reversers vs brakes
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To those of you asking about stopping distances with regard to use of reversers; remember that the auto-brake selects a deceleration RATE, hence the total stopping distance will be the same with or without the use of reverser. Applying the standard 70% reverse will simply reduce the amount of braking applied, hence reducing wear and brake temps.
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the airplane must be able to stop on 60% of the available runway
.. but be aware that, on a limiting runway, you had best push the pedals through the floor to achieve something approaching the raw data figures ...
Most who have had any exposure to performance landing tests have a jaundiced view of such comments ..
I'd really hate to get near the far end of the seal and then find out I have a problem .. as opposed to putting some effort into slowing down early and then finding out that I don't have a problem ... but call me a dinosaur, I guess.
Company economics are important .. but will not cut much slack for you at the enquiry after you bend the bird ... as a very wise checkie observed at the debrief years ago .. the book usually has the most important words left off the front page .. 'to be read with a bit of commonsense'
.. but be aware that, on a limiting runway, you had best push the pedals through the floor to achieve something approaching the raw data figures ...
Most who have had any exposure to performance landing tests have a jaundiced view of such comments ..
I'd really hate to get near the far end of the seal and then find out I have a problem .. as opposed to putting some effort into slowing down early and then finding out that I don't have a problem ... but call me a dinosaur, I guess.
Company economics are important .. but will not cut much slack for you at the enquiry after you bend the bird ... as a very wise checkie observed at the debrief years ago .. the book usually has the most important words left off the front page .. 'to be read with a bit of commonsense'
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Note about drag chutes
Fully agree with you, John T...!
xxx
To the few of you flying some airplanes without reversers, such as Learjets, with drag chutes... just a word...
If you think you need to use the drag chute, do it right after touch-down...
They work great... except with bad crosswinds.
xxx
Obviously, you do not want to have to go out of the airplane to pack the drag chute after use...
What a mess of dirt, mud, and... all that on your new uniform...
Then you trip face down with your legs tangled in the ropes...
As a general rule, the junior pilot is on duty to pick-up the fabric...
xxx
I wrote "deploy immediately after touch down"... but you did not read that...
So, you wait until 50-60 knots to decide, and the end of the runway is coming...
Too late...! At that speed, the drag chute has same effectiveness as a piece of wet Kleenex tissue.
But you will keep the uniform clean... Just dirty shoes with the mud beyond the end of the stopway.
xxx
Happy contrails
xxx
To the few of you flying some airplanes without reversers, such as Learjets, with drag chutes... just a word...
If you think you need to use the drag chute, do it right after touch-down...
They work great... except with bad crosswinds.
xxx
Obviously, you do not want to have to go out of the airplane to pack the drag chute after use...
What a mess of dirt, mud, and... all that on your new uniform...
Then you trip face down with your legs tangled in the ropes...
As a general rule, the junior pilot is on duty to pick-up the fabric...
xxx
I wrote "deploy immediately after touch down"... but you did not read that...
So, you wait until 50-60 knots to decide, and the end of the runway is coming...
Too late...! At that speed, the drag chute has same effectiveness as a piece of wet Kleenex tissue.
But you will keep the uniform clean... Just dirty shoes with the mud beyond the end of the stopway.
xxx
Happy contrails
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The regulations (in the US, anyway) don’t require the use of reverse thrust to calculate landing distances or accelerate-stop distances, but the rules don’t prohibit that use either. What they do say, in both situations, is that another “means” may be used other than wheel brakes provided whatever is used is safe and reliable; is used so that consistent results can be expected in service; and is such that exceptional piloting skill is not required to control the airplane.
In fact, you might recall the Southwest accident at Midway airport some time back? At that time SWA used an “on-board” calculator (some said it was a laptop computer that the F/O lugged around) to calculate the landing distance required using reverse thrust – meaning that it had to work. I’m under the impression that they don’t use that same procedure anymore.
In fact, you might recall the Southwest accident at Midway airport some time back? At that time SWA used an “on-board” calculator (some said it was a laptop computer that the F/O lugged around) to calculate the landing distance required using reverse thrust – meaning that it had to work. I’m under the impression that they don’t use that same procedure anymore.
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Interesting thread...
Of course, your company's SOP is what matters...nothing else.
At my company, the 'normal' landing uses max reverse thrust...sans local noise requirements and any other special airport requirements, MEL, non-normals, etc.. However, it IS in our SOP that the captain can elect to use idle reverse, if he feels conditions are such that it is prudent to do so.
