V1 question.
 

I always thought V1 was calculated as the last point in the take off roll that the take off could be safely abandoned and the aeroplane brought to a stop on the runway. Therefore, a problem occuring after V1, unless catastrophic, would be taken into the air and dealt with once airborn. It follows (I'd have thought) that the V1 calculation must take into account available runway length (is there enough length left to stop in?).

Talking to a retired 747 / 777 BA pilot the other day it transpired this is not the case. He though this was daft, and it does seem so.

If you are taking off from a dry salt lake bed with 20 miles of 'runway' available, surely you've never reach V1 as you could abandon the take off and safely stop right up to the point of rotation and lift off.

So, what's V1 really about, then?

So, what's V1 really about, then? In regard to the question, it’s about safety.
Runway length is a practicable limit and must be considered; we don’t have the luxury of infinite runway length.
Safety is about risk and managing risk. Stopping at speeds above V1 involves proportionately greater risk, and limits such as tyre speed and brake energy have to be considered irrespective of runway length.
Talk to your ‘Captain’ about brake temperatures, likelyhood of fire, tyre burst; then what risk is there in infinite operations – perhaps infinite risk unless it is bounded; V1 is just one boundary.

It is. Or at least, it should be:rolleyes:

Sometimes it's more length to stop, V1 is not always limited to runway length, e.g. it can't be greater than VR. No margin = balanced field.

Some performance charts or performance calc computers tell you the margin which is left stopping at V1.

It is what you thought it was, have you not ever had V1 and VR just about the same speed? Glad this captain is retired.

That is only half the definition.



If those two calculations end up with the same number, you have a balanced field.

Indeed, but this wasn't the question.

Similar subject....

http://www.pprune.org/rumours-news/4...eport-out.html

Never. I don't use V speeds in the lovely Chipmunk, hence my question. :O

Other issues such as maximum tyre speed, rotate and V2 also impose their own limitations on V1 and don't forget that this is just a decision speed, not a maximum speed. So on most take-offs an abandonment would result in you stopping well before the runway end. And if you want to continue the 'infinite runway' argument, you'll find that an aircraft will also have to meet a minimum climb gradient when airborne so before dispatch, the thing has to be able to fly at some time... then the limits above start becoming relevant.

PM

Shaggy, i like your way of thinking. I like to use extreme cases to validate hipotheses.

In the infinite runway case, you still have limits: tyre speed, for instance. Not maximum brake energy speed, because all you would have to do is retard the throttles and smoke a cigar while you slows down.

In not so big airplanes such as B737, A320, etc... optimum V1 is just the same as VR, meaning that you have more runway than you need to stop the airplane (basically you have "infinite runway length" in case of stop). In these airplanes you will hear something like "V1, Rotate" in most of the take offs.

In the big birds, however, you will hear something like "V1", ".......", then "Rotate".

It is in the big birds where V1 is truly meaningful. At the V1 call they remove the hands from the levers. From then on they are going to fly whatever happens.

V1 also means that the airplane will be able to lift off, maintain v2 and then reach 35 ft by the end of the take off distance available. At V1 you can either stop or go safely. Depending on the circumstances, most of the times you can choose a V1 within a speed band. The trick is choosing the optimum one (the one giving max take off weight or max assumed temperature).

Then you have the climb and obstacle clearance consideration, which can have effects in the determination of the optimum V1, too.

OK, so V1 can never be greater than rotate speed (after that, it's irrelelvant as you'll be flying). But at rotate speed, given sufficient runway length remaining, surely the power could be pulled off, reverse selected, and the brakes used normally (not agressively) to bring the aircraft to a gentle stop.

Most runways in the real world are not long enough to allow that, hence my assumption that V1 takes into account available runway length. Does it?

In the 'infinate runway' scenario, wouldn't V1 always be rotate speed, and therefore not relevant as a descision speed?

Yep, that's right. Not relevant, we can say.

we posted simultaneously

When V1 and VR are the same, runway lenght is not limiting ("infinite") for the stop case. And we are using the maximum possible V1, which is the best for the Go case (shorter take off distance). So performance is very good, runwaywise. But you can still have marginal performace for climb and obstacle clearance, though.

refer to my other post, if you like
regarding the rejecting take off after VR... Never an option. In the infinite runway it would be a quick take off and landing. Once you rotate, you never ever reject take off.

Yes :ok:

Thanks guys, that makes sense.

As a point of interest, why is a T/O never rejected after rotate but before lift-off?

And remember that Vr is not a fixed speed either, it can be moved to enable a better climb performance after take off (obstacle limit). With all the performance tools available you can use a 4000m runway completely even with a lightweight 737 (our ATC brethren really hate it when we do that).

When you calculate V1 you are using an unreliable fudge to find the point on a balanced field runway from which you may stop or accelerate to Vr before the end.
I say 'fudge' because, ideally, you'd have a mark on the runway as I seem to recollect was used by the V Force.
The use of IAS doesn't take into account an unnoticed power shortfall or a dragging brake which will place your V1 point too far down the runway.

Like Vmcg and Vmca there are woolly areas but they are the best we have at present.

A slight drift but have been in an aircraft (light and poorly loaded) that "auto rotated" before VR. The thrills of flying ACMI freight!

When V1 and VR are the same, runway lenght is not limiting ("infinite") for the stop case

Nit picking, I guess, but it might be better to say that the runway length may not be limiting. V1=VR certainly doesn't preclude the takeoff's being ASD limited.

why is a T/O never rejected after rotate but before lift-off?

Procedural protocol. In addition, operational technique may become critical from the viewpoint of repeatability for distances.

And remember that Vr is not a fixed speed either, it can be moved to enable a better climb performance after take off

Not really the case. VR generally is predicated on V2 and provides a suitable delta V such that, for the AFM rotation technique, VR will produce V2 - OEI - as the aircraft passes screen height. V2 may well be increased in some situations to take advantage of the improved climb gradient capability associated with an overspeed V2 takeoff.

When you calculate V1 you are using an unreliable fudge to find the point on a balanced field runway from which you may stop or accelerate to Vr before the end.

Not at all unreliable. Considerable effort goes into the flight test program and, for the conditions inherent in V1's establishment, the distance figures are sensibly repeatable. V1 does not necessarily presume BFL and can, for those aircraft whose AFM permits, apply for an unbalanced field length scenario.

I trust, for BFL, that you mean achieve OEI/AEO screen before the end rather than VR ? The latter might be quite disconcerting in many situations.

Well, at least the 737 performance software from boeing tries to optimize every take off, to do that it usually does not use the lowest V2/Vr combination but rather slightly higher to much higher speeds. Over 95% of all take offs have an increased V2/Vr combo. Difference between Vr and V2 is usually only 2 to 3 kts anyway which is not enough even for the OEI case to just reach V2 at screen height, therefore the recommendet technique is to fly the actual speed of V2 to V2 + 20 and usually it is at the higher end of the airspeed band.

So in all practical application V2 and Vr are not fixed values, they can be changed considerably to provide best possible performance or highest possible thrust reduction.

Several decades ago, a BAC-111 (Dan?) captain rejected T/O at LGW above Vr - the a/c had been misloaded and the nosewheel stayed firmly on the runway when he pulled back - a/c stopped inside the available length no problem.

A few years later an Iberia DC-10 captain rejected T/O at Malaga above V1 with a burst nosewheel tyre and mucho vibration - went off the end resulting in a burnt-out hull and significant loss of life.