Blinx 17 Posted November 18, 2015 Share Posted November 18, 2015 Hi! Say for example you are climbing out of an airport, target FL370 AP ON, A/THR ON, all is well. Then change of plans, you disconnect the AP (A/THR is still ON) and go out of managed speed to selected Speed 250Knots. If I now pitch down, the aircraft will not decelerate and leave engine thrust at near full for the climb. Only if I change the altitude knob to something below I am currently flying will the engines spool down to near idle and reduce to the selected speed. Is this normal? Link to comment Share on other sites More sharing options...
skelsey 103 Posted November 18, 2015 Share Posted November 18, 2015 In a word, yes. Airbus Golden Rule #5: FMA, FMA, FMA! Think about what modes you are in: Assuming everything as per a normal departure, you will be in THR CLB | CLB | NAV | AP1 1FD2 A/THR (white) after acceleration altitude. If you pull speed you have not changed the vertical and/or thrust mode - you will still be in THR CLB | CLB. The only difference is that the speed bug instead of being magenta will now be blue and the aircraft will pitch to maintain the selected speed at climb thrust. If you now disengage the autopilot (and do nothing else) you are now in: THR CLB | CLB | NAV | 1FD2 A/THR (white). If you fail to follow the FD at this point you will run in to the problem you describe: the thrust and vertical modes remain THR CLB | CLB -- i.e. climb thrust and speed controlled by pitch, and the F/D will command pitch to maintain 250kts. If you ignore the FD then all bets are off. If you are not intending to follow the FD then you should turn it off. This will cause the A/THR to revert to SPEED mode. The better solution would be to select the altitude you desire and pull for OP DES (or V/S or whatever) and put the aircraft in to the correct mode. Link to comment Share on other sites More sharing options...
Blinx 17 Posted November 18, 2015 Author Share Posted November 18, 2015 Zitieren Airbus Golden Rule #5: FMA, FMA, FMA! I didn't comply Zitieren Assuming everything as per a normal departure, you will be in THR CLB | CLB | NAV | AP1 1FD2 A/THR (white) after acceleration altitude. Exactly. Zitieren If you now disengage the autopilot (and do nothing else) you are now in: THR CLB | CLB | NAV | 1FD2 A/THR (white). Exactly. Zitieren If you fail to follow the FD at this point you will run in to the problem you describe: the thrust and vertical modes remain THR CLB | CLB -- i.e. climb thrust and speed controlled by pitch, and the F/D will command pitch to maintain 250kts. If you ignore the FD then all bets are off. If you are not intending to follow the FD then you should turn it off. This will cause the A/THR to revert to SPEED mode. I understand now! I usually don't come across this, since in approach mode the FD does accept the SPEED mode? I only ever turn the FD off when the green lines are misleading on some visual approach. Thank you! Link to comment Share on other sites More sharing options...
skelsey 103 Posted November 18, 2015 Share Posted November 18, 2015 No problem! Mode reversion is a big topic ;-) 11 minutes ago, Blinx said: I understand now! I usually don't come across this, since in approach mode the FD does accept the SPEED mode? I only ever turn the FD off when the green lines are misleading on some visual approach. That is because most of the time on the approach you will be in either V/S (or FPA) or G/S mode, both of which are modes where speed is controlled by thrust and not pitch. Therefore in this situation when you disengage the A/P you are already in SPEED mode (indeed, one of the items on the landing checklist is AUTOTHRUST............SPEED (or OFF) to prevent the reverse situation to that you describe -- descending in THR IDLE, disengage the A/P, ignore the F/D and pitch up whilst the thrust stays at idle and the speed decays...) It is not the FD that accepts (or otherwise) the SPEED mode -- for instance, in the cruise with A/THR, A/P and FD engaged you will be in SPEED (probably MACH) | ALT CRZ | NAV | AP1 1FD2 A/THR (white) It is the vertical mode (largely) that determines whether the A/THR operates in SPEED or THR mode. However, one of the effects of turning off the FDs (with A/P disengaged) is to cause the A/THR to revert to SPEED mode. The key is, as Airbus say, to watch the FMAs carefully and ensure that they are reading as you want at all times. http://theflyingengineer.com/projects/airbus-tech/autoflight/ is worth reading if you want to spend some time getting your head around the autoflight system and mode reversions! Best, Simon Link to comment Share on other sites More sharing options...
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