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Topic: Control And Stability Augmentation Systems (CSAS)
Username: Airbuske
Posted 2009-01-27 20:44:19 and read 13219 times.

I am aware that the function of these systems is to artificially modify (augment) the apparent stability and control characteristics of an airplane in the event that it fails to meet the requirements in some way (which is mostly the case).

My question is this : on large commercial airliners, are pilots aware that for a particular flight maneuver, even though the controls they are inputting brings about the desired/anticipated aircraft response, their perceived control surface deflection may not be entirely accurate?

For example, the aircraft is flying straight and level and now the pilot wants to pitch up 5 degrees. He would pull back on the yoke/stick until his pitch attitude was 5 degrees. The associated control surface deflection required to bring about this aircraft response would be the elevator deflecting upwards for the entire period that the yoke/stick is being pulled. To my understanding, in reality, the elevator would constantly be deflecting upwards and downwards (as being commanded by the augmentation system) to prevent the aircraft from going into an unwanted dynamic state of motion (in this case, a phugoid). Is the pilot aware that the elevator is not fixed in position and is actually moving to make corrections? And can a pilot turn such an autopilot control system off?

If I have grossly misunderstood the entire concept, I am sincerely sorry.

Jinal

Topic: RE: Control And Stability Augmentation Systems (CSAS)
Username: Flypig687
Posted 2009-01-27 20:50:53 and read 13216 times.

I am not 100% sure if your description of a SAS is correct.

Either way in a plane with Fly-by-wire (FBW) control the stick will feel nothing at all without the computer telling it to feel something. Most FBW systems will use active sticks to give the pilot some feedback from the airplane relating to their input.

Also most likely in any large plane where servos/sas are in use I would imagine that they are not aware of the control surface moving in all sorts of directions, they are just aware of the final results being a commanded pitch/roll/yaw rate.

Topic: RE: Control And Stability Augmentation Systems (CSAS)
Username: Tdscanuck
Posted 2009-01-28 16:48:19 and read 13158 times.



Quoting Airbuske (Thread starter):
My question is this : on large commercial airliners, are pilots aware that for a particular flight maneuver, even though the controls they are inputting brings about the desired/anticipated aircraft response, their perceived control surface deflection may not be entirely accurate?

It's part of the training, and you can see it if you pull up a flight control synoptic display, but there's no tactile or normal display feedback to tell them that it's happening. So, although they're mentally aware of it and can look at it if they want to, it's designed to behave as if it's invisible to the flight crew.

Quoting Airbuske (Thread starter):
Is the pilot aware that the elevator is not fixed in position and is actually moving to make corrections? And can a pilot turn such an autopilot control system off?

It's not an autopilot function, it's a flight control system function, but both Airbii and Boeings can be dropped into direct control mode, at which point you would turn the stability augmentation off and the flight controls would just follow the position of the crew controls.

Tom.

Topic: RE: Control And Stability Augmentation Systems (CSAS)
Username: Airbuske
Posted 2009-01-29 19:04:15 and read 13108 times.



Quoting Flypig687 (Reply 1):
I am not 100% sure if your description of a SAS is correct.

Where do you think I'm misunderstanding?

Quoting Tdscanuck (Reply 2):
It's not an autopilot function, it's a flight control system function

You are correct in that it isn't an autopilot function but it is an autopilot system that is governing the servo signal input. Would you happen to know where in the cockpit you can turn on/off the CSAS system? Pics would be nice.

Thanks,

Jinal

Topic: RE: Control And Stability Augmentation Systems (CSAS)
Username: Tdscanuck
Posted 2009-01-29 22:44:04 and read 13090 times.



Quoting Airbuske (Reply 3):

Quoting Tdscanuck (Reply 2):
It's not an autopilot function, it's a flight control system function

You are correct in that it isn't an autopilot function but it is an autopilot system that is governing the servo signal input.

No, it's not. The autopilot gives commands to the primary flight control system just like a pilot does. It's the PFCS that actually commands the servos. Autopilot signals go through the envelope protection and augmentation system the same way that pilot commands do.

Pg. 2 of this PDF shows the basic architecture:
http://www.smartcockpit.com/pdf/plane/airbus/A320/systems/0010/

Quoting Airbuske (Reply 3):
Would you happen to know where in the cockpit you can turn on/off the CSAS system? Pics would be nice.

On a 777, it's the PFC disconnect switch on the P-5 (overhead) panel. It's in the top left of this photo, right beside the Thrust Asymmetry Compensation (TAC) switch:
http://www.airliners.net/photo/Ameri...lines/Boeing-777-223-ER/0734534/L/

On an A320 there are separate switches for each computer. You can see them in the second panel down on the left here (the panel is labelled FLT CTL):
http://www.airliners.net/photo/Myair/Airbus-A320-212/1030643/L/

On a large FBW airliner that doesn't have manual reversion, the FBW system is still active even if the control laws drop to direct mode...exactly what kinds of control modification you get in direct mode vary from airplane to airplane.

Tom.

Topic: RE: Control And Stability Augmentation Systems (CSAS)
Username: Airbuske
Posted 2009-02-08 21:22:07 and read 12891 times.

Thanks for your input Tom. Tech Ops wouldn't be as interesting without you  Wink

P.S. - sorry for the late reply, been really busy. But I do appreciate you taking the time out to answer my questions.


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