Does calculating V1 really take into consideration the length of the runway?

Question

i mean if V1 is at 140 knots, and the runway is (let's just imagine) about 10 miles long, you could stop even after V1 right? So i don't think that when calculating the V-Speeds, the runway lengths don't play a role, right?

Answer 1

Yes, you are correct.  V1 applies only if you are taking off from a "limiting length" runway.

Here's how you calculate your minimum length runway:

1.  Calculate the distance required to begin the takeoff, lose an engine at V1, and continue the takeoff up to 35 feet AGL.

2.  Calculate the distance required to accelerate to V1, lose an engine, and stop.

3.  Calculate the distance to do a normal takeoff (i.e. no engine failure)  up to an altitude of 35 feet AGL.  Add 15% to this number.

The longest of these numbers is your minimum runway length.  It's only if you're operating from a runway at (or close to) this length where strict observence to V1 is critical.

There are some complicating factors.  For example, in distance #1 the actual runway only needs to extend to the point where the aircraft is able to lift off the ground.   You can use something called a "clearway" to climb to 35 feet.  (Think of a clearway as an obstruction free area that extends past the runway).

Distance #2 allows the aircraft to use a "stopway" to bring the airplane to a complete stop.  A stopway is an extension of the runway that isn't strong enough to withstand daily runway operations but can support the weight of an aircraft if required.

In practice we don't actually calculate required runway lengths and we sometimes get in trouble for it.  We have a company that evaluates every runway we use and they calculate maximum takeoff and landing weights for each one.  Factors such as headwind/tailwind, temperature, etc are taken into account.  As long as the airplane is below the maximum weight we are legal (but not necessarily safe) to operate from it.  The reason I say we aren't necessarily safe is that no determination is made for a runway condition worse than "wet".  If there's ice on the runway and braking action is poor we just assume it's "wet".  Ask the Northwest crew who rolled off the end of 7R in Milwaukee how well this works.

Answer 2

On a "short" runway, V1 is less than Vr, so runway length IS a factor in calculating V1.

However, as you have realized, on a very long runway you could theoretically stop from a speed higher than Vr (rotate speed). However, on a perf A aircraft, aborting the take off after rotate is not considered an option, so V1 can never be greater than Vr. In this case V1 is set equal to Vr, but it is not a "real" V1 in the runway performance limiting sense.

Answer 3

You are essentially right, runway length does not determine V1, it's the otherway around, although runway conditions and slope do play a part. The distance required to reach V1 and the momentum you are carrying at that point determines two important runway and "clearway" factors however. As I'm sure you know, V1 is take off safety speed, meaning that if an engine quits before you reach that number you probable cannot accelerate further to attain a safe climb speed, so you must abort. What determines required runway length are called the Accelerate-Stop and Accelerate-Go distances. The first is the distance required if you abort the takeoff just prior to reaching V1, and it is dependent upon weight, density altitude, wind and runway conditions (slope, wet pavement etc). The second is the distance required to reach 50 feet if you reach V1, lose an engine, and continue the takeoff. This second one may include "clearway" space required beyond the end of the runway.

Edit: For an extra safety margin, due to some recent accidents, under FAR Part 135 and 121 "commercial carrier" regulations, 15% must now be added to the calculated accelerate-stop and accelerate-go distances.

Answer 4

No, they are about safe stopping.  After calculating V1 you calculate required runway length.  There aren't any 10 mile runways... so though in principle you are correct, in practice it does not come up.

Answer 5

Yes, you are right, and there are some pretty good explanations above.  One major consideration to think about though is V2 (in addition to Vr)

V2 MUST be higher than V1, so after takeoff, the aircraft is pitched to V2 for the climbout.  If V1 is too high, V2 will result in an airspeed that does not allow a climb.

For this reason, there is actually a published number on all our takeoff data sheets called a "Max V1"

Max V1 is exactly that.  the highest number V1 can be.  There are a variety of things that can determine Max V1, like weight, runway length, density altitude, rising terrain, runway slope, etc.  One of those is the ability to outclimb terrain at a certain diverging angle. (V2 must allow the airplane to outclimb rising terrain by a certain angle)    If V2 is set too high, the aircraft will not outclimb terrain at all.  And it is possible to end up with a V2 that would not allow the aircraft to climb period.  ALSO, V2 cannot be more than 20% above Vso.  Meaning that since V1 cannot be greater than V2, it is also limited to less than 20% above Vso

V1 is then limited by V2.