Optimum V1

[Scoobywill]

Gentlemen.
I know this type of thread has been done numerous time however after searching I have failed to find a definition for Optimum V1.
I understand the Vstop, Vgo, balanced field, balanced V1 principles however when we are given optimum V1 speeds from one off calculations what exactly are we being given? I realise that one off calcs. takes into account environmental conditions/weight/runway length etc but why do they provide an 'optimum V1' and what is the definition of it?

[Skyjob]

V1 is the takeoff decision speed, following critical engine failure, from which a decision to continue results in:

a takeoff distance tp a height of 35ft at V2 that won't exceed the usable takeoff distance
the speed from which a decision to bring the aircraft to a full stop will not exceed the accelerate-stop distance avaialble


For some takeoff performance calculation modules, when the parameter for V1/Vr Choice is set to 0, a range of v1 is calculated. Most output options however only present one v1, so a choice has to be made. By default, the printed v1 will be the balanced field v1 when it's within range. If a balanced v1 does not lie in the valid range, either minimum or maximum v1 will be printed, whichever is closer.

When a range of v1 is available the 'user' (performance department or IT engineer) can specify which v1 to print based on a scale of 0-1, thus:
v1(printed) = (scalar x (v1 max - v1 min)) +v1 min

Output Calculation Example:
MTOW 75000 based on:
Field limited 78438
Tire Speed Limited 86183
Climb Limited 75000 (2nd)

Obstacle Clearance Limited 75426(V1 min), 77380(V1 bal) 77899(V1 max)
Brake Energy Limited 86183(V1 min) 86183(V1 bal) 86183(V1 max)

Speeds for MTOW:
V1 min = 141.8
V1 bal = 148.8
V1 max = 150.6
Vcmg = 99.3
Vmbe = 168.9
Vr = 150.6
V2 = 155.7

Notes:
Minimum V1 is limited by obstacle clearance
Maximum V1 is limited by Vr

One Engine Inoperative Runway Distances:
Takeoff Run 8266 (V1 min) 7390 (V1 std) 7176 (V1 max) 8765 (Available)
Takeoff Distance 9030 (V1 min) 8154 (V1 std) 7940 (V1 max) 9065 (Available)
Accelerate-Stop Distance 7344 (V1 min) 8154 (V1 std) 8376 (V1 max) 8965 (Available)

[Chris Scott]

Yes, when there's a range of legal V1s (i.e., on the vast majority of T/Os, sometimes even when using reduced thrust by the assumed-temperature method on a very light a/c), it depends on what you mean by "optimum"!

On a 4-engine a/c, at an airfield with no engineering support, you might choose the minimum V1. At base, you might choose the maximum. Discretion suggests giving yourself a bit of "fat" in either case, however. :}

[FE Hoppy]

Simply put it is the V1 which allows the highest Field length limited take off weight.

[Centaurus]

why do they provide an 'optimum V1' and what is the definition of it?

Since accident investigation research reveals that high speed aborted take offs are inherently dangerous, I would have thought the lower the V1 the better; especially on a wet runway. An "optimum" V1 would therefore suggest the lowest V1. I accept it is purely a personal opinion and there would be a storm of contrary views.:ok:

[de facto]

A clue....an optimum v1 is derived from a serie of optimizations..:E
Have a read ...

[No Fly Zone]

Sir,
Please know that I am NOT a current pilot and even when current I never flew the Big Machines for which V1 and V2 are such apparently critical numbers. That said, I do understand the physics behind these calculations. The basic formulas do include a modest fudge factor, always in your favor. Despite that fudge factor, and understanding the rather simple math, I'd 'fudge' V1 a bit lower and V2 a half-a-bit higher. Of course the Flight Operations Departments have other ideas and pilots may be called to task for even minor variations of their calculations. Your question is about safety, so 'error' on the safe side and give yourself that extra second of wiggle room when (if) it goes to hell at the last instant. If the performance limits are already being stretched (Hi and hot with old engines) hike up your already too tight shorts and delay your departure, dump some cargo or pax or whatever will improve your numbers enough to make you comfortable. In other words, please don't fly the margins as far too many do. Have the [anatomical parts] to say NO when you don't like the numbers. How do you know that both of those engines will perform at 100% or 105% when you most need it? (I'd suggest that most will not - and it is not worth your life and that of your 100 - 500 pax to find out. If you're not 110% comfortable, please do not fly or wait until conditions improve. And it really is that important. Ten percent can make the difference, fudge factor or not. -C.

