Calling V1 early
Alex:
Thanks for that; I had no idea that Danny demanded a "handle" but I can now see why.
For the moment we can consider the hatchet well and truly buried but that is not to say that we might have a disagreement in the future!
Thanks for that; I had no idea that Danny demanded a "handle" but I can now see why.
For the moment we can consider the hatchet well and truly buried but that is not to say that we might have a disagreement in the future!
JW411
Well, already married and she has been "Mrs deeceethree" for over 18 years now. I must admit to using more than one alias on this forum (so don't let joining dates or other such nonsense fool you). Its simpler that way and has the advantage of keeping over-zealous folk off my back - you know the kind, those that don't want 'company' linen aired in public. Worked a treat so far.
Anyway, pleased to see that the hatchet is well and truly buried. Very gracious of all concerned. PPRuNE is a valuable place for us all to air our thoughts and, perhaps, blow the whistle when the need arises. As such, anonymity has its uses.
In closing, I believe it is fair to say that Alex would still be a very valued member of the active cockpit fraternity (!) if it wasn't for unfortunate circumstances. I certainly owe my some of my licence to his fantastic teaching style - SALUTE!
Well, already married and she has been "Mrs deeceethree" for over 18 years now. I must admit to using more than one alias on this forum (so don't let joining dates or other such nonsense fool you). Its simpler that way and has the advantage of keeping over-zealous folk off my back - you know the kind, those that don't want 'company' linen aired in public. Worked a treat so far.
Anyway, pleased to see that the hatchet is well and truly buried. Very gracious of all concerned. PPRuNE is a valuable place for us all to air our thoughts and, perhaps, blow the whistle when the need arises. As such, anonymity has its uses.
In closing, I believe it is fair to say that Alex would still be a very valued member of the active cockpit fraternity (!) if it wasn't for unfortunate circumstances. I certainly owe my some of my licence to his fantastic teaching style - SALUTE!
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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
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.
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Well said Old Smokey. Although this has been a very interesting topic, it has become a discussion between the theoreticians and the practitioners. My own advice as a long-time practitioner is stick to your company's S.O.P.'s, do not make up your own nor introduce habits from previous jobs. When V1 is achieved, my hand comes off the throttles is if they have become electrified and GO is the only option.
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skiesfull, I think that you've nailed the source of the disagreement between the parties involved here, that is, a disagreement between the "theoreticians and the practitioners". I have had a fortunate career in having simultaneously lived in both the theoretical and the practical world, and I remain confident in my statement that THE NUMBERS WORK, USE THEM!
This has been proven to me again and again in certification test flying, Performance Engineering to "put the numbers together", and then the sometimes heart stopping moment of taking the performance data that I've produced into a real aircraft with real live people in it, and proving that it works - IT DOES. The non-stressful bit is the other part of my regular job of line flying, non-stressful because I'm comfortable that the unmeddled with figures work.
It worries me that in this area, there is very small latitude for error or adjustment in the OEI situation, and any fiddling with the figures may lead to disaster.
Regards,
Old Smokey
This has been proven to me again and again in certification test flying, Performance Engineering to "put the numbers together", and then the sometimes heart stopping moment of taking the performance data that I've produced into a real aircraft with real live people in it, and proving that it works - IT DOES. The non-stressful bit is the other part of my regular job of line flying, non-stressful because I'm comfortable that the unmeddled with figures work.
It worries me that in this area, there is very small latitude for error or adjustment in the OEI situation, and any fiddling with the figures may lead to disaster.
Regards,
Old Smokey
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Most airfields I operate out of have runways which offer the option of Improved Climb performance using Improved Speeds.
Oft times, were I to elect to use this method to get airborne (for whatever reason), I might increase my V1 by anything up to 30kts.
Executing an RTO, for instance, at PRG off R/W 24, using Improved Speeds, allows me to reject a T/O at a speed far in excess of V2 using Normal Speeds!
There seems to be an additional margin here to be exploited which is well worth bearing in mind when considering a reject using Normal Speeds...
Oft times, were I to elect to use this method to get airborne (for whatever reason), I might increase my V1 by anything up to 30kts.
Executing an RTO, for instance, at PRG off R/W 24, using Improved Speeds, allows me to reject a T/O at a speed far in excess of V2 using Normal Speeds!
There seems to be an additional margin here to be exploited which is well worth bearing in mind when considering a reject using Normal Speeds...
Old Smokey. Thank you for a really first class piece of writing on the subject. It is now in my Favourites file. You mentioned that pilots should be "coiled" to stop up to V1. Interesting word use. I have observed many pilots using the rather strange technique during the take off run of coiling their hand over the front of the thrust levers in quite an unnatural grip, signifying apparently that they are poised with claw like grip like a bird on a branch, to rip back the levers against the stops if something happens to trigger the abort reflex.
Others simply open the throttles normally with a normal hand grip and take what comes if it happens. I confess to being of the latter. Someone "coiled" for an abort makes me uneasy especially on a limiting runway with the sea dashing against the rocks at the end of the stopway and the circling fins of white pointers occasionally appearing in the surf on the extended centre line.
