Originally Posted by
NFFuture
Hey Folks!
What would be the benefit of a lower V1 on a wet runway if the take off is not field length limited at all? I would be talking about a lightly loaded E-Jet on a 4000m runway on a standard day. The runway has no significant CWY or SWA.
The performance tool always gives quite a low V1 and thus a big V1/Vr-split (usually around 10kts) as soon as a wet runway is selected (same Vr with almost V1=Vr on dry runway). Sadly there is no option to select/force a higher V1. Sadly the tool doesn’t display the stop margin at all, which makes it tougher to comprehend its calculations.
The Ops Performance Tool may provide for the functionality of having the reduced V1 by a margin, no more than 10kts, to improve the STOP case. The additional margin that is gained is the time/distance of the continued acceleration from the reduced V1 to the normal V1, and the deceleration time/distance difference for the stop case. Roughly, most aircraft are accelerating at around 3kts/sec +/- towards the higher end of the TO roll, dependent on thrust/weight/temp etc, and about a bit over double that for the deceleration at the higher speeds, assuming that the anchors have been applied in an eye pleasing manner. So for the difference in buffer, take about ~4.5 seconds at the speed of V1-5kts... roughly divide your IAS by 2 for meters a sec... 150kts = 150/2=75 meters/sec approx. 4.5 x speed... 337m... close to 1,000' all up.
However, the reason that the reduced V1 is being applied is due to the historical stop case being compromised by wet conditions... so the effective "buffer" doesn't really apply, you are achieving an increase in confidence of the normal margins.
The GO case is results in a reduction of the required screen height at the end of the TODR from 35' to 15', which may have knock on issues where there is a second d segment issue as well.
Note:
1. In general, for a normal takeoff without a failure, the aircraft should exhibit a height at the end of the TODR well above screen height... If you are seeing anything close to the "35' screen height" at the end of the TO roll, without a failure, then it is a good thing you didn't have an engine failure... The maths is simple to determine, just consider the acceleration difference from V1 (- 1 second... ) through to Vr, between the normal case and the OEI case... The rotate time to achieve liftoff is slightly longer OEI, in the real world, but is assumed to be about the same in most performance analysis, the liftoff occurs at a higher body attitude, at the lower speed. The exercise of doing the maths may lose belief in the tooth fairy, but may also lead to more discipline in runway use optimisation, thrust setting techniques etc... For 2 engine aircraft, you should not be very close to the fence at all unless you have had a bad day for cause... For 3/4 engines, it is still a big margin that should be seen for normal ops at the end of the TODA... if you aren't at around 180-200' AGL at the end of the TODA for mid to heavy weights, then you might want to get out a calculator and have a look at the geometry.
2. Vanilla planes will generally achieve the V2 speed just before the wheels coming off the ground, in a normal takeoff rotate rate... which meets the V2 + margin at 35' (15' wet). For the OEI case, the wheels will come off the ground a little earlier, and minimum of V2 is achieved by 35'. The margin of control above Vmca is provided by the definition of Vr IAW FAR25/CS25. An abused Vr may get sporty for a low weight take off, but will only compromise tail clearance (Vmu) for mid to heavy weight cases.