Interesting thread with no simple answer, although previous posts have covered the main considerations ...
Some thoughts as I read through the posts ..
(a) certification data don't give you any guarantees .. rather the AFM suggests that, if you do certain things under certain conditions, then there might be a reasonable chance that the observed performance will be such and such ..... of course, at the enquiry, the bewigged ones will probably take a diametrically opposed point of view ...
(b) temperature and wind profiles, humidity and just where what devilry is going to happen to you, are all a bit hard to predict with great precision at the time you advance the throttles/power levers/whatever
(c) the operating company ought to publish sufficient guidance as to the bases of scheduled performance data so that crews can make intelligent decisions (beyond SOP considerations) as to how such data might best be applied in the real world
(d) the accountants might not want to see you err too much on the side of conservatism .. while your wife/husband/partner and the people down the back might prefer a more risk averse approach to life
(e) whatever you do, it is comforting to have a defensible position to argue at the enquiry, court case, whatever. It is definitely not a good thing to have opposing counsel ask a question which leaves you with only a facial blank look of horror response option ...
(f) wind is addressed for the climb out ... certainly in all the AFMs I have worked with ... usually by the performance engineer nominating the wind to go into the calculation (by one means or another) ..... just how closely the presumed wind in the calculation might compare with the real world wind is another consideration altogether ... clearly if you know that there is a significant low level wind shear, it is a bit silly to blast off, having ignored that observation, with brain in neutral
(g) as an aside, for near surface wind with height variation, the usual presumed profile is
W1/W2 = (H1/H2)^(1/7)
This is reasonably representative of typical real world conditions in a steady wind field and in the absence of ground obstructions. It is the usual formula built into your AFM charts ... however, I wouldn't use it much above 100 - 200 ft or so
(h) the 35 ft clearance bit relates to a calculated basis for the RTOW (sorry, RTOM for the purists).
The certification test data are fudged a bit to give the AFM "gross performance" figures which account for a reasonable variation amongst individual aircraft. The gross data are then fudged a bit further to give "net performance" figures which are used in keeping the calculated net flight path above the hard bits sticking out of the ground. The calculations made by the performance guys and gals are predicated on a minimum of 35 ft clearance between the terrain/obstacle profile and the calculated net flight path.
In the real world, normally you don't have an engine failure and the aircraft goes like a cut cat ... with twins doing better than tri-motors which, in turn, do better than big birds. These latter observations are a consequence of the physics and the gross to net margin requirements.
If you do have a failure your performance, under any half reasonable conditions, probably will be at, or a little better than gross performance expectations.
When you consider the gross to net decrement, this becomes very significant to the pilot in the early stages of the takeoff because the decrement hasn't had enough distance from end TODR to give you much in the way of fat. As the aircraft progresses through the segments, the decrement progressively gives you an increasing height delta between gross and net.
(i) as to whether the V1 failure scenario is critical depends very much on the planned flight path. If the normal departure is the same as the escape path, then fine. If, however, the two are different, then it is necessary to consider failures at all relevant points along the normal flight path until a safe height has been achieved ... I know that some companies DO NOT do this and ONLY consider the V1 failure case.
... and, as Richard 4Dogs observes above ... at the end of the day it is not feasible to prescribe away all the worries by having ever bigger Operations Manuals ... sometimes the guy or gal in the left seat gets to earn his or her salary. The old saying remains ever true ... the captain doesn't get paid for having a routine flight .. rather he/she earns the salary every (infrequent) once in a while.
Last edited by john_tullamarine; 17th April 2004 at 02:29.