"Coiled" means tensed and tensed can sometimes mean reacting instantly to sight or sound in an inappropriate manner. I prefer a "watching brief" up to V1 - it sounds less frightening.
Others simply open the throttles normally with a normal hand grip and take what comes if it happens. I confess to being of the latter. Someone "coiled" for an abort makes me uneasy especially on a limiting runway with the sea dashing against the rocks at the end of the stopway and the circling fins of white pointers occasionally appearing in the surf on the extended centre line.
"Coiled" means tensed and tensed can sometimes mean reacting instantly to sight or sound in an inappropriate manner. I prefer a "watching brief" up to V1 - it sounds less frightening.
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Great post, Old Smokey! I like the energy explanation leading up to V1. I also like the idea about safety margins applied to Vmo and I assume also Mmo. I’m printing this one and saving it. Also, John T, your post on V2 speed was excellent as well. I still have instructors telling me to slow down to V2 if they fail the engine just above V2. That one is getting printed as well.
If remember reading the Boeing/FAA Takeoff Safety Training Aid correctly, their recommendation was for the PNF/PM to call V1 speed at a point where the call would be completed at V1 speed. This was to preclude completion of the “V1” call above V1 speed. An event occurring simultaneously with the call could result in the initiation of the RTO above V1 speed. Since certification requirements, past and current, specifies a minimum 1 second delay between the engine failure and V1 (Vef ≤ 1 second ≤ V1) and with acceleration near V1 speed at approximately 3 to 6 knots per second, many operators have taken to initiating the call “V1”, so that the call is complete and the PF’s hands are off the thrust levers at V1 speed. This is important since V1 speed is the maximum speed the RTO can initiated, e.g., brakes applied, T/L to idle, etc, whatever is specified as the first step in the RTO procedure. I don’t believe the intent was to transition to the “GO Phase” 5 knots before V1 speed.
It’s interesting reading the different discussions on recognition time, reaction time, and time delays with regard to V1 speed. As I’ve tried to emphasize when I speak to people on this subject, the pilot needs to understand the certification basis applicable to their aircraft, which rule amendment is applicable. In the US, AC 25-7A, provides an excellent discussion on the certification rules, past and present, and how they are applied to V1/accelerate-stop determination.
Smokey, correctly me if I’m wrong, but in the US, there are three changes to FAR 25 that effect V1 speed and accelerate-stop determination (ref AC 25-7A):
FAR 25 – V1 = Critical Engine Failure Speed
V1 and Vmcg can be the same (Vmcg ≤ V1). Even though V1 speed is defined as “critical engine failure speed” this is a misnomer. In accelerate-stop distance determination, engine failure occurred 1 second prior to V1 speed (engine failure ≤ 1 second ≤ V1) even though Vef speed was not yet defined in the certification regulations.
After V1, a 1 second time delay was added between each step of the remaining RTO procedures (brakes ≤ 1 second ≤ T/L Idle ≤ 1 second ≤ Spoilers extend). If a command is required to actuate the deceleration device, then a 2 second time delay is applied instead of the 1-second delay. The time delay assumes no aircraft deceleration during application.
FAR 25-amd 48 (1978) – V1 = Takeoff Decision Speed
Three major changes occurred. First, Vef was established as critical engine failure speed with the requirement that Vmcg ≤ Vef. V1 speed was determined by adding the speed gained during the time interval (1 second minimum) between the instant the engine is fails and the instant at which the test pilot recognizes and reacts as indicated by the application of the first retarding means. Therefore, the distinction between engine failure (Vef) and first pilot RTO action (V1) was also established. V1 not only ends the decision process, it established the speed at which the pilot needs to perform the first action to stop the aircraft.
Second, a 2 second time delay is applied between V1 and the first stopping action during which the aircraft continues to accelerate with all engines operating at takeoff thrust. Additional time delays between device activations are not applied for the first 2 devices.
For any number of devices beyond 2, however, an additional 1-second should be included for each device.
Third, the requirement to determine all-engine accelerate-stop distance was added.
FAR 25-amd 92 (1998)
This change again revised the accelerate-stop criteria and added requirements for:
a) The stopping capability of the aircraft as effected by brake wear (fully worn limit of their allowable wear range)
b) Wet runway as well as dry runway A/S distance.
c) Removed reference to “Takeoff Decision Speed” from definition of V1 speed
d) Replaced the “2 seconds of continued acceleration beyond V1” from FAR 25-48 with a distance increment equivalent to 2 seconds at V1 speed.
e) A/S distance must account for the highest speed reached during the RTO maneuver including speeds higher than V1.
f) Permits credit for reverse thrust in determining wet runway A/S distance and explicitly deny use of reverse thrust credit for dry runway
g) Require a maximum brake energy A/S test with not more than 10% of the allowable brake wear range remaining on each brake assembly.
Accelerate-stop distance 2-second delay is only a method to calculate the required distance increment and is not part of the A/S braking sequence. AFM accelerate-stop performance data must account for any residual acceleration that occurs after V1 while the airplane and its systems are stabilized in the braking configuration.
