CMD A versus CMD B and A/T Disagreement Questions

[Bman2006 6]

Hey all-

 

I had a good flight last night over in eastern Australia. I have a few things crop up that got my attention but I managed to complete the flight and all was well. However, I am curious to know how to correct the situation should it rise again:

 

1) After departure and good level flight / trim, I punched on the LNAV / VNAV and CMD A/B. The flight director was flipped on captain side then F/O side. I noticed a few minutes later that the CMD A had turned off and the controlling FD was on the F/O side. If I flipped on CMD A, CMD B would turn off and the F/D would move back to the captain's side. Odd. I couldn't figure out why they both wouldn't stay - am I losing my mind, should A and B be on?

 

2) During various segments of my flight the A/T would disengage due to asymmetrical throttle. The real question is why was I getting such strong asymmetrical throttle? Nothing was being done with the physical quadrant so I can only surmise it was the weather engine (ASE) give some serious fits. Wind was around 100 knots 30° off the nose of the plane.

 

3) The speed of the plane was erratic (tied to #2 above)? I would notice the speed tape clicking up up and up until high speed warning. However, the A/T was at and idle state and the plane would continue to accelerate in level flight. In spite of the low N1 the plane wouldn't begin to slow do again until the high speed warning annunciated. Why it was doing this... not really sure. I think again it's tied to the weather engine.

 

All things considered - it seems I had weird gremlins in the system somewhere. I have not had these problem before. I am going to do another flight tonight but perhaps you guys have some insight on what is going on.

 

I am using FSUIPC (weather smoothing on and all control mapped through FSUIPC). SPAD is also being used. Again, it has worked just fine for the past 50 hours of flight so I am not really sure what might be up.

 

Thanks-

 

Benjamin

[Fabo 213]

If I flipped on CMD A, CMD B would turn off and the F/D would move back to the captain's side. Odd. I couldn't figure out why they both wouldn't stay - am I losing my mind, should A and B be on?

 

That is correct behaviour. MA light notes the active master computer, which is either the one on active autopilot side, or, if AP was not turned on, on the side of FD that was turned on first. You want to have the master computer on your side. The reason why MA flipped to F/O side is that when you turned on CMD B, CMD A switched off and relegated control to right system.

 

Both A and B systems are only on in a single case - that is an autoland.

 

 

2) During various segments of my flight the A/T would disengage due to asymmetrical throttle. The real question is why was I getting such strong asymmetrical throttle? Nothing was being done with the physical quadrant so I can only surmise it was the weather engine (ASE) give some serious fits. Wind was around 100 knots 30° off the nose of the plane.

 

Weather should not be a factor, after all, the airframe is moving within the air mass itself. As long as yaw damper is operating correctly, there will never be any sideslip and therefore no uneven airflow to the engines. (note this is valid inflight). If you have separate axis for throttles, it might just mean that your quadrant is up for recalibration (and maybe cleaning).

 

 

3) The speed of the plane was erratic (tied to #2 above)? I would notice the speed tape clicking up up and up until high speed warning. However, the A/T was at and idle state and the plane would continue to accelerate in level flight. In spite of the low N1 the plane wouldn't begin to slow do again until the high speed warning annunciated. Why it was doing this... not really sure. I think again it's tied to the weather engine.

 

Possibly tied, however, might be an effect of bad weather program behaviour, either erratic wind injection, or some weird turbulence interpretation. Hard to tell. Maybe it was just having a bad day.

 

 

All things considered - it seems I had weird gremlins in the system somewhere. I have not had these problem before. I am going to do another flight tonight but perhaps you guys have some insight on what is going on.

 

Well the first one is, obviously, wrong operation. Is it possible that you were flying an older plane recently, with separate roll/pitch AP switches? I found that that might lead to similar errors. Not dangerous in this direction, but more dangerous in the other one, if the pilot engages only single axis thinking he has engaged whole system.

 

 

all control mapped through FSUIPC

 

Might be the culprit. If problem with AT and speed persists, try running the throttle axis directly to FS (no FSUIPC axis processing at all).

[rsvit 16]

1) Both autopilots can be used only for APP mode. CMD A makes captain's FD master and F/O's FD becomes slave. Vice versa with CMD B.

 

2) Crosswind has no effect on TL's symmetry. You can try to turn off A/T override in PMDG options and what happens.

 

3) It might be change of velocity of wind. Something like headwind increasing from 30 to 100kt, FSUIPC smooth it 2kt per second and you get additional 2kt of IAS every second. Just my guess, but I'm using AS2012 and don't have such problems.

[Guest BeaverDriver]

On the A/T thing, if you're using FSX weather, you get horrific wind shifts and shears (totally unrealistic). If your headwind component suddenly shot up by a hundred knots or so and the A/T tried to very quickly bring the thrust back to idle, that might be enough to kick it offline. I'd recommend getting AS2012 to combat that sort of thing, assuming that's what caused it (you'll be glad you got it anyway!).

[tazisdylan 2]

Yeah I too have experienced the shocking sudden wind shifts and shears with the subsequent throttle 'issues' on other aircraft if using FSx weather as Glenn refers too. Give it a miss and use something else AS2012 or, I use REX Essential without issue. See if that helps.

[Bman2006 6]

Thanks guys -

 

1) Duh - I don't know why I thought the contrary. I must have had autoland on the brain from previous flight. The plane navigated just fine so I had the sinking feeling that I was not with the game plan in the ways of correct operation. Thanks for the clarification.

 

2) I will try again without Active Sky X. I have Essentials so I might try that.

 

3) I think you might have pegged the speed issue with the FSUIPC smoothing. 2 second intervals sounds about right. I think ASE might have pumped in a large Wx shift and FSUIPC just upped the speed 2 seconds at a time until I reached the correct wind speed. I did notice the winds aloft went from darn near head on to strong tail wind so it would make sense if the weather was smoothed out but at a faster rate than the plane could slow down. It wasn't like the speed shot up instantly so I like this theory. Nice work.

 

Thanks again.

 

Benjamin.