V1 Factors

uwochris

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Hey guys,

Here is a question from an ATP workbook:

Which of the following factors will increase V1 speed:
a) increase in weight.
b) reverse thrust capability.
c) snow/slush on the runway.
d) an increase in tailwind.

The correct answer is "a." When I try to think about it, it doesn't really make much sense to me- for instance, given a specified runway length, a heavier a/c will require a higher Vr and will use up more distance for the takeoff run. Since V1 is the maximum speed where the pilots must take action to stop the a/c within the LDA, it would make sense that a heavier a/c would need to take such action sooner in the t/o roll in order to stop within the specified distance (i.e. heavier a/c=more momentum=more distance to stop). This seems to imply that the decision to stop/continue should be made sooner in the t/o run, meaning a lower V1 speed.

What's wrong with my logic?? Also, is it true that any factor affecting your takeoff distance (weight, runway slope, temp, pressure, etc) will also affect your V1?

Thanks in advance,
Chris.
 

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Keep in mind that a V1 speed is often calculated to "Balance" the field... V1 doesn't take into account using the entire runway at lighter weights...
 

Gulfstream 200

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I think you need to think about BFL (balanced field length)

Dont you need to be able to stop OR climb to 35ft by then end of the runway? higher weights require more to climb to that 35ft one one engine than to stop....hence my guess at the higher V1.

You will be runway limited by weight due to the climb requirements usually...



:eek:

Oh heck, I dont know...I swear I knew all this at one time..but have forgotten the useless stuff and just fly by numbers. Maybe I need to go back to school.

:(
 
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viper548

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uwochris said:
Hey guys,

Here is a question from an ATP workbook:

Which of the following factors will increase V1 speed:
a) increase in weight.
b) reverse thrust capability.
c) snow/slush on the runway.
d) an increase in tailwind.

The correct answer is "a." When I try to think about it, it doesn't really make much sense to me- for instance, given a specified runway length, a heavier a/c will require a higher Vr and will use up more distance for the takeoff run. Since V1 is the maximum speed where the pilots must take action to stop the a/c within the LDA, it would make sense that a heavier a/c would need to take such action sooner in the t/o roll in order to stop within the specified distance (i.e. heavier a/c=more momentum=more distance to stop). This seems to imply that the decision to stop/continue should be made sooner in the t/o run, meaning a lower V1 speed.

What's wrong with my logic?? Also, is it true that any factor affecting your takeoff distance (weight, runway slope, temp, pressure, etc) will also affect your V1?

Thanks in advance,
Chris.
a. At V1 you have to be able to reject or continue. If your weight increases, V1 will need to increase also in case you need to continue a takeoff. Suppose at light weight Vr is 105 and V1 is 100. If you increase the weight Vr will increase also, say to 110. If V1 is left at 100, the aircraft would need to be able to accelerate to Vr and continue the takeoff. At a higher V1 it will take more distance to stop, but it would take a lot more distance to accelerate on one engine to Vr.
b. I don't think reverse thrust is taken into account for V1, but i'm not certain.
c. a contaminated runway will increase stopping distance, so V1 needs to be reduced to be able to bring the a/c to a stop from V1.
d. a tailwind will increase stopping distance, so V1 needs to be reduced to stop the a/c from V1.
 

eljefe

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uwochris said:
What's wrong with my logic??
Thanks in advance,
Chris.
The flaw in your logic is that at a heavier weight V1 is increased but so is the runway length required. It will take longer to reach V1 but there will be more runway available for an abort or to continue after V1.

The way it generally works in the real world is you have a certain amount of runway available and want to take off at a certain gross weight. V1 is dertermined for those conditions.

If you want to weigh more, say to take more fuel or carry more payload, V1 will increase but so will the required runway. In some situations you can help yourself by choosing a different runway, or waiting until the temperature gets cooler later in the day.

If those options are not available to you, your V1 will be fixed at a certain value that is determined by weight, temperature, runway available, etc.

If you have options to increase the runway length available you have the option of also weighing more which will increase V1. If those options are not avaiable to you then you're runway length limted and V1 will be whatever it is at a balanced field length situation for the runway available.

Keep in mind, btw, that V1 is the speed at which the decision to continue or abort must already have been made.
 

MauleSkinner

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eljefe said:
Keep in mind, btw, that V1 is the speed at which the decision to continue or abort must already have been made.
well...it depends on the rules that the airplane was certificated under...

For a Part 25 airplane, the first action to abort must be taken by V1, the engine failure happening earlier at Vef.

For a CAR4 airplane, the engine is assumed to fail at V1.

Kind of a nit-picky difference, but if you have an engine failure on a runway limited takeoff, it can make a big difference...and in my personal experience, 50% of engine failures in jets happen at V1.

Fly safe!

David
 

EagleRJ

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A heavier weight will require a higher V1 to increase the available lift at decision speed. Of course, this will also increase the required runway. The performance charts on most turbine aircraft will work the problem backwards, giving you a maximum takeoff weight for a given runway, taking into account both TORA and TODA.

B- Reverse thrust is not accounted for in computing takeoff distance and runway required, as RTO numbers assume it is unavailable due to an engine failure. Of course, for an emergency other than an engine failure, you're going to use it, and you can even use it carefully on one engine. The effect it has on shortening your distance to come to a stop is just gravy- you can't take credit for it.

C- Snow or rain on the runway (a "contaminated" runway) will reduce V1 due to comprimised braking in the event of a RTO. You're also going to take a weight hit, of course.

D- Tailwinds will decrease V1 in some aircraft, since you're eating up runway faster than normal. There's also a weight hit. Headwinds normally increase V1 slightly, since your groundspeed is slower and you have more effective runway to work with.
 
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