Understanding the differences between and more importantly the limitations inherent with these certification rules is necessary for pilot to completely understand V1 & accelerate-stop distance.
Rejecting a takeoff above V1 has significant consequences, as does taking an aircraft into the air that’s not airworthy. Continuing a takeoff following an engine failure below Vef also has consequences. As pointed out, the screen height will be less than 35 ft (15 ft on wet runways, FAR 25-92 aircraft). As one who is now familiar with the growing problem of close-in obstacles, those that penetrate a 62.5:1 takeoff surface (1.6% NTOFP), there are many airports where lower screen heights pose a significant risk.
Just my two cents worth on the subject
Rich Boll
Wichita, KS
If remember reading the Boeing/FAA Takeoff Safety Training Aid correctly, their recommendation was for the PNF/PM to call V1 speed at a point where the call would be completed at V1 speed. This was to preclude completion of the “V1” call above V1 speed. An event occurring simultaneously with the call could result in the initiation of the RTO above V1 speed. Since certification requirements, past and current, specifies a minimum 1 second delay between the engine failure and V1 (Vef ≤ 1 second ≤ V1) and with acceleration near V1 speed at approximately 3 to 6 knots per second, many operators have taken to initiating the call “V1”, so that the call is complete and the PF’s hands are off the thrust levers at V1 speed. This is important since V1 speed is the maximum speed the RTO can initiated, e.g., brakes applied, T/L to idle, etc, whatever is specified as the first step in the RTO procedure. I don’t believe the intent was to transition to the “GO Phase” 5 knots before V1 speed.
It’s interesting reading the different discussions on recognition time, reaction time, and time delays with regard to V1 speed. As I’ve tried to emphasize when I speak to people on this subject, the pilot needs to understand the certification basis applicable to their aircraft, which rule amendment is applicable. In the US, AC 25-7A, provides an excellent discussion on the certification rules, past and present, and how they are applied to V1/accelerate-stop determination.
Smokey, correctly me if I’m wrong, but in the US, there are three changes to FAR 25 that effect V1 speed and accelerate-stop determination (ref AC 25-7A):
FAR 25 – V1 = Critical Engine Failure Speed
V1 and Vmcg can be the same (Vmcg ≤ V1). Even though V1 speed is defined as “critical engine failure speed” this is a misnomer. In accelerate-stop distance determination, engine failure occurred 1 second prior to V1 speed (engine failure ≤ 1 second ≤ V1) even though Vef speed was not yet defined in the certification regulations.
After V1, a 1 second time delay was added between each step of the remaining RTO procedures (brakes ≤ 1 second ≤ T/L Idle ≤ 1 second ≤ Spoilers extend). If a command is required to actuate the deceleration device, then a 2 second time delay is applied instead of the 1-second delay. The time delay assumes no aircraft deceleration during application.
FAR 25-amd 48 (1978) – V1 = Takeoff Decision Speed
Three major changes occurred. First, Vef was established as critical engine failure speed with the requirement that Vmcg ≤ Vef. V1 speed was determined by adding the speed gained during the time interval (1 second minimum) between the instant the engine is fails and the instant at which the test pilot recognizes and reacts as indicated by the application of the first retarding means. Therefore, the distinction between engine failure (Vef) and first pilot RTO action (V1) was also established. V1 not only ends the decision process, it established the speed at which the pilot needs to perform the first action to stop the aircraft.
Second, a 2 second time delay is applied between V1 and the first stopping action during which the aircraft continues to accelerate with all engines operating at takeoff thrust. Additional time delays between device activations are not applied for the first 2 devices.
For any number of devices beyond 2, however, an additional 1-second should be included for each device.
Third, the requirement to determine all-engine accelerate-stop distance was added.
FAR 25-amd 92 (1998)
This change again revised the accelerate-stop criteria and added requirements for:
a) The stopping capability of the aircraft as effected by brake wear (fully worn limit of their allowable wear range)
b) Wet runway as well as dry runway A/S distance.
c) Removed reference to “Takeoff Decision Speed” from definition of V1 speed
d) Replaced the “2 seconds of continued acceleration beyond V1” from FAR 25-48 with a distance increment equivalent to 2 seconds at V1 speed.
e) A/S distance must account for the highest speed reached during the RTO maneuver including speeds higher than V1.
f) Permits credit for reverse thrust in determining wet runway A/S distance and explicitly deny use of reverse thrust credit for dry runway
g) Require a maximum brake energy A/S test with not more than 10% of the allowable brake wear range remaining on each brake assembly.
Accelerate-stop distance 2-second delay is only a method to calculate the required distance increment and is not part of the A/S braking sequence. AFM accelerate-stop performance data must account for any residual acceleration that occurs after V1 while the airplane and its systems are stabilized in the braking configuration.
Understanding the differences between and more importantly the limitations inherent with these certification rules is necessary for pilot to completely understand V1 & accelerate-stop distance.
Rejecting a takeoff above V1 has significant consequences, as does taking an aircraft into the air that’s not airworthy. Continuing a takeoff following an engine failure below Vef also has consequences. As pointed out, the screen height will be less than 35 ft (15 ft on wet runways, FAR 25-92 aircraft). As one who is now familiar with the growing problem of close-in obstacles, those that penetrate a 62.5:1 takeoff surface (1.6% NTOFP), there are many airports where lower screen heights pose a significant risk.
