Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Was MCAS needed?

Tech Log The very best in practical technical discussion on the web

Was MCAS needed?

Old 2nd Feb 2021, 18:44
  #101 (permalink)  

Only half a speed-brake
Join Date: Apr 2003
Location: Commuting home
Age: 43
Posts: 3,138
Guess all left here is saying thanks for setting me straight? I do, appreciated. Happy and excited to read your contributions, fdr.

Last edited by FlightDetent; 2nd Feb 2021 at 22:29.
FlightDetent is offline  
Old 2nd Feb 2021, 22:26
  #102 (permalink)  
Join Date: Mar 2006
Location: England
Posts: 919
Bergerie, ' a better solution'
We might assume that a STS type of fix was considered / tested.
STS as I understand is only a single trim input, also like the 747 example it uses speed as the sensor; loss of sensor was not critical.
MCAS 'downfall' involved the dynamics of the situations; the need of AoA as a sensor and more than one trim input. Introducing these was critical to the MAX deficiency (type similarity / certification), which enabled the opportunity, with corporate pressures, to poorly engineer MCAS where loss of sensor was critical.
PEI_3721 is offline  
Old 3rd Feb 2021, 01:36
  #103 (permalink)  
Join Date: Jun 2001
Location: 3rd Rock, #29B
Posts: 899
Originally Posted by FlightDetent View Post
Guess all left here is saying thanks for setting me straight? I do, appreciated. Happy and excited to read your contributions, fdr.
FD, forums provide the opportunity for bouncing ideas around and give a quick option for exploring concepts and justifications. If I espouse a position or hypothesis, I have to be open to questioning and should either justify a position or amend my position if there is no proof or supporting rationale for the position.

Sorry for taking up so many column inches, you covered a lot of ground, and my response may have spread the kitty litter a bit wide.

The cartoon of the bandaids is close to reality. For years I used 3M 8661HS, 8672, and similar until we developed better materials with St Gobain. We used EPDM foam on blades and on the rotors, with a cluster of experimental guys out on the left coast enjoying the performance changes on their experimental aircraft. we ran dozens of different materials on the fan blade POC test and showed the benign nature of stuff not working. The left coast guys went short of materials and really did go to Ace Hardware, and tried local products that did not work, but then they went out of the cat tray and tried velcro. dang if that didn't work a treat. running short of the half they were using, the hook side, one guy used the fur bit, and got the same performance shift. the fur stands up to liquid and particulate impact amazingly well, as in, essentially indefinitely. sticking an aluminum angle on a prop TE will fail promptly, and spectacularly, foam lasted until abraded out but was benign, velcro sticks like... well.... velcro. Curious observations; tabbing a blade increases the flutter boundary for normal aoa, never did test out reverse though, ran out of time and airframes. Below 200KCAS it was neat, and unfortunately, we stopped playing in that sandpit at 280KCAS, so wanted to go back there and run faster. (we did run higher on a single flight on a modified P51 look alike, which was able to match it's normal performance but had a sick merlin that was way, way down on power, but never ran with comparative power settings. The fan blade stuff even when only half the fan disk was modified had lower vibration than the clean fan blades, which was neat to observe. We flicked foam all over that little blender and proved that radial acceleration centrifuges stuff very effectively. Little engines are a pain to get to the N1 blade, and the CFM56 was barely better with the -3 blades. Doing a play with a fan pretty much red tags the engine from the manufacturer's POV, so until a spare clapped out Trent-type blender and a testbed comes to hand, we won't get to try getting around the fixed blade angle design of the fan engine.

The current (or at least recent) SME at NASA on separated flow control was one of the authors of the first paper that showed the bizarre shift of Cl and Cd from a tab, back in the 80's... and fan blades are the most significant application of that weirdness if you believe a fan is essentially a fixed pitch blade... which our experience with the props and JT15 did strongly suggest, as does the SFC achieved by fans in operation. Nothing that a billion or two won't sort out an STC for. We showed 30% increase of static thrust, which was more than expected, but the cruise SFC the charts suggest considerably better outcomes. On the T28, testing stopped for external reasons at 12% and 280KCAS, well before getting to a parametric based configuration. The P51 ran with a boost limited engine that matched performance with 20" less boost, have no idea of what 20" on a souped-up merlin is worth. I digress.
fdr is offline  
Old 3rd Feb 2021, 03:06
  #104 (permalink)  
Join Date: Jun 2001
Location: 3rd Rock, #29B
Posts: 899
Originally Posted by Bergerie1 View Post

Thank you (and all you other knowledgeable posters here too). As you say fdr, the 747 stalls very nicely, I did many on the -100s and -200s during CofA check flights. When the 747 was put on the British register, D. P Davies required a stick nudger (not a pusher) to restore longitudinal stability shortly before the clean stall at aft CG. I quote his words from his book, Handlng the Big Jets:-

"With trim speed of 1.3Vs, after a small elevator force to start the speed reduction, the stick force falls to zero while the aeroplane quietly progresses all the way to the stall on its own .... It is common knowledge that the UK attitude to stalling is quite firm and this degree of instability, although slight, was declared unacceptable..... ."

Here was a simple cheap and reliable fix which had already been used on Boeing aircraft. It would have required no training with only a mention in the manuals. Would this not have been a better solution?
K, for those wanting to stall any of these aircraft, a note first up, there is a maintenance procedure for inspections after stall buffet encounter on most if not all of these aircraft. The amount of buffet that occurs varies from type to type and with configuration. Clean is usually quite light buffet and associated loads on the tail, but heavy buffet entry in the MD11 in the cruise, at high Mach pretty mush shredded the elevators on both sides, and came close to two ripples on the pond. A fully configured stall buffet can be quite impressive, as are the loads to the tail and flap tracks.

