There's no actual formula for this, at least not an easy one a pilot cold generally use.

The jet takeoff is composed of these segments:

Accelerate to V1 then engine failure
Accelerate to Vr and rotate
Climb to 35 ft while accelerating to V2

It could be the case that the distance from V1 to 35 ft are half the runway. As the performance of the plane drops from high, hot, heavy, the part from Vr to 35 ft gets longer and longer. At some super extreme point, the plane can't get to 35 ft at all, too little performance with one engine to do it. This is usually never an issue in practical terms with jets. Conversely, if the plane is low, light, and cold, the distance from V1 to 35 ft could be very short.


This rule is seriously flawed.

It assumes that you have constant acceleration throughout the takeoff run. Thus to reach takeoff speed at the end of the runway, you have to be at 70% by half of it. This is basic Newton 2nd law, F = ma. Here's the speed profile versus distance:

But the acceleration is not constant. The drag is increasing as you build speed, and the prop is losing thrust as you build speed. So if you have exactly 70% by 50%, you are already behind and won't make it.

Oddly, the rule works best for fixed pitch props and not as well for constant speed props. A fixed pitch prop is gaining power as you go, engine RPM increasing, which means more thrust as you get faster. The constant speed prop is losing thrust as you go since engine power is constant.

Lastly, the law only tries to get to takeoff speed at the VERY END of the runway, no margin left.

I would suggest it would be better if you had 80% by 50% to give yourself some margin, particularly in a constant speed prop airplane.

Mike C.

501SP doesn't need SPE, so no issue there.

As far as I know, there is no SPE equivalent in Europe for the part 25 Citations.


I am told there are. Here's a map of the factory service centers, plus there are two MSU (mobile service units, trucks).

Thee are undoubtedly a lot of independent shops which can perform service on a 501SP throughout Europe.

Mike C.

I found this chart that could be interpreted to mean that thrust for this engine drops from 3,050 lbs to 2,700 lbs from brake release to rotation. So, thrust is dropping (and of course drag is increasing) during the takeoff roll.

But to get down to my basic question, do you monitor acceleration at all with respect to runway remaining or are you only checking N1 and looking for airspeed to hit VR?

I found this chart that could be interpreted to mean that thrust for this engine drops from 3,050 lbs to 2,700 lbs from brake release to rotation. So, thrust is dropping (and of course drag is increasing) during the takeoff roll.

But to get down to my basic question, do you monitor acceleration at all with respect to runway remaining or are you only checking N1 and looking for airspeed to hit VR?

Like these guys? https://en.m.wikipedia.org/wiki/Emirates_Flight_407

I scale the units to make them more manageable.

PPW per year
MU2: 0.652 lbs
C560V: 6.667 lbs (~10x MU2)

PPW per 1000 hours
MU2: 4.4 lbs
C560V: 20.4 lbs (~4.6x MU2)

PPW per 100,000 nm
MU2: 1.6 lbs
C560V: 5.5 lbs (~3.3x MU2)

Roughly speaking, the MU2 produces a new sheet of paperwork every 620 nm and the C560V produces a new sheet of paperwork every 180 nm. This assumes standard office paper, letter size.

Mike C.

Like these guys? https://en.m.wikipedia.org/wiki/Emirates_Flight_407

Assuming rate of paperwork accumulation per year remains the same one average as before, and this applies to empty weight and not gross:

I will be able to fly my MU2 in 10,500 years.

I will be able to fly my C560V in 1,410 years.

Good thing I bought the Citation, I will get to fly it much sooner.

Mike C.

The 501 AFM lists a certain minimum takeoff runway length for a certain set of conditions. There is a unique V1 given such that you can accelerate to V1, abort at V1 and stop by the end of the given field length. It seems to me that in the process of determining V1 the aircraft manufacturer must have also determined the runway remaining at V1 because they need that number to make sure the brakes can stop the airplane within that distance. All I'm asking is what is that number because if I'm not at V1 by that amount of runway remaining, something is not right with the calculations or the airplane performance.

The 501 AFM lists a certain minimum takeoff runway length for a certain set of conditions. There is a unique V1 given such that you can accelerate to V1, abort at V1 and stop by the end of the given field length. It seems to me that in the process of determining V1 the aircraft manufacturer must have also determined the runway remaining at V1 because they need that number to make sure the brakes can stop the airplane within that distance. All I'm asking is what is that number because if I'm not at V1 by that amount of runway remaining, something is not right with the calculations or the airplane performance.

Bad data can create bad results. Balanced field length is the published distance. There are not published distances for V1 and Vr. It's important for the airplane to pass preflight and have takeoff calcs properly figured.

There have been a lot of beer cans made from airplanes that used bad data.

No doubt! Garbage in... garbage out.

If you can’t do the data right you will end up like this.

Andrew

You don’t know and it doesn’t matter does it?

It’s the greater of start-stop or V1 to 35ft. What’s it matter? If you calculate on the worst case which one it is?

Andrew