Do you teach Rejected TakeOff?

[MauleSkinner]

There's a rule of thumb that I don't remember, but it's something about if you haven't reached 60-odd percent of your rotation airspeed by the time you hit midfield, you're not gonna make it.

Close...70% of LIFTOFF speed by midfield.

Accelerate-stop matters for 135 and 121 ops. So does accelerate-go, but that's not an option for singles.

Again, a bit of digression, but can you show me where a single-engine airplane, or even a light twin, needs accelerate-stop runway under 135?

Fly safe!

David

 

[TMetzinger]

Close...I'll give a little background info for the benefit of those who may not know...

Brakes absorb tremendous amounts of energy in the process of stopping an airplane. This energy is then released by the brake in the form of heat. Lots of it.

If all this heat gets released near the wheel and tire (which it does), the wheel, tire, and the air inside heat up. Increasing the heat of a contained gas increases its pressure, and eventually something's going to give. Jets generally have "fusible plugs" in the wheels. These are a plastic plug that melts at the proper temperature and lets the air out slowly, rather than allowing pressures to build up to where the tire explodes.

We also have "brake energy limits", which equate to the point where as you roll to a stop, your brakes are so hot that they no longer have any effectiveness.

There's a note in the Hawker manual that basically states "if you abort a takeoff at V1, and are brake energy limited, the fusible plugs will release the air in the tires."

So...combining all this, we abort the takeoff, the brakes heat up, the wheels heat up, and the tires go flat (although not right away).

Now we have a flat tire with the hot brake resting directly on the tire. This, then, results in the brake setting the tire on fire. Put the flames out, they come back because the hot brake is still resting on the tire. It'll happen several times. Depending upon the airplane, these flames can be large enough to lick at the bottom of the wing. (Again, don't ask, but it wasn't an RTO ) Of course, there's also the danger of the wheel or brake itself shattering, sending shrapnel all over the place if you give it a good shot with the fire extinguisher while trying to put the tire out. (No, THAT one I haven't experienced.)

OK, so technically you're not always brake energy limited on your takeoff. But say you're in Las Vegas, and get to taxi from the FBO on the northwest corner of the airport all the way to runway 25R...that's about 3 miles, all downhill, at least half of which is with a tailwind. Guess what? You're using your brakes, and you're heating them up. You really have no idea how much brake energy you have left. You could have problems on an aborted takeoff at fairly light weights. As I indicated earlier, it just isn't that nice, neat package that we like to believe.

Fly safe!

David

Thanks for the information.

There's a really good segment on the 777 documentary (21st Century Jet, I think) that shows their rejected takeoff tests for certification. To pass, they had to go at max gross, go to V1, reject the takeoff, come to a stop, and then wait a certain amount of time before the fire department was allowed to come in and put out the brake/tire fires.

I remember the test pilot was annoyed because the night before Boeing decided to increase the weight, because they were anticipating a stretch (maybe the -300) using the same brakes, so they wanted to use THAT projected weight for the test.

 

[TMetzinger]

Close...70% of LIFTOFF speed by midfield.

Again, a bit of digression, but can you show me where a single-engine airplane, or even a light twin, needs accelerate-stop runway under 135?

Fly safe!

David

Lemme look at the regs. It may not be in the reg itself, but it's in the op specs of at least 1 135 operator using PC-12s. Back shortly.

Edit: I couldn't find any requirement in part 135. So I asume the operator put that into his specs as a risk-reduction technique, maybe to help with insurance, since the PC-12 did specify an A-S distance. Balanced-field is a part of transport category certification, right?

I'll also note that this was an operator flying out of KMTN and it was over five years ago. I stopped by to pick up a manual and got a PC-12 ride and a discussion with a pilot who was new to the operation, and he told me about the training he went through and the operator's procedures - the accelerate-stop thing was new to him at the time - it was his first flying job after instructing.

PS - what's liftoff speed in a single?

This is a good way to spend a day while Hanna turns my lawn into a pond.

 

[MauleSkinner]

PS - what's liftoff speed in a single?

Well, it's "rotation" speed if you don't accelerate between rotation and liftoff...it's something higher if you accelerate as you rotate before you lift off.

