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FBW pitch/thrust compensation

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FBW pitch/thrust compensation

Old 18th Oct 2016, 12:33
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FBW pitch/thrust compensation

I am wondering how Boeing and Airbus software compensates for pitch/thrust coupling?

Does it simply monitor inertial rates and apply pitch control/trim to null them/adjust to the demanded input?

Is there also some form of commanded/actual N1 or similar input to allow the system to anticipate high compensation requirements such as go arounds &c in the same way as config trim changes are handled?

If anyone has any good in depth links to discussions of particularly the Boing but also the Airbus FBW software implementation I would be v interested in reading them.
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Old 18th Oct 2016, 14:47
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Here you go:

http://www.icas.org/ICAS_ARCHIVE/ICA...PAPERS/605.PDF

http://www.rvs.uni-bielefeld.de/publ...ft_Protect.pdf
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Old 18th Oct 2016, 14:48
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In airbus FBW pitch changes are achieved through the side stick as g demands. It is linear from 1g with stick neutral to 2.5g stick full back. As long as stick is out the elevator and stab move to give you the g and once achieved, through feed back mechanism maintain that position. When the stick is returned to neutral it is 1g demand and computer will move the elevator and stab to achieve that. Any g effect that is not ordered through the side stick either through thrust/weight or environmental the computer resists as it tries to maintain 1g within it's authority.
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Old 18th Oct 2016, 19:39
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After a few astonishing landings made using only thrust for control I believe a software program was developed and tested with success on a jet airliner. It was an old tech a/c. I've not heard the evolution of this software or any introduction into line a/c. How would this thrust control software be modified to FBW a/c where there is no pitch/roll/yaw effect? Given that all future a/c (within 30 years) will be FBW has that technology already been adapted for FBW, or has it been shelved?
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Old 18th Oct 2016, 20:47
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Jan-Paul - much appreciated, exactly what I was after thanks!

RAT - NASA is actively working on this in fighter aircraft - the Intelligent Flight Control System - there is some really interesting archive stuff on the early tests.

I imagine this sort of thing will come with the next clean sheet FBW system.
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Old 19th Oct 2016, 15:59
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Boeing FBW Thrust Control not Integrated with Path Control

Unfortunately the historic separation between flight control surface control and engine control on Boeing commercial airplanes has carried over into the FBW era. As such, it is unlikely that thrust control specifically designed to provide pitch control will be coming any time soon.

As noted in entries above, the pitch axis maneuver augmentation provided by C* and C*U control laws treats pitch disturbances generated by thrust changes as a disturbance and uses the elevators and horizontal stabilizer to compensate without need for any pilot input. The relatively low bandwidth (i.e., slow acting) nature of thrust changes allows for the C* and C*U systems to very effectively counter thrust pitching moments via integral feedback control.

As long as commercial transport pitch control systems are designed such that loss of adequate pitch control is extremely improbable, I doubt that there will be any value seen in designing engine controls in such a manner as to provide backup pitch control.
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Old 19th Oct 2016, 16:07
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As long as commercial transport pitch control systems are designed such that loss of adequate pitch control is extremely improbable, I doubt that there will be any value seen in designing engine controls in such a manner as to provide backup pitch control.

There have been more than you think occasions where thrust control saved the day. The loss of pitch control was not a failure of the pitch system; it was because of an external cause such as a mid-air collision or explosion on board. Totally unpredictable. It's happen many times, going back to first time I heard of this demonstration of great skill on a Constellation that had 2 fins and most of the elevators chopped off. Sods law is still in force.
I thought an F16 (FBW) had demonstrated this system and then a B727?? Someone will know for sure.
I suppose it will all come down to risk assessment and cost v risk.
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Old 19th Oct 2016, 16:40
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Sioux City DC-10

Rat5 - I concur that there will be events such as the DC-10 that managed to "land" at Sioux City in 1989 after a center engine rotor burst took out all hydraulics to the tail surfaces. Al Haynes and the rest of the crew on that one managed to get it to the runway using just thrust control of the two remaining wing mounted engines. In order for that to be successful you need to have an airplane that exhibits a fair degree of inherent speed stability such that engine pitching moment can be balanced by pitching moment due to changes in speed. I think that today's configurations that have been optimized for performance with much less inherent stability would present a significantly greater challenge to the likes of Capt. Haynes or any control system designed for this purpose. The bottom line is that if you don't have control inputs that can independently control thrust and pitching moment there is no control system magic that will be able to bring you home.

