B747-200: Vref, V2 and Vmcg
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Maybe I have presented it badly, but I will try again.
Consider two airplanes on takeoff with an engine failure for one of them at V1. Up until the engine quits the performance of both is the same. At V1 the all engine airplane and the three engine one both reject takeoff. The only difference will be the reduced availability of reverse for the three engine airplane, and since reverse is not considered, to all intents and purposes the distance needed to stop will be the same. The ASDR will be the same, except that the all engine case is subject to a 15 percent penalty, ie the required runway (plus stopway) for the all engine case is always longer. The case is similar for the continued takeoff; even though the all engine airplane will perform better than the three engine one, it is rarely more than 15 percent better, since only the time from V1 to the end of the TOD is considered.
In my second para, I was referring to the way many new pilots are surprised at the closeness of the end of the runway, as it approaches at 180 knots or so just prior to liftoff. Many have commented to me that it would be diffcult to stop should a problem develop at that point. But the only time it is relevant to check the runway remaining would be at the V1 speed, since a reject is not guaranteed above that speed. The mindset should change to GO when past V1.
Where have I gone off the rails?
LOM, the Vref with Flap 30 is around 140, so I doubt you would have 146 or 152 with Flap 1. But a good point. Must try it in the sim.
Consider two airplanes on takeoff with an engine failure for one of them at V1. Up until the engine quits the performance of both is the same. At V1 the all engine airplane and the three engine one both reject takeoff. The only difference will be the reduced availability of reverse for the three engine airplane, and since reverse is not considered, to all intents and purposes the distance needed to stop will be the same. The ASDR will be the same, except that the all engine case is subject to a 15 percent penalty, ie the required runway (plus stopway) for the all engine case is always longer. The case is similar for the continued takeoff; even though the all engine airplane will perform better than the three engine one, it is rarely more than 15 percent better, since only the time from V1 to the end of the TOD is considered.
In my second para, I was referring to the way many new pilots are surprised at the closeness of the end of the runway, as it approaches at 180 knots or so just prior to liftoff. Many have commented to me that it would be diffcult to stop should a problem develop at that point. But the only time it is relevant to check the runway remaining would be at the V1 speed, since a reject is not guaranteed above that speed. The mindset should change to GO when past V1.
Where have I gone off the rails?
LOM, the Vref with Flap 30 is around 140, so I doubt you would have 146 or 152 with Flap 1. But a good point. Must try it in the sim.
Moderator
Boofhead,
It is quite possible that I may have missed something here ...
(a) which design standard imposes the 15 percent pad on AEO ASD ?
(b) a significant difference between AEO and OEI reject is the maximum speed achieved during the acceleration and reject sequence. In the AEO case, the reject will see a higher maximum achieved speed and longer distance to stop .. one of the main hazards in rejecting from high speed for other than a failure on a limiting runway.
(c) while the current rules require the 2 second pad, which provides a bit of a buffer, I would question any suggestion that a reject from V1 on a limiting real world runway involves any sort of guarantee that you will actually get away with it .... especially on a pre-amendment 42 certification aircraft .....
It is quite possible that I may have missed something here ...
(a) which design standard imposes the 15 percent pad on AEO ASD ?
(b) a significant difference between AEO and OEI reject is the maximum speed achieved during the acceleration and reject sequence. In the AEO case, the reject will see a higher maximum achieved speed and longer distance to stop .. one of the main hazards in rejecting from high speed for other than a failure on a limiting runway.
(c) while the current rules require the 2 second pad, which provides a bit of a buffer, I would question any suggestion that a reject from V1 on a limiting real world runway involves any sort of guarantee that you will actually get away with it .... especially on a pre-amendment 42 certification aircraft .....
Boofhead,
As I said above, the Vmca-2 speeds with flap 1 are promulgated with respect to a 2-engined go around where you would be climbing at Vref + 60 with flap 1. The Boeing procedures do not address any other scenario with 2 engines inoperative and so I suppose that is why no other figures are published i.e. the 2-engined go-around procedure is the only case where you have to worry about Vmca.
Rgds
L
As I said above, the Vmca-2 speeds with flap 1 are promulgated with respect to a 2-engined go around where you would be climbing at Vref + 60 with flap 1. The Boeing procedures do not address any other scenario with 2 engines inoperative and so I suppose that is why no other figures are published i.e. the 2-engined go-around procedure is the only case where you have to worry about Vmca.
Rgds
L
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LOM, my point was that Boeing does not address the possibility of a double engine failure on takeoff or soon after. Such a scenario is considered statistically impossible and is not covered in their manuals.