Bottom line, the critical number is where you break ground. You may roll off the end if you're close, and use "rotation" speed in your formula.

Fly safe!

David

 

[wabower]

Yes, we're digressing a bit. And yes, I think a student should practice that.

There's a rule of thumb that I don't remember, but it's something about if you haven't reached 60-odd percent of your rotation airspeed by the time you hit midfield, you're not gonna make it. Practicing the rejected take off manuever, say at 40 knots, or 50 (make sure your runway is long enough) is probably a good thing. Have the long runway since you DON'T want to brake the way you would in a real rejection.

How do you teach them to practice digressing? Make them join this forum?

I think Sparky says 70% by midfield. I'd be interested to see how many pilots can get their act together on this one, both figuring and executing.

Seems to me that this subject is somewhat paradoxical in that it's both a lot simpler and a lot more complex based on a number of factors.

"If the motor quits on takeoff roll, what are you going to do?"

"Uhh, stop straight ahead, hopefully on the runway?"

"What if there's not enough runway?"

"Uhh, try to keep it rolling, right-side up?"

"If you are at Telluride (show photo) would you try to ground-loop or go off the end?"

"Are you kidding?"

"OK, lets talk about your final checks for line up and take-off roll, so you will have the best chance (or maybe a better chance) of identifying a problem that might lead to an abort. What do you do now?
What else could you do that might give you better odds?"

 

[TMetzinger]

How do you teach them to practice digressing? Make them join this forum?

I think Sparky says 70% by midfield. I'd be interested to see how many pilots can get their act together on this one, both figuring and executing.

Seems to me that this subject is somewhat paradoxical in that it's both a lot simpler and a lot more complex based on a number of factors.

"If the motor quits on takeoff roll, what are you going to do?"

"Uhh, stop straight ahead, hopefully on the runway?"

"What if there's not enough runway?"

"Uhh, try to keep it rolling, right-side up?"

"If you are at Telluride (show photo) would you try to ground-loop or go off the end?"

"Are you kidding?"

"OK, lets talk about your final checks for line up and take-off roll, so you will have the best chance (or maybe a better chance) of identifying a problem that might lead to an abort. What do you do now?
What else could you do that might give you better odds?"

I think that's what I was aiming for when I started the whole digression - a recognition that there are lots of things that will affect your decision to reject a take off, and they aren't all engine-related, they can happen at different times, and things like runway length, terrain, and other stuff all matter.

The more complete a job we do thinking about it before we push the throttle forward, the more informed, and (hopefully) better our decisions and outcomes will be.

 

[wabower]

10-4.

I think that's what I was aiming for when I started the whole digression - a recognition that there are lots of things that will affect your decision to reject a take off, and they aren't all engine-related, they can happen at different times, and things like runway length, terrain, and other stuff all matter.

The more complete a job we do thinking about it before we push the throttle forward, the more informed, and (hopefully) better our decisions and outcomes will be.

 

[gismo]

In the example, it's rejecting the takeoff at rotation speed.

The point I think you're missing is that without knowing the accelerate-stop distance, you might assume that a 2800 runway is a "good" runway for departure.. It's 500 feet longer than your obstacle clearance distance (2300), right?

So you're tooling along, and right before you pull back - BANG - your prop flies off and your engine is on fire. You stand on the brakes, and overrun the end of the runway by at least 150 feet.

If you'd KNOWN your accelerate-stop distance, then you would have probably chosen a longer runway, and likely stopped while still on the runway, where evacuation and other things would be easier.

YES, if your engine fails after rotation, you are sca-rewed regardless. But I hope you see that knowing the distance can affect your decision-making.

The point I think you are missing (or more accurately I'm not explaining well enough) is that the AS distance only applies to that one unlikely event (catastrophic power loss at rotation). If the prop flew off 150 or more feet sooner the shorter runway is fine, if it flew off 50 ft later you're in the same uncomfortable boat. And in any case when the "prop flies off" my choices are rather limited. regardless of how much runway remains or how long of a runway I'd chosen.

