Habsheim

[Chris Scott]

Possible benefit of "Zoom Climb", using any available airspeed

Owain Glyndwr's translation of a BEA conclusion:
"Other calculations show that if the elevator had been brought back faster or earlier during the accident flight, an AoA greater than 15° would have been obtained before the impact on the trees. They also show that without an earlier thrust increase such a manoeuvre, which would have led to an increase in the instantaneous drag of the aircraft would not have permitted (the aircraft) to avoid impact with the trees."

I notice that OG has so far not specifically disputed rudderrudderrat's calculation that a trade of kinetic energy by zoom climb from an airspeed of 112 kt to 107 kt (assuming a steady wind) would provide an altitude gain of 52 ft. (I assume that the 112 kt was based on the IAS recorded at DFDR time-frame of 333.0 secs, which is only 1 or 2 seconds before the defined point of impact.)

awblain has also taken up the cudgels (my emphasis):
"You're right. They could have exchanged another 5kt (2.5m/s) of airspeed, from 110 to 105 (~55m/s). for height h (in m) ~ v.Delta-v/g ~ 55(2.5)/10 ~ 14m.
"The key question is… did they have that 5kt of airspeed to exchange (FBW doesn't change the physics), while avoiding a stall. "

The short answer is in the negative. The GW was about 59 tonnes, with "Flaps" (config) 3 and L/G down. For an A320-100 (no winglets), the VS1G is a CAS of 114 kt. Given that VS1G is said to be defined on the A320 as the steady airspeed at the alpha-max in this confiuration of 17.5 deg (not the CL-MAX), provided the Nz (normal acceleration) is 1G, that begs the question of why the AoA coincident with 112 kt on the Habsheim fly-past was only +14.

My suggestion is that the a/c was in a slight bunt-manoeuvre. The Nz at that stage is around 0.93G, and the pitch has been reducing. Although the IAS had fallen from 116 in the previous second, the ground-speed remained the same (112 kt). That suggests a mini-tailwind shear in that second. The following second shows an IAS of 114 at an AoA of +15 with 1.00G. However, it appears that either the a/c was overperforming slightly, or the ZFW (zero fuel weight) may have been lower than calculated on the loadsheet.

I think it might be more realistic, therefore, to postulate a zoom climb initiated at time frame 329.0 sec; 5 or 6 secs before impact. That is the point at which TOGA thrust had been commanded, at IAS 122. Therefore, the PF was already anxious to expedite his go-around from that point. If rudderrudderrat's energy calculation is good, that should provide at least as much height-gain as he calciulated for his.

As for the zoom climb per-se, I would be very interested to hear the views of Owain Glyndwr, John Farley, and others as to whether it is in any way analogous to the ski-jump concept used operationally, 6 years earlier, to increase the take-off performance of the Hawker/BAC Harrier to permit - in effect - overweight take-offs in STO mode.

I'm wondering if the gain in altitude achieved at the expense of IAS, followed by a semi-ballistic trajectory - maintaining alpha-max at an Nz below 1G - might have provided two advantages, compared with maintaining height/altitude into the treetops.
(1) The initial impact would be avoided (and the engines were rapidly spooling up).
(2) The semi-ballistic segment would delay the stall, providing even more time for the engines to add energy to the total-energy equation. In the case of the Harrier, the engine is already at its TOGA thrust (and, admittedly, with thrust-vectoring). The A320 at Habsheim was light, of course, with very great surplus performance at TOGA, and less than two seconds short of achieving TOGA thrust at the time of impact.