V1 vs BFL

- James Albright (a former G450 driver)

Updated: 2017-11-18

If you come from big jets, say a Boeing 707 or larger, being able to adjust your V1 is a foreign concept. If you come from a smaller jet, say a Citation, having to worry about V1 may be a foreign concept. In the G450 we have the right combination of power to weight to allow us this luxury.

The G450 will default to V1MIN on a less than dry runway but there are times you may want to override its decision making:

• V1BFL — Your default mode, your abort distance is equal to your takeoff distance following an engine failure at V1.

• V1MIN — Wet (or worse) runways, you leave yourself with the most runway to abort. You are going to have a low V1, you are more likely to fly because you are afraid to stop.

• V1MAX — Obstacles, gets you to climbing sooner to beat an obstacle. You are going to have a high V1, you are less likely to fly because you are afraid of an obstacle.

Everything here is from the references shown below, with a few comments in an alternate color.

 V1 vs BFL

Overview

The best explanation of the relationship of accelerate-go, accelerate-stop relationship is in, of all places, the Honeywell portion of the operating manual. They do a good job of showing how one trades off from the other and meet at where the field becomes balanced. They did, however, get a part of the explanation wrong. . .

[G450 Aircraft Operating Manual §2B-26-50 §3.C]

Accelerate-stop distance increases with an increase in V1 speed. As V1 is increased, additional runway is needed to stop the aircraft due to the increased aircraft speed.

The effects V1 speed has on distance for accelerate-go are not as obvious. If the V1 speed is low, more runway will be required compared to using a higher V1 speed. This is due to the requirements for accelerate-go distance. Accelerate-go distance is defined as the distance required for the aircraft to accelerate to VR and takeoff, if an engine failure occurs at the V1 speed. With a large difference between V1 and VR (small V1), more time is required for the remaining engine to accelerate the aircraft to V1 and then continue the takeoff. This additional time results in additional runway being used for takeoff. This is why the accelerate-go distance increases with the reduction of V1.

The default value is V1 balanced field length (BFL). If V1BFL is not available, the FMS selects either V1MAX or V1MIN as the default. The FMS selects the V1 that gives the closest solution to a BFL takeoff.

All of that is correct except where accelerate-go is defined, which is a bit muddled. A better explanation is in the AFM:

[G450 AFM, §05-01-20, para;2] ACCELERATE-GO DISTANCE - the required distance to accelerate with both engines operating to the critical engine failure speed, sustain an engine failure, and continue the takeoff to a height of 35 feet AAL on a dry runway or 15 feet AAL on a wet runway.

Is this important? Perhaps. While accelerate-stop is defined in 14 CFR 25, ¶25-109, the feds leave accelerate-go up to the eye of the beholder. What is important is to realize that adjusting V1 up or down will have an inverse impact of one on the other.

Effective Runway Length

[G450 Airplane Flight Manual §5.2] The takeoff field length charts in the AFM are based on the concept that the shortest takeoff field length required will exist when the accelerate-go and accelerate-stop distances required are balanced (i.e., when the accel-go distance is exactly equal to the accel-stop distance). A balanced field length condition cannot exist for every set of takeoff conditions or configuration, however. For example, at high gross weight, high altitude and/or high temperature conditions the minimum accel-go distance may exceed the accel-stop distance required even at V1/VR = 1.0 and a balanced condition is not possible. Hence, the shortest takeoff field length required is referred to as an "effective runway length required" rather than a balanced field length.

Adjusting V1

[G-450 Airplane Flight Manual §5.2, pg. 5.2-1] The takeoff field length charts seen in this section are based on the concept that the shortest takeoff field length required will exist when the accelerate-go and accelerate-stop distances required are balanced (i.e., when the accel-go distance is exactly equal to the accel-stop distance). A balanced field length condition cannot exist for every set of takeoff conditions or configuration, however. For example, at high gross weight, high altitude and/or high temperature conditions the minimum accel-go distance may exceed the accel-stop distance required even at V1/VR = 1.0 and a balanced condition is not possible.

This can be confusing because the factors cited — gross weight, high altitude and/or high temperature conditions — do not impact reference accelerate-stop distance available or reference accelerate-go distance available. But the distance is likely to increase due to runway condition and slope, winds, ground spoilers, and anti-icing.

[G-450 Airplane Flight Manual §5.2, pg. 5.2-5] Lower values of V1/VR can be used to reduce wear on the brakes if a stop is necessary. For obstacle clearance, higher values of V1 may be used to reduce the reference accelerate-go distance which determines reference zero. Moving the reference zero point closer to brake release results in a greater horizontal distance to clear the obstacle.

In most cases, the V1/VR pretty much sorts itself out and what you see in the performance computer is what you want to do. There are, however, cases where you would want to adjust V1 towards V1MIN or V1MAX:

• V1MIN gives you the best possible scenario for aborting a takeoff, which would come in handy on a slippery surface. If you lose an engine at V1MIN, the aircraft will spend a longer period accelerating to continue the takeoff.

• V1MAX gives you the best possible scenario for getting off the ground with enough speed to beat an obstacle. If you lose an engine just prior to V1MAX, you will have a challenging stop.

• More about this: Departure Obstacle Avoidance.

 Example

Let's say you are trying to leave a wet runway at fairly high gross weight with no obstacle considerations:

• Environment: 10°C, Sea Level, no wind

• Runway: 6,000' length, no slope

• Aircraft: 65,000 lbs, 20° flaps, no anti-icing required, ground spoilers operative

Your effective runway length required will be 4,500'.

 Accelerate Stop

There is no impact to accelerate-stop distance available, it remains equal to the runway length of 6,000'.

 Accelerate - GO

There is no impact to accelerate-go distance available, it remains equal to the runway length of 6,000'.

 V1/Vr Adjustment

• The first step is to draw a line from the reference accelerate-stop distance up to intersect with the reference accelerate-go distance. (In blue in the figure.) You must remain within these lines.

• The second step is to draw the effective runway length that remains within the earlier constraint. (In red in the figure.) Your choice of V1/VR will be along this line.

• Stopping is a priority. (Black line on the figure.) You can select a V1/VR ratio of 0.80 to use V1MIN and that will result in a stopping distance of 4,300' if you abort, but it will require 4,600' to continue the takeoff.

• Obstacle clearance is a priority. (Orange line in the figure.) You can select the highest available V1/VR ratio of 0.96 to use V1MAX and that will get you off the ground in only 3,500' but that means you will need all 6,000' of runway to abort.

 Performance Computer Selection

The performance computer can make these calculations for you.

Select page 2 of Takeoff Data in the MCDU:

Select the V1 prompt (LSK 1L)

From here you can select V1MAX, V1MIN, V1BFL, or manual enter any number between the MIN and MAX numbers.

 References:

14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013