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View Full Version : A330 goes from MACH to SPEED mode when changing flight levels


evalf
3rd Sep 2012, 09:33
The other day during the cruise ATC descends us from FL380 to FL340. Flying in managed speed, at mach 0.81, the autopilot modes before the descent are MACH ALT CRZ NAV. We descent in V/S -1000, and for some reason shortly after starting the descent the speed mode changes from MACH to SPEED. When leveling off at FL340 The speed mode went back to MACH.

I can't think of any reason why the speed mode would change from MACH to SPEED at such an altitude. Usually in the climb it changes from SPEED to MACH around FL280. Even though we were still in the cruise phase, the speed selected in PERF DESCENT was .81/300.

Can anybody shed some light on the reason behind this mode change?

Bearcat
3rd Sep 2012, 11:35
The 330 is a cow to revert to ias very early in descent mode and chase the ias at higher levels when in reality it should be in Mach.

My 2c is that your a330 reverted to DES mode on the going to the FL340 and went back into CRZ mode when level.

I use selected a lot to save this b/s of reverting into speed mode early in descents.

evalf
3rd Sep 2012, 11:38
OK, I'll try to have a look at the MCDU PERF page next time that happens to see what it does, thanks!

JockMcToff
3rd Sep 2012, 15:27
IMHO, I think you are confusing the change from speed (IAS) to mach in the climb (or descent) due to Mach crossover (which is automatic) with the A/THR mode. Two different things. [If you are in selected speed in the descent, passing mach crossover, the value in the FCU will automatically change from Mach to IAS, the FMA A/THR mode will not change]

From the FCTM (Normal Operations, Descent):
If the pilot wishes to shallow the descent path, V/S can be used. A/THR reverts to SPEED mode.

So the A/THR will signal FADEC to obtain and maintain the target speed to give you the selected V/S, hence SPEED on the FMA.

:ok:

evalf
4th Sep 2012, 12:43
The ATHR mode does not change in descent or open descent at the mach crossover because it is in THR DES or THR IDLE. However when descending in V/S the ATHR mode is initially MACH as the ATHR maintains the selected or managed MACH, and I don't see a logical reason why it would go into SPEED unless the crossover altitude is crossed. If anything, if for some reason the crossover altitude had been crossed during the descent then I would have expected the ATHR to remain in SPEED after the level off, not go back to MACH.