Just my two cents worth on the subject
Rich Boll
Wichita, KS
Last edited by richjb; 18th Feb 2006 at 21:58.
As a matter of interest does anyone know why hands are removed off the throttles at the V1 call? Someone once told me it was a symbolic gesture to prove that an abort was out of the question. I don't much care for symbolic gestures in flying - I prefer technical reasons. From the time I learned to fly in the Air Force it was drummed into me that one always kept one hand on the throttles including during take off and climb. Maybe the friction nuts weren't so good in those days....
Someone else came up with the theory that rotating with one hand caused a wing drop. Absolute balls to that one. Certainly I know of one 737 opertor that insists on the PF using both hands on the wheel for flare and landing. Not quite sure of the logic of that gem.
Someone else came up with the theory that rotating with one hand caused a wing drop. Absolute balls to that one. Certainly I know of one 737 opertor that insists on the PF using both hands on the wheel for flare and landing. Not quite sure of the logic of that gem.
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Calling V1 Early
JT, Mutt, Smokey, Alex, et al,
Thank you for arguably the best discussion I have read on pprune. This is a fascunating issue and it is clear that there are lots of pilots out there whose knowledge of takeoff performance remains hazy to say the least.
Smokey said it all when he said BELIEVE THE NUMBERS. Don't attempt to second guess either the manufacturer's data or your company's Flight Ops/Performance people. Stick to your company's script and you will not go far wrong. If you are unhappy with the SOPs try to get them changed. In my flying experience (25+ yrs) your Chief Pilot will take notice of a well-argued letter on any technical/operational topic from one of his pilots. He may not agree with you but should tell you why if he doesn't. Remember the collective wisdom of the Flight Ops department is almost certainly greater than your particular hot topic.
For what it's worth, the biggest issue in any RTO is to get the MAX RETARDATION AVAILABLE. This means max reverse (a bonus on dry runways) consistent with directional control and MAX BRAKING. Boeing did a study some years ago on the amount of braking applied in RTOs by a range of pilots ranging from their own certification test pilots to the average line guy. It was interesting that the only group who consistently used the max braking available to them was the test pilot group. This is not surprising when we rarely operate our aircraft to the absolute max and we are conscious of hot brakes etc for the next departure. An old instructor of mine used to teach us to "Get your ar*e up the back of the seat" to drive this message home to us that we needed to STAND on the brakes. Nowadays you would need to check whether the RTO function of your autobrake system applies absolute max braking or whether max manual effort would provide a higher level. In any case your non-pnormal SOPs should cover this area.
The Boeing study referred to above was published in either the Boeing Airliner magazine of the IFALPA mag (or both?). If anyone can tell us where it can be found, it would make interesting reading for today's pilots.
John, a wonderful discussion. Many thanks.
Thank you for arguably the best discussion I have read on pprune. This is a fascunating issue and it is clear that there are lots of pilots out there whose knowledge of takeoff performance remains hazy to say the least.
Smokey said it all when he said BELIEVE THE NUMBERS. Don't attempt to second guess either the manufacturer's data or your company's Flight Ops/Performance people. Stick to your company's script and you will not go far wrong. If you are unhappy with the SOPs try to get them changed. In my flying experience (25+ yrs) your Chief Pilot will take notice of a well-argued letter on any technical/operational topic from one of his pilots. He may not agree with you but should tell you why if he doesn't. Remember the collective wisdom of the Flight Ops department is almost certainly greater than your particular hot topic.
For what it's worth, the biggest issue in any RTO is to get the MAX RETARDATION AVAILABLE. This means max reverse (a bonus on dry runways) consistent with directional control and MAX BRAKING. Boeing did a study some years ago on the amount of braking applied in RTOs by a range of pilots ranging from their own certification test pilots to the average line guy. It was interesting that the only group who consistently used the max braking available to them was the test pilot group. This is not surprising when we rarely operate our aircraft to the absolute max and we are conscious of hot brakes etc for the next departure. An old instructor of mine used to teach us to "Get your ar*e up the back of the seat" to drive this message home to us that we needed to STAND on the brakes. Nowadays you would need to check whether the RTO function of your autobrake system applies absolute max braking or whether max manual effort would provide a higher level. In any case your non-pnormal SOPs should cover this area.
The Boeing study referred to above was published in either the Boeing Airliner magazine of the IFALPA mag (or both?). If anyone can tell us where it can be found, it would make interesting reading for today's pilots.
John, a wonderful discussion. Many thanks.
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It is interesting how much difference can be in the performance data you have to use. Im flying the old 733 for 6 years now and we changed the way we calculate our performance only once with interesting differences in what kind of figures we got before and after.
We used to use BA performance where we used a BA performance manual and a speed booklet to obtain values for V1/r/2. V1 was always the same as Vr except a very few cases especially with contanimated runway or reduced braking action.
