737 V1 adjustment

[steelbird]

I've been wondering about this for a while but have not been able to figure out why the V1 adjustment increases as weight decreases on the on 737 slippery/slush/standing water V1 adjustment advisory info table.Could it be the acceleration rate to Vr after engine failure? http://www.pprune.org/ubb/NonCGI/confused.gif
Thanks.

[mutt]

It has to do with the drag of the slush during the accelerate stop. The deeper slush builds up infront of the wheel and assists you in stopping.

Mutt

[steelbird]

Yeah that's true with increasing depth of slush/water/...,but you'll find that with the same contaminant depth the V1 reduction increases with lower weights.
Why is there a bigger V1 reduction for a lower weight for the same contaminant depth?

[FurryDice]

I figure it is due to the fact that the person who is working out the performance wants you to GO, not STOP - i.e. the manufacturer has determined that passed V1 the aircraft WILL be able to accelerate to Rotate and Take off, whereas the stop case is less certain. This is easier at lighter weights than when heavier, hence a bigger split when you are light.

Errm, I think anyhow! :)

[mutt]

I supposed that I should RTFQ before hitting the reply button…..

The Contaminated Runway V1 reductions are greater for a lower weight/lower V1, because it is presumed that your weight is runway limited. Therefore the runway is shorter than if you were to have a higher weight/higher V1. The V1 adjustments are calculated for that shorter runway.


Mutt ;)

[steelbird]

Thanks mutt that makes sense,but if you have a look at the FPPM/QRH you'll find that the field/obstacle limit weight is taken care of in the first table (Weight adjustment) before one goes into the V1 adjustment table.Therefore it's possible even at the higher weights to be taking off r/w limited.
Surely the adjustment at higher weights as well as lower,caters for this.
I think there might be more to this,what do you think? :)

[skynet737]

noticed the same thing the other day...
the way i figure it is that its for the same reason that ure v1 is lower at a lower weight...

which is because....

[mutt]

Steelbird

The contaminated runway corrections for all weights are based on runway limit weights. Looking at the long winded method in an AFM, you have to enter with the runway length to find the applicable corrections.

There was a very interesting procedure in an AFM dated 1973, it doesn’t appear in the modern AFM, it states:

At a speed of VR-10 Kts, lift the nose wheel off the runway, accelerate to VR in this configuration to reduce drag. Does anybody actually do that??

Mutt :)

[steelbird]

Mutt
Okay,we agree that the table refers to T/O at r/w limited weights.That much is clear.
Therefore shouldn't the higher weight t/o have a greater V1 adjustment(due to inertia,..)compared to a lower weight t/o?
The fact that the lower weight t/o is assumed to be fm a shorter r/w shouldn't be a factor as they are both r/w limited.(Re:your earlier post)

P.S Are you saying they encouraged rotating before Vr?(Vr-10 probably would be below V1 on balanced field t/o.) ;)

[mutt]

Steelbird

Sorry for the last response, the reason why the V1 adjustments are larger with the lower V1 speeds has to do with runway friction.

It is assumed that the wet braking Mu is half of the dry runway Mu. At higher weights the dry Mu starts to decrease and so the margin between the dry Mu and wet Mu gets smaller. (The wet Mu is a constant.) therefore so do the corrections!

As for the statement in the AFM dated 1-5-73.

Takeoff from precipitation covered runways

At rotation speed minus 10 kts (VR-10Kts) lift the nose wheel out of the precipitation by rotation to a three to five degree nose up attitude. Maintain this attitude until reaching the computer VR, then rotate to normal takeoff attitude.

Nose wheel and impingement drag will be greatly reduced by use of this procedure.


Mutt ;)