Takeoff decision point

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woxof

Pre-takeoff checklist
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woxof
I have read about these reference points that can be made for GA light aircraft ops over the years. Recently, I read this in an article. I thought I might post it and see if anyone had any other words of wisdom....

"There is an oft-quoted rule of thumb about aborting a takeoff if a certain percentage of rotation speed is not achieved halfway down the runway. Unfortunately, this rule cannot guarantee that 100 percent of rotation speed is available at a safe point along the runway. Because air resistance increases with speed, each additional knot of airspeed will take longer to achieve than the one before. For example, accelerating an airplane from 50 knots to 60 knots will take less time (and distance) than accelerating from 60 knots to 70 knots. Furthermore, the rule ignores the distance needed to stop if a safe takeoff cannot be made. Identifying a landmark along the runway by which to rotate is an easier and far safer strategy."

https://www.aopa.org/news-and-media/all-news/2019/june/pilot/proficiency-making-it-happen
 
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Abort point at rotation gives less chance of an overrun than "x% of speed by halfway". However, the halfway point can give you an earlier indication of a problem and then you will be nailing the brakes at a lower speed with greater margin. The flip side is that the halfway point might not be accurate.

Of course, you could use both as abort reference points.
 
For larger aircraft, the term used for this is an “accelerate-stop distance”.

It can be calculated or estimated for any aircraft that doesn’t have charts / tables for different conditions.

And of course there’s V1/V2... etc.
 
“A certain percentage” (70) takes into account acceleration changes. The rule of thumb works. As stated in the AIM, it does not account for variances in runway surface or any obstacles.

The author is correct that it makes no guarantees about stopping...just like the author’s suggestion for identifying the rotation point makes no guarantees about stopping.

The rule of thumb is designed to give a go/no-go decision point when the takeoff distance is very close to the runway available. It works quite well for that. Unfortunately, since very few people put themselves in that situation knowingly, the rule of thumb gets mis-applied (as the author did) and gets a bad rap because it doesn’t do things it’s not intended to do.
 
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I also notice that the author pulled the rotation decision distance she’s planning on directly from her ass...basically, “my charts say 1500 feet, so I’m going to arbitrarily add 500 feet.” If she had any clue about how her normal technique compares to book numbers, she’s not letting on.
 
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AOPA is trying hard to come up with content it seems

The whole article could basically be summed up in one sentence: "Review the takeoff and landing distance performance charts for for your departure with the correct atmospheric and weight and balance figures; if the airplane is not performing as it should abort the take off"

I'm not even sure what to make of this...:
"
Make an abort plan. With an estimated ground roll of 1,500 feet and, in similar conditions, a landing distance (ground roll) of 1,000 feet, I decided to identify a landmark 2,000 feet from the beginning of the runway as a decision point to abort or continue the takeoff. If we are not off the ground before the landmark then performance is significantly less than expected and there will be ample distance to retard the throttle and stop the airplane on the runway. You may choose a different value, but ensure that the sum of the distance to rotation, the distance to stop, and a safety buffer is less than the available runway length.
"
A.) how long is said runway?
B.) if the plane needs a 1,500 ft ground roll to take off, and 1,000 ft to stop.. then where the hell did she come up with the 2,000 ft figure? This mathematics professor pulled in from thin air
C.) you should know within the first 1-3 seconds whether your plane is performing as it should... are you making correct RPM / manifold pressure / percent power / etc? I've never heard of this "I-don't-seem-to-be-going-fast-enough-and-I'm-well-beyond-the-point-where-I-should-have-left-the-ground-so-the-plane-isn't-working-right" method

I know it's a nominal fee and it goes to "supporting GA" but reading fluff like this does not make me feel like I'm missing out on value added content from AOPA
 
Go back a ways, and you’ll find an article that states every takeoff should have enough runway for takeoff to 50 feet plus landing from 50 feet, as this will give the same level of safety the airlines use. :rolleyes:
 
Fun Bit of Trivia, I recently attended a presentation by Mission Aviation Fellowship that have been operating a remote strips for many years. They were discussing the procedures they use and talked about abort points. They showed an example of landing Kodiak on a highly sloped airstrip. IIRC it was something like 900ft long and 12-15% slope. talking point about landing is don't stop early as it requires everyone in the village to push the plane to the top of strip, as you won't be able to taxi there. The abort point on the takeoff is at Brake Release. Once rolling you are committed to taking off.