What I'd really like to know is the exact point along the runway where I'm better off aborting for reasons that actually make the abort a choice e.g. door pop, unusual vibration/sound etc. In your example with a 3000 ft runway that would be at rotation assuming the plane and pilot perform as well as the factory's test plane and pilot. But what if the runway was 3500 ft or 2500 ft? In either case the point in the takeoff roll where the choice to abort without damage exists will be different than the point of rotation. As I said before, up to a point well beyond the AS distance runway length (can't call it a balanced field length can we?) adding length improves the odds at about the same rate as subtracting length below the AS distance. What's so magical about this number? In a multi-engine airplane the A/S distance runway length is much more meaningful (again assuming perfect airplane/pilot performance) because it's supposed to mean that you actually will have a choice to make.

Even on the 3000 ft runway you'd likely go off the end if the brakes were dragging during the takeoff roll or some turbine wear/damage reduced your thrust by a difficult to detect 10%, so what good is there in "knowing" you're OK aborting at rotation?

 

[gismo]

I think that's what I was aiming for when I started the whole digression - a recognition that there are lots of things that will affect your decision to reject a take off, and they aren't all engine-related, they can happen at different times, and things like runway length, terrain, and other stuff all matter.

The more complete a job we do thinking about it before we push the throttle forward, the more informed, and (hopefully) better our decisions and outcomes will be.

I gotta agree with all that!

 

[TMetzinger]

Even on the 3000 ft runway you'd likely go off the end if the brakes were dragging during the takeoff roll or some turbine wear/damage reduced your thrust by a difficult to detect 10%, so what good is there in "knowing" you're OK aborting at rotation?

I think for me it was more along the lines of knowing I wasn't ok aborting at rotation on a 2800 foot runway that otherwise seemed more than adequate. Make sense?

 

[SkyHog]

I like the way it was done in the old Discovery Wings show "Learning to Fly" where the instructor pulled the mixture just after rotation.

Pulling throttle isn't the same.

 

[gismo]

How do you teach them to practice digressing? Make them join this forum?

Works for me!

I think Sparky says 70% by midfield. I'd be interested to see how many pilots can get their act together on this one, both figuring and executing.

That's based on a simple mathematical model of acceleration at a constant from zero V:
distance = V^2/(2* accel)
This implies that if you cut the distance to reach a certain velocity in half you will attain that velocity divided by the square root of 2 or approximately 70.7%.

There are a lot of incorrect assumptions in this ROT but in practical terms it's a pretty good approximation as long as the acceleration is relatively constant. If you're takeoff roll is through mud or deep snow that simply won't be the case and the ROT fails. IMO a much better concept is to estimate the distance you need to bring the plane to a stop at liftoff speed and reject any takeoff where you get almost that close to the end before breaking free of the ground. This distance is reasonably close to the published landing ground roll distance (adjusted for conditions) under most circumstances. The more difficult part is determining when you get there, either by eyeballing it (least reliable), using runway strip marks (better), or pacing it off before attempting a takeoff when things might be tight (best).

Seems to me that this subject is somewhat paradoxical in that it's both a lot simpler and a lot more complex based on a number of factors.

"If the motor quits on takeoff roll, what are you going to do?"

"Uhh, stop straight ahead, hopefully on the runway?"

"What if there's not enough runway?"

"Uhh, try to keep it rolling, right-side up?"

"If you are at Telluride (show photo) would you try to ground-loop or go off the end?"

I have to wonder if ground looping on a dry paved surface would actually result in stopping in less distance than max effort braking. If the gear remains intact this would probably mean that sliding the tires sideways produces more friction than they can with threshold braking straight ahead, something I know isn't true with automobiles. And if the gear collapses it doesn't seem likely the sliding airframe would decelerate faster unless some parts of the plane actually penetrated the surface. OTOH, if you managed a complete 180 (and then stopped the rotation) you'd plow into whatever tail first which seems more survivable than head on.

"OK, lets talk about your final checks for line up and take-off roll, so you will have the best chance (or maybe a better chance) of identifying a problem that might lead to an abort. What do you do now?
What else could you do that might give you better odds?"

Well since we're "practicing digression" (this was about rejected takeoffs not takeoff prep), that would have to include the aformentioned determination of a safe reject point, checking for airspeed, RPM, and oil pressure early and often in the takeoff roll, not to mention passing on the whole touch and go idea.

