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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 18th Mar 2019, 15:03
  #1921 (permalink)  
 
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Originally Posted by Chesty Morgan
Yes but reducing thrust on the 737 creates a large nose down moment (pitch/power couple) so as Vessbott stated they would have just hit the ground sooner.

My experience of this is unusual attitude recovery during manual reversing air tests. Reducing thrust, if you're already using everything you have to keep the nose up, WILL result in a further nose down moment and no reduction in speed. They'd have been better off sticking the speed brakes up.

And in fact Boeing DO publish this in the QRH.
In fact the forward and higher placement of the engines that required MCAS because of their aerodynamic lift at high AoA, has also significantly reduced the pitch/power couple as the engines are closer to the vertical midline. In any case the idea is not to get to the full thrust position in the first place. As stated up thread the aircraft might have been controllable in pitch at a reduced speed.
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Old 18th Mar 2019, 15:05
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Originally Posted by Vessbot
You ever try taking off at idle thrust?
I always wondered if people who give that hardline analogy have never flown an airplane? When I get asked "what controls airspeed?" My answers is always -It depends.
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Old 18th Mar 2019, 15:05
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Originally Posted by Sailvi767
They were at high speed, reducing thrust would have had almost no effect on pitch. Itís done every single day on every airline flight. The stabilizer at high speeds is extremely powerful in control of pitch. Engine thrust is negligible.


Are you aware of the relationship between excess thrust and climb angle? How much do you suppose they had if they hit the ground?
​​​
Pulling the power from takeoff power to cruise power for 250 would not even be noticed. In normal ops maybe 1 click of trim moving the stab .01.
Yet they were so hard up for trim that they hit the ground. So I'm not swayed by an argument that X amount is so little that they should have given it up toward an unimportant goal. Any amount of trim they could use should have obviously gone toward pulling up (which would also have had the effect of slowing down)


I think you are getting confused with recovering from flight near stall with a nose up attitude and high AOA. In that specific situation slamming the power to max will cause the nose to pitch up further because the stabilator has far less control authority.
Again, I'm not talking about the thrust pitch couple. But it we include it, it would only worsen the consequences of your proposed action.
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Old 18th Mar 2019, 15:11
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Slowing from 320 to 250 at reduced thrust and reduced climb angle is beneficial over slowing from 320 to 250 at full thrust and a steep climb angle,... Why? What is the benefit? When did altitude stop being our friend? Especially when struggling to maintain control and maintain altitude when close to the ground?
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Old 18th Mar 2019, 15:16
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Originally Posted by Ian W
But more speed when your stab trim is too nose down is your enemy. Reduction in speed would reduce the force to counter the nose down trim so 'piloting' would dictate reducing speed.
I concur. It is also important to realize that at cruise speeds and higher the elevator actuators are not able to supply enough force to push the elevators to their travel limits. Elevator travel at these speeds is hinge moment limited - the faster you go the less you get when applying full hydraulic pressure to the elevator actuators. No amount of crew pulling on the column can get around this. If you are running out of pitch control authority at moderate to high speed, additional speed is not you friend.
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Old 18th Mar 2019, 15:24
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Originally Posted by Ian W
In fact the forward and higher placement of the engines that required MCAS because of their aerodynamic lift at high AoA, has also significantly reduced the pitch/power couple as the engines are closer to the vertical midline. In any case the idea is not to get to the full thrust position in the first place. As stated up thread the aircraft might have been controllable in pitch at a reduced speed.
All fair points. I'm sure it would have been more controllable at lower speed but the fact is they got fast with a large nose down trim input they couldn't counter. Reducing thrust at that point would only make things worse.

