|Quoting Lemurs (Reply 7):|
Quick follow-up Q...are the numbers based on Vr, or 35ft AGL at the runway threshold? I remember from the IL-96 post a few weeks ago people saying that operating rules in the US were 35' AGL at threshold, so you couldn't really run up to the end of the runway, could you?
Vr is the rotation speed applicable for all engine or one engine inoperative. It is selected relative to V1 (engine failure recognition speed) so that the airplane is 35 ft. above the end of the runway for an engine inoperative takeoff if the engine inoperative continued takeoff is the limiting case (usually true for twins). Note that TOFL is also determined based on 115% of the all engines takeoff distance (often more limiting for quads).
The only time you will be at the end of the runway is if the pilot doesn't recognize the engine failure at V1 and/or is slow in initiating a stop.
Stopping distance is calculated as follows:
- All engine acceleration distance to Vef (engine failure speed)
- One engine acceleration distance to V1 (allows one sec. for the pilot to recognize an engine has failed)
- Two seconds of constant speed (V1) distance.
- Stopping distance from V1 to stop with allowance for pilot reaction time to apply the brakes, chop the throttles, deploy the thrust reversers and extend the speed brakes.
The two seconds of constant speed distance acts a "pad" in the stopping calculation.
Also if the airplane was certified prior to FAR
part 25, Amend. 98, no credit is allowed for use of reverse thrust. On a dry runway, reverse isn't a factor as the airplane is usually stopped before the reversers deploy and the engines spin. Reversers do improve performance on wet, snowy or icy runway though and Amend 98 was the FAA's acknowledgement of the UKCAA's provision for reverse thrust performance improvement under these conditions.
Let's not talk about the sidetrack that Amend. 45 put us down.
From the above, you can see that takeoff calculations involve a number of factors and are very complex. I've seen past postings that indicate takeoff performance is predicated on second segment climb capability (engine inoperative climb gradient from gear up to a minimum of 400' AGL) and obstacle clearance requirements. This is far too simplistic and ignores all the other factors that go into calculating a limiting field length.