Interestingly, we are required to use autobrake on every landing (except for MEL and other situations that require otherwise). The logic is that, with carbon brakes, the number of brake applications, primarily, determines brake wear. And, by requiring autobrakes on every landing, ostensibly only one brake application is used for each landing.
Personally, I use max reverse on every landing, except where prohibited (as stated above). However, I've noticed...and this is just a non-scientific observation...that the leading edges of the engine fan blades seem to get less chewed up when guys use idle reverse. (I may be totally full of XXXX on this one...so, I qualify this...feel free to take pot shots at me!
) Most of us try to do rolling takeoffs for this reason, as well.
Fly safe,
PantLoad
At my company, the 'normal' landing uses max reverse thrust...sans local noise requirements and any other special airport requirements, MEL, non-normals, etc.. However, it IS in our SOP that the captain can elect to use idle reverse, if he feels conditions are such that it is prudent to do so.
Interestingly, we are required to use autobrake on every landing (except for MEL and other situations that require otherwise). The logic is that, with carbon brakes, the number of brake applications, primarily, determines brake wear. And, by requiring autobrakes on every landing, ostensibly only one brake application is used for each landing.
Personally, I use max reverse on every landing, except where prohibited (as stated above). However, I've noticed...and this is just a non-scientific observation...that the leading edges of the engine fan blades seem to get less chewed up when guys use idle reverse. (I may be totally full of XXXX on this one...so, I qualify this...feel free to take pot shots at me!
) Most of us try to do rolling takeoffs for this reason, as well.Fly safe,
PantLoad
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MFS has queried a comment I made here (and now have deleted) regarding certification aspects of reverse thrust .. as I can't lay my hand on the regulatory derivation, it is best that the comment be removed to avoid confusion. It may be that my memory has placed the requirement in the wrong jurisdiction ...
Certified landing distance = from 50 feet above the landing surface in the landing configuration at an airspeed of 1.3Vslg to where the aircraft comes to a stop using maximum wheel braking
Landing distance (Dry) = 1.67*certified landing distance
Landing distance (Wet) = 1.15*dry landing distance
Reverse thrust is not used in these calculations because it is considered as an additional safety factor.
Letter from Boeing to QANTAS July 1997
…..Additionally, some of the techniques we have heard discussed, such as reduced landing flap settings and the use of idle reverse thrust, have a negative impact on airplane stopping performance. Therefore, these techniques are not recommended as standard practice.
Extract from report into QANTAS overrun Bangkok 23 September 1999.
The flight crew did not select (or notice the absence of) full reverse thrust. The use of full reverse thrust would have substantially reduced the landing distance on a runway with poor braking action. The failure of the crew to consider the use of full reverse thrust during the landing roll appeared to be primarily due to the high workload they were experiencing. Had the crew received more training in the importance of reverse thrust on water affected runways, or recent experience in the use of reverse thrust, it is reasonable to expect that the crews awareness of the importance of reverse thrust (and therefore the likelihood of them selecting full reverse thrust) would have been greater.
QANTAS at the time had no simulator exercise requiring the use of full reverse thrust. Idle reverse was the ‘standard’ procedure.
Letter from Boeing to QANTAS April 2000
Boeing does not consider the standard practice of going to reverse idle (idle detent) only to be patently unsafe, but does think that it reduces the existing performance margins. It is acceptable pilot technique to do this (using good judgement) as an exception to the normal procedures when landing on a long, dry runway. We perceive, however, that there is a human factors issue of developing a habit pattern of not using reverse thrust beyond the idle detent. The pilot may then fail to respond quickly when such reverse thrust is needed during an RTO (rejected takeoff) or landing in some type of performance critical situation. We therefore do not provide a “No Technical Objection” for this as a standard operating policy.
If the reverse idle technique is adopted, it should be taught as the exception rather than the rule. Further, we would encourage simulator drills to be incorporated into the transition and recurrent training courses that would require pilots to use judgement to use full reverse thrust as the best successful means of stopping the airplane. This would periodically reinforce this concept of using that capability when needed.
Do the Boeing manuals not advocate the use of full reverse thrust as the BKK report suggests?
The best option is to use IDLE reverse when operationally safe and feasible. This is the best balance of cost vs. options for safety. This allows the selection of more reverse if required for unforeseen circumstances without having to wait for them to deply
Idle reverse is probably better in the high speed regime than no reverse but not by much. You can count on hot brakes when using just idle reverse. The Boeing 737 FCTM states that the use of minimum reverse thrust as compared to maximum reverse thrust can double the brake energy requirements and result in brake temperatures much higher than normal.