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Old 18th Mar 2019, 15:26
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BEA press release confirming the earlier announcement of the successful recovery of the FDR and CVR data:



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Old 18th Mar 2019, 15:26
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Originally Posted by Vessbot
Slowing from 320 to 250 at reduced thrust and reduced climb angle is beneficial over slowing from 320 to 250 at full thrust and a steep climb angle,... Why? What is the benefit? When did altitude stop being our friend? Especially when struggling to maintain control and maintain altitude when close to the ground?
I just realized youíre not a pilot. If they had the ability to get the nose up and trade airspeed for altitude they certainly would have done so. In fact if they had that ability they would have simply climbed out at 250 and never got fast. Your all hung up on thrust causing pitch changes because you read news articles by reporters with zero knowledge. Thrust changes are a non issue at normal speeds! If you have a trim runaway you need to slow down to reduce the effectiveness of the stab and reduce control column loads. L/D max or probably about 215 knots would have been my target airspeed with trim issues.
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Old 18th Mar 2019, 15:29
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Originally Posted by Sailvi767


I just realized youíre not a pilot. If they had the ability to get the nose up and trade airspeed for altitude they certainly would have done so. In fact if they had that ability they would have simply climbed out at 250 and never got fast. Your all hung up on thrust causing pitch changes because you read news articles by reporters with zero knowledge. Thrust changes are a non issue at normal speeds! If you have a trim runaway you need to slow down to reduce the effectiveness of the stab and reduce control column loads. L/D max or probably about 215 knots would have been my target airspeed with trim issues.
Don't forget that slowing to about 250 knots also allows the control system to use full elevator travel. At speeds above that "blowdown" or "blowback" (choose your desired label) occurs and the system is not able to push the elevator to its full travel limits.
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Old 18th Mar 2019, 15:36
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Originally Posted by Ian W
Where did the steep climb angle come from? That was their problem
What steep climb angle, they were level about 1000 feet AGL and left the power up so the aircraft accelerated rapidly. They had no climb angle.
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Old 18th Mar 2019, 15:39
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Originally Posted by FCeng84
Don't forget that slowing to about 250 knots also allows the control system to use full elevator travel. At speeds above that "blowdown" or "blowback" (choose your desired label) occurs and the system is not able to push the elevator to its full travel limits.
Bingo, you are reducing the effectiveness of the miss trimmed stab and increasing elevator effectiveness.
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Old 18th Mar 2019, 16:22
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Apologies if already posted

Sensor cited as potential factor in Boeing crashes draws scrutiny

March 17 at 7:47 PM

https://www.washingtonpost.com/busin...=.b9df74af9cb0


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Old 18th Mar 2019, 16:35
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Sounds like they were stuck between a rock and a hard place

Reduce thrust - immediate reaction is a slight reduction in pitch due to thrust coupling but maybe they would slow down and recover enough elevator authority to begin a stable climb eventually
vs.
Maintain / increase thrust - maybe a slight pitch increase to start with but they will gain speed with limited elevator travel and no chance of overcoming the nose down moment from the stab

Option 1 probably works nicely at when you have altitude on your side...
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Old 18th Mar 2019, 16:52
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Question for all the pros: if MCAS is supposed to AVOID stalls by putting the nose down, does anyone know how many times it actually DID this on all airlines since it was introduced? Is there some sort of log of when it was activated, and is there any analyses of valid/invalid activation? Or, is this asking too much? Seems to me this would be extremely relevant history.
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Old 18th Mar 2019, 16:53
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"Prosecutors, Transportation Department Scrutinize Development of Boeing’s 737 MAX"

https://www.wsj.com/articles/faas-73...d=hp_lead_pos1
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Old 18th Mar 2019, 17:11
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Originally Posted by Running Ridges
Sounds like they were stuck between a rock and a hard place

Reduce thrust - immediate reaction is a slight reduction in pitch due to thrust coupling but maybe they would slow down and recover enough elevator authority to begin a stable climb eventually
vs.
Maintain / increase thrust - maybe a slight pitch increase to start with but they will gain speed with limited elevator travel and no chance of overcoming the nose down moment from the stab

Option 1 probably works nicely at when you have altitude on your side...
They had a much easier choice, comply with the QRH for runaway trim, disconnect the trim system, manually trim and fly to destination as airline crews have been doing since the invention of electric trim.
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Old 18th Mar 2019, 17:13
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Automation has been taking over cockpits for decades. Flight engineers have all but disappeared and the second officers filling their seats are gone too, along with the high time, well seasoned first officers who were often more experienced than their captains due to mergers and takeovers. These days, it must feel pretty lonely up there on a bad night with a copilot new to the airplane and the game. Automation, now so essential but not always cooperative or fully understood, only adds to the percentage of recent accidents due to confusion over who or what had control of the airplane.The writing is clearly on the wall. According to at least one source, Boeing believes eighty five percent of all accidents are due to pilot error, and there are those who think the sooner the day comes when the AI does the all work and the pilot does all the cross checking, the better.

