This has to be one of the better topics to have run on Tech Log, John_Tullamarine must be in his element here, and yours truly has to go on holidays whilst his favourite topic is running.
It's interesting, sometimes a little frightening, to see the various approaches taken by our fraternity in the interests of increasing Takeoff safety.
Some, most, I believe, observe the AFM performance criteria and procedures as written, but then quite a number exist who take the performance figures to be rubbery, to be stretched and altered (sometimes by company policy) to suit their own perception of increased safety.
A major contributory factor to the confusion of many is that, in the past, earlier aircraft operated to a much more demanding set of parameters, Mutt has named a few of these aircraft, B707, B727, DC9 etc. In the modern era, increased recognition and reaction times have been factored into the Takeoff calculation. An additional 1 second may not seem a lot, but in more recently certified aircraft, it does more appropriately address fairly realistic pilot recognition and reaction times. Another factor is that, in the older aircraft, V1 was always calculated on the basis of engine failure, it still is, but older certiication rules, in considering ONLY engine failure, did not recognise the effects of an all-engines RTO, where the idle thrust of the (assumed failed) engine was a negative in the RTO deceleration. Modern aircraft DO consider this, so, although the high speed RTO still represents one of the more hazardous phases of flight, it is, in newer aircraft, safer than it was previously.
IMHO, I believe that a lot of the "Older Aircraft" thinking has been carried over in pilot thinking to the modern era. The major manufacturers, Boeing, Airbus, BAe, all provide good guidance to aid the decision making process for a high speed abort. I agree with them, at higher speeds we should be more "GO Oriented", but, having said that, there are still circumstances where an RTO must be made, even 1 second before V1. Highest on the "STOP" list is engine failure. Other serious events, such as control jam or loss also warrant a STOP decision at high speed. The manufacturers of newer aircraft not only advise against high speed RTO for less than serious events, they've also introduced various forms of Takeoff Warning Inhibit, so as to inhibit warning of non-serious events above an arbitrary speed. The same manufacturers of modern aircraft have introduced an automated "V1" call, not at some threshold below V1, but precisely AT V1.
To my mind the Takeoff is in 3 phases -
Phase 1 - From Start of Takeoff up to the speed which approximates half of the kinetic energy associated with V1. This is approximately 70% of V1, e.g. for an aircraft with a V1 of 140 Kt, the 50% K.E. speed is passed at 98 Kt. An RTO where only 50% of the Kinetic Energy must be dissipated during the abort is a very safe affair with enormous safety margins. Below this speed, RTO for less serious failure (such as PFD) is justified, and safe. The manufacturers have recognised this in their guidance material for pilots, and in determining the threshold for Takeoff Warning Inhibit.
Phase 2 - Between the 50% K.E. speed and V1, RTO is a much more serious matter. Newer aircraft have warning systems for "non-serious" failures inhibited, and pilots of older aircraft are trained to reject only for serious matters such as Engine Failure, Serious Warnings (e.g. Fire) and anything that may adversely affect the aircraft's capability for safe flight (e.g. Control Jam or Failure).
When either V1 is called or the pilot notes it on his/her ASI, Phase 2 is ALREADY OVER. Stopping is NOT an option. We could, in fact, make a good case to change the "V1" call to a "GO" call, because it's already too late to stop.
Phase 3 - V1 has been called or noted, and Takeoff MUST continue, except in the most dire of circumstances such as control loss or jam.
As I mentioned earlier in this thread, operating to an artificially low V1 WILL improve RTO safety, there is no doubt of that. There is also no doubt that it dramatically DECREASES continued Takeoff safety after the 'low V1' call, because, after that call, you're comitted to GO.
Aircraft have many limits, and usually there is no problem or illegality in observing a more conservative limit than that in the AFM. A common example is most operator's limiting normal operating speeds to something like 20 knots below Vmo - Good! V1 must also be recognised as a limit speed, it is the upper speed limit by which RTO must already have been initiated, but simultaneously it is the LOWER speed limit for continued Takeof. There is no conservative side of the V1 limit, too low, and continued Takeoff is compromised, too high and Accelerate-Stop is compromised.