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747 reversers and brakes
OK... I will quote my "bible" here, FCTM 747-200, for JT9D-7Q engines.
At the end, I will pass along some personal techniques, and suggestions as I am active as check-captain.
xxx
(FCTM) Reverse Thrust
To obtain maximum reverse thrust, after the interlock release, pull the reverse levers up to the stops. Maintain maximum reverse thrust until the airspeed approaches 60 knots. Start modulating the reverse thrust approaching this speed so that the reverse levers are moving down at a rate commensurate with the deceleration rate of the airplane. A reverse thrust level of 60% N1 should be reached by 40 knots. This reverse thrust level may be maintained to a safe taxi speed or a full stop if required. It is important that the reverse thrust levers ve modulated towards the lower thrust settings at the designated airspeed to prevent engine surging.
(...)
Changes of command to the trust reversers while in the mid-stroke between reverse and forward thrust or vice-versa can damage the actuating mechanisms. They require approximately 2 seconds to extend, and 5 seconds to stow to forward thrust.
(...)
The use of reverse thrust below approximately 40 knots should be avoided (unless required for adverse conditions) due to possible surging and foreign object ingestion. - CAUTION - Do not attempt a go-around after reverse thrust has been initiated. Factors dictating this are - 5 seconds are required for a reverser to stow in the forward thrust position, and a possibility exists that a reverser may not stow in the forward thrust position.
(...)
FCTM - Auto Brakes
MAX - This setting should only be used when a minimum stop distance is mandatory. The distance to a full stop will be comparable to the actual certified landing distance on a dry runway. Operation of this setting results in a full 3000 psi being applied to the anti-skid modules. Reverse thrust does not result modulation of auto brake pressure but is additive to the total deceleration level achieved.
MED - This setting should be used for wet or slippery runways or when positive braking is imperative, and for all landings where rollout distance is limited. Operating at this setting limits applied brake pressure to 1800 psi, and will provide a wet or slippery rinway deceleration with optimum anti-skid performance. Thrust reverser operation will result in modulation of the applied brake pressure.
MIN - This setting will provide a moderate deceleration effect suitable for all routine operations. Thrust reverser operation will result in modulation of the applied brake pressure.
(...)
My notes in briefings...
We generally fly approaches at Vref+5 Kts, with 25 flaps for reduction of noise and fuel burn. I stress the use of 30 flaps when runways are wet/slippery for touch down. When landing is assured, at 50 feet over the threshold, it is acceptable to select flaps 30 if approach was performed with 25 flaps. Let the speed bleed off, with no power increase. Further speed reduction from Vref+5 to Vref when 30 feet above runway (start of power reduction and flare) is acceptable, probably achieving a Vref-5 touchdown.
xxx
I personally suggest immediate pull of the 4 reversers upon touch down, which is indicated by deployment of speed brakes. I suggest immediate full operation of inboard reversers (little concern for asymetric power) then careful operation to full reverse of the outboard engines, in case of power asymetry. In case of engine out (such as in training) for an outboard engine, werecommend deployment of opposite engine to idle reverse only. For the case of inboard engine, we recommend partial reverse thrust.
xxx
Minimum speeds for use of reverse, as copied above from the FCTM. My experience has shown that these are "inboard engines" limitations, as the reverse flow of inboards disrupts the inlet of outboard engines, which are located further aft of the inboards. When moduling power to reduce the reverse thrust, I fully abide by the reduction, however I "deduct" 10 knots for the outboard engines... in practice, reducing the inboards as published to achieve 60% N1 by 40 knots for the inboards, but leaving more power ("1 extra knob worth") on the outboards, to achieve 60% N1 by... 30 knots.
xxx
A few years ago, on a slippery short runway in Chicago ORD, one of our 747 ended with nosewheel beyond the end of the runway, because the PF slammed the reversers from high power setting reverser (to a full stop) but got a burst of unexpected foward thrust, after the reversers did stow. So, my personal technique, is to stay with idle reverse, until airplane taxiing speed (and turnoff, if applicable) is achieved.
xxx
Happy contrails
At the end, I will pass along some personal techniques, and suggestions as I am active as check-captain.
xxx
(FCTM) Reverse Thrust
To obtain maximum reverse thrust, after the interlock release, pull the reverse levers up to the stops. Maintain maximum reverse thrust until the airspeed approaches 60 knots. Start modulating the reverse thrust approaching this speed so that the reverse levers are moving down at a rate commensurate with the deceleration rate of the airplane. A reverse thrust level of 60% N1 should be reached by 40 knots. This reverse thrust level may be maintained to a safe taxi speed or a full stop if required. It is important that the reverse thrust levers ve modulated towards the lower thrust settings at the designated airspeed to prevent engine surging.
