Same acf, different Vspeeds
 

Hi all,

Happen to have 2 different Vspeed tables for the 757 from
different sources. Both from 757 operators.
What I don't understand is that they have different speeds.
The top one is from 1996, the bottom one from 2001.

Below is a sample cut.
As you can see, V2 is about the same, but V1 and Vr
are larger in the lower newer table.

Any ideas? Temp, alt etc is the same.

http://www.xplanefreeware.net/morten/DOCS/Vspeeds.jpg

Cheers,

XPM

Different Engine Thrust?

Wet perfomance v1 and vr are lower than dry perhaps that's the reason.

As you said, it is only the V1 and VR that are different, V2 is unchanged.

V1 cannot be lower than Vmcg for obvious reasons, so often, especially at low speeds as shown in your example, V1 is limited by the Vmcg speed. If the aircraft has a higher thrust setting, Vmcg will increase and therefore V1 must also be increased.

This explanation would explain the changes to V1 at the lower weights and speeds, but not the higher end. Therefore maybe there is another difference between the two aircraft such as the brakes used by each operator. A better brake system would change the Accelerate Stop Distance Required, and therefore alter V1.

VR is chosen as a suitable speed that is equal to or greater than V1 but less than V2, therefore if V1 was increased, VR would also increase.

Finally V2 is defined by aerodynamic considerations, therefore would not have changed with the higher thrust setting.

I don't believe the data indicates a higher thrust for the second aircraft.

Firstly, Vr is not just chosen as any speed below V2 and above V1. Vr and V2 are a matched pair - rotation at Vr should see you at V2 at 35ft (adjust per rules and runway conditions). You can't move Vr relative V2 without good reason.

Now, the Vr-v2 "spread" is much smaller in the second set of data - almost zero at the MTOW case. Which leads me to suspect that the thrust in the second case is significantly lower.

The V1 being driven by V1mcg seems highly unlikely - I'd expect to see a near-constant V1 at the minV1=v1mcg for a range of weights. The fact that V1 varies considerably with weight implies that there isn't a v1mcg effect here. I think the braking system being different and rebalancing between ASDA and TODA is far more likely.

Balanced versus un-balanced runway performance, possibly.
Also, overspeed takeoffs (improved climb) are sometimes used (runway permitting) in the obstacle case, although, this also affects V2, in most cases.