The Velocity Vector is not Enough
I have to admit this accident/incident bothers me tremendously as I have thought about this issue of performance assessment on the takeoff roll for some time.
The velocity vector is a useful "secondary tool" but until the specific performance specifications using the V.V. are documented and mandated by the manufacturer as a check item on takeoff roll, to my mind its' use is "vague" at best. In fact, to my mind it is only slightly better than the "seat of the pants" judgment many experienced Captains currently use to judge the rate of acceleration on a given takeoff roll on a given day.
The V. V. is based the aircraft's computed future speed based upon a current rate of change of acceleration over a given time frame projected forward ("XX speed" will be achieved in "XX time" projected forward based upon current the current rate of acceleration).
This vector is totally dependent upon the performance data criteria selected by the crew in the flight management computers given the actual a/c weight and current runway length, field elevation and airfield wx (temp, wind and QNH).
Case in point:
If I were to take off at MTOW in any given aircraft (A-319 through to A-380 or Boeing equivalent up to 747-400) on a 20,000' runway at sea level at ISA wx conditions, you would expect a max flex t/o (irrespective of weight given the runway length) and a minimal "benign/subdued/non-aggressive/etc?" velocity vector displayed on the PFD during the take off roll through 100 kts given the conditions.
Take that same a/c and put it on an 8000' runway at 6000' above sea level and the velocity vector for the same given weight on that runway would be considerable different. How much V.V. on the PFD is right at that weight on that runway length at that temp at that elevation on that day? How much is "not enough"?
Under a different scenario, what if you take the same a/c on the same day at a "simple airport" (long runway, near sea level, normal temps, etc) and make a gross input error (due to crew fatigue/distraction/etc) in the computer inputed ZFW or max TOW data to calculate the performance figures?
Unless experience tells you to expect that based upon the given aircraft type, field and wx conditions, and actual t/o weight you should expect a V1 of "XXX", a configuration of "XX" and a V.V. above 100 kts of "XX", you might be happy to see "100 kt + your comfortable "XX" amount" on the V.V. on the takeoff roll and be totally wrong in your assessment.
My point is that using strictly V.V. without some specific manufacturers numbers to match up your "seat of the pants" assessment of it's use is to my mind incorrect and should not be relied upon to assess performance takeoff criteria.
The real question is why have the manufacturers not published the performance data using the velocity vector as it is something that is easy to document (at the manufacturers level) and the display of the data ( the Velocity Vector) is already a part of the modern airline cockpit?
Why have Airbus and Boeing not provided airline crews with this easily produced and readily available data?
To my mind that is the key issue in this incident and one that Emirates, all other airlines and the regulatory authorities in every country must contemplate in assessing fault for this incident.
Dune