(...)
Changes of command to the trust reversers while in the mid-stroke between reverse and forward thrust or vice-versa can damage the actuating mechanisms. They require approximately 2 seconds to extend, and 5 seconds to stow to forward thrust.
(...)
The use of reverse thrust below approximately 40 knots should be avoided (unless required for adverse conditions) due to possible surging and foreign object ingestion. - CAUTION - Do not attempt a go-around after reverse thrust has been initiated. Factors dictating this are - 5 seconds are required for a reverser to stow in the forward thrust position, and a possibility exists that a reverser may not stow in the forward thrust position.
(...)
FCTM - Auto Brakes
MAX - This setting should only be used when a minimum stop distance is mandatory. The distance to a full stop will be comparable to the actual certified landing distance on a dry runway. Operation of this setting results in a full 3000 psi being applied to the anti-skid modules. Reverse thrust does not result modulation of auto brake pressure but is additive to the total deceleration level achieved.
MED - This setting should be used for wet or slippery runways or when positive braking is imperative, and for all landings where rollout distance is limited. Operating at this setting limits applied brake pressure to 1800 psi, and will provide a wet or slippery rinway deceleration with optimum anti-skid performance. Thrust reverser operation will result in modulation of the applied brake pressure.
MIN - This setting will provide a moderate deceleration effect suitable for all routine operations. Thrust reverser operation will result in modulation of the applied brake pressure.
(...)
My notes in briefings...
We generally fly approaches at Vref+5 Kts, with 25 flaps for reduction of noise and fuel burn. I stress the use of 30 flaps when runways are wet/slippery for touch down. When landing is assured, at 50 feet over the threshold, it is acceptable to select flaps 30 if approach was performed with 25 flaps. Let the speed bleed off, with no power increase. Further speed reduction from Vref+5 to Vref when 30 feet above runway (start of power reduction and flare) is acceptable, probably achieving a Vref-5 touchdown.
xxx
I personally suggest immediate pull of the 4 reversers upon touch down, which is indicated by deployment of speed brakes. I suggest immediate full operation of inboard reversers (little concern for asymetric power) then careful operation to full reverse of the outboard engines, in case of power asymetry. In case of engine out (such as in training) for an outboard engine, werecommend deployment of opposite engine to idle reverse only. For the case of inboard engine, we recommend partial reverse thrust.
xxx
Minimum speeds for use of reverse, as copied above from the FCTM. My experience has shown that these are "inboard engines" limitations, as the reverse flow of inboards disrupts the inlet of outboard engines, which are located further aft of the inboards. When moduling power to reduce the reverse thrust, I fully abide by the reduction, however I "deduct" 10 knots for the outboard engines... in practice, reducing the inboards as published to achieve 60% N1 by 40 knots for the inboards, but leaving more power ("1 extra knob worth") on the outboards, to achieve 60% N1 by... 30 knots.
xxx
A few years ago, on a slippery short runway in Chicago ORD, one of our 747 ended with nosewheel beyond the end of the runway, because the PF slammed the reversers from high power setting reverser (to a full stop) but got a burst of unexpected foward thrust, after the reversers did stow. So, my personal technique, is to stay with idle reverse, until airplane taxiing speed (and turnoff, if applicable) is achieved.
xxx
Happy contrails
Moderator
Judd,
Your observation probably wasn't terribly helpful, either in its own right or in thread context ... as indicated by the subsequent chastising responses.
Perhaps we might ask that you consider the content of your posts with a little more contemplation than seen in your last ?
Your observation probably wasn't terribly helpful, either in its own right or in thread context ... as indicated by the subsequent chastising responses.
Perhaps we might ask that you consider the content of your posts with a little more contemplation than seen in your last ?
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CENTAURUS
Would you be so kind to explain why the amount of reverse thrust (ie. force) markedly decreases with decreasing speed? and why "markedly so below 80kts"????
Me thinks the reverse force remains roughly equal during deceleration. Of course, the impact on landing distance is hugely dependent on what speed you initiate reverse.... but thats a different story and it applies to any type of retarding force.
From 100 knots down, reverse thrust is less effective and markedly so below around 80 knots.
Me thinks the reverse force remains roughly equal during deceleration. Of course, the impact on landing distance is hugely dependent on what speed you initiate reverse.... but thats a different story and it applies to any type of retarding force.