[Skyjob]

I'll try to elloborate regarding the V1 min , max and optimum in easy words.

When doing a takeoff calculation some objectives need to be achieved:

Not run out of runway
Be able to stop on remaining runway at V1
Be able to clear all obstacles airborne


To calculate the first two, it is a simple accelerate-stop scenario.

The brake capability, therefore the deceleration rate, is known by the performance engineers.
The acceleration however depends on thrust availability which is of course a variable due to potential of derated thrust and assumed temperature methods or flex methods to reduce engine thrust at takeoff, designed to extend engine on wing lifetime.
Due to the variable in takeoff thrust availability, the acceleration thus varies.
The least amount of thrust required to achieve liftoff and still reach V2 and achieve screen height and obstacle clearance can then be calculated.
Due to the now reduced thrust availability the aircraft now however does not climb as well as it would with full rated thrust and bleeds off, the xxK rated engine thrust limit.
Therefore obstacle clearance may not be achieved, so in order to achieve that weight must be reduced at this trust setting or additional thrust may be required to achieve these clearances.

It now becomes an issue of finding the minimum speed which will at a predefined thrust availability:

allow you to still get airborne on the runway length available and clear the obstacles in your path
be able to stop in the ruway distance remaining


Look closely at the example I posted from a calculation output.
You will see a relationship between different areas of assessment, runway length, obstacles, brake energy, ...

Optimum merely defines a calculation midpoint between the max and min as predefined by the software in the Operations IT department using a set of criteria, like i said using a scalar, or additive of difference between max and min added to the V min.

[FE Hoppy]

https://lh3.googleusercontent.com/-itG1mV8eMuI/T1CkVtzcjzI/AAAAAAAAABU/PkT_El3j10Y/s640/V1.001.jpg


The V1 equal to the point when the lines cross would be optimum.

[Skyjob]

:ok: FE Hoppy :D

[slam dunk 4]

Hi to all,

can anyone explain this for dummies please:

As a pilot, we do have take off calculation tool in our EFB. When we do our take off calculation that tool gives us some time the choice, for a give weight, a min. V1 and max. V1.
What exactly am i doing, when i decide to take the min. V1 for that take off or the max. V1. Why or for what reason should i decide to take the lower V1 and for what reason should i take the higher V1.

[Sidestick_n_Rudder]

There's no rights and wrongs here, each V1 within the range is good enough and satisfying performance requirements.

However, choosing lower V1 will improve your stopping performance, so perhaps it's advisable if the runway is wet/slippery, or the stop margin is low. Some people say it is better to be 'go minded' and use lower V1, to enable them to continue the t/o rather than stop in case of engine out.

Higher V1 OTOH will enable you to stop until higher speed, but with a lower stop margin. Also, you will probably use less runway for continued takeoff, have better screen height, perhaps attain higher speed for climb (even though V2 is fixed) and better climb performance. Could be a good idea if obstacle, or climb gradient are the limiting factors.

Anyhow, it is up to the Captain's judgement, there's no black and white here

[aterpster]

I was active in the industry many years ago when the policy changed to make V1 as low as possible (but not less than VMCg). The policy was predicated on it is generally safer to continue takeoff with an engine failure than abort at high speed.

[Skyjob]

Sidestick_n_Rudder is correct, no right nor wrong.
Some airlines in OPT show only one V1, not a min and/or max. This one V1 is then defined by airline settings, which could be either, an intermediate or optimum...

