tick tock tipms topms

[3WE]

Note that V1 is not useful as a go-nogo decision in multi-engine jets either for the case of reduced performance.

Huh?

[Gabriel]

I concur that Dan’s rule is BS.

Light planes need very different rules DEPENDING on the situation.

For the typical long runway- if your acceleration is off and you miss your liftoff point YOU STOP. The one engine really needs to be working right.

Now…for truly short fields where accelerate stop is an issue…you need a speed vs distance check THAT ALLOWS YOU TO STOP.

There’s also a top secret method of a nice check before you are too committed, that was alluded to by a respected airline pilot.

Of course, Boeing Bobby’s well-trained a$$ and sixth sense is also pretty darn good.

Footnote: It’s possible that Dan’s rule provides decent protection, but it’s based on nothing concrete. Perhaps the general disdain that one of our friends has for him is well founded?

Doing some basic physics and math I found out that, with constant acceleration, and starting from rest, you achieve 70% of any speed in 50% of the distance needed to achieve 100% of that speed. I suspect that that's the basis for this "rule" which, in that case, should be in "survivor's" version, not Dan's. You must achieve 70% of the take-off speed in 50% of the take-off run. If not, you stop.

Something nice about this is that, even if you attempt the take-off in a runway that exactly equal to the take-off run, by the time you decide to abort you still have 50% of the runway remaining. And for sure you will be able to stop the plane from a given speed (especially a general aviation piston single) in less distance that it took to achieve that speed in the first place.

But there are problems. First, it assumes that the acceleration is constant, which is not. Not only because drag builds up as you gain speed, but also thrust reduces as you gain speed. Second, it assumes no wind. If you have a headwind component, as you would in most take-offs), you would achieve 70% of your liftoff GROUND speed in 50% of the take-off run distance, For example, say that a plane takes 1000ft to accelerate to the take-off speed of 60 knots. Then it should achieve 42 kts (70% of 60 kts) by the 500ft mark. Now say that there is a 18kts headwind. The take off run to achieve 60 kts now is 500 ft. But the decision point should not be stated now as "250 ft to achieve 42kts -70% of 60 kts-". But 250 ft to achieve 47 kts. Why 47 kts? Because that airspeed corresponds to a ground speed of 29 kts, which is 70% of 42 kts, which will be your ground speed when your airspeed reaches 60 kts (due to the 18 kts headwind).

So the algorithm should be:
1- Calculate the distance it will take the plane to achieve the take-off airspeed.
2- Calculate 50% of that distance, that will be the "decision point".
3- Subtract the headwind from the take-off airspeed to convert it into ground speed
4- Calculate 70% of that ground speed
5- Add the headwind to convert that 70% groundspeed into airspeed. That will be your decision criteria.

Now, again, all of the above is with constant acceleration. I know that acceleration will not be constant, it will decrease as speed increases, but I don't know if it would be significant or negligible.

[Gabriel]

Note that V1 is not useful as a go-nogo decision in multi-engine jets either for the case of reduced performance.

Huh?

The V1 model assumes that the performance is normal until something happen and you make a decision to continue with one engine less (but nothing else affecting performance) or stopping (with a normal stopping performance). It will not prevent you from running out of runway before Vr because the engine thrust was less than it should or because you are dragging your brakes, because there is no single decision event where you would say "ok, now we need to decide if we stop or go: Are we above or below V1?" You actually could run off the end of the runway without ever achieving V1 to begin with.

[3WE]

Note that V1 is not useful as a go-nogo decision in multi-engine jets either for the case of reduced performance.

Perhaps I read it differently than your intent.

I’m thinking a V-1 is calculated for DELIBERATE reduced-performance takeoffs and that it works, as intended, for that scenario.

If performance is less than ASSUMED, then yep, it’s invalid.

Good information on the 70-50 rule…Given that acceleration is “actively decaying” I’m thinking Dan Juan’s rule depends on strong braking being available…yes?

[Gabriel]

Good information on the 70-50 rule…Given that acceleration is “actively decaying” I’m thinking Dan Juan’s rule depends on strong braking being available…yes?

In my limited experience in GA piston singles, at (almost?) any speed on the ground deceleration due to braking is greater than acceleration due to engine power. So if you could accelerate to any given speed in 50% of any given distance, you can stop from that speed in the other 50%.

[3WE]

(almost?) any speed on the ground deceleration due to braking is greater than acceleration due to engine power.

Maybe not_on glaze ice.

[Gabriel]

Maybe not_on glaze ice.

Good point, although I was thinking the "almost" more on the line of quite powerful singles that have a relatively slow take-off speed, like a Cessna 182. Especially if lightly loaded, it will get to 60 knots in a rush and by that time, with the positive deck angle that it has on the ground (to avoid prop strike) and flaps 10, the lift will be a good fraction of the weight, leaving not a lot of weight-on-wheels, so I am not sure that you would be able to stop from 60 knots in the same or less distance that it took to get to 60 knots in the first place.

[3WE]

Good point, although I was thinking the "almost" more on the line of quite powerful singles that have a relatively slow take-off speed, like a Cessna 182. Especially if lightly loaded, it will get to 60 knots in a rush and by that time, with the positive deck angle that it has on the ground (to avoid prop strike) and flaps 10, the lift will be a good fraction of the weight, leaving not a lot of weight-on-wheels, so I am not sure that you would be able to stop from 60 knots in the same or less distance that it took to get to 60 knots in the first place.

Complex Gabrielian thought process which is kind of interesting.

I guess retract the flaps…and use measured pull ups to put what weight you can on the MLG…avoiding a formal rotation.

[Not_Karl]

Virgin Australia B738 takes-off after overrunning available runway into a closed section.

https://avherald.com/h?article=502215e7&opt=0

[Evan]

Virgin Australia B738 takes-off after overrunning available runway into a closed section.
https://avherald.com/h?article=502215e7&opt=0

Methinks this was less a case of performance miscalculations and more a case of not getting (or not reading) the NOTAM.

[3WE]

https://www.facebook.com/share/r/k4zwLDtn6vk3inFF/?mibextid=qDwCgo

[Evan]

https://www.facebook.com/share/r/k4z...ibextid=qDwCgo

Thank you for that lesson in basic holdmybeermanship.