Mutt,
Many thanks for taking the time to produce those sample figures - I have to say they do raise eyebrows amongst the pilot body.
Opinions on performance vary considerably depending on viewpoint. There seem to be three basic ones; commercial, Ops/Performance Engineering and the pilots actually strapped to the pointy object.
Now pilots genuinely do appreciate the commercial and safety benefits of reduced thrust/assumed temp. procedures. Destress the engine and thereby maintain its performance margins for longer and in turn extend life on the wing.
A significant number of us however do get very twitchy about taking it all the way to field length limitations. Here are some of the reasons why but first please be sure that this is not taking a personal poke at you or your colleagues. We're educating each other or at least passing on our differing perspectives.
As a professional you're taking data which you can only assume is proffered to you in good faith by the suppliers. You run the computations and come up with the figures. They have been factored for safety reasons to a level required by a controlling authority or agency.
A significant number of the readers of your post are horrified at the results. You have a professionally derived and, to you, entirely logical set of calculations which are legal and produce a a final safety net of
25 feet before going into the over-run, water or bundu.
Pilots take issue with this type of calculation on several counts.
The performance requirements are the result of down and dirty compromise between commercial interests and safety regulators. It is significant that many other authorities take a much more conservative stance than the FAA.
The performance figures have not been derived by real world pilots on normal commercial runways - they take no account of the oil and rubber residues completely coating the 600 to 200 metre section at the upwind end of the runway where we're trying to get max deceleration.
FDR and CVR analysis of real life problems and failures show the recognition and action time as certified is deeply flawed. Boeing effectively underline this with their papers on being Go minded.
The basic performance figures derived may not be glossy sales material for airshow goers but they are an utterly vital sales tool when dealing with the people who actually sign for the aircaft. Now, I'm sure this doesn't seem rational to you and your figures but the view we have ahead of us at V1 does not inspire us with confidence when field limited. Why?
Well, the data has been prepared without the extended turns and taxi times we have in the real world. The simple truth is this. The tyres will not hold out and even the best anti-skid unit in the world has a problem stopping on the wheel rims. Wild speculation? No!
A glance at the papers on the sites run by the tyre manufacturers clearly tell every one of us that when taxi times are greater than ten minutes all bets are off because of hysterysis in the sidewalls producing very high temperatures before we even begin the roll. Repeated turns have an even more deteterious effect. Yes, available brake energy may be factored in, tyre speed ratings as well but not the temperature and thus the integrity of our tyres when we begin the roll.
I hope we get the chance to discuss this over the long planned pint or two Mutt - but in the meantime I'd really like you to have a think about the points I've raised.
Performance Requirements created by all sides of industry.
Certification figures as a sales tool.
Reaction/action times close to V1
Rubber and oil Contaminated upwind braking areas
Tyre manufacturers disowning their products over extended (10 minutes!!)taxi times and turning prior to takeoff.
Please have a think about it, talk to your powerplant engineers about a cost/benefit analysis on derates versus deep derates. I think you'll find it's the first few percentage points of reduction that truly extend the engine margins - all else is willy waving to look good in meetings
Please give your guys a bit more concrete for the day they have to stop quite quickly. I'd be interested to see the the views of others on a 25 foot margin...........
Regards from the Towers
Rob Lloyd
[email protected]
[This message has been edited by PPRuNe Towers (edited 29 May 2001).]