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V1 question

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C172Heavy

Active member
Joined
Nov 26, 2001
Posts
36
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
 
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.
 
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.
 
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.
 
V1 doesn't depend on runway length, but helps establish required runway length. It depends on weight, and density altitude.
 
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
 
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.
 

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