Ethiopian airliner down in Africa
Join Date: Dec 2006
Location: Florida and wherever my laptop is
Posts: 1,349
Originally Posted by diclemeg
And yet...it's the MD95, also known as the Boeing 717, that never has had a fatality..... McDonnellDouglas made a great plane...
Join Date: Jun 2001
Location: 3rd Rock, #29B
Posts: 805
Originally Posted by yanrair
Well spotted. You trim nose down during acceleration because centre of lift moves forward. From memory.
read my follow up comment, any further questions feel free to PM.
CsubP moves forward on a section as AoA increases, however it is dependent on section camber, and also to an extent the section LE radius. A section that has a laminar separation bubble will have a slight wobble in moments for lift and pitching but not a big deal. Mach tuck occurs due to reduction in lift inboard, on a swept section this loads the tips giving a pitching moment, as well as a movement rearwards of Cp and the resultant pitching moment from that change, Cp follows normal shock movement on the section. As the wing section and T/C result in shock formation being more pronounced inboard than outboard, the total result is an increase in pitching at higher Mach. At speeds below Mcrit, lift and section a-slope, and inflow to the tail result in pitch moments leading to nose down trim being needed as speed rises above trim speed. Shock formation is not relevant to JT or ET’s events, they had normal pitching moments going on other than the MCAS establishing a major trim error for desired speed.
The annoyance of MCAS is that it has authority to schedule trim for a period of more than 3 times the miss-trim case considered for certification, per 25.255, which seems like a lousy concept.
Join Date: Nov 2018
Location: Brisbane
Posts: 20
Originally Posted by fdr
back on planet earth, where Newtonian physics still reigns, you may find that for an aircraft to be statically stable, mass is always ahead of the center of pressure.
Agreed with the rest of your post, but as a counter-example to the statement above, consider a 737 NG loaded to it's absolute rear limit (36%MAC), and trimmed to fly at high but nonstalling AoA. It's CP will be relatively forward and close to but not reaching the wing aerodynamic centre of 25%MAC (will be approximately 32%MAC). CP is hence ahead of the CG and the tail is lifting slightly to maintain trim. However, the aircraft is still longitudinally stable, as although the tail is lifting it's local AoA is less than the wing. Of course this is an edge case for stability, more often CP is behind the CG in all flight regimes and the tail is always in downforce.
Join Date: Jun 2001
Location: 3rd Rock, #29B
Posts: 805
Originally Posted by LEOCh
Agreed with the rest of your post, but as a counter-example to the statement above, consider a 737 NG loaded to it's absolute rear limit (36%MAC), and trimmed to fly at high but nonstalling AoA. It's CP will be relatively forward and close to but not reaching the wing aerodynamic centre of 25%MAC (will be approximately 32%MAC). CP is hence ahead of the CG and the tail is lifting slightly to maintain trim. However, the aircraft is still longitudinally stable, as although the tail is lifting it's local AoA is less than the wing. Of course this is an edge case for stability, more often CP is behind the CG in all flight regimes and the tail is always in downforce.
25% is a convention for measurement of moments, so a wing with zero pitching moment will have a Cp at 25% chord. Refer to Abbot and VonDoenhoff to look at the moments that occur on a section. Next time walking around your brand A or B plane have a look at the section of the stab, it is an inverted cambered section, which means it has a zero lift line, ZLL, that is considerably beyond a zero stab LE up limit, which is usually around 2 to 3 degrees up dependent on flavour. The stab resides in an area of down wash on standard tails, less so for T tails or cruciform tails. The stab on an A or B brand does not get to a point in normal use of trimming to an up force. The elevators of course may result in a change above the stab limit, but would be untrimmed. For your vanilla flavoured brand, the neutral point is way, way further back, around a center of mass aft of 40% for a plane with an aft envelope limit of 32%. Long time back we looked at a B744 that achieved 43.5% in flight... vs a 32% envelope. The plane was marginally statically stable (being generous), and appeared to be slightly dynamically unstable, the autopilot coped with the mis load, but dang if the elevators weren’t working their passage, they oscillated for the whole flight. To achieve that level of load error, the nose wheel was not on the ground at 80kts, and on landing, the plane sat with the nose wheels off the ground, unable to steer for taxi. Even at that case, the tail was producing a slight down force.
Join Date: Jun 2007
Location: Highbury, London
Age: 63
Posts: 61
MCAS... “not a stall-protection function and not a stall-prevention function,” says Mike Sinnett, Boeing Commercial Airplanes vice president of product development and future airplane development. “It is a handling-qualities function. There’s a misconception it is something other than that.“
Join Date: Dec 2015
Location: Cape Town, ZA
Age: 59
Posts: 424
Originally Posted by 3rd_ear
Eeeeew! That gives me the heebies, it reeks of disingenuity. Is he justified in what he says?
