V1 = Vr, Balanced field???

[Old Smokey]

Alpine Pilot,

I’m not familiar with the Dornier 328, so a lot of what I say here will be generic, and possibly not specific to your aircraft.

It concerns me that “some bright person” reduced the V1 for your aircraft without verification from the AFM or the manufacturer. If we look at the Accelerate-Stop manoeuvre in isolation, we can reduce it to anything we like, 20 knots if you like, and the Accelerate-Stop performance will be impressive. Unfortunately, we cannot look at the Accelerate-Stop manoeuvre in isolation, as V1 is both a “Stop” and a “Go” speed. V1, Vr, and V2 are variable speeds, they may be adjusted by the performance engineer as necessary to achieve the desired performance, but there are both Lower and Upper limits to this variation. Whilst reducing V1 will have obvious benefits in improving Accelerate-Stop performance, for the “Go” case from V1 there are two major considerations –

(1) V1 must be equal to or greater than VMCg, to ensure directional control for the continued Takeoff, and

(2) The aircraft must be able, with one engine inoperative, to accelerate from V1 to Vr, become airborne, and achieve V2 by the screen height.

Arbitrarily reducing V1 by 6 Kt may well place the aircraft below VMCg, and directional control for the continued Takeoff may not be assured. John_Tullamarine can provide some pretty graphic illustrations of such a circumstance. Acceleration to an additional 6Kt of speed with 1 engine inoperative may consume a prodigious amount of runway, not considered in your certified Takeoff performance data.

On very short runways, Accelerate-Stop performance will obviously be limiting, and V1 must be as low as possible (but not below the minimums specified by the AFM). This will require a weight reduction, which will provide a “Go” performance better than required, and acceleration from V1 to Vr to V2 presents no problem. As the runway becomes longer, V1 and the weight may be increased, Accelerate-Stop performance is retained because of the longer runway, but, at the higher weights, continued acceleration from V1 to Vr for the “Go” case becomes more limiting, and the V1/Vr delta must be reduced to allow for this acceleration within the available Takeoff distance. If we continue with lengthening the runway, V1 may continue to increase until it reaches, and passes Vr. As we cannot have a V1 greater than Vr, the upper limit of V1 then becomes Vr, i.e. V1=Vr.

During all of the processes described here, Balanced Field performance may be retained, i.e. ASD=TOD, by manipulation of the speeds, V1, Vr, and V2. Now, if you throw in a Stopway, or a Clearway, we have Unbalanced field performance. Accelerate-Stop and Accelerate-Go performance must now be considered in isolation, and the operation performed at the lesser of the 2 limiting weights.

At the end of the day, it gets down to the individual performance characteristics for each aircraft. One earlier aircraft that I did the numbers for had a superb stopping performance, and a mediocre continued Takeoff performance. V1 was always equal to Vr. The last aircraft that I number crunched had a mediocre stopping performance, and a superb Go performance. There was always a significant V1/Vr split for this aircraft.

Regards,

Old Smokey