I need some advice,what could be a good setling time?given that the horizontal nominal speed is 250m/s and the weight of the aircraft is 280,000Kg.Is the settling time the time needed for the aircraft to settle on the desired altitude,or is it just the time that the controller needs to operate without any significant error?if anyone can get back to me on that I'd appreciate it.

 

I would like to offer the benefit of my 30 years experience as an engineer and private pilot. The first assumption you should make is that the altitude correction will be less severe and easier to accomplish the quicker you start it. Settling time is probably the wrong question.

Why not try to approach the problem from facts available to you? A skilled pilot can manually hold altitude to within +/- 50 feet for an extended period. This assumes no CAT occurances (Clear Air Turbulence). You can determine the lag time of a typical altimeter (call altimeter manufactures). However, even light planes have instantaneous vertical speed indicators. These permit the pilot to sense a vertical movement in time to initiate a correction before a significant change in altitude occurs. Even then the human response time limits how fast the pilot can correct. As I recall from a study of helicopter wire avoidance sensors, the pilot takes 3-5 msec to recognize a change of instrument output. This means he must initiate a correction and look for the desired aircraft response which will take several seconds. Your digital altitude hold should operate as fast as the pilot or you will cause the passengers to become ill from motion sickness ha ha. Is any of this making sense? I hope you get the idea I intend to convey. I worked on DC-10s which are similar to the Boeing 747. You will want to sample the instantaneous vertical speed to give you a trend or drift of altitude. The sample time can be your choosing (within limits) since the mass of the aircraft will dampen out the short changes like 1 to 2 secs of drift. And whatever sample time you pick, you will need a feedback loop to prevent you from ripping the flight control surfaces off the wings by too sever movements. Check out the F-16 fly-by-wire aircraft to see how fast the control surfaces move.

Another point you would be wise to consider. No aircraft holds an altitude exactly. All fluid dynamics is statistical in nature. Your job is to head-off trends before they translate into large altitude fluctuations. Does this help? E-mail [email protected] for further dialog.

 

The reponse time of 25 minutes is more than adequate (IEEE Std. 867.11h U.S military standard)

Steven McPhee, PhD.
McGill University