Edit: I meant it is not an 'elegant' solution, more a band-aid. And the nose-down trim still needs to be explained to pilots.
Edit #2. Forgot to add that when MCAS 'unwinds' the trim that it previously applied, this is another time delayed effect.
Last edited by GordonR_Cape; 10th Apr 2019 at 22:45.
Join Date: Jul 2007
Location: Germany
Posts: 533
Originally Posted by 3rd_ear
Eeeeew! That gives me the heebies, it reeks of disingenuity. Is he justified in what he says?
Bernd
Join Date: Dec 2013
Location: South Coast, UK
Age: 63
Posts: 54
Sadly I haven't been able to keep up with all the posts in this thread so apologies if this is a duplicate.
How did the MEL change with respect to AoA sensors with the introduction of MCAS?
My understanding from this:
http://fsims.faa.gov/wdocs/mmel/b-737_rev%2057.pdf
is that only one AoA sensor is required before despatch?
How did the MEL change with respect to AoA sensors with the introduction of MCAS?
My understanding from this:
http://fsims.faa.gov/wdocs/mmel/b-737_rev%2057.pdf
is that only one AoA sensor is required before despatch?
Join Date: Jul 2007
Location: Germany
Posts: 533
Originally Posted by GordonR_Cape
As a computer programmer and former engineer, the time-delay between the AOA input, and the 10-15 second activation of MCAS, could lead to oscillatory behaviour. Pilot input may still be needed, along with some training.
Edit: I meant it is not an 'elegant' solution, more a band-aid. And the nose-down trim still needs to be explained to pilots.
Edit: I meant it is not an 'elegant' solution, more a band-aid. And the nose-down trim still needs to be explained to pilots.
Bernd
Join Date: Jul 2004
Location: Found in Toronto
Posts: 610
Originally Posted by Chris2303
SLF here
Could the pitch up problem caused by the new engine position be ameliorated with the use of strakes such as those on the CF6 attached to the DC10?
Could the pitch up problem caused by the new engine position be ameliorated with the use of strakes such as those on the CF6 attached to the DC10?
Join Date: Jul 2007
Location: Germany
Posts: 533
Originally Posted by catch21
Sadly I haven't been able to keep up with all the posts in this thread so apologies if this is a duplicate.
How did the MEL change with respect to AoA sensors with the introduction of MCAS?
My understanding from this:
http://fsims.faa.gov/wdocs/mmel/b-737_rev%2057.pdf
is that only one AoA sensor is required before despatch?
How did the MEL change with respect to AoA sensors with the introduction of MCAS?
My understanding from this:
http://fsims.faa.gov/wdocs/mmel/b-737_rev%2057.pdf
is that only one AoA sensor is required before despatch?
The line referring to the alpha vanes explicitly only lists Original and Classic models (-100/-200/-300/-400/-500). Not NG (or MAX).
Bernd
Join Date: Jun 2007
Location: Highbury, London
Age: 63
Posts: 61
Originally Posted by bsieker
Yes, and this has been discussed a dozen times here. Of course one reason for the certification requirement for certain control forces at increasing angles of attack is there to make it harder to stall the aircraft inadvertently, but the primary reason for MCAS is to fulfill very specific criteria for control forces (14 CFR, §25.173), and even more specific instructions on how to demonstrate their fulfillment (14 CFR, §25.175).
Ok, I'm with you - I've read most of this thread and yes, the intent of MCAS seems clear. Perhaps I wouldn't have reacted if he'd said "intended to be a handling qualities function" instead of "is...". Maybe wrongly, I mentally separate emulation from underlying functionality (computer background) and that doesn't apply here? As if, in this case, "sure, it's just like the NG but boy, we had to screw with the kernel to get that working".
Join Date: May 2016
Location: Nantes
Posts: 63
Originally Posted by yanrair
my apologies. Misunderstanding. I was just referring to 737 needing AND trim all the time during acceleration.
all you say is correct. The NNP for runaway stab. Is predicated on staying ahead of trim requirements. To avoid the last ditch heave up , and unload- TRIM TRIM TRIM NU routine. Use trim switches NU until stab back in trim - not blip blip , but major NU input over several seconds. STAB OFF. TRIM Manually thereafter. In runaway stab in previous variants by the time you notice, the stab. will be a couple of divisions AND. Or more and requires a sustained ANU input.
Recent training handbook also recommend using the speed for which the aircraft is trimmed, i.e. AND => increasing speed
In the case of ET 302, it is clear that INCREASING speed had no chance of success and lead to loosing control.
