V1 question

[C172Heavy]

Not that it matters much flying a 172 but I had a question that I'm sure someone can enlighten me on.

Does V1 depend somewhat on runway length?

The way I understand it, V1 is the go no-go speed. I think??? that it is the speed at which you can still safely abort the takeoff with the runway remaining. Please correct me if I'm wrong.

Do you have to factor runway length into this equation? Say you are taking off on an 10000' runway and abort at V1 and end up at the end of the runway after stopping. Wouldn't it spell potential disaster if you did the same thing on an 8000' runway?

Thanks for any input

 

[FlyinBrian]

V1 does not depend upon runway length. It depends more upon aircraft weight. But, you have to check your performance data to make sure you can get the amount of weight that you're carrying off any given runway. The performance charts will prohibit a takeoff if you are so heavy that you will not be able to accelerate to V1 and then stop on the remaining runway. This is a simplified description, but I think it should answer your question.

In your example, a takeoff would be prohibited (under part 121 at least) from the 8000 foot runway. You also take weight penalties for contaminated runways and what not.

 

[Andy Neill]

Since V1 is a decision speed (should I stay or should I go?), it does indeed depend on runway length on the short side. The theory is that, if I am at V1 and have an engine failure, I can continue the takeoff or abort the takeoff with equal liklihood of success. If V1 is too low, it will be no problem aborting, but takeoff may be dicey. If it is too high, the takeoff is not jeapardized but the abort may be problematic.

The typical solution to this is adjusting weight. The question asked is, "at what weight can I takeoff such that I can reach V1 and abort before the end of the runway or takeoff and get to a safe takeoff altitude before the end of the runway". For a given set of circumstances (runway length, slope, temperature, wind, braking capability, etc.) there is a given V1 that will let you take off with the maximum possible weight for the circumstances.

The variables to this would be the operator limitations on whether or not overruns and clearways can be considered in the equation.

 

[FlyinBrian]

Andy is 100% correct, sorry if I confused you.

 

[Andy Neill]

Brian,

We posted at nearly the same time. I was going to come back and say YOU had the right answer. Thanks.

 

[FlyinBrian]

I think both answers are technically correct. But mine almost made it sound like runway length does not in any way affect V1. In essence our answers hit the same point from different directions. I just didn't want to confuse the original poster. Runway length does affect V1 in that you may have to take a weight hit in order to take off from the shorter runway. The new weight is going to change your V1 for that runway.

 

[avbug]

V1 doesn't depend on runway length, but helps establish required runway length. It depends on weight, and density altitude.

 

[Airplane]

V1

What about configuration? Flaps or no flaps...

 

[ERJCapt]

Flaps

Wow, is this getting technical. You gotta love it.

The use of flaps would lower V1 for a given weight, thus reducing runway length requirements. It's a common practice to use the lowest available flap setting for takeoff, and notch it up one when it's needed.

The other posters have really answered the whole V1 question quite well. There are a host of factors that go into a V1 speed, but from the pilot perspective, it's quite simple. At my airline, we check each runway for a specific weight and temperature, and then use V1 speeds derived by cross-referencing weight and temperature.

True, as mentioned previously, a host of other factors come together to make a specific weight/temperature combination allowable for takeoff from a specific runway.

Pete

 

[RichardFitzwell]

C172Heavy,

As the useable runway length increases -- V1 (decision speed) also increases until V1 equals Vr (rotation speed). V1 will never be higher than Vr.

R.F.

 

[TurboS7]

Let me try.V1 is the speed at which you can abort the takeoff and apply maximum breaking with spoilers and no reverse(two engines) and stop at the end of the runway. At V1+.00000000001 you can continue the takeoff, rotate at Vr and cross the end of the runway at 35 feet.1st segment of a takefoff is from V1 to 35 feet or gear retraction 2nd segment is from 35 feet or gear retraction to 400 feet, 3rd segment or cruise segment is from 400 feet to 1500 feet. The aircraft must be able to maintain a given climb gradient for 121 and 135 on one engine assuming standard gear and flap retraction. In the case of a propeller airplane engine feathered or auto-feathered with a maximum 5 degree bank into the good engine. That's from memory after a long flight whoa, the climb gradient I think is 2.8%.

 

[Groucho]

V1 an other stuff.

The above posters are correct when they state that V1 is not runway length dependant.

However new technology and computers have adjusted the V1 scenario somewhat. Database derived performance data used by some of the larger carriers will give various V1 data for the same runway and flap data. Improved climb situaitions, used in places like LAS and other high density altitude airports, allow for increasing V1 with minimum flap settings to allow for increased second segment climb requirements. The MD-80 can have up to a 40kt V1 split from a min V1 used in a contaminated runway condition to a very high speed long and dry runway improved climb situaition.

The FAA definition of V1 does not consider runway length as a variable factor. With the new computer driven real time performance data available V1 is a highly variable speed for any given runway length. Weight, runway condition, temperature, density altitude, slope and rated or derated power settings will all change the V1 the computer puts out.

We read it off the print out and set the bugs to it. I don't know of any pilot who can explain all the factors and formulas that go into the computer models, the performance engineering department takes care of all that for us. Of course the old way of looking at the weight and looking at the flip chart speeds always works.

I hope this confuses things further.

 

[TurboS7]

We have a policy at our airline if the numbers don't work call dispatch with all the particulars, they will run all the details through the computer and then give you the go/no go. If it is a go they will give you all the supporting data via fax.

 

[FlyinBrian]

I'd be weary of accepting a "go" decisinon from dispatch without understanding how they figured their data. They don't die in the crash, and at the hearing, "dispatch said we should go" is probably not going to hold much water.

Aeronautical Decision Making philosophy:
"When in doubt, start at the hearing and work backwards"

 

[avbug]

We just go down the runway until the airplane starts to feel light. Then we pull back. If we go far enough that we start to feel queesy or tense, we pull back the thrust levers instead. If we just flat-out go too far, then we pull back the cabin door and evacuate.

Airspeed alive! 90, and crosschecked. Vee One!

Vich one?

Vis von. Over veer.

Votcha.

Votate.

Veer up! Vos vate, avter vakeoff vist, pees.

Why V1 doesn't work in Dutch cockpits.