Questions about V1?

Question

I have several questions about V1. I know what it is but I dont understand this-

Picture a fully laden jet. Taking off from a 7,000ft runway. It uses 5,000ft to get to 5 knots below V1. The the pilot decides to abort. How can he stop in 2,000ft of runway? And V1 doesnt vary much from runway to runway. maybe 5 knots tops. What if the runway was 6000ft??

Answer 1

The only relationship to runway length regarding V1 is whether or not the runway is long enough to stop from V1 or continue the take off and be at thirty five feet by the end of the runway. On a balanced field, the accelerate stop and the accelerate go distance is equal, or balanced. So if it takes 5000 feet to reach V1 and you can either stop in the next two thousand feet or continue to thirty five feet, you are legal. If your performance charts say you need more than that, you are dumb to attempt a takeoff. Either you can stop in the remaining runway length or you can't. So if the runway is shorter than the BFL or balanced field length, you find another runway or offload something.

We practice rejected takeoffs at V1 every six months in the simulator with a runway length exactly equal to the computed BFL. It becomes very obvious that if you are at V1 and an engine quits, continuing the takeoff is the best idea even though test pilots have shown that you have enough runway to stop. But I ain't a test pilot am I?

Answer 2

V1 actually doesn't vary at all according to runway length. It varies principally according to aircraft weight and possibly elevation and flap setting. The runway length requirement is a separate calculation or a separate chart. In other words, you can calculate V1 without knowing available runway. But, bottom line, the drill is below V1 you discontinue the takeoff. What I find more mind-bending than your hypothetical is takeoff on a 15,000 foot runway or on a hypothetically infinite runway, with respect to V1. On a theoretically infinite runway, or on the 15,000 foot runway for most jets, you WOULD have enough runway to stop even above V1. Yet the drill is to continue the t/o. In reality, runway length is essentially independent of (the pilot's calculation of )V1 despite the definition of V1, with runway length requirement a different (but obviously important) consideration. Your hypo is not really a V1 question---it's a runway length requirment question. Suggest you flip through some charts for the jet of your choice and see the dependence/independence of these calculations and the factors affecting them. You'll see that, okay, youve got the V-speeds posted, now what's it say for required minimum runway? It's not, okay, we've got this much runway, what's V1 for this runway length.

Answer 3

Runway length is NOT used to determine V1. It's the other way around. On the plane I fly before takeoff I must consult charts that show my V1,V2 and Vr. These charts are based on field elevation, takeoff configuration (flap setting), gross takeoff weight, and air temprature. I find the chart that shows these numbers for my current situation and it tells me what my Vspeeds are. It also tells me the REQUIRED runway length at these speeds for me to takeoff leagally. If I don't have enough runway available I must change something and recalculate. (offload fuel or passengers/cargo, wait till the temp cools down, try a higher flap setting. I can't do anything about the field elevation). So the relation is a given V1 (under specified conditions) yeilds a given runway length requirement. (This is a minimum length) In your example my guess is when you consulted the charts you would see you needed more than 7000 feet and you wouldn't be able to takeoff leagally (or safely).

Answer 4

My understanding was V1 did vary with runway distance. V1 is the decision speed below you can safely stop, above runway is long enough to take off if you lose an engine. I been wrong before though.

Answer 5

There is a concept with certain aircraft called "Balanced field length". It deals with just exactly the scenario you have stated.
Basically what is says is you must be able to accelerate to a certain speed, and either stop on the remaining runway, or continue the take off if some malfrugalty occurs.
If you cannot do either in the type, you cannot legally use that runway.

Answer 6

John is dead right. All I can add is to explain Accelerat-stop and Accelerate-go distance a little deeper.

Accelerate-stop= this is the distance the aircraft will take to stop at a RTO at V1. ie if your V1 is 145 knots, you can stop completly in the Accelerate stop distance.

Accelerate-go= this is the distance from 0 to 35 ft above the ground, assuming engine failure after V1.

Both these distaces must be within the airplanes preformance limits. If not, you are breaking FAR's.

Answer 7

cj1pluspilot answer is very accurate. It has to do with weight, outside air temperature, airplane configurations.

As with infinite runway length, all can I say is, the V1 will be the same as Vr, which can be infinite as well, but then the take-off run will be limited by tire-speed limit, which is around 190mph (?).

Answer 8

John B is the man... you cant explain it anybetter than that. It looks Charles M needs to hit up the books. You cant get anymore in depth than that