Brian
CFIIG/ASE
 
^I imagine Lukla, Courcheval, and perhaps others, have similar operating standards
 
I suspect this is rather facetious. Most of us will be assured of a successful takeoff on most flights. But things can pile up. Sshort or short(isn) strip, DA, wet grass, all sorts of things. Perhaps we all should whip out our slide rules and do lots of engineering calculations right there and then, but the 70% rotation speed halfway down the runway is going to work for about 99% of the situations. And if the situation is so extreme that it doesn't work, odds are you'll know it beforehand and either do the calculations or maybe take off when conditions are better.
 
steingar said:
whip out our slide rules and do lots of engineering calculations right there and then
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The best part about the article though, is that there wasn't even lots of math.. just "gee, I'll add 500 ft, that should be good! to hell with the POH and everything I've been taught and the work other engineers did!"
 
MauleSkinner said:
just like the author’s suggestion for identifying the rotation point makes no guarantees about stopping.
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...unless you calculate the stopping distance at Vr and set that as the rotation point.
 
Obsticles need factoring in as well... is a 2500 runway with obstacles really a longer runway than 1700 in the middle of the plains?

if i make my conservative calculations I may use a runway i need almost all of to get airborne and over a fence 100 yards away vs a longer one with hills or trees... can i really be 70% halfway down a medium length runway with obstacles and be okay? Maybe, Maybe not...

Rules of thumb have their place but are not to be confused as absolutes, and sometimes they simply get taken as that...
 
For piston engine aircraft POH numbers are are to be taken with several shakers of salt. If you are flying a practically brand new airplane, using a dry level paved runway, in good weather, in weight and cg limits, with good technique, and there are no obstacles limiting your path you have a good estimate as to the performance you can hope for. Hope for but not expect.

The performance of piston engines and propellers is not easily determined. Age, leaning, density altitude, and many factors reduce performance and are not easy to characterize. Vrotate, V1, and V2 apply to multi-engine aircraft but not single engine. Trying to apply these to single engine aircraft is a waste of time and energy.

From an engineering perspective few POH's contain information to help you determine whether you can clear obstacles in your path. Basically you can get a feel for whether you might be able to take off then clear a 50 foot obstacle and if the aircraft is loaded within limits.

You should perform take off and landing performance but keep in mind your predictions may be very misleading...as the saying goes your mileage may differ.
 
asicer said:
...unless you calculate the stopping distance at Vr and set that as the rotation point.
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...or any number of other things that the author chose to ignore.

keep in mind that I also think the fixation with stopping on the runway has its dangers...I’ve seen two airplanes that I think were needlessly wrecked because of that fixation.
 
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jonvcaples said:
You should perform take off and landing performance but keep in mind your predictions may be very misleading...as the saying goes your mileage may differ.
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Which is why you should do enough of it to have a really good idea how your takeoffs and landings compare to book numbers. I don’t believe most pilots have a clue.
 
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MauleSkinner said:
keep in mind that I also think the fixation with stopping on the runway has its dangers...
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Agree. I'd much rather smoosh into a fence at 5kts/0AGL than stall it into the top of a 100ft tree.
 
asicer said:
Agree. I'd much rather smoosh into a fence at 5kts/0AGL than stall it into the top of a 100ft tree.
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One of the guys I saw went off the end at 50 or so with brakes locked & nose strut compressed, having retracted flaps for a “ short field go around”.

of course, that was after the REAL screwup.
 
Choosing when to abort based on distance requires you to survey the runway and identify these points. It becomes difficult at unfamiliar runways, or at night. It is better to watch the RPM and manifold pressure and make a decision based on that. Alternatively, you could use a timer. Figure out how many seconds it should take to reach 70% speed, and abort based on that.
 
Lots of variables....runway length...paved runway or grass...level, upslope or downslope...wind direction and speed....empty weight...obstacles near departure end of runway...density altitude. Then you have to be familiar with your airplane and your piloting ability. Do not take chances if there is any doubt.

I learned to fly off a 1400 foot grass runway...power line ran across one end. Some days we went flying...sometimes, we did something else.
 
sarangan said:
Choosing when to abort based on distance requires you to survey the runway and identify these points. It becomes difficult at unfamiliar runways, or at night. It is better to watch the RPM and manifold pressure and make a decision based on that. Alternatively, you could use a timer. Figure out how many seconds it should take to reach 70% speed, and abort based on that.
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The timer thing sounds like an engineering problem to me.


If you can count to 10 reliably, it’s pretty easy to determine long enough distances on a lighted runway.

For unlighted runways, learning to pace off accurate distances isn’t very difficult.
 
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