The benefits of using a lower than scheduled V1 for the RTO are obvious, but consider the following points for the Continued Takeoff -
(1) V1 must not be less than Vmcg, and on shorter runways (the ones that pilots most fear the RTO) V1 will inevitable be equal to Vmcg. At V1 minus 5 Kt, directional control may well be lost. In certification flying that I've done, this is the most frightening of ALL manoeuvres. Vmcg is established in Zero Crosswind (or 7 Kt in some cases) with the Nose-Wheel Steering Inoperative, so you might have NWS available to reduce the Vmcg, but what of those aircraft that lose the NWS operating hydraulic system with the loss of the engine, or those aircraft that severely reduce NWS authority (most) as speed increases? Now, throw in the (allowed) 35 knot Crosswind from the same side as the failed engine. Such a situation may be borderline in control capability even at the correct V1.
(2) The V1/Vr/V2 relationship must not be tampered with. The increased distance One Engine Inoperative (OEI) from V1 minus 5 Kt through to Vr and onwards to V2 would not be achievable at limiting weight if Takeoff Distance limited. In a Max TOW situation, I can envisage rotation being required probably 5 Kt below Vr as the Runway end approaches, becoming airborne well below the screen height, and 'dragging out' at V2 minus 5 knots. V2 is on the 'back' side of the Drag Curve, where drag rises very steeply below V2, and at V2 minus 5 Kt, you would probably consume all, or more, of the 0.8% buffer between Net and Gross Climb Gradient. If your Takeoff is over water, you'll live to fly another day, if obstacle limited you cannot lose gradient by lowering the nose to accelerate to V2, and you cannot hope for obstacle obstacle clearange gradient at V2-5, a complete 'NO WIN' situation.
For a 2 engined aircraft, perhaps the most demanding phase is the OEI acceleration from V1 through to Vr and V2. Even when the correct speeds are flown, Runway Slope may exceed the climb to Screen Height capability, and be the restrictive factor in the MTOW determination. For one aircraft type that I did the Performance Engineering for, the 0.9% UP slope on Melbourne (Australia) Runway 34 was the restrictive factor, being most obvious in the V1 to V2 phase.
Please, please, don't try to re-invent the wheel - THE NUMBERS WORK! They're tried and proven. You do have some small margins, which will be consumed in an instant if inappropriate speeds are flown. For a Wet Runway Takeoff, where the runway margins are reduced to NIL (for the RTO), and abyssmal for the continued Takeoff, you would be unlikely to survive a continued OEI Takeoff from V1 minus 5 knots.
Finally, if you do not find that the AFM data provides sufficient safety, then THE POWER IS IN YOUR HANDS TO CORRECT IT, but don't tamper with the V1/Vr/V2 relationship. IF RTO is your main concern (as it seems to be to most people, and statistically justifiably so), then, determine the additional stopping distance margin that you desire, reduce the ASDA by that amount, and determine a new (lower) RTOW from the AFM data. For the new MTOW, use the V1/Vr/V2 speeds as published, You'll have your margin.
If you think that the answer lies in a 5 Kt lower V1, then , having obtained the V1 for the limiting weight, run your finger up or down the page to a 5 knot lower V1, and apply the associated weight as the limit weight, and again, use the new V1/Vr/V2 exactly. Again, You'll have your margin.
If you're flying one of the older "1 second" aircraft, and want the benefits of the more modern 2 seconds, figure the Ground Speed at V1 (TAS minus half the Headwind or plus 1.5 times the Tailwind) in Feet or Metres per second. Apply that distance as an ASDA reduction and figure a new Takeoff weight from the modified ASDA. (This method may be necessary at lower weights where V1 is constant due to being equal to Vmcg). I used this method for one client when the Regulatory Authority forced the "2 second rule" on a "1 second" aircraft.
In all of the fore-going, Vmcg remains my major concern, using the correct V1 goes a long way towards alleviating an often unconsidered flaw in the certification system. There are other flaws to be addressed.
I remain in complete agreement with Boeing and Airbus in being "GO minded" for the high speed portion of the Takeoff roll to V1, and continue to plagiarise their words in writing Operations Manuals. A pilot should be like a coiled spring when approaching V1, rapid and correct assessment of a STOP or GO decision is vital, with different criteria than for the low speed phase.
A GO decision for an engine failure recognised at any time below the correct V1 is a disaster in the making.
Fly Safe - Fly the numbers! THEY WORK!
Regards,
Old Smokey
Last edited by Old Smokey; 16th Feb 2006 at 02:42.