Following an uneventful flight from Lyons the crew prepared for a descent and approach to Strasbourg. At first the crew asked for an ILS approach to runway 26 followed by a visual circuit to land on runway 05. This was not possible because of departing traffic from runway 26. The Strasbourg controllers then gave flight 148 radar guidance to ANDLO at 11DME from the Strasbourg VORTAC. Altitude over ANDLO was 5000 feet. After ANDLO the VOR/DME approach profile calls for a 5.5% slope (3.3deg angle of descent) to the Strasbourg VORTAC. While trying to program the angle of descent, "-3.3", into the Flight Control Unit (FCU) the crew did not notice that it was in HDG/V/S (heading/vertical speed) mode. In vertical speed mode "-3.3" means a descent rate of 3300 feet/min. In TRK/FPA (track/flight path angle) mode this would have meant a (correct) -3.3deg descent angle. A -3.3deg descent angle corresponds with an 800 feet/min rate of descent. The Vosges mountains near Strasbourg were in clouds above 2000 feet, with tops of the layer reaching about 6400 feet when flight 148 started descending from ANDLO. At about 3nm from ANDLO the aircraft struck trees and impacted a 2710 feet high ridge at the 2620 feet level near Mt. Saint-Odile. Because the aircraft was not GPWS-equipped, the crew were not warned.Why this is interesting is that fuel economy drives the use of many of the modes in the Airbus:
On a jet, the most fuel efficient way to descend is of course to stay in the optimal cruising altitude as long as possible, then cut the thrust and glide with a certain glide speed to the destination deceleratate for the approach, and, ideally you only advance throttles as far as the outermarker to spool up for go-around.While I don't know details, I wouldn't be at all surprised that this was the intersection of a complicated cockpit control design and corporate policies that made pilots think twice before taking corrective action.
Simply put, the speed (i.e. steepness of your glide/dive ) calculation is based on economic fuel to flight time ratio (the cost index). So all you have to do is make a good decison about the top of descent point observing all the speed and altitude constraints ahead. Which is, obiously the tricky part.
For this regime the autopilot interface has a mode selector named Level Change (B) or Open Descent (A).
Should you find yourself short of the field, either you slow down (shallow your descent) - not good as you accumulate extra time on the flight, or add thrust - fuel penalty, the lower the worse.
Certainly Airbus has a history of their computerized control systems confusing pilots - a record not shared by Boeing. Me, I much prefer flying in a Boeing rather than in a "Scairbus", but that's just me.