V1 rejects with decreasing performance

BobDDuck

BobDDuck

Island Bus Driver
All this talk of speeds and certification reminded me of a question I asked the training department here and never got a good answer for.

For simplification purposes, lets assume a V1 speed off of a certain runway is based solely on being able to accelerate to V1 and then reject and still be able to stop at the end of the runway. Obviously that V1 number is going to shift based on weight, runway available and runway surface conditions that effect braking action. For our ops we simply select a runway on the ACARS, put in the current aircraft weight and any runway condition reports and hit send and Aerodata sends us back a V1 speed (and a bunch of other stuff not pertinent to this discussion).

My question stems from the fact that as you approach the departure end of a runway while decelerating, you start to encounter rubber buildups from aircraft which have landed on the opposite direction runway. While landing (so I'm pretty slow by the time I get to these deposits) I've started to skid slightly at this point. So I guess my question is, do most performance data companies take into account decreasing brake performance during the course of a reject? I suppose over the course of a reject the brakes heat up as well which results in decreased performance also.

So many of our takeoffs involve calling V1 at a point at which I seriously wonder if we could stop in time if we had to. Throw in less than forecast braking performance and stuff could get bad in a hurry.
 
Your stopping performance is predicated (by the mfgr) on average breaks and average pilot technique, ie nothing special required. The AFM takes into account the energy your breaks are abosorbing when it gives you accelerate stop numbers (or field length for takeoff). As for runway conditions, I would guess the airport is responsible for keeping the runways in a condition that meet a minimum friction standard. The runway may look covered in rubber but I think it is cleaned from time to time and the look is more cosmetic than it is detrimental.

It would be interesting to see just how accurate the AFM numbers are in real world ops, I just don't have the brass golf balls to go our and try it....
 
I do know that my one real high speed reject that I duplicated in the sim ended up taking about 1000 feet less in the made up world than it did in the real world.
 
BobDDuck said:
I do know that my one real high speed reject that I duplicated in the sim ended up taking about 1000 feet less in the made up world than it did in the real world.
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Eh, might be a sim-ism, but replicating a real high-speed abort at the exact brakes-on speed with the exact same brake/reverser application in the exact same conditions at the exact same weight (you get the idea) wouldn't be easy. That could explain a good part of the discrepancy.
 
dasleben said:
Eh, might be a sim-ism, but replicating a real high-speed abort at the exact brakes-on speed with the exact same brake/reverser application in the exact same conditions at the exact same weight (you get the idea) wouldn't be easy. That could explain a good part of the discrepancy.
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Also, you're expecting it in the sim. In the day to day grind of things, you're probably not planning on an a high speed abort on every takeoff. Yes you should, but honestly, human reaction time and processing time included I'd suspect that there's a bit of a delay between "Oh, XXXX" and "BUCKETS AND BRAKES!"
 
ppragman said:
Also, you're expecting it in the sim. In the day to day grind of things, you're probably not planning on an a high speed abort on every takeoff. Yes you should, but honestly, human reaction time and processing time included I'd suspect that there's a bit of a delay between "Oh, XXXX" and "BUCKETS AND BRAKES!"
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Correct! These performance numbers are based on "real world" application of the procedure by a test pilot that knows he's going to abort the takeoff. A "padded" number is generated based on the actual numbers to compensate for the "Oh, XXXX" moment.

Also, V1 may or may not be the same for accelerate-stop and accelerate-go. Whether you can stop on the runway is not necessarily At the V1 point.
 
dustoff17 said:
Correct! These performance numbers are based on "real world" application of the procedure by a test pilot that knows he's going to abort the takeoff. A "padded" number is generated based on the actual numbers to compensate for the "Oh, XXXX" moment.

Also, V1 may or may not be the same for accelerate-stop and accelerate-go. Whether you can stop on the runway is not necessarily At the V1 point.
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Can you explain what you mean there?
 
I know we don't use a balanced field length V1 for our numbers and the accelerate stop distance can include the runway clearway which is (I think) 1000 feet beyond the end of usable "runway". I don't know if this takes into account runways that don't have legal clearways (which I know we operate into several of).
 
V1 "guarantees" you will stop on the avaiable runway, we just treat V1 as a go speed in more cases than not. I thought the clearway thing was done away with for stopping, but could be considered for the 35 foot clearance thing??
 
dasleben said:
We do a "soft" V1 call 5 knots prior to the computed V1 for this very reason.
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If you have an engine failure at your "soft" V1, how do you know you have enough capability on one engine to accelerate and take-off in the remaining distance? Let me use an extreme example: normal V1 at 100 knots, Vr at 120 knots. Let's say we decide to use 80 knots as a "soft" V1, what are the chances that we can accelerate from 80 knots to 120 knots, rotate, and lift-off in the remaining runway?
 
dustoff17 said:
Correct! These performance numbers are based on "real world" application of the procedure by a test pilot that knows he's going to abort the takeoff. A "padded" number is generated based on the actual numbers to compensate for the "Oh, XXXX" moment.
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I'm pretty sure that every aircraft I've flown already has a certain delay built in to the published numbers. In other words, the distances used to calculate V1 are based on the test pilot waiting a specified number of seconds after the engine lose in order to duplicate (as near as possible) what happens in real life. They do NOT cut the throttles and stand on the brakes right at V1.
 
