DC-ATE;
Sometimes discussing techniques and even how the 320 works is like watching a roomful of economists come to an agreement. Sometimes they're talking about the same thing but the language gets in the way and sometimes mistakes in knowledge, judgement or memory are made. The airplane "behind the panel" is complex but flying it is not.
NoD is correct in his responses as far as they go, (including the similarity to Boeing in V/S in the circumstances described and the Alpha modes as Lemurian indicates), but in descent, the airplane operates as described and this is an important point because in "IDLE - OPEN DESCENT" mode, the airplane does not operate like a Boeing.
The speed is indeed "on the elevator" in descent, (thank you again NoD for the clarification in the climb case - my year-and-a-half retirement is catching up with me I think!) and the engine power will not increase until and unless the altitude set in the altitude window on the Flight Control Unit panel, (glareshield panel), is captured.
If the pilot pulls back slightly on the stick, the speed will decrease and continue to do so until reaching a speed just above the "lowest selectable" speed, (Vls in the graphic) at which time the descent mode will change from Idle-Open Descent to Vertical Speed and the autothrust reverts to "SPEED". This "mode reversion" occurs in other autothrust-manual flight circumstances as part of the protections under "Normal (fbw) Law".
This design was the result of an accident at Bangalore, India in which the altitude selected was set too low to be captured during the approach and the autothrust stayed in the "Idle - Open Descent" mode until ground contact. The Flight Director system was similarly change so that if either or both FD's were switched off, the A/T system reverted to SPEED, to bring the power up. Previous to this accident, this was not the case.
The prevention of this accident was straightforward, and the same as any other airplane - take over the airplane, in this case by pushing the thrust levers forward, commanding a thrust increase from the engines to arrest the rate of descent - no different than the Boeing or a C152. The crew failed to do this and the airplane performed exactly as commanded by the crew.
Normally in descent when approaching the cleared altitude, one reaches for the throttles and pushes them up as a natural reaction (on the '8 for example), but not in the 320 (and Boeing with A/T's on, for that matter) - the autothrust will bring up the engines when the altitude is captured. But if the airplane is leveled off above the selected altitude as described, the 320's engines will not accelerate. To demonstrate the speed decay a bit I would ask transition line indoc students on the airplane to fly manually and "level off" just above the altitude selected in the FCU window. When the speed began to decay a few knots, one could pull the speed knob which maintained the current speed or one could turn the F/D's off to revert to the SPEED mode. The importance of this was to show why IDLE-OPEN DESCENT mode was never to be used below 1000ft AGL and if it did occur, how to handle it - fly the 320 like a 152.
In my view, airline managments and training departments alike do not emphasize nearly enough the fact that the 320 is just another airplane and can and should be disconnected entirely from the automation - it hand-flies beautifully. I know crews today are still reluctant to disengage the autothrust because they are not taught properly how to and are reluctant to grab a mitt-full of throttle and push when needed. It's still an airplane - clearly, Scully is keenly aware of that fact.
The airplane is indeed complex and difficult to explain in text in a forum - afer one flies the aircraft for a period of time, say six months, it becomes second nature.
Last edited by PJ2; 8th February 2009 at 00:59.