REDUCING speed would have caused a pitch down moment induced by static stability and by reduced thrust, which AMHO could have been counteracted by elevator, until the moment when the aerodynamic loads would have been alleviated enough to permit manual trimming. If manuel trim were still jammed, aft forces required on the column to maintain level flight would have been lower (due to logic of feel and centering system) with no risk of elevator blowback anymore.
Join Date: May 2016
Location: Nantes
Posts: 63
Originally Posted by 3rd_ear
Eeeeew! That gives me the heebies, it reeks of disingenuity. Is he justified in what he says?
MCAS was not there to prevent stall (which was already very unlikely) it was there to improve already excellent (though not certifiable) flying qualities
Join Date: Jul 2007
Location: the City by the Bay
Posts: 531
These accidents cast doubt not only on the max but also on Boeing itself and on the FAA. The aljazeera expose on the 787 didn’t put Boeing in a good light about the 787. Now I wonder about the 777x. What did they cock up on that plane that we don’t know about yet? After all who the heck heard of MCAS before? Now it’s Boeing’s 4 letter word as in “ don’t MCAS it up now Jimmy”.
if that plane turns up broken or poorly thought out then Boeing will have nothing but flawed planes. I’m less proud and trustful of Boeing now than I was before. Same goes for the FAA being the authority it used to be.
killing Ralph Naders niece on the Egyptian flight was also bad for PR . Watch out he doesn’t come out with a new book.
“ unsafe at any speed -Boeing’s Corvair”.
Think i I will avoid the Max and the new 777x for a while
if that plane turns up broken or poorly thought out then Boeing will have nothing but flawed planes. I’m less proud and trustful of Boeing now than I was before. Same goes for the FAA being the authority it used to be.
killing Ralph Naders niece on the Egyptian flight was also bad for PR . Watch out he doesn’t come out with a new book.
“ unsafe at any speed -Boeing’s Corvair”.
Think i I will avoid the Max and the new 777x for a while
Last edited by armchairpilot94116; 10th Apr 2019 at 15:31.
Join Date: Jul 2007
Location: Germany
Posts: 533
Originally Posted by Vilters
What I don't understand is why they keep on using these "obsolete" AOA vane sensors with so many alternatives available.
"Someone had this brilliant idea and it's obviously so much better!" is not sufficient.
Neither is "it doesn't suffer from this particular problem, therefore it's always better."
Bernd
Join Date: Jan 2012
Location: Welsh Marches
Posts: 66
Originally Posted by armchairpilot94116
killing Ralph Naders niece on the Egyptian flight was also bad for PR . Watch out he doesn’t come out with a new book.
“ unsafe at any speed -Boeing’s Corvair”.
Think i I will avoid the Max and the new 777x for a while
Alchad
Join Date: Dec 2001
Location: Brisvegas
Posts: 2,971
https://utcaerospacesystems.com/prod...-data-systems/
https://www.swiss-airdata.com
https://www.flightglobal.com/FlightP...20-%202855.PDF
The "alternative to AoA vanes" have been in use on commercial jet aircraft for quite a while. A moving vane is old style.
https://www.swiss-airdata.com
https://www.flightglobal.com/FlightP...20-%202855.PDF
The "alternative to AoA vanes" have been in use on commercial jet aircraft for quite a while. A moving vane is old style.
Join Date: Nov 2018
Location: Vancouver
Posts: 67
Originally Posted by deltafox44
Here is what I was referring to (published by satguru) :
Recent training handbook also recommend using the speed for which the aircraft is trimmed, i.e. AND => increasing speed
In the case of ET 302, it is clear that INCREASING speed had no chance of success and lead to loosing control.
REDUCING speed would have caused a pitch down moment induced by static stability and by reduced thrust, which AMHO could have been counteracted by elevator, until the moment when the aerodynamic loads would have been alleviated enough to permit manual trimming. If manuel trim were still jammed, aft forces required on the column to maintain level flight would have been lower (due to logic of feel and centering system) with no risk of elevator blowback anymore.
Recent training handbook also recommend using the speed for which the aircraft is trimmed, i.e. AND => increasing speed
In the case of ET 302, it is clear that INCREASING speed had no chance of success and lead to loosing control.
REDUCING speed would have caused a pitch down moment induced by static stability and by reduced thrust, which AMHO could have been counteracted by elevator, until the moment when the aerodynamic loads would have been alleviated enough to permit manual trimming. If manuel trim were still jammed, aft forces required on the column to maintain level flight would have been lower (due to logic of feel and centering system) with no risk of elevator blowback anymore.
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Or, you can watch most of it here...
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