Originally Posted by
Jet Jockey A4
- The crew did not program the right flight path into the aircraft's FMCs because they were below the temperature corrected flight path.
That seems unlikely.
From the TSB report:
"As the aircraft descended, the actual flight path diverged from the desired profile as a result of wind variations. The divergence continued to increase throughout the approach. The airspeed was constant, and the vertical descent speed ranged between 700 and 800 feet per minute (fpm)."
The report analyses the calculations of the crew and finds them correct; the crew computed and presumably set a FPA of 3.5° (for an actual angle of 3.08°), but the FDR shows their actual path vs. indicated altitude was 3.77°. On a normal visual approach, the pilot will compensate as she flies the approach from MDA when the autopilot is turned off, so it's not usually an issue. TSB report Section 1.18.2 discusses the Airbus FPA guidance mode and the possible deviation.
The interesting part is that the airplane flew an actual 3.50° descent. Since the ADIRS also uses inertial inputs to determine height, and these are not affected by temperature, it is possible that the unit did actually do as programmed and steered the plane down an actual 3.5° slope, effectively ignoring the barometric deviation; this would be contrary to published documentation. (Undocumented behaviour is not uncommon in the software realm.)
The report also mentions in 1.18.3 that Airbus intially got the procedures wrong, as was discovered in 2009, and it was supposedly corrected by Air Canada and Airbus under Transport Canada's supervision.
I'm sure somebody at Airbus knows if my theory (ADIRS working inertial and non-barometric) or the TSB's "wind perturbation" theory is correct.