25.173 requires 1lb/6kts average gradient over the desired window being assessed, which is within the flight envelope from stall to upper limits. There are two main methods of determining compliance, but basically, taking the load on the control, coming off the trimmed speed, (1.23 or 1.3 depending on the rules applied earlier in the Part, the year etc... ) and slowing down as well as speeding up around that datum. For a stall speed around 100kts, the minimum trim point is Vref... so the loads being measured total are around 4lbs to 6 lbs of elevator force, which is quite light. The requirement is for the average to be that, and the guidance material in AC25.07 shows clearly that a reduction in force can occur towards an extreme of the test points, and still be acceptable. That is open to interpretation in so far as a zero gradient for the last 10kts could be fully compliant, but considered objectionable from an HQ view by the testing driver. My recollection is that the stick gradient in the mid-trim condition was way higher than that, and never encountered reversal, which is where a rationale for a SAS system would arise. D.P. Davies did good work, and perhaps at aft CG limit -0.5% at light weight, high thrust, he encountered an undesirable that Boeing didn't. It is possible. Is it likely? TBC did a comprehensive testing program on the 747 under the gaze of Joe Sutter and coming in the wake of the Trident, and some 727 funnies, they were not without awareness of what was needed. A spring balance to the controls could detect that level of force in the day, today, some of the FBW systems record the forces applied, or force sensing gloves can be applied and calibrated to give the info. (make your own, go grab some Phidgets... ).

Last edited by fdr; 3rd Feb 2021 at 07:19.
fdr is offline  
Old 3rd Feb 2021, 03:28
  #105 (permalink)  
Join Date: Jul 2013
Location: Everett, WA
Age: 65
Posts: 3,131
Gums, I joined this forum about 3 years before I retired (basically right after the Asiana 777 crash-landing at SFO). Several co-workers knew of my presence, but I was very, very careful about what I posted to avoid anything that might be considered sensitive, proprietary, or anything that would otherwise get me crossed up with management. More than once, someone posted something that I knew was total BS, but couldn't respond because my knowledge was considered to be Boeing proprietary (one example that comes to mind was a discussion regarding 747-8 fuel burn - of which I had first hand knowledge but couldn't talk about).
Since retirement, I don't need to worry as much but still need to be careful about posting info that could still be considered sensitive. Much of what I know about MCAS is based on an off-the-record discussion I had with a Boeing test pilot friend shortly after the Ethiopian crash. I felt obligated not to post any of that information as not to betray his trust. Most of it has since been reported in the Seattle Times, so I'm confident discussing it - but a few things I still need to keep under wraps...
tdracer is online now  
Old 3rd Feb 2021, 17:42
  #106 (permalink)  
Join Date: Dec 2020
Location: Southwest
Posts: 34
"Most of it has since been reported in the Seattle Times"

I knew people who worked for Airbus who greatly enjoyed Byron Acohido's pieces in the Seattle Times.
I often wonder whether Boeing would have preferred to have him silenced (in a friendly, painlesss, way).
osborne is offline  
Old 4th Feb 2021, 18:19
  #107 (permalink)  
Join Date: Mar 2002
Location: near an airplane
Posts: 1,889
Lecture on the 737 MCAS story by Chris Brady kicks off in just over 10 mins: https://www.solent-raes.org.uk/lectures
Free to join (I'm just another listener, nothing to do with it so I hope it's allright to post this).
Jhieminga is offline  
Old 4th Feb 2021, 18:27
  #108 (permalink)  
Join Date: Feb 2010
Location: attitude is nominal
Posts: 1,241
Thanks the harp plays already.
Less Hair is offline  
Old 4th Feb 2021, 19:45
  #109 (permalink)  
Join Date: Mar 2002
Location: near an airplane
Posts: 1,889
PPRuNe got mentioned!
Jhieminga is offline  
Old 4th Feb 2021, 22:26
  #110 (permalink)  
Join Date: Feb 2010
Location: attitude is nominal
Posts: 1,241
It was worth it. Thanks for telling us right on time.
Thanks to RAeS for sharing this event. Most appreciated.
Less Hair is offline  
Old 5th Feb 2021, 07:52
  #111 (permalink)  
Join Date: Jan 2008
Location: Reading, UK
Posts: 12,634
Originally Posted by Jhieminga View Post
Lecture on the 737 MCAS story by Chris Brady kicks off in just over 10 mins: https://www.solent-raes.org.uk/lectures
Free to join (I'm just another listener, nothing to do with it so I hope it's allright to post this).
Recording still available on the above link. Lecture starts at around the 15:55 mark.
DaveReidUK is offline  
Old 6th Feb 2021, 12:13
  #112 (permalink)  
Join Date: Nov 2003
Location: LPL, UK
Posts: 472
Recording still available on the above link
That link will disappear soon (if not already) but it will remain available at this link:
CaptainSandL is offline  
Old 6th Feb 2021, 15:18
  #113 (permalink)  
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,829
vilas is offline  
Old 6th Feb 2021, 15:34
  #114 (permalink)  
Join Date: Nov 1999
Location: UK
Posts: 1,594
Great presentation by Chris Brady. Many thanks to him for an informative, comprehensive and unbiased lecture.

Most particularly, how MCAS can look like normal THS operation and therefore not necessarily lead the pilots to Runaway Stab Trim.
Uplinker is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.