Originally Posted by
Klauss
Hi, fdr,
could you please explain why you exclude a wind change from considerations ?
Used to be that introducing a tailwind into takoff-calculations carried a substantial penalty in terms of possible t/o weight.
Having unforeseen and thus uncalculated winds = possibly, a huge takeoff problem.
How was the acceleration from brake release/ start of takeoff roll to the point where rotation was attempted ?
The report gives in the discourse the IAS for Vr and V2, and they are consistent with the TAS/GS of the data. For a windshear case, then at the time near rotate, there would have to be a large difference between IAS and TAS/GS, which is not evident. Consider picking up an unexpected 30kt tailwind as you start the TO roll, which suddenly stops (is not evident) approaching rotate... for the takeoff, your performance would actually be improved over calm conditions. (don/'t try it... ) Same goes for forgetting to set flaps and then setting them on the roll. The performance is actually improved, but your heart rate gets a work out, and your hearing gets belted by the GPWS warnings. If a wind shear occurs, then the GS as indicated would reflect a change from the basic curve, and that is not there. IT is much easier to assess when the IAS and GS are both recorded and displayed, at which time any change in the wind shows as a change to the IAS curve, and an opposite change to the GS curve, do the differential becomes very evident.
Tail winds do give a severe penalty to TO performance, in actual outcome and calculation. The calculation takes 150% of the component as the effective wind, providing a buffer for error.
How was the acceleration from brake release/ start of takeoff roll to the point where rotation was attempted ? That is the right question, and the answer is given in the report, yet gets no weight. The crews rotate rate was slow, no doubt about that, but they didn't get an excessive IAS as a result so something was going on. The report on p36 gives the distances for TOR, and notes an excess of 806m for the event, yet the rotate was only 9.7s from rotate to liftoff, vs between 4.5-5 s for the certification case. That is about 5s at... 75m/s, which is less than 806m by an amount that has not been accounted for. That is part of the performance error, and it speaks as well to the point:
On any takeoff that fails to achieve an appropriate screen height including the amount additive for not having the engine failure in normal operations, the reject case has been compromised to some extent, which is identifiable in the recorded data, but no carrier bothers to track that data to assure continued performance. Pretty large Ooops.
The sensor data is available to record the location on the runway where various events occur, just as in the report, and there are specific points where they much occur to meet the certified performance of the aircraft. Looking at the data closely is not a negative to the carriers, it would indicate where issues exist that need to be taken account of, such as wind changes, or erroneous temperature info, but verifying the outcome would increase safety margins. It is also possible that monitoring the performance will detect weight errors of the aircraft, and where performance is actually better than schedule (it does happen, just not often). The performance is originally based on degraded engines and brakes etc, and would a suitable equivalent safety case, one could argue (might even be successful) in optimising the performance based on actual. Hard sell, but it is a selling point to the beancounters as they have less interest in operational safety alone.