Today I decided to try the managed speed feature for the first time.
There's either a bug or I'm doing sthing wrong.
pre-conditions:
- one of the A319's, but the same applies to all other family members
- standard FOB 12000lbs, GW 130000lbs
- MCDU: CI 40, FL330
- AP panel: push SPD and NAV buttons to let AP do all the business
- initial speed set by autopilot was 157
climb phase (seems perfect to me):
- after lift-off I selected AP1
- 157IAS was maintained until 3000ASL, then 250IAS commanded (automatically)
- after crossing 10000, 300IAS was commanded
- as ALT increases, IAS decreases and MACH increases slowly:
- FL180/290IAS/M.61
- FL250/280IAS/M.68
- FL300/275IAS/M.73
- slightly before reaching FL300, the speed AP dial automatically switched from SPD to MACH
cruise:
- FL330
- M.75, GS/TAS 402, 265IAS, headwind around 40kts, maybe a bit slow, but still ok...
descend (not as expected):
- FOB 7400lbs, GW 125400lbs
- dialed 10000 and pressed the ALT knob
- as expected, IAS increases, MACH decreases slowly UNTIL M.703
- once MACH 0.703 reached, it remains set constantly, AP speed dial still switched to MACH!
- result - the plane descends, IAS increases (as M.7 is constant) up to 360+IAS (at around FL150)....where I manually leveled off and slowed-down the plane
I performed immediately another climb to FL330 followed by a second descend to FL13:
- during the climb, MACH was selected (at FL around FL150 I expected SPD instead of MACH), M.703 commanded (way too fast for this altitude)
- above FL250 the commanded MACH speed started to increase slowly (as expected) up to M.075 at FL330 (cruise FL/speed)
- cruise seems to be correct at FL330, M.75, 265IAS, 430GS, 428TAS (side-wind 40 from 080)
- but on descend, again, speed was decreaesing until M.703, then remains constant
- during the descend I tried to pull the SPD knob, dial 270IAS, wait for slow-down and then push it again to re-activate managed speed - it automatically switched to MACH and commanded M.703 (although I was well below FL150!!)