And here's another issue related to takeoff prep: Do you think it's a good idea to switch tanks at any time between engine start and takeoff assuming you positioned the tank selector properly before starting? My concern is that there's a significant delay between changing the selector and fuel from the newly selected source actually reaching the engine. Enough delay in many airplanes that a slug of contaminant from the tank just switched to might reach the engine when you are about 100 AGL.

 

[wabower]

Wo

And here's another issue related to takeoff prep: Do you think it's a good idea to switch tanks at any time between engine start and takeoff assuming you positioned the tank selector properly before starting? My concern is that there's a significant delay between changing the selector and fuel from the newly selected source actually reaching the engine. Enough delay in many airplanes that a slug of contaminant from the tank just switched to might reach the engine when you are about 100 AGL.

The T-210 has "header tanks" for each side under the floor, so it was doubly-dangerous to switch tanks. Just enough to run for a while, then empty just after rotation.

 

[TMetzinger]

Good question - on the low-wing airplanes, the tank change occurs before run-up, but the ones I fly don't have significant storage the central point - so the run-up process does show good fuel flow.

How do I know? I switched the selector to OFF once and ran out pretty quickly.

 

[gismo]

Good question - on the low-wing airplanes, the tank change occurs before run-up, but the ones I fly don't have significant storage the central point - so the run-up process does show good fuel flow.

How do I know? I switched the selector to OFF once and ran out pretty quickly.

On a fuel injected airplane turning the selector off will kill the engine quickly but that has no bearing on the time it takes for air or water to reach the engine from the tank selector. When you turn the valve off, the flow stops with the lines between the valve and the engine full of fuel. When you switch to a tank with enough water in it to get picked up the water has to make it all the way from the tank to the engine or at least from the valve to the engine and that can take a long time at idle and many, many seconds at takeoff power. Same for air in the line except that in some cases the air will slip past the fuel in the lines once about half the fuel is gone from there.

If you really want to learn how long it takes in your plane, run one tank completely dry then switch to the other long enough to purge the air from the lines between the valve and engine (several minutes at idle, a few minutes at high power). Then at the appropriate power setting (idle or full power) switch back to the empty tank and count the time until the engine quits. You may be surprised how long it takes, especially at idle. I'd add 50-100% to that for a safety margin and double that for contaminated fuel. If you have a carburetor this still works but the times will be longer both for the valve switch to an empty/contaminated tank and for switching to off. On my 65 HP taildragger the engine will continue to run for 2-4 minutes at idle after shutting the fuel supply off.

 

[gismo]

I think for me it was more along the lines of knowing I wasn't ok aborting at rotation on a 2800 foot runway that otherwise seemed more than adequate. Make sense?

Sure, I get that, but it's just one datapoint in the how far down the runway can I stop without worrying about damage (or worse). If you had the choice between a 3000 ft runway and a 3500 ft runway would you choose the 3000 footer knowing it met the A/S length?

If the longest runway was only 2800 ft (again ignoring additional fudge factors) would you reduce the load or wait for conditions that reduced the A/S distance to 2800 ft? Or would you just depart and plan to not abort unless absolutely necessary (fire, engine failure, etc) a bit before reaching "rotation" speed? (that would be my choice).

 

[TMetzinger]

If I were solo, I'd depart off 2800. Wouldn't do it with my kid (or yours) in the back.

There are risks I'll accept solo but not with pax. There are risks I can't mitigate. Those that I can, I do.

 

[gismo]

If I were solo, I'd depart off 2800. Wouldn't do it with my kid (or yours) in the back.

There are risks I'll accept solo but not with pax. There are risks I can't mitigate. Those that I can, I do.[/quote ]

OK we're back to the additional risk of using a runway that's shorter than the POH A/S distance (in a single) and IMO there's just as much additional risk in using a runway that exceeds that number vs one that's another 1000 ft longer. Obviously you disagree I guess we'll just have to leave it at that unless you can explain why you see a difference.

 

[wabower]

I have concluded that in every activity I undertake, from checking into the hotel (have you ever read that blurb on the back of the door?) to all that goes before and after, that whoever is making up the rules that day may not be doing so in what I consider to be my best interest.