After BA sold our company we changed to a swedish performance data supplier and now get very different V1 figures, most of the time around 15 to 20 kts lower than Vr and quite often regulated by Vmcg (which wasnt limiting in most of BAs performance). Nothing has changed with the airplane, our SOPs or the route network we fly but we still use very different V1 figures now.
We used to use BA performance where we used a BA performance manual and a speed booklet to obtain values for V1/r/2. V1 was always the same as Vr except a very few cases especially with contanimated runway or reduced braking action.
After BA sold our company we changed to a swedish performance data supplier and now get very different V1 figures, most of the time around 15 to 20 kts lower than Vr and quite often regulated by Vmcg (which wasnt limiting in most of BAs performance). Nothing has changed with the airplane, our SOPs or the route network we fly but we still use very different V1 figures now.
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Copy of Boeing/FAA Takeoff Safety Training Aid
I have a PDF copy of the Boeing/FAA Takeoff Safety Training Aid. If anyone would like a copy, email me at [email protected] and I will send it to you.
Rich Boll
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Centaurus;-
Having 'failed' an engine just after V1 in the sim. I stand by my action to remove my hand when V1 is achieved and preferably called - I have witnessed several "knee-jerk reaction" rejects above V1, simply because the crews were expecting a high speed reject at some point during the detail!
Throttle-friction takes me back a bit though!
Having 'failed' an engine just after V1 in the sim. I stand by my action to remove my hand when V1 is achieved and preferably called - I have witnessed several "knee-jerk reaction" rejects above V1, simply because the crews were expecting a high speed reject at some point during the detail!
Throttle-friction takes me back a bit though!
Skiesfull. Point taken re knee jerk reaction . Which is why I have never liked V1 and Vr being the same figure and much prefer to have a healthy spread where possible of 10 knots between the two to avoid the knee jerk abort at VR.
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Old Smokey,
I like your approach so I have a little question for you.
There is a tendency to feel more comfortable with low V1. As a consequence one sees that people are inclined to use the wet RTOW in conditions that approach but are not quit like the wet scenario.
Taking into account that the wet screen height is much lower and the role the reversers play in the wet case, how would you explain how and/if any safety margin is eroded by this practise?
I like your approach so I have a little question for you.
There is a tendency to feel more comfortable with low V1. As a consequence one sees that people are inclined to use the wet RTOW in conditions that approach but are not quit like the wet scenario.
Taking into account that the wet screen height is much lower and the role the reversers play in the wet case, how would you explain how and/if any safety margin is eroded by this practise?
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Gentlemen,
A very interesting thread indeed.
With regard to the screen height - I believe it is limiting only if the Takeoff is performed under a Field Limited scenario.
Where Takeoff is Climb Limited, Obstacle Limited - the screen height is less of a problem - you will cross the end of the runway with ample margin - simply because the weights that limit you will be below the Field Limit i.e. stop at end or cross end at 35 feet(dry). On majority of Takeoff's - field is not the limit - unless operating out of really short fields.
By using a wet V1 on a dry runway - I believe you will still cross the end of the runway at 15 feet. The Wet V1 was used to improve the stopping performance not the Go criteria which was calculated to cross the end at 15 feet. Contaminated would be different as the V1 is calculated for the slower retardation and also slower acceleration should you choose to continue.
A very interesting thread indeed.
With regard to the screen height - I believe it is limiting only if the Takeoff is performed under a Field Limited scenario.
Where Takeoff is Climb Limited, Obstacle Limited - the screen height is less of a problem - you will cross the end of the runway with ample margin - simply because the weights that limit you will be below the Field Limit i.e. stop at end or cross end at 35 feet(dry). On majority of Takeoff's - field is not the limit - unless operating out of really short fields.
By using a wet V1 on a dry runway - I believe you will still cross the end of the runway at 15 feet. The Wet V1 was used to improve the stopping performance not the Go criteria which was calculated to cross the end at 15 feet. Contaminated would be different as the V1 is calculated for the slower retardation and also slower acceleration should you choose to continue.
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Since my earlier posts, there seem to have been a few questions pointed my way, good manners and a healthy interest in PPRune’s best ever running thread compel me to reply. I’m still on leave, so perhaps the replies are a little disjointed and out of time synch.
Centaurus, my reference to the pilot being like a coiled spring approaching V1 was in the vein of being MENTALLY like a coiled spring, the decision must be made and executed in a very short time indeed. There’s nothing symbolic in removing one’s hands from the thrust levers at V1, it’s a positive action to prevent the inadvertent ‘knee-jerk’ Thrust Lever closure by the ‘mentally coiled pilot’ when hearing the V1 or Rotate call. It was my own discipline to do this long before it became a SOP, my own mental preparation was “We are going to Reject”, before V1 was reached, immediately becoming “We are going to Continue” when V1 was called, as I raised my hand from the Thrust Levers. Your description of some pilots with a vice-like grip on the Thrust Levers is a bit worrying.
No problem with V1 and Vr being the same figure, simply call Rotate”, and dispense with the V1 call, it’s now redundant anyway. A little akin to my suggestion that there is a case for replacing the V1 call with a GO call. (We did this (the Rotate call that is) on the B727).
