Originally Posted by
machtuk
You seem to miss the point. ALL the numbers are minimums, V1 is the only exact number because it's known as a decision speed, clear cut, Vr & V2 do NOT have to be adhered to as long as they are not acted upon or flown LESS than calculated. If perf allows a healthy climb greater than minimum V2 (you can remain at the current speed if an engine failure occurred above the min V2 speed during initial climb out) then there is no law to state you must be at V2 but usually is V2 + 10/20 in order to achieve the required climb gradient. Anyway I've always been shown during all my type endo's that stay on the Rwy till the A/C is tracking straight even if this means going beyond the MINIMIN Vr speed, its worked for all the 20.7.1B A/C I have been involved with. An Eg when NOT to rotate at Vr if V1 & Vr are the same. You have an engine failure just as you get to V1 (& you go) & the A/C heads bush momentarily (due the startle factor) say off Rwy Hdg by 15 degs towards the grass, are you going to rotate as soon as you heard Vr from the PNF if they are both the same numbers? No bloody way!
Maybe... maybe not.
There is no specific margin for delaying the rotate, to do so will compromise the TODR/TODA for the actual engine failure case. What time you have is dependent on the relationship of ASDR and TODR.
There is a widespread training position that ensuring the aircraft is stable prior to rotating is acceptable, and that becomes best practice. While that is great when there is spare TODA remaining in front, there is no legal requirement for that to always be the case. Good practice? probably but with a major warning, that any time delaying rotate potentially compromises the go case. In rough terms, in an absolute limiting Go case, with a failure at Vef, and a go decision, that 35' screen height is around 3 seconds of interval, so probably needs to be used wisely. If a wet condition has resulted in a V1 reduction and a commensurate screen height reduction to 15', that is about 2 seconds of play time before you may be catching stuff in the gear. Note if you use the V1 wet reductions that while that is permitted in 25.113, it doesn't rate a mention in 25.115 so it is an open question whether your second segment calculations actually incorporate wet margins. On any given day, you may have enough spare runway (probably will have) in order to be stable for the rotate, but that is not an absolute given. Getting airborne out of control may be a great definition of excitement, but it is not the expected outcome from a trained pilot applying a correction in a timely manner following failure. The good news is that if there is a limited split between V1 and Vr, then logically, the stop case would have additional distance required, (ASDA) and that ensures that there should be some margin in the time to commence rotate. If there is a large split between V1 and Vr, then the issue should not arise, the failure occurs with a fair time to ensure the flightpath is under control. 4 Engine aircraft will have the most limiting conditions in the go case, but then the V1 will also usually have a large split from Vr. The 2 engine aircraft looks great on 2, so the 115% margin is not the limiting case, it is the engine out case, and that may look great until it happens. The 2 engine aircraft will also usually have a lower split in V1 and Vr compared to the 3 or 4 engine case, unless policy is to use a minimum V1 for all performance which can be elected by the operator.
Practically, in a simulator few failures are given at exactly Vef, as the outcome is uncertain, the crew may respond with a go or a stop, and the result could easily be a failure of the check ride. In real planes, training risk management would usually preclude operating anywhere near a limit condition for an practice power loss, and again it won't occur at Vef, the results have too much chance of an adverse outcome. Playing with a Part 25 aircraft with practice engine failures is not an ideal situation, but it still happens. [A while ago, I had a twin jet type check where the DPE slapped the thrust lever back to idle at V1 on a short runway, as we did our initial departure towards the airport that was long enough to do V1 cuts with some margin. Flicking the thrust lever back resulted in the lever going to cuttoff, and the ensuing single engine climb out over a major downtown area was rather quiet in the cockpit. At a safe height, and cleaned up, we got the restart part of the type out of the way. 12 months later, another DPE managed to get dead along with another 2 pilots doing same sort of thing in another corporate jet this time getting a TR deployment, The engine shutting down DPE also managed to become an ex DPE when the wing on the aircraft in a test decided to part company from the rest of the plane in level unaccelerated flight. R.I.P.].
Bottom line is any delay of rotate results in some level of compromise of the takeoff flight path, you as the pilot are paid the big (or small) bucks to make wise decisions on what you see on the day. Whatever you do, you need to achieve the screen height at the end of the TODA, to achieve terrain clearance. Good news is that in the real world most engine failures don't occur at Vef. A failure in the rotate will often give a wild ride, particularly if there was a strong x wind. A failure at low speed in low vis is always interesting as well.