Difference in Wet and Dry Screen Height
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Well I happen to have 5 minutes available
Hypotheses : 4 knots per second during acceleration (or 2m/sē) and +0.4G deceleration
Calculation based on v1 at 120 knots, giving an ASD of X = 1430 m
Then computation for RTO initiated at 125kt : what is speed at length X ?
58kt !!!
Total ASD for rejection at 125kt : 1545m
I am shocked !!
However, 1430m of ASD sounds pretty short, any runway used by commercial aircraft will be longer than that.
Would anyone care to criticize my hypothesis, confirm whether or not the usual ASD (no margins at all) for your A/C will be this short ? (I tried to be representative of a medium jet)
The runway exit speed as a function of rejection speed looks like this :
https://i.gyazo.com/695a1bd56a8a324f...ac6cbf903b.png
There is a beginning to the curve because runway exit speed does not exist if you reject under V1, by definition of V1.
Sharp increase in the beginning, then the steepness of the curve decreases but you're already at high speeds.
There is an end to the curve : the speed you reach at the end of the normal ASD without ever braking.
Next question is how well can an airliner handle a runway exit at higher speeds. Even on grass.
Hypotheses : 4 knots per second during acceleration (or 2m/sē) and +0.4G deceleration
Calculation based on v1 at 120 knots, giving an ASD of X = 1430 m
Then computation for RTO initiated at 125kt : what is speed at length X ?
58kt !!!
Total ASD for rejection at 125kt : 1545m
I am shocked !!
However, 1430m of ASD sounds pretty short, any runway used by commercial aircraft will be longer than that.
Would anyone care to criticize my hypothesis, confirm whether or not the usual ASD (no margins at all) for your A/C will be this short ? (I tried to be representative of a medium jet)
The runway exit speed as a function of rejection speed looks like this :
https://i.gyazo.com/695a1bd56a8a324f...ac6cbf903b.png
There is a beginning to the curve because runway exit speed does not exist if you reject under V1, by definition of V1.
Sharp increase in the beginning, then the steepness of the curve decreases but you're already at high speeds.
There is an end to the curve : the speed you reach at the end of the normal ASD without ever braking.
Next question is how well can an airliner handle a runway exit at higher speeds. Even on grass.
Hi KayPam,
Your sample figures are interesting in supporting galaxy flyer's voice of experience! Actually, however, the acceleration of 4 kt/sec approaching V1 might be a little optimistic at higher weights. Then, of course, the V1 differential we are considering on this thread is typically, IIRC, about 8 - 10 kt IAS, depending on the aircraft type?
Your sample figures are interesting in supporting galaxy flyer's voice of experience! Actually, however, the acceleration of 4 kt/sec approaching V1 might be a little optimistic at higher weights. Then, of course, the V1 differential we are considering on this thread is typically, IIRC, about 8 - 10 kt IAS, depending on the aircraft type?
Chris,
Welcome back! It looks like you haven't aged a day since this old thread. I dug it out thus morning over coffee.
http://www.pprune.org/tech-log/32726...ter-v1-11.html
Classic thread
Welcome back! It looks like you haven't aged a day since this old thread. I dug it out thus morning over coffee.
http://www.pprune.org/tech-log/32726...ter-v1-11.html
Classic thread
Kay Pam,
Thanks for your efforts, much obliged. Not to worry about the 1430m runway length, the "test" is a situation where runway available equals accelerate-stop distance of the plane in question. Your numbers would be correct for a mid-weight Global Express business jet, perhaps V1 being a bit high, but in the right area for that 1430m ASDA.
Go to this thread and look at posts 250-254, please
http://www.pprune.org/tech-log/32726...ter-v1-13.html
Thanks for your efforts, much obliged. Not to worry about the 1430m runway length, the "test" is a situation where runway available equals accelerate-stop distance of the plane in question. Your numbers would be correct for a mid-weight Global Express business jet, perhaps V1 being a bit high, but in the right area for that 1430m ASDA.
Go to this thread and look at posts 250-254, please
http://www.pprune.org/tech-log/32726...ter-v1-13.html
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Obviously anything built up will be a catastrophy if met at a high speed.
Chris : actually the problem lies in the supplementary distance traveled when you're above v1. Since you're fastest, its when you lose the most runway per unit second (by definition of speed, we call that a Lapalissade in French).
If you were applying full power instead of v1 not during 2 seconds just after v1, but during 2 seconds at 60kt in deceleration, then i'm pretty sure the speed values when exiting the runway would be much more low.
Sure the acceleration would not help neither...
Galaxy flyer : you could abort if you were pretty sure that you can achieve more braking power than the braking power taken into account in the v1 calculation.
I've seen airliners decelerate at +0.8G, that's almost as much as crushing your car brake pedal.
If you had this kind of deceleration (0.8G instead of 0.4) then the few knots would not be a problem.
Obviously what lies beyond the runway should be taken into account as well..
For instance i'm not sure whether full reverse thrust is taken into account ?
I think upslope isn't taken into account as well in the ASD calculation ?
Is it the same for headwind, I think you can only account for 50% of the headwind ?
Kay Pam
Slope is taken into account. I'm not sure about commercial ops, but USAF we assumed zero wind unless needed then the 50% of the headwind component. In corporate ops (Global Express, pour moi) we never accounted for the wind, but performance was rarely a limiting factor.
Going back I see you used 0.4G deceleration, I think the 0.8G is better, but dependent on the crew fully applying the brakes. The referenced thread spoke of random tests and pilots typically only achieved 75% of max line pressure as the habit patterns of normal landings caused a bit of "laziness" on brake application. One really has to stand on the pedals to achieve full anti-skid braking.
Merci beaucoup
Slope is taken into account. I'm not sure about commercial ops, but USAF we assumed zero wind unless needed then the 50% of the headwind component. In corporate ops (Global Express, pour moi) we never accounted for the wind, but performance was rarely a limiting factor.
Going back I see you used 0.4G deceleration, I think the 0.8G is better, but dependent on the crew fully applying the brakes. The referenced thread spoke of random tests and pilots typically only achieved 75% of max line pressure as the habit patterns of normal landings caused a bit of "laziness" on brake application. One really has to stand on the pedals to achieve full anti-skid braking.
Merci beaucoup
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underfile:
Not if the airplane is using a valid OEI procedure, and all planning and performance requirements were complied with by the operator and the flight crew.
If you are at 15' or even 35' at the end of the runway, you have far bigger issues to be concerned about....
Quote from underfire:
"If you are at 15' or even 35' at the end of the runway, you have far bigger issues to be concerned about...."
You've evidently lived a sheltered life? Try a B707-320B/C at MTOW out of LAX in summer, bound for London. And, by the way, we are talking about the end of the clearway, not the runway...
"If you are at 15' or even 35' at the end of the runway, you have far bigger issues to be concerned about...."
You've evidently lived a sheltered life? Try a B707-320B/C at MTOW out of LAX in summer, bound for London. And, by the way, we are talking about the end of the clearway, not the runway...