SR71, Yes the very long runway far in excess of requirements does pose a philosophical argument making an abort decision above V1 more acceptable. I have had the “pleasure” of an abort above V1 with a complete control jam, but the runway was some 2000M in excess of that required. All of my fore-going arguments have related to the aircraft being Field and/or obstacle limited, if both of these are spare, then there is no problem, but the pilot has no way of knowing just what is to spare, and therein lies a problem.
A further major problem is that when there is field length “to spare”, we then make it limiting by using Flex / Assumed Temperature thrust, making every takeoff a limiting one. A reject above V1 when using Flex / Assumed Temperature thrust would be most inadvisable. And 'one for the road', does your reject above V1 consider Vmbe?
SIDSTAR, a good post, you make several good remarks –
(1) ”Stick to your company's script and you will not go far wrong. If you are unhappy with the SOPs try to get them changed”. Yes, we could make every V1 equal to Vmcg (or Vmcg plus a bit for comfort) if we wanted (I wouldn’t), it would cost a lot of money from Boeing, Airbus et al, because then the continued Takeoff would become much more limiting, and have to be re-certified at great expense. Denti, I think that your new owners would have had to do this when they introduced the new much lower V1s, at least I hope that they did! Balanced Field principal would be thrown out the window (now that’s a good idea, J_T may agree, but Mutt is a much more balanced individual).
(2) ”Boeing did a study some years ago on the amount of braking applied in RTOs by a range of pilots ranging from their own certification test pilots to the average line guy. It was interesting that the only group who consistently used the max braking available to them was the test pilot group”. There are three ways to fly an aircraft – (A) Smoothly and accurately (Normal line flying), (B) Assertively (Engine out, show the aeroplane who’s boss, but as accurately as possible), and (C) Aggressively, using maximum effort and adrenaline. RTO Brake application and collision avoidance fall into the latter category.
Richjb, I think that you said it all. Some good extra points arise from your discussion. “critical engine failure speed” this is a misnomer”, heartily agreed, it might have been good in it’s day, but if we’re looking for a realistic critical engine failure speed, then look to Vef. That’s where the process begins. Nor is it the ‘Decision Speed’, by the time that V1 has been called, the decision has already been made, back at Seattle, Toulouse, Wichita, et al.
I think that your best remark was “Understanding the differences between and more importantly the limitations inherent with these certification rules is necessary for pilot to completely understand V1 & accelerate-stop distance”. It is VITAL for a competent operating pilot to fully understand and comprehend the basis upon which the performance criteria have been created. FAR 25 (and all of it’s equivalents) is an unusual piece of legislation. It is the law, but an unusual law in that it is also the criteria by which the Performance Engineering folk generate the real-world data with which we comply. In most instances in life we stay within the law, in the FAR 25 case, we take the law all the way to the limits.
Streamline, “There is a tendency to feel more comfortable with low V1”, whilst I heartily endorse being “GO minded” at higher speeds approaching V1, I feel uncomfortable when V1=Vmcg (as it often is). Test flying has shown me the EXTREME directional control problems that can arise here, my mate John_Tullamarine can provide some pretty graphic accounts of testing in this regard. I’m much more comfortable when V1 is a little above Vmcg. This is one area seriously in need of a fix.
”one sees that people are inclined to use the wet RTOW in conditions that approach but are not quit like the wet scenario”. You’d better have a good lawyer, a very very good one. The law requires that for NORMAL operations, a screen height of 35 feet be achieved, and that one means of retardation be kept in reserve for the RTO calculation. Wet runway operations (when the AFM has clearly spelled out what constitutes a wet runway), allow for a legal dispensation to use a 15 foot screen height and full credit for Reverse Thrust. In short there is NOTHING in reserve for the RTO, and a dramatically reduced screen height (and obstacle clearance height). Using this dispensation on a ‘less than AFM defined’ Wet runway is patently illegal. If you really like the lower V1s that go with the Wet Runway data for use on an ‘unwet’ runway, do two things –
Firstly, reduce the ASDA by 15%. That’s the reduction required by the FARs for an aircraft without a reserve means of retardation. It’s not in FAR 25, but in the FAR defining the requirements for aircraft certification (I’m on leave and don’t carry the FARs with me).
Second, calculate the 1st segment gradient, and subtend this gradient (typically about 0.8%) for the extra 20 feet screen height required back along the TODA to find the TODA reduction. That’s about a 2500 ft (762M) reduction for 0.8% gradient, worse for an Up-Sloping runway). With the new ASDA and TODA, go back to your Wet Runway charts to find the new MTOW, and you’re legal! Sorry Mutt, another unbalanced field.
This ignores the effects of reduced acceleration and deceleration considered for the Wet runway, so this will be a small bonus if the runway is less than ‘legally wet’. If you use Contaminated Runway data, as Marcellus Wallace has discussed, then the additional safety margin will be even greater.
