it likes", or until it hits its hard limit of 2.5 stabilizer trim units (0.6 units at higher speeds).
We don't know exactly what the trigger point is, so I'm just guessing here, but let's assume MCAS was designed to activate before the stick shaker, and you are in a terrain escape maneuver when MCAS activates.
The result would be that the angle of attack reduces with the same stick deflection, and the pilot would have to apply more and more deflection (and force) to reach the stick shaker limit. At some point MCAS would reach its 2.5 units of trim limit and stop adjusting the stabilizer. Then, assuming the elevator has enough authority to counter those 2.5 units of stabilizer trim, the pilot will finally be able to reach the stick shaker limit.
Now, at this point, with the old version, if the pilot blipped the trim switches, 5 seconds later MCAS would reactivate, it would again not like the AoA, and it would feel free to adjust the trim another 2.5 units. Now, would anyone want up to 5 units of additional nose down trim during a terrain escape maneuver?
What I'm trying to say is that, depending on where its trigger AoA point was configured in the original version, MCAS might have been a problem even when the AoA sensors worked properly. Of course, a lot of what I said above is speculation, but MCAS doesn't smell right to me, at least not when described as a "stick force augmentation system".
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Another question comes to mind.
What would have happened in a real terrain avoidance maneuver with the stickshaker active ? Wouldn't it have run for 9-10 seconds, adding 2.5 units of nose down trim ? And so overwhelming the elevator authority ?
What's the pilot to do in that case ? Counter trim, with a risk of negating the MCAS raison-d'etre ?
Flip the trim cutout switches on the pedestal ?
If you have to counter/cut off the MCAS at the very moment it is supposed to be useful, why not have the freedom to counter it just by pulling on the yoke ?
What would have happened in a real terrain avoidance maneuver with the stickshaker active ? Wouldn't it have run for 9-10 seconds, adding 2.5 units of nose down trim ? And so overwhelming the elevator authority ?
What's the pilot to do in that case ? Counter trim, with a risk of negating the MCAS raison-d'etre ?
Flip the trim cutout switches on the pedestal ?
If you have to counter/cut off the MCAS at the very moment it is supposed to be useful, why not have the freedom to counter it just by pulling on the yoke ?
Of course I'm sure the whiz bang kids in their safety analysis will have covered such a case ??? Trust but verify comes to mind !!
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Thank you ! I realize that normally TCAS only provides up/down altitude change alerts supposedly at a range/time sufficient to avoid collision. As I understand it- MCAS only works/engages if/when AP is off, flaps up, etc. And that normally, the AOA sensor provides input to AP. But the disconnect/removal of the yoke/stick switch which has been standard for decades such that a pull or push in opposition to stab/trim input under a TCAS maneuver which would normally cut electric power to stabilizer might be a surprise to many. Seems to me if HAL( MCAS) is/was G limited by design - but no longer is - then a conflict between pilot and HAL is probable ??
But, I am at a total loss at how a situation where an aircraft is noted, by the pilot, to have stick force lightening, while it is approaching the stall, with changing speed, alpha, pitch rate, elevator angle, etc, etc and determine that is a static stability problem. The aircraft is not static, it is manoeuvering.
The problem with FAR 25 and the AC's is that the information that you are seeking may well be elsewhere.
But you could try FAR 25.143, and review the guidance material of AC 25.7d for 25.143 and 25.255.
Last edited by zzuf; 26th Jun 2019 at 05:36.
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Yes, but only for the purposes of MCAS activation. The control column cutout switches work as normal for all other applications.
Could MCAS interfere with the terrain escape maneuver? I think that could be the worst moment for MCAS to activate.
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Icarus It's fine with me if MCAS is a system designed to prevent excessive AoA, which could result in stalls, just like I don't have a problem with the Airbus alpha floor protection. If MCAS activates to prevent reaching an AoA higher than the safe limit, it sounds like a nice feature to have. Of course, assuming MCAS was designed properly with the adequate redundancy in its inputs.
My main problem is the insistence of some people that the purpose of MCAS is just to adjust the stick feel, and it isn't related at all to stall prevention. That may be the case, but Boeing seems to have nevertheless implemented a system that focuses on limiting AoA rather than just adjusting stick feel. So, whatever their original intent was, it seems they actually implemented a stall prevention system.
My main problem is the insistence of some people that the purpose of MCAS is just to adjust the stick feel, and it isn't related at all to stall prevention. That may be the case, but Boeing seems to have nevertheless implemented a system that focuses on limiting AoA rather than just adjusting stick feel. So, whatever their original intent was, it seems they actually implemented a stall prevention system.
which is a crude way to affect up feel - and in failure mode can overpower a pilot.