increased V2

[EPLS]

Hi
I can see the advantage of using an increased V2 but what are the penalties involved. (CAP 385 and flying the big jets are not helping me)

Answers on a postcard please.
Thanks

[Crossunder]

When using an increased V2, the take-off speeds are also increased (acceleration to a higher speed is done on the runway). This means that your V1 will be sky high, thus requiring a very long runway.

[411A]

Higher speed (and longer runway), yes. But "sky-high"...hardly,
L10 & B707 were in the vicinity of 3-4 knots.
Higher V2 increases rate of climb but brings you closer/sooner to obstacles.

[sky9]

EPLS
All you are doing by using an increased V1 is increasing a take off performance that is limited by climb gradient rather than ASD (hope I am correct) and increasing the V1 to a point that the ASD is limiting, thereby being able to increase the climbout speed and therefore gradient.
The disadvantage (or as you put it the penalty) of using increased V1 is that on a limiting takeoff an engine failure just before V1 would mean that the aircraft should come to a halt on the end of the runway.
(At this point a performance expert is going to write "not strictly correct and give you the legal explanation).

[Dan Winterland]

Close in obstacle clearance is worse - laregly due to the later liftoff point. It's the distant obstacle clearance only which is improved.

[john_tullamarine]

MinV2 takeoff is limited either by Vmca or stall considerations and, typically, is considerably less than the speed for maximum climb performance in the relevant takeoff configuration. This permits some flexibility with juggling increased V2 against available "spare" runway to obtain a climb benefit.

Ignoring runway balancing considerations .. while noting that unbalancing often will give higher limiting weights, the main considerations with "overspeed V2 takeoff" (Boeing-speak) and "improved performance takeoff" (Airbus-speak) are -

(a) if the sole aim is to achieve a better second segment WAT-limited TOW (ie plenty of runway and nothing to note in respect of obstacle problems) then the prime concern is to push the V2 up from minV2 to gain an improvement in second segment climb gradient. While the speed increase to achieve the best gradient can be quite significant, it is usual to limit the speed increase to a modest amount where the gradient return (benefit) compared to the speed increase (distance penalty) is highest .. due to the shape of the climb gradient by speed curve, one soon gets to a situation of diminishing returns and the distance penalties become a little silly.

V1 can be selected from anywhere within the permitted range depending on company policy, available TORA/ASDA/TODA, etc....

(b) if the aim is to get a better obstacle-limited weight where the minV2 takeoff is limited by a mid-distance obstacle from a long runway, then one would expect the V2 to be pushed up to the optimum distance vs gradient for the obstacle while keeping within runway distance limits.

As the aim, in this case, is to get to a higher useful V2 in the shortest distance, it would be normal to schedule the maximum practical V1 to minimise the speed difference between V1 and Vr (ie minimise the OEI TODR).. obviously, acceleration (and distance) on all engines is better than OEI.

As to the range of useful overspeed V2, this will vary between aircraft and specific runways.... for those aircraft with which I have had an ops eng involvement, the typical maximum overspeed is going to be less than 15-20 kts.

It should be noted that a similar benefit is obtained by scheduling very low flap takeoffs where there is plenty of runway.

One of the problems which can occur if an operator vigorously pursues overspeed schedules to maximise TOW is that the training regimes may routinely follow the typical line situation. If the operator's rules permit minV2 takeoffs in appropriate circumstances, then there is a significant "gotcha" if a crew, trained principally or totally on overspeed takeoffs, makes a minimum weight minV2 takeoff and suffers an engine failure ... there is a very real risk of a Vmca departure if the bank is not well controlled due to the pilot's not having had adequate sim practice in this quite different handling regime.