If a range is given, it merely indicates the range between lowest and highest V1, both enable the aircraft to stop in the remaining distance and/or to fly off the tarmac and clear obstacles. Depending on conditions you may wish to use lower or higher V1. Obstacle clearance may be one additional factor, using an improved climb by using the higher V1...

aterpster also has a point that the lower V1 was some years ago preferred due incidents at the time requiring a rethink in using a higher V1 or not. Some EU operators have defined their OPT to always use minimum V1 in most airports, such that this enables the best margin in case of a stop and to prevent risk of an overrun (we are after all not all test pilot material that can stop in minimum distance and perform all required actions when being startled for real, but we are all trained to fly the tube with problems every few months in the simulator)

[172_driver]

We operate the 737 on short sectors, generally quite light. I often see accelerate-stop margains in excess of 1000 meters. Under those circumstances I see no reason why not to use max V1, even on wet. That way I don't have to bring any problem into the air, while at the same time giving plenty of room for the abort (incl. the crappy execution). I'd say that even on slippery an increased V1, away from balanced V1, could have some positive effects. If you're taking off from a long piece of tarmac I can sacrifice some accelerate-stops distance to move the V1 away from Vmcg. That should make controllability easier if you have to continue. Every situation is what it is, no one size fits all...

[megan]

V1 is the takeoff decision speedLets not forget that the FAA does not define it as a decision speed, but rather the time at which the first stopping action takes place, the decision to stop, or go, having taken an FAA defined time prior to V1.

[Jwscud]

Of course it goes the other way if you are approaching the TOPL.

My employer routinely uses improved climb speeds, which tend to be ~10kts faster than the FMC speeds. This apparently makes money as it allows for a slightly increased MTOW and thus greater payload when not obstacle limited.

However, in all the other company and industry literature, the danger of a close to V1 stop and possibility of an erroneous stop decision is emphasised. The company to make minuscule amounts of money now routinely expose all their heavies to an extra 5-10s on the ground with a greatly reduced stop margin because they say statistically the chances of encountering a stop-go decision is greatly reduced.

I'm just a line grunt, but there is a long history of statistically unlikely events happening to crews, and I would far rather a bit more fat if I'm doing an RTO close to limits than coming to a halt right at the end of the available tarmac at Vmbe with 350 punters down the back.

The company will respond and say "its legal..." but I personally would prefer they only use improved climb performance where it's actually operationally required rather than seeing us on the ground at 170kts and seeing how uncomfortably little runway is reminding as the V1 call is made on every long sector.

[john_tullamarine]

optimum ?

Unless otherwise defined, presume maximum weight for the conditions.

If your ops eng group has an operational background and is in cahoots with the CP and the two aren't too beholden to the bean counters .. a bit of fat might just be injected into the data.

For instance, when QF started out with reduced thrust takeoffs (a LONG time ago), the then Ops Eng boss, Wal Stack, mandated (as I recall) 1000ft ASD pad as a flight crew sweetener.

[FlightDetent]

Sorry for not reading all of the above properly, but maybe a noteworthy remark - terminology.

In the world of Airbus computerized calucaltions (FOVE, LPC NG, FlySmart, EFRAS, TOPAZ ... it is actually AFM Octopus implementaions) there is no OPTIMUM V1. If the results provide V1 speeds range (see FEHoppy above), then the choice is to display one of the following to the user:
- available V1 speeds range
- V1 min
- V1 max
- V1 mean value

The OPTIMUM term is typically used for (increased) V2 value, but not for V1. In line with 172_driver's Every situation is what it is, no one size fits all... on a given day it might be optimal for crew to choose one of the extremes. Thus, for the educated pilot's airline, displaying the full V1 range is the optimal choice.

[aterpster]

Lets not forget that the FAA does not define it as a decision speed, but rather the time at which the first stopping action takes place, the decision to stop, or go, having taken an FAA defined time prior to V1.

I guess it depends upon a reading of the Part 1 definition:

V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V1 also means the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance.