Altitude Hold/Capture Still Needs Tweaking

[tatfsn 2]

Hi,

I recently did a flight with the beta 3.4 model.   One issue that I've consistently observed with the Falcon is that it

tends to drift a bit above and below the desired altitude when the autopilot is on.     During turns, and when the

aircraft rolls out of turns, the issue seems particularly acute.   Once the plane drifted down about 60 feet, and then

drifted up to nearly 100 feet above the altitude set in the autopilot.  On my latest flight I had selected 17,000 for

a cruise altitude, and the aircraft did not remain at that altitude, but drifted between 16,970 feet and 17,030 feet 

through the entire albeit brief flight.

Otherwise this is truly fabulous work by Flysimware.   Like all their previous releases, I've no doubt that it

will continue to improve.   This is just another item to add to the list.

 

[ark 1,199]

25 minutes ago, tatfsn said:

Hi,

I recently did a flight with the beta 3.4 model.   One issue that I've consistently observed with the Falcon is that it

tends to drift a bit above and below the desired altitude when the autopilot is on.     During turns, and when the

aircraft rolls out of turns, the issue seems particularly acute.   Once the plane drifted down about 60 feet, and then

drifted up to nearly 100 feet above the altitude set in the autopilot.  On my latest flight I had selected 17,000 for

a cruise altitude, and the aircraft did not remain at that altitude, but drifted between 16,970 feet and 17,030 feet 

through the entire albeit brief flight.

Otherwise this is truly fabulous work by Flysimware.   Like all their previous releases, I've no doubt that it

will continue to improve.   This is just another item to add to the list.

 

As I understand it, FSW wrote custom Altitude Hold code, instead of using sim default code, because the custom code allows directly activating Altitude Hold without using Altitude Select, and so better models the real world autopilot.  In other words, you can just directly  'hit' the ALT HLD button to capture the altitude you are at.  In your example above, drifting up or down 30 ft out of 17,000 is an error of less than 0.2 percent.  If I compare this to the required accuracy of an altimeter, I see the FAA FARs allow an altimeter error of + or - 80ft at  10,000 ft, or an error of 0.8 percent. And of course, any weather issue will cause some altitude 'drifting' as well. So I don't see a significant issue with Altitude Hold at this point. However, I don't have any real world experience flying at high altitudes with an autopilot, so if someone out there does please comment on what should be expected with respect to altitude hold.

Al

[JRBarrett 2,702]

It’s an FAA violation if an aircraft deviates by more than 300 feet plus or minus from an assigned altitude. In the US ATC system, the controllers’ radar tracking software will generate an alert if an aircraft’s Mode C altitude deviates by more than 200 feet from assigned altitude.

Alitmeters (air data computers) used in RVSM airspace between FL290 and FL410 must demonstrate accuracy to within +/- 80 feet, at any altitude within that range, but that can only be tested on the ground using a calibrated air data test set connected to the aircraft pitot/static system, which generates very precise static reference pressures corresponding to different test altitudes.

The amount of drift allowed in altitude hold mode would be found in the documentation of the specific make and model of autopilot. I would think drifting by more than plus or minus 100 feet would be cause for concern. I had a Gulfstream G200 in maintenance that was written up for “porpoising” above and below selected altitude in altitude hold mode. That particular issue turned out to be caused by the tension being too loose on the control cables connecting the autopilot pitch servo to the elevator.

[tatfsn 2]

Thanks for the feedback, guys.

I'm comparing the Falcon issue with that of the other Prepar3d  and Flysimware aircraft that I have.  The

Lear 35 for example, remains nailed exactly on the altitude that I select with barely a foot of deviation.   That's

even in sharp turns.  I've been impressed by it's ability to hold altitude even in pretty robust turbulence.   

Hopefully Flysimware will address this issue with the Falcon.   I can't remember the last flight simulator or P3D

aircraft I've had that deviates from selected altitude this way. 

[Cool 27]

I noticed also that during a descent to the ALT SEL in VS mode, after the ALT alert sound I decreased the ALT  Because of the new clearance, I did not do anything else. When the plane arrived at the previous selected ALT,  he maintained it. I thought that he would not stop to the previous one and descent to the new selected ALT?

Pascal

[Wink207 78]

Personally, I kinda like a bit of imperfection, if that is what it is, in these maneuvers. Especially the altitude captures and the bank angle losses. It makes me a more attendant pilot in planning my maneuvers to maintain stability, and stop the brandy snifters back aft from sliding off the tables. 🙂