Sorry if I’ve seemed to have hijacked the thread, colleagues, back to my leave now. I'll leave you in peace
Regards,
Old Smokey
Centaurus, my reference to the pilot being like a coiled spring approaching V1 was in the vein of being MENTALLY like a coiled spring, the decision must be made and executed in a very short time indeed. There’s nothing symbolic in removing one’s hands from the thrust levers at V1, it’s a positive action to prevent the inadvertent ‘knee-jerk’ Thrust Lever closure by the ‘mentally coiled pilot’ when hearing the V1 or Rotate call. It was my own discipline to do this long before it became a SOP, my own mental preparation was “We are going to Reject”, before V1 was reached, immediately becoming “We are going to Continue” when V1 was called, as I raised my hand from the Thrust Levers. Your description of some pilots with a vice-like grip on the Thrust Levers is a bit worrying.
No problem with V1 and Vr being the same figure, simply call Rotate”, and dispense with the V1 call, it’s now redundant anyway. A little akin to my suggestion that there is a case for replacing the V1 call with a GO call. (We did this (the Rotate call that is) on the B727).
SR71, Yes the very long runway far in excess of requirements does pose a philosophical argument making an abort decision above V1 more acceptable. I have had the “pleasure” of an abort above V1 with a complete control jam, but the runway was some 2000M in excess of that required. All of my fore-going arguments have related to the aircraft being Field and/or obstacle limited, if both of these are spare, then there is no problem, but the pilot has no way of knowing just what is to spare, and therein lies a problem.
A further major problem is that when there is field length “to spare”, we then make it limiting by using Flex / Assumed Temperature thrust, making every takeoff a limiting one. A reject above V1 when using Flex / Assumed Temperature thrust would be most inadvisable. And 'one for the road', does your reject above V1 consider Vmbe?
SIDSTAR, a good post, you make several good remarks –
(1) ”Stick to your company's script and you will not go far wrong. If you are unhappy with the SOPs try to get them changed”. Yes, we could make every V1 equal to Vmcg (or Vmcg plus a bit for comfort) if we wanted (I wouldn’t), it would cost a lot of money from Boeing, Airbus et al, because then the continued Takeoff would become much more limiting, and have to be re-certified at great expense. Denti, I think that your new owners would have had to do this when they introduced the new much lower V1s, at least I hope that they did! Balanced Field principal would be thrown out the window (now that’s a good idea, J_T may agree, but Mutt is a much more balanced individual).
(2) ”Boeing did a study some years ago on the amount of braking applied in RTOs by a range of pilots ranging from their own certification test pilots to the average line guy. It was interesting that the only group who consistently used the max braking available to them was the test pilot group”. There are three ways to fly an aircraft – (A) Smoothly and accurately (Normal line flying), (B) Assertively (Engine out, show the aeroplane who’s boss, but as accurately as possible), and (C) Aggressively, using maximum effort and adrenaline. RTO Brake application and collision avoidance fall into the latter category.
Richjb, I think that you said it all. Some good extra points arise from your discussion. “critical engine failure speed” this is a misnomer”, heartily agreed, it might have been good in it’s day, but if we’re looking for a realistic critical engine failure speed, then look to Vef. That’s where the process begins. Nor is it the ‘Decision Speed’, by the time that V1 has been called, the decision has already been made, back at Seattle, Toulouse, Wichita, et al.
I think that your best remark was “Understanding the differences between and more importantly the limitations inherent with these certification rules is necessary for pilot to completely understand V1 & accelerate-stop distance”. It is VITAL for a competent operating pilot to fully understand and comprehend the basis upon which the performance criteria have been created. FAR 25 (and all of it’s equivalents) is an unusual piece of legislation. It is the law, but an unusual law in that it is also the criteria by which the Performance Engineering folk generate the real-world data with which we comply. In most instances in life we stay within the law, in the FAR 25 case, we take the law all the way to the limits.
Streamline, “There is a tendency to feel more comfortable with low V1”, whilst I heartily endorse being “GO minded” at higher speeds approaching V1, I feel uncomfortable when V1=Vmcg (as it often is). Test flying has shown me the EXTREME directional control problems that can arise here, my mate John_Tullamarine can provide some pretty graphic accounts of testing in this regard. I’m much more comfortable when V1 is a little above Vmcg. This is one area seriously in need of a fix.
”one sees that people are inclined to use the wet RTOW in conditions that approach but are not quit like the wet scenario”. You’d better have a good lawyer, a very very good one. The law requires that for NORMAL operations, a screen height of 35 feet be achieved, and that one means of retardation be kept in reserve for the RTO calculation. Wet runway operations (when the AFM has clearly spelled out what constitutes a wet runway), allow for a legal dispensation to use a 15 foot screen height and full credit for Reverse Thrust. In short there is NOTHING in reserve for the RTO, and a dramatically reduced screen height (and obstacle clearance height). Using this dispensation on a ‘less than AFM defined’ Wet runway is patently illegal. If you really like the lower V1s that go with the Wet Runway data for use on an ‘unwet’ runway, do two things –
Firstly, reduce the ASDA by 15%. That’s the reduction required by the FARs for an aircraft without a reserve means of retardation. It’s not in FAR 25, but in the FAR defining the requirements for aircraft certification (I’m on leave and don’t carry the FARs with me).