The flying public are unlikely to accept security guards and night watchmen minding the machinery however, so real pilots will be with us for the foreseeable future.. Despite the shift in perception, away from the status and prestige of airline pilots in previous generations, they need to be a lot smarter in ways we old stick and rudder guys would never have dreamed of. Know your airplane is as important now as it ever was, but training is apparently too expensive and the task is not made any easier when the builders donít think the pilots are up to it. Until they can design airplanes with pilots completely out of the loop, they need to be completely within it. Right up until the last one out of the cockpit turns out the lights.
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Old 18th Mar 2019, 17:14
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Originally Posted by Sailvi767


They had a much easier choice, comply with the QRH for runaway trim, disconnect the trim system, manually trim and fly to destination as airline crews have been doing since the invention of electric trim.
Yes, we know...
The billion $ question is: Why went two crews into that deadly trap?
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Old 18th Mar 2019, 17:24
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Originally Posted by Sailvi767
They had a much easier choice, comply with the QRH for runaway trim . . .
Except that MCAS activation doesn't present as runaway trim (not continuous and can be briefly interrupted by operation of column trim switches), so it wasn't recognized by at least two crews. Then there's the problem of trimming manually, perhaps from full nose-down, at high speed and low altitude, while trying to fly the airplane -- with any number of other distractions bombarding the senses and causing distraction and confusion.

This has been discussed exhaustively and compellingly in these threads, but some still insist that the problem is pilot error. I suppose it's OK for individual pilots to think that way, but, when engineers, manufacturers, regulatory authorities, etc. do, the result is all too likely to be catastrophic loss.



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Old 18th Mar 2019, 17:53
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Devil

Originally Posted by FGD135
Quite commonplace, really, for critical airborne systems to use only one sensor, and only raw data at that. Examples:

Turkish Airlines flight 1951, 25 Feb, 2009. B737-800 where one radar altimeter was malfunctioning. The data from the sensor went to zero, the computers thought the plane was on the ground, so they reduced the engine power to idle. The result was a stall at low altitude where many occupants were killed. The idiotic thing was that not only were the computers using just one radar altimeter sensor, they were making no effort to inspect it for reasonableness or filter it against spikes. The data was showing valid heights but then instantly started showing zeros!

Qantas flight 72, 7 Oct, 2008. Airbus A330. Pilot's side air data computer had a momentary spike in the angle of attack data. Silly computers took this as indicating the aircraft was suddenly stalling, and at a speed of about 450 knots, pushed the nose down. Passengers were thrown into the ceiling and many were seriously injured. The idiotic thing was that the computers were using completely raw data and could therefore believe that the angle of attack could, in the space of one second, change from sensible values to a stalling angle. Also idiotic that the computers would happily perform a manouever of such violence.
Well I've only been an aviation professional for 52 years, so what do I know. Seriously though, the Turkish Airlines case illustrated poor failure monitoring within the RADALT (Not to MENTION complete crew unawareness and monitoring) of IAS. RAD ALT inputs to AFCS typically is used for gain gearing and throttle retard and usually have good self monitoring (But not in this case of course).
The QANTAS A330 case has yet to be completely and adequately explained ADIRU fault, the suspect was a data labelling issue. (GREAT airmanship by the QANTAS crew however). The ADIRU functions certainly are monitored and compared.
For ANY flight control system, ALPHA as well as Ps & Pt inputs are invariably monitored.. The absence of an Alpha disagree function of MCAS is at the very best negligent to the extreme, as has been the whole frantically rushed throwing together of this appalling system. Pilots and passengers deserve FAR better than this.
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