SteveC said:
If you have an engine failure at your "soft" V1, how do you know you have enough capability on one engine to accelerate and take-off in the remaining distance? Let me use an extreme example: normal V1 at 100 knots, Vr at 120 knots. Let's say we decide to use 80 knots as a "soft" V1, what are the chances that we can accelerate from 80 knots to 120 knots, rotate, and lift-off in the remaining runway?
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By the time you even say "V1" at V1-5 and the other guy hears it, you're already there. I don't have the math on hand to go into it deeper unfortunately, but that procedure comes straight from Boeing. I'll see what else I can find.
 
I'm guessing that maybe we are sort of on the same page, but not using the exact same language.

Let me put it another way. If you take your calculated V1 speed (however you derive it) and reduce it by 5 knots, do you know whether or not you are above Minimum V1?

To clarify even further, let's use AC 120-62 as a point of reference, if that's OK with you?
 
SteveC said:
I'm pretty sure that every aircraft I've flown already has a certain delay built in to the published numbers. In other words, the distances used to calculate V1 are based on the test pilot waiting a specified number of seconds after the engine lose in order to duplicate (as near as possible) what happens in real life. They do NOT cut the throttles and stand on the brakes right at V1.
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If I'm reading right, the delay is either 1 second or the delay that the test pilots demonstrated, whichever is longer.
 
I think you're both on the same page. V1 is V1. Das is saying he just makes the V1 call five knots prior to V1, at whatever speed that may be isn't relative, it's just V1-5. Some thing I have done for quite a while. Even if the numbers are padded slightly for reaction time, you are still using it as a descision speed. If you call V1 as you see the airspeed indicator hit it, by the time you see it, call it out, the other guy hears it, you're easily V1+5, and in some A/C at Vr. It makes since to me to use V1-5 as a point to announce it. In fact there are some articles floating around that recommend this procedure.
 
PositionAndHold said:
I think you're both on the same page. V1 is V1. Das is saying he just makes the V1 call five knots prior to V1, at whatever speed that may be isn't relative, it's just V1-5. Some thing I have done for quite a while. Even if the numbers are padded slightly for reaction time, you are still using it as a descision speed. If you call V1 as you see the airspeed indicator hit it, by the time you see it, call it out, the other guy hears it, you're easily V1+5, and in some A/C at Vr. It makes since to me to use V1-5 as a point to announce it. In fact there are some articles floating around that recommend this procedure.
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Exactly. If SOP is to call V1 at the exact calculated speed, what happens if we're V1-1 and have an engine failure? By the time either pilot recognizes what's happening, we're a few knots past V1 and committed, even if the failure occurred prior to V1. V1-5 provides a buffer for reaction time; by the time the other pilot processes what you've just said, you're at the calculated V1 instead of past it. Again, I don't have the math to back up any (presumably minute) decrease in balanced-field performance, but I'd like to read one of these articles you've mentioned to get a better understanding myself.

At any rate: V1-5, V1-1, whatever. Measure with a micrometer, mark with a crayon, cut with a chainsaw. :D Steve, it's a little late here, but I'll dig into those links tomorrow.
 
SteveC said:
I'm pretty sure that every aircraft I've flown already has a certain delay built in to the published numbers. In other words, the distances used to calculate V1 are based on the test pilot waiting a specified number of seconds after the engine lose in order to duplicate (as near as possible) what happens in real life. They do NOT cut the throttles and stand on the brakes right at V1.
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Steve, you and I are saying the same thing here. I'm using the word "padded" and you're saying that a delay is built in.

The published numbers take into account the reaction of the test pilot plus a human factors delay.

The big difference is that the test pilot KNOWS he's going to get the failure, so even the "built in delay" might not be enough in the real world.
 
Dugie8 said:
V1 "guarantees" you will stop on the avaiable runway, we just treat V1 as a go speed in more cases than not. I thought the clearway thing was done away with for stopping, but could be considered for the 35 foot clearance thing??
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V1 will NOT guarantee that you'll be able to stop on the runway. V1 is only a reference speed that you can use to make the decision to abort the takeoff and take your chances with available runway or to continue the takeoff and take your chances in the air. This is why V1 is often called the "decision speed". Your computations for Accelerate-Stop and Accelerate-Go will provide the runway distances you'll need to know IF you can stop at V1.

The US Federal Aviation Administration defines it as: V1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V1 also means the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance

You're right about the clearway. It is used for computations of Accelerate-Go but is rarely used for Accelerate-Stop.
 
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