Second, calculate the 1st segment gradient, and subtend this gradient (typically about 0.8%) for the extra 20 feet screen height required back along the TODA to find the TODA reduction. That’s about a 2500 ft (762M) reduction for 0.8% gradient, worse for an Up-Sloping runway). With the new ASDA and TODA, go back to your Wet Runway charts to find the new MTOW, and you’re legal! Sorry Mutt, another unbalanced field.
This ignores the effects of reduced acceleration and deceleration considered for the Wet runway, so this will be a small bonus if the runway is less than ‘legally wet’. If you use Contaminated Runway data, as Marcellus Wallace has discussed, then the additional safety margin will be even greater.
Sorry if I’ve seemed to have hijacked the thread, colleagues, back to my leave now. I'll leave you in peace
Regards,
Old Smokey
Last edited by john_tullamarine; 22nd Feb 2006 at 20:44.
Moderator
.. no more leave for OS .. he instantly goes into punny mode ...
I’m much more comfortable when V1 is a little above Vmcg
This is a point which we try to keep in the line pilot fraternity's mind. Because the very great majority of us don't get to play around at Vmcg with a failure, there is a tendency for pilots to fall into the trap of believing that the bird will always behave like it is "on rails". This is made worse by the problem with poor fidelity in some/many simulators.
In the real world, the onset of directional problems typically can be very sudden in terms of heading/centreline deviation as a function of failure IAS - for the continued takeoff - generally the accel-stop is not such a problem.
It really is a case of now you get away with it ... now you roll yourself up into a ball on the grass. We might be talking here in terms of needing only a few knots to go from relatively easily controlled to runway side departure and once we get onto that roller coaster, it is only a few knots to make the departure one which could be described as exciting ...
In the loss of control case the mental workload associated with the pilot's having to assess the problem and then, while the aircraft is pirouetting off the side of the runway, having to close the throttles to get some control back (if you are lucky) is a big ask .. remember this is the guy who is primed up to GO !! and it just isn't working out the way he expected to see it happen.
The problem I see is that the very great majority of pilots have NEVER seen the problem and have a tendency to under-rate the significance of a Vmcg-limited continued takeoff. It is for this reason that, for sim endorsement training, I always exposed the crews to min speed/weight, aft CG failures (unless the particular sim was dreadfully unrealistic).
As an aside, there is one thing in the pilot's favour .. while the weight may be low and the speed schedule at minimum, normally one isn't going to see CG at the aft limit as well and the effect of CG on Vmcg (real world as opposed to certification) is significant. But, if the holes all line up, then beware.
.. perhaps we should talk about crosswind effect on Vmcg again .. ?
I have taken the liberty of blanking out several words in OS' post to protect my identity a little .. talk about putting my name in defacto lights ...
I’m much more comfortable when V1 is a little above Vmcg
This is a point which we try to keep in the line pilot fraternity's mind. Because the very great majority of us don't get to play around at Vmcg with a failure, there is a tendency for pilots to fall into the trap of believing that the bird will always behave like it is "on rails". This is made worse by the problem with poor fidelity in some/many simulators.
In the real world, the onset of directional problems typically can be very sudden in terms of heading/centreline deviation as a function of failure IAS - for the continued takeoff - generally the accel-stop is not such a problem.
It really is a case of now you get away with it ... now you roll yourself up into a ball on the grass. We might be talking here in terms of needing only a few knots to go from relatively easily controlled to runway side departure and once we get onto that roller coaster, it is only a few knots to make the departure one which could be described as exciting ...
In the loss of control case the mental workload associated with the pilot's having to assess the problem and then, while the aircraft is pirouetting off the side of the runway, having to close the throttles to get some control back (if you are lucky) is a big ask .. remember this is the guy who is primed up to GO !! and it just isn't working out the way he expected to see it happen.
The problem I see is that the very great majority of pilots have NEVER seen the problem and have a tendency to under-rate the significance of a Vmcg-limited continued takeoff. It is for this reason that, for sim endorsement training, I always exposed the crews to min speed/weight, aft CG failures (unless the particular sim was dreadfully unrealistic).
As an aside, there is one thing in the pilot's favour .. while the weight may be low and the speed schedule at minimum, normally one isn't going to see CG at the aft limit as well and the effect of CG on Vmcg (real world as opposed to certification) is significant. But, if the holes all line up, then beware.
.. perhaps we should talk about crosswind effect on Vmcg again .. ?
I have taken the liberty of blanking out several words in OS' post to protect my identity a little .. talk about putting my name in defacto lights ...
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Whoops! Sorry J_T, upon reflection, my description of the Time/Place/Events/Scenario could have compromised your valued anonymity.
Is that an 'in-facto' personal identification as opposed to the de-facto' type.
OK, OK, I'll go back on holidays now, just gotta get a PPRune 'fix' now and then. (Why do you run the best threads whilst I'm away?)
Regards,
Old Smokey
Is that an 'in-facto' personal identification as opposed to the de-facto' type.
OK, OK, I'll go back on holidays now, just gotta get a PPRune 'fix' now and then. (Why do you run the best threads whilst I'm away?)
Regards,
Old Smokey