Contrary to comments about not using reversers at lower speeds, US operators use reverse thrust in some cases to back the aircraft away from the gate without external assistance, in a procedure known as "powerback".
In Europe this seems unknown, and when you first see it done in the US you can't quite believe it. The power level seems very high for the slow rearward speed attained - slower than a tug might push at. Why is this acceptable in the US but not elsewhere ?
In Europe this seems unknown, and when you first see it done in the US you can't quite believe it. The power level seems very high for the slow rearward speed attained - slower than a tug might push at. Why is this acceptable in the US but not elsewhere ?
Would you be so kind to explain why the amount of reverse thrust (ie. force) markedly decreases with decreasing speed? and why "markedly so below 80kts"????
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Because the amount of reverse thrust consists of two components: the ram drag (which as usual is a function of the amount of air going in the front end, and its speed) and the gross thrust (which, unusually, is being directly 'vaguely' forwards, with various engines doing so to varying degrees of effectiveness).
The gross thrust term is more-or-less independent of the forward speed, and depends on basic engine parameters. Important here are the amount by which the flow is directed forward - often not much, indeed some designs don't divert the core flow at all. This is one reason why powerback is often a problem (the other being reingestion).
The ram drag term is basically the mass flow into the intake times the forward velocity. Since the mass flow is also dependent on the velocity, this makes it a velocity^2 term. So the ram drag term at 100knots is (10/8)^2= 56% higher than at 80knots, and (10/6)^2=178% higher than at 60knots. Or, if you prefer, if 100knots=100% reverse thrust, then I only have 64% at 80knots and 36% at 60knots.
Since the ram drag term can be a significant proportion of the total reverse thrust, I can easily have lost 50% of my reverse thrust by 60knots or so. That means a potentially significant drop in deceleration.
Next, what really matters in terms of stopping isn't deceleration - that's just a means to the end, which is distance. From the simple equations of motion, distance covered, s, between speeds u and v at a constant acceleration is:
s= (v^2 - u^2) /2a.
Between, say, 110 and 90 knots, the bit in the brackets is -4000.
Between, say, 80 and 60, it's -2800.
So if I could gain, or lose, 10% of my decel, it would be worth 40/28 times more in terms of distance if I could arrange to get the extra decel over the higher speed range. Basically, it's like compound interest; every little counts at the start of the decel run, but by the time you're 'trundling', the distance isn't so sensitive to the exact accel/decel.
So, two things at work:
1. Due to the way most reverse thrust is actually implemented, you get more actual decel at higher speeds; and
2. Any decel at high speed is much more effective in reducing landing roll than the same decel at lower speeds.
The gross thrust term is more-or-less independent of the forward speed, and depends on basic engine parameters. Important here are the amount by which the flow is directed forward - often not much, indeed some designs don't divert the core flow at all. This is one reason why powerback is often a problem (the other being reingestion).
The ram drag term is basically the mass flow into the intake times the forward velocity. Since the mass flow is also dependent on the velocity, this makes it a velocity^2 term. So the ram drag term at 100knots is (10/8)^2= 56% higher than at 80knots, and (10/6)^2=178% higher than at 60knots. Or, if you prefer, if 100knots=100% reverse thrust, then I only have 64% at 80knots and 36% at 60knots.
Since the ram drag term can be a significant proportion of the total reverse thrust, I can easily have lost 50% of my reverse thrust by 60knots or so. That means a potentially significant drop in deceleration.
Next, what really matters in terms of stopping isn't deceleration - that's just a means to the end, which is distance. From the simple equations of motion, distance covered, s, between speeds u and v at a constant acceleration is:
s= (v^2 - u^2) /2a.
Between, say, 110 and 90 knots, the bit in the brackets is -4000.
Between, say, 80 and 60, it's -2800.
So if I could gain, or lose, 10% of my decel, it would be worth 40/28 times more in terms of distance if I could arrange to get the extra decel over the higher speed range. Basically, it's like compound interest; every little counts at the start of the decel run, but by the time you're 'trundling', the distance isn't so sensitive to the exact accel/decel.
So, two things at work:
1. Due to the way most reverse thrust is actually implemented, you get more actual decel at higher speeds; and
2. Any decel at high speed is much more effective in reducing landing roll than the same decel at lower speeds.
Reverse thrust to back out of the gate
Question answered. you end of backing away from the reverse efflux not running into it. So no increased FOD problem unless you power up enough to suck a vortex off the ramp before you move
... There is also the question of FOD.
