News:

Precision Simulator update 10.173 (24 February 2024) is now available.
Navburo update 13 (23 November 2022) is now available.
NG FMC and More is released.

Main Menu

Time between V1 and Vr

Started by United744, Sat, 3 Apr 2021 14:24

United744

Hi,

I saw some videos recently of 747 max weight takeoffs (FB has been analyzing my screenshots of PSX it would seem and started recommending videos and pages despite never directly referencing PSX or the 747).

I didn't sit with a stopwatch, but it seemed the time between V1 and Vr in the video took an eternity, compared to what we experience in the sim?

Is this anything to do with the fact we use more generic speeds, vs. runway-specific speeds?

Can V1 be quite a bit lower than we see calculated? As I understand it, there can be some difference, but it tends to only be small. The videos however, seem to make it appear significant.

V2 I think is the only value that is pretty fixed, due to aerodynamics and required climb performance, so the only thing I could come up with was some airlines are using lower V1 (in part to reduce probability of rejected takeoff, as it was found stopping can be more dangerous than continuing).

I'm also curious whether this apparent increased delay is actually psychological.

Will

With an infinitely long runway, V1 and Vr should be the same. V2 doesn't depend on runway length. Leaving every other variable the same, as the runway shortens, V1 gets smaller while V2 stays the same.

The runway distance required for takeoff is the longest of three distances: the all-engine go distance, the accelerate-stop distance, or the engine out accelerate-go distance.

The all-engine go distance is 115% of the actual distance required to accelerate, liftoff, and reach a point 35 feet above the runway with all engines operating. The accelerate-stop distance is the distance required to accelerate with all engines operating, have an engine failure (or some other event) at least one second before V1, recognize the event, reconfigure for stopping, and bring the airplane to a stop using maximum wheel braking with the speed brakes extended (but without thrust reversers). The engine out, accelerate-go distance is the distance required to accelerate with all engines operating, have one engine fail at least one second before V1, continue the takeoff, liftoff, and reach a point 35 feet above the runway surface at V2.

So V1 will be farthest from V2 when the accelerate-stop distance is limiting, and when the difference between the accelerate-stop distance and the engine out, accelerate-go distance is maximal. All the V speeds increase with weight, and the distance between them increases proportionally. So you'd expect the maximum spread between V1 and V2 to be found at comparatively shorter runways with heavier aircraft.
Will /Chicago /USA

Jeroen Hoppenbrouwers

#2
Additionally, since it does not make sense to put Vr below V1, and pilots like to do things in the same way all the time, the convention is to never call Vr before V1 (or leave V1 out). So in many cases, Vr sort of pushes V1 down and you get them essentially at the same moment. Only when the weight of the aircraft is so significant that V1 (the last moment to hit the brakes) needs to be called before you can actually fly, you will see V1 and Vr drift apart.

Imagine this: if your runway is infinite, you can rotate, fly, have an engine failure, and land again on the same runway. Cessna-style. Vr is calculated to guarantee you can fly even with one engine out as you have sufficient lift and rudder authority to correct the imbalance. So an engine failure after Vr is "irrelevant" (you can still royally screw it up, of course).

An engine failure between V1 and Vr is the grand prize, because you cannot stop any more, and you cannot fly yet.

Hoppie

andmiz

#3
Rough calculation is that on average we'll have a 20 knot difference between V1 and VR.  For instance, real-world calculation; ISA conditions at MTOW on an ERF, 12000ft TORA/ASDA provides a 25kt difference between V1 and VR.  Often at least a 10kt split to VMCG, but on wet runways (and particularly in the sim) our V1 will equal VMCG.  Do some reading into balanced V1's and unbalanced V1's. 

dhob

An infinite runway doesn't imply V1 equals VR, but that a range of possible V1 speeds are available, from V1=VMCG to V1=VR.

Before the V-speeds are determined, the Takeoff Field Limit Weight must be determined. This weight is derived from either the All Engine Takeoff corrected distance, or the engine inoperative corrected runway distance, whichever is the least.
These corrected distances account for slope, HW/TW, clearway, and/or stopway.

In most cases, the engine inoperative corrected runway distance is the most restrictive. This distance is determined from either a computerized program such as the PET, or in older airplanes like the 747-400, from a Runway Length and V1/VR chart (called a X-Plot). This chart uses the engine inoperative takeoff distance (y-axis), and the corrected accelerate-stop distance (x-axis). Where these two lines intersect equates to the engine inoperative corrected runway distance AND a V1/VR ratio. These charts are Flap and Region dependent, where the Region is a function of Temp and Pressure.

Assuming the engine inoperative corrected distance is the most restrictive, then this distance, along with OAT, PA and Flap setting, is used in another chart in the AFM to determine the Maximum Takeoff Field Limit Weight (MTFLW).

Once the MTFLW is determined, this weight is then applied to another chart to determine VR and V2. Multiplying VR by the V1/VR ratio gives the V1 speed. As implied earlier, V1 must be equal to or greater than than VMCG (V1MIN) and less than or equal to VR (V1MAX).

Note that the actual airplane gross weight is not considered in the determination of the MTFLW. Obviously if the Actual takeoff gross weight exceeds the MTFLW, then takeoff isn't allowed.

If the actual takeoff gross weight equals the MTFLW, then the V1 determined above is the only V1 available for takeoff, and is considered a balanced field V1 speed (usually). However, for many takeoffs, the actual takeoff weight is LESS than the MTFLW, in which case there are a range of V1 speeds available (V1MIN to V1MAX).

This is where operators have a choice, and most choose to derate the thrust as much as possible to save engine maintenance. As the thrust is derated, the Actual Takeoff weight distance extends toward the MTFLW distance. The maximum thrust reduction  is 25% from a rated thrust, such as 25% from TO2.

If light enough, even after maximum thrust reduction, excess runway may still be available. Operators at this point typically choose to use an unbalanced V1 speed. In our performance calculations, the V1 is always unbalanced toward V1MIN, with a cap of 15-20 knots less than the balanced V1 typically.

It is interesting to note that regardless of runway length, if the corrected engine inoperative takeoff distance is equal to the correct accelerate-stop distance (balanced field), the V1/VR ratio is always approximately 0.91 when operating in Temp/Pressure region A. Thus when the actual takeoff weight is at the MTFLW, then V1/VR = 0.91. The FMC generated V1 Speed is always a balanced V1 speed (based on the TOGW in the FMC), as such the FMC V1/VR ratio should be approximately 91 to 94 percent.

As the Temp/Pressure increases (or the thrust is decreased), then the V1/VR ratio increases somewhat. For example, using TO2 (15% reduction), ATM of 55C, the balanced field V1/VR ratio is approximately 0.94. For the 747-8, with more thrust available and a derated thrust of TO2 at 20%, the V1/VR ratio is approximately 97%.

Of course Takeoff Field Limit Weight is only one of several limits that must be determined, such as Obstacle Limit Weight, Climb Limit Weight, VMBE limit etc., any one of which may be more restrictive than Takeoff Field Limit weight.

Wet and contaminated runways complicate the process further, and as mentioned in a previous post, V1 may need to equal to VMCG.