In the Sioux City event, I believe Al and his crew found that they had to fly rather fast to be able to maintain pitch trim. I also recall that they were only able to turn in one direction so had to be creative with their line-up. That event is a huge tribute to how quickly and effectively pilots who understand the physics of their airplanes can innovate on the fly and make do with a very crippled bird. It is unfortunate that the DC-10 ended up cart-wheeling on the runway, but it was amazing that about half of those on board survived.

To design a control system that would be able to do much good in such a situation it would need to either have extensive information as to what surfaces and controls were still available and what had been lost or the control algorithm would have to be adaptive such that it could quickly learn what works and what does not.
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Old 19th Oct 2016, 18:48
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Originally Posted by FCeng84 View Post

In the Sioux City event, I believe Al and his crew found that they had to fly rather fast to be able to maintain pitch trim. I also recall that they were only able to turn in one direction so had to be creative with their line-up. That event is a huge tribute to how quickly and effectively pilots who understand the physics of their airplanes can innovate on the fly and make do with a very crippled bird.
Capt Haynes seemed to think it was an even bigger tribute to sheer luck.
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Old 19th Oct 2016, 20:19
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Luck always goes better with some help

I think Capt. Haynes is being modest. Sure it took a degree of luck, but it also took significant skill to work carefully within the range of control available to do the best that they could to get it down onto the airport. To have learned that they could only turn in one direction and to manage their line-up and approach accordingly is the stuff heroes are made of.
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Old 21st Oct 2016, 07:01
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There was a NASA program called "Propulsion Controlled Aircraft" - PCA - that was intended to bring battle-damaged military planes home rather than save civil aircraft, but the principle holds.


Propulsion Controlled Aircraft (PCA) | NASA


There's also a monograph on the subject entitled "Touchdown: The Development ofPropulsion Controlled Aircraft at NASA Dryden" in the "Monographs in Aerospace History" series, I had downloaded the pdf but can't find the link anymore. The monograph contains a very touching photo of Al Haynes making a PCA landing in the simulator....
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Old 21st Oct 2016, 11:03
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John Farley 8.sept2016 in thread "Loss of control recovery" :
While going through these steps it is common sense to close the throttle if the nose is low and add power if the nose is high.
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Old 21st Oct 2016, 12:26
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Read the NTSB report. Compare the FDR vs the CVR. Captain Denny King (RIP) walked into the cockpit several minutes after the event started. He saw the yokes full back and full right. Immediately looked at the hydraulics gauges and realized they had no hydraulics. That is when control of the a/c improved as CA King assumed controls of the thrust levers. CA Haynes, and crew, tried throughout the event to use systems that required hydraulics (a/p, slats). CA King kept having to correct them. It's in the CVR transcripts. It's also part of CA King's refusal to close the throttles. BTW, he'd basically sacrificed himself by squatting by the center console and he flew into the crash without a seatbelt. He's the hero.
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Old 22nd Oct 2016, 00:09
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"Capt King's" name is actually Fitch, but carry on.
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Old 22nd Oct 2016, 09:12
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The monograph contains a very touching photo of Al Haynes making a PCA landing in the simulator....


Interesting link and discussion on PCA, Fizz. Thank you. PDF of the monograph can now be found here: Touchdown: The Development of Propulsion Controlled Aircraft at NASA Dryden
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Old 22nd Oct 2016, 16:52
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There is more to this scenario than Sioux City. It's the one which has the most publicity. There was also the B747 in Japan with the blown out rear bulk head; the DHL missile strike in Baghdad; going back to the Constellation in 60's where I first read about this type of event and they landed it in a field. No doubt a trawl though the archives will reveal many more from a variety of causes.
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