My responsibility is to be sure that the playing field is at least level, and if possible to tilt the odds in my favor to the extent possible. I have also determined that saying anything about how I accomplish that goal may not always be a good strategy. For me and many other tail-dragger folks, concrete is concrete. What's painted (or not painted) on it is less important than the outcome of the maneuver.

If I were solo, I'd depart off 2800. Wouldn't do it with my kid (or yours) in the back.

There are risks I'll accept solo but not with pax. There are risks I can't mitigate. Those that I can, I do.[/quote ]

OK we're back to the additional risk of using a runway that's shorter than the POH A/S distance (in a single) and IMO there's just as much additional risk in using a runway that exceeds that number vs one that's another 1000 ft longer. Obviously you disagree I guess we'll just have to leave it at that unless you can explain why you see a difference.

 

[midlifeflyer]

Yeah, but there are other reasons you might want to stop besides engine failure.

True. Might be oil pressure, abnormal temperature indications, a DG that's just spinning round and round and round, an open door, an ASI that isn't moving or a good number of other things that we should be looking to be "okay" on the roll or in the initial climb.

I think the principle is still the same. We look for certain parameters and abort if they are not within tolerance. It still seems to be that this is a normal part of every takeoff and I'm not sure that a PTS task for it is really necessary any more than a "watch out for towers" requires a specific task, unless its because its absence means that it's not being covered at all.

 

[midlifeflyer]

What caused the RPM drop?

It was a good number of years ago and I can't remember exactly what it turned out to be.

 

[TMetzinger]

OK we're back to the additional risk of using a runway that's shorter than the POH A/S distance (in a single) and IMO there's just as much additional risk in using a runway that exceeds that number vs one that's another 1000 ft longer. Obviously you disagree I guess we'll just have to leave it at that unless you can explain why you see a difference.

I'll give it one final shot: For takeoff in a single, I have a magic number - that number is the highest of A/S (if known), Takeoff Roll, and Obstacle Clearance distances. Add a fudge factor too if you like.

I think the risk of using a runway shorter than that magic number is higher (marginally,slightly,whatever, but HIGHER) than the risk of using a runway that is longer than the magic number. Once we're above that magic number distance then extra runway MAY mean more options. But less runway than that magic number DEFINITELY means LESS options.

 

[Areeda]

Just a couple of comments on what's been said so far.

I believe I can teach my students how to handle the plane on the runway after you've decided to abort the takeoff. We practice high speed taxi and pulling the throttle just after lift off on a bit less than a 4,000' runway in small Cessnas and Pipers.

We discuss and run scenarios on when one would decide to abort the takeoff. We play these what-ifs to encourage good decision making but we just can't teach it.

There are times when you have plenty of runway ahead of you and the vaguest uncertainty is a good reason to abort.

There are times after lift-off in an urban area where there are no good choices except to pray that the old bird has enough power in her to get you to a safe landing area.

The times in between are a pure judgement call. I'd much rather hit the fence at 20 kts and pay the penalties than hit the trees at 67 kts and pay those.

As far as pulling the mixture instead of the throttle, well it is a training exercise and power can fix a bunch of mistakes. I think a student will learn much more making a mistake and having the instructor take over than having the instructor make sure they are not given enough freedom to make the mistake.

Joe

 

[poadeleted20]

There's not a lot of challenge if you kill the engine by mixture or throttle. The challenge is getting the trainee to make a good decision when a problem is injected as well as the execution of the RTO. Killing the engine makes the decision for the student, which takes half the learning experience out of the lesson.

 

[Areeda]

There's not a lot of challenge if you kill the engine by mixture or throttle. The challenge is getting the trainee to make a good decision when a problem is injected as well as the execution of the RTO. Killing the engine makes the decision for the student, which takes half the learning experience out of the lesson.

Ron - how do you teach RTO?

Joe

 

[bbchien]

I was curious because I thought for sure RTOs had to be in the PTS. I found out that the are in the multiengine PTS but not in the single engine section.

Yup I brief 'em every time, too.

SE pilots need to at least time their t/o runs. Something like 14 (whatever yours really is) secs is average on a standard density alt. day.....

 

[poadeleted20]

Ron - how do you teach RTO?

I'll inject something -- deer running across the runway, oil on the windshield, etc.