Wet V1
I've recently received a PM relating to Wet V1, which I have taken the liberty of "cutting and pasting" below. The anonymity of the originator is preserved, as good manners would require of a Private Message. As I believe that the question posed is worthy of open discussion, I have added my reply, and open the thread to greater minds than mine -
The Message :
I know and understand the principal behind why V1 is reduced, the stopping distance is increase because breaking efficiency is reduced on contaminated rwy. What I have fail to understand is the screen height reduction. Surely wet or dry VR is at the same point and your rotation is still 10 to 12 degrees NU, therefore the thrust is the same and your angle of climb is the same. So why is there a 15ft and 35ft screen height. I see the screen height more as a marker to indicate the end of the take off and the start of the climb. I know JAR states, it is the min height achieved over the rwy before the end of the clearway should an engine fail on takeoff. But why a reduced V1 should change the height.
My reply :
Your understanding of reduced V1 for a wet runway is essentially correct, so no further comment is necessary in this respect. The answer lies in the Accelerate-Go considerations.
As you've alluded to, Vr and V2 will be essentially the same, with a much larger difference between V1 and Vr than for Dry Runway operations. In the Accelerate-Go case, which is predicated upon engine failure at Vef (below V1), the aircraft must be accelerated from Vef to Vr with one engine inoperative.
Taking some sample figures of V1=150 : Vr=155 : V2=160 for DRY runway operations, the aircraft only needs to accelerate 5 knots with OEI. If, on the other hand Wet runway data is used, more typical speeds are V1=140 : Vr=155 : V2=160. The aircraft now has to accelerate 15 knots to Vr with OEI. For a 2 engined aircraft, OEI acceleration will typically be less than 10% of normal, so the distance from Vef to Vr is much longer. (Newtonian laws apply : V^2=U^2 + 2AS), but to simplify linearly, the distance from Vef to Vr will be 3 times that for the dry runway case. Thus, Vr will occur MUCH further down the runway than for the normal dry case.
At and beyond Vr, OEI performance (as you've indicated) will be the same.
So, with all other things being equal, if "Wet" V1s are used, rotation, followed by essentially the same "Vr onwards" performance will be much further down the runway, and screen height much lower because rotation was initiated much later. There are 2 solutions -
(1) Leave the 35 ft screen height in place for all operations, and suffer a huge loss in payload (and I mean HUGE), or
(2) Legislate allowing for a lower screen height (15 ft) for Wet Runway operations.
The legislative process has opted for the 2nd choice, i.e. the 15 ft 'line in the sand' Vs the 35 ft 'line in the sand'.
By accident, you've posted to someone who has suffered a legislative authority which accepted Reverse Thrust credit for the Wet Accelerate-Stop, but insisted upon retaining the 35 ft for the GO case (Option 1). Not a difficult Performance Engineering task (just put an artificial 20 ft obstacle at the Runway End, and use Wet data), but the performance penaltys were, as stated, HUGE.
One of the toughest jobs that we ask of the remaining engine/s following failure at Vef is the acceleration from Vef to Vr. If the V1 is reduced too far, it becomes a very big 'ask' of the remaining engine/s.
Awaiting better responses, which is why I've thrown this to the open forum. The question was too good to remain private!
Regards,
Old Smokey