If the sim is any guide, an outboard engine failure requires full rudder or almost to maintain directional control, although a little bank assists when airborne. The Vmca is not published but is probably close to Vmcg. The loss of two engines on thesame side would exceed the rudder authority of the airplane and it would be likely that the outboard engine would have to be throttled back to maintain directional control. A figure of 186 knots was given to me as the Vmca for this situation and I have no reason to doubt it.
For a two engine go around the thrust is increased on the inboard to full thrust and the outboard only as far as directional control will allow. Acceleration to Flap 1 speed is achieved in the descent and by the time the Flap 1 speed is reached full thrust can be applied to both engines and a climb commenced. I hope the next time I get a chance to experiment in the sim I can resolve this and will report back.
Unless someone out there has already done it?
John..
a. None. My mistake, sorry.
b. Agreed. This does, though, show that the all engine case is still the limiting one so far as the performance charts are concerned.
c. Most Operators recommend that the V1 speed not be used as a decision speed but the speed at which the decision has already been made and the reject commenced. As line pilots we have to assume that the experts have done their job well and that if we follow procedures properly we will not get into trouble. If the runway is wet, the runout area is covered in rubber, there is no stopway or there are close-in obstacles, it would be prudent to reduce the V1, but since this will affect the GO case it should be considered carefully. The contaminated runway speeds can be a guide for this although I do not know the legality of using these speeds on a dry runway.
If the sim is any guide, an outboard engine failure requires full rudder or almost to maintain directional control, although a little bank assists when airborne. The Vmca is not published but is probably close to Vmcg. The loss of two engines on thesame side would exceed the rudder authority of the airplane and it would be likely that the outboard engine would have to be throttled back to maintain directional control. A figure of 186 knots was given to me as the Vmca for this situation and I have no reason to doubt it.
For a two engine go around the thrust is increased on the inboard to full thrust and the outboard only as far as directional control will allow. Acceleration to Flap 1 speed is achieved in the descent and by the time the Flap 1 speed is reached full thrust can be applied to both engines and a climb commenced. I hope the next time I get a chance to experiment in the sim I can resolve this and will report back.
Unless someone out there has already done it?
John..
a. None. My mistake, sorry.
b. Agreed. This does, though, show that the all engine case is still the limiting one so far as the performance charts are concerned.
c. Most Operators recommend that the V1 speed not be used as a decision speed but the speed at which the decision has already been made and the reject commenced. As line pilots we have to assume that the experts have done their job well and that if we follow procedures properly we will not get into trouble. If the runway is wet, the runout area is covered in rubber, there is no stopway or there are close-in obstacles, it would be prudent to reduce the V1, but since this will affect the GO case it should be considered carefully. The contaminated runway speeds can be a guide for this although I do not know the legality of using these speeds on a dry runway.
Moderator
Boofhead,
(a) all's well ... just thought that I must have missed something along the way
(b) people need to be aware that the current rules address AEO reject ... and it would be reasonable to presume that this is applied to new TCs ...... however most of the aircraft out there were certificated to earlier standards which DID NOT ..
(c) my point precisely .. the real world boundary conditions are, in general, quite different ... and usually unconservative, with respect to the certification presumptions. The book figures are fine if the conditions and techniques are replicated ... in the real world situation this usually is not the case ... so we need to be a little circumspect about rejects from near V1 speeds from an ASD-limiting runway ....
(a) all's well ... just thought that I must have missed something along the way
(b) people need to be aware that the current rules address AEO reject ... and it would be reasonable to presume that this is applied to new TCs ...... however most of the aircraft out there were certificated to earlier standards which DID NOT ..
(c) my point precisely .. the real world boundary conditions are, in general, quite different ... and usually unconservative, with respect to the certification presumptions. The book figures are fine if the conditions and techniques are replicated ... in the real world situation this usually is not the case ... so we need to be a little circumspect about rejects from near V1 speeds from an ASD-limiting runway ....
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Boofhead,
This gives me the impression that you dont reduce the V1 on Wet Runways, where did you get this policy from?
Mutt.
If the runway is wet, the runout area is covered in rubber, there is no stopway or there are close-in obstacles, it would be prudent to reduce the V1, but since this will affect the GO case it should be considered carefully. The contaminated runway speeds can be a guide for this although I do not know the legality of using these speeds on a dry runway.
Mutt.
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Company policy and the Boeing manuals spell out what to do in the case of a wet/contaminated runway. My point was that you might be able to reduce the V1 even if the runway is dry, if there is a good reason for doing so. Using the QRH tables will give you a guide, rather than making a random reduction.
