What's V1?
 

I understand this is speed, on take off, above which the aircraft is committed to fly (if possible) despite any failure occurring. You take the failure into the air.

I always assumed this speed was calculated taking available runway length into account, and if you try to stop after V1 you are in serious danger of over-running the runway and going off the end. Hence the need to fly with a failure after V1.

However, I was talking to a couple of retired Big Aeroplane driver mates the other day and they told me this was not the case! V1 is calculated for weight and other factors, but runway length isn't one of them. As one of these guys said "if you were taking off from Bonneville Salt Flats the calculated V1 would be the same as from any airfield. I'd probably stop after V1 if there was no doubt about available length to stop in, but it would get me a fail on a sim check".

What's the point of V1, then?

You can read more in this article which defines V1 as:

The required runway length for take-off depends on both the distance required to stop the aircraft in case of an aborted take-off and the distance required to reach a safe height over the runway end in case of continued take-off after engine failure.

A low v1 speed means that the stopping distance in case of engine failure will be short, but the continued take-off distance will be long as the aircraft must accelerate longer with one engine out. A high v1 means the opposite. The v1 speed where the stopping and continuing take-off distances are equal is the optimum v1 and will give the shortest required runway length (the "balanced field" length). Clearly both v1 and the required runway length will depend on factors such as aircraft mass, weather and runway conditions...

Thus the important thing for the pilot is to ensure that the actual runway length is at least as long as the required runway length. There is no need to change v1 depending on actual runway length.

That is not to say that you couldn't change v1 if the runway is longer than the minimum required but there is usually no reason to and I don't think pilots generally have the performance data to do such calculations.

But if you were taking off from the Salt Flats with a 'runway' many, many miles long, you could ignore V1 and stop any time up to just after Rotate Speed?

And even then, in the event of, say, a serious engine fire you might consider chopping the power and landing straight ahead rather than continuing flight?

Not necessarily. There is a lot of kinetic energy in an aircraft at Vr. The wheel brakes must be able to absorb this energy without overheating or catching fire. Thus there is a maximal possible v1. Of course, this assumes maximum braking so if you are braking carefully you could stop from higher speeds. But then you would be in unknown territory as regards stopping distance. (Wheel/brake fires are not unheard of in emergencies...)

Sure, but again this is all very hypothetical. A transport category aircraft is moving really fast and runway will disappear behind it quickly. Since you have no figures, you would need very very large margins to do this safely.

It is not like the spamcans we PPL types usually fly where you could safely make several take-offs/immediate full stop landings in sequence on a 3000 m runway.

Vmbe being the speed I implicitely referred to here:

Whilst they are correct in one way, what will be the case on some airfields is that the runway length would restrict the TAKE OFF MASS, this take off mass would then dictate the V1, so you might have the same V1 at Bonneville as a shorter runway, but only up to a more limited Max TO on the shorter runway. Also, on a longer runway you can have a range of V1 whereas if the TO mass is restricted by runway length there will only be the one V1.

I guess that would be calculated by the operator rather than the pilot? Or using assumed lower take-off performance?

Yeah but... At the Salt Flats you have effectively an infinitely long runway. No need to use the brakes at all!

If we discount Vmbe for our "infinite runway case", V1 must not exceed the tyre speed limit.

Of course; I use The Salt Flats example merely to make a point, as it is effectively not runway-length limited.

That point is that as runway length plays no part in calculating V1, then pilots surely have some leeway as whether they do indeed 'always take a post-V1 failure into the air'. I had, prior to my chat with my ex-BA mates, assumed that VI took runway length into account and therefore trying to stop after V1 would probably lead to an over-run accident.

Wasn't there a 748 some years ago that suffered a catastrophic engine explosion with massive fire after V1 and the pilot elected to stop straight ahead? Everyone walked away. It would have been unlikely the aeroplane and pax would have made it round even a very tight visual circuit had he flown.

Yes - and he was a member of our illustrious aero club and I might :rolleyes: have had a hand in teaching him..........:oh:

SSD

I somewhat doubt they meant that and I wonder if there's been a misunderstanding.

On one airline I know of on the line the V1 (amongst other things) is calculated by entering weight and other factors, as you put it, into either a computer program or by reference to a (thick) book which contains a page for every single runway on the network. If you're using the computer you enter the runway identifier and entry point, not the length into the "computer", if using the book you use the page appropriate to runway and entry point. In neither case do you as the pilot, actually enter the numeric runway length in feet or metres, since the "computer"/book has that inormation so it is a value most assuredly involved in determining the value of V1.

I suspect that is what your Big Aeroplane pilots were driving at ( at least I hope that was what they meant, otherwise :\ )

No, our conversation was quite specific and 'runway length not a factor in V1' was the whole basis of our discussion. In fact it was one of these ex-BA captain chaps who came up with the Salt Flats scenario to illustrate that V1 can be meaningless in some circumstances (i.e. you may be OK to stop when really at any point after V1 you should fly. There's a judgement to be made if it happens).

No, runway length restricts MTOW and the TOW then determines V1. V1 cannot occur after TORA for that reason.:ugh:

TORA I guess is Take Off Run Available. So how can anything 'occur after TORA'? :confused:

It's worth remembering that,most of the time, there is a range (sometimes a very wide range) of possible values for V1, because the first moment at which continuing becomes possible occurs before the last moment at which stopping is an option. The manufacturer and/or operator chooses a value to be used operationally.

as runway length plays no part in calculating V1

Not quite the case and very misleading without some caveats.

As with most performance limit calculations, the pilot/operator has to ensure that a number of considerations is addressed with the result that one will become the limiting case for the particular set of circumstances - normally we are concerned with determining the maximum weight for the takeoff. Indeed, change the situation a bit and the limiting case may/will change.

In respect of runway length

(a) if the runway is longish compared to the particular aeroplane's needs, it may not become the limiting consideration for figuring a suitable V1. However, it will still be addressed as part of the process to determine the limiting factor as that process usually is addressed as sausage machine calculation.

(b) if the runway is shortish compared to the particular aeroplane's needs, runway length may, quite definitely, become a significant input into the determination of an appropriate V1.

It can be meaningless in the situation that your V1 is the same as your Vr ie the runway left is more than your breaking distance required from Vr speed.

Light TP's which are pref A often have V1/Vr being exactly the same speed. And basically if the runway is longer than 2000m they don't even bother giving you performance tables. You just go straight into your climb performance.

The best book to get your hands on is Aircraft Performance Theory for Pilots by Swatton.


Or he has a second addition out but its a bit pricey at 40 quid new.

You need to go through what all the different distances are and what they all mean. As well as what the different speeds relate to.

Many thanks. At a tenner inc. postage I ordered a copy 'in 1st class condition' from Amazon.

As a SEP pilot it'll only be of academic interest, but so is this thread I started!