Idle reverse policy on landing
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Idle reverse policy on landing
For many reasons, individual company SOP may direct idle reverse after touch-down - unless of course good airmanship dictates otherwise. It may be for fuel conservation, perceived engineering costs or noise-restrictions.
In the CFM 56 series (737), idle reverse N1 is the same as idle forward thrust N1 (nominally 23% N1) - except it takes a bit of fiddling to get the reverse thrust levers at idle reverse without inadvertently slipping into forward thrust. The theory of selecting idle reverse only is that instant full reverse is available should things look a bit dodgy and the end of the runway is coming up faster than planned.
The problem then arises of the relatively long delay in spool-up to max reverse from idle reverse (23% N1) Try it in the simulator and you are looking at 6-8 seconds before full reverse is reached. Because of the extended spool-up period, by the time full reverse is attained the airspeed has probably dropped below 80-100 knots when reverse is not as effective.
I understand N1 remains at high (in-flight) idle with thrust levers closed at touch down and if reverse has not been selected within four seconds after touch-down, the power automatically decays down to ground idle, which works out to be around seven percent N1 less than flight idle.
Presumably this is based on the assumption by the designer that if you haven't selected reverse by four seconds after touch-down then you aren't going to use it and so the power drops back to ground idle (less residual thrust).
The point is this. If company policy is to use only idle reverse for landings then crews should ensure the reverse levers are correctly positioned to give at least (say) ten percent above ground idle in order to permit rapid spool up to max reverse if needed. The spool-up time from ground idle reverse (nominally 23% N1) to full reverse, is significantly longer than the spool-up time from (say) 35% N1 reverse thrust. By selecting (say) 35% N1 reverse thrust after touch-down, the landing roll safety factor is increased with little discernible increase in noise footprint and/or fuel consumption.
Any thoughts?
In the CFM 56 series (737), idle reverse N1 is the same as idle forward thrust N1 (nominally 23% N1) - except it takes a bit of fiddling to get the reverse thrust levers at idle reverse without inadvertently slipping into forward thrust. The theory of selecting idle reverse only is that instant full reverse is available should things look a bit dodgy and the end of the runway is coming up faster than planned.
The problem then arises of the relatively long delay in spool-up to max reverse from idle reverse (23% N1) Try it in the simulator and you are looking at 6-8 seconds before full reverse is reached. Because of the extended spool-up period, by the time full reverse is attained the airspeed has probably dropped below 80-100 knots when reverse is not as effective.
I understand N1 remains at high (in-flight) idle with thrust levers closed at touch down and if reverse has not been selected within four seconds after touch-down, the power automatically decays down to ground idle, which works out to be around seven percent N1 less than flight idle.
Presumably this is based on the assumption by the designer that if you haven't selected reverse by four seconds after touch-down then you aren't going to use it and so the power drops back to ground idle (less residual thrust).
The point is this. If company policy is to use only idle reverse for landings then crews should ensure the reverse levers are correctly positioned to give at least (say) ten percent above ground idle in order to permit rapid spool up to max reverse if needed. The spool-up time from ground idle reverse (nominally 23% N1) to full reverse, is significantly longer than the spool-up time from (say) 35% N1 reverse thrust. By selecting (say) 35% N1 reverse thrust after touch-down, the landing roll safety factor is increased with little discernible increase in noise footprint and/or fuel consumption.
Any thoughts?
Last edited by Centaurus; 21st May 2012 at 13:07.
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Not a good analysis, IMO.
Reverse thrust is most effective at high forward speeds, so you should not try to rely on more reverse thrust as a last-ditch attempt to save a too-long/fast rollout. Also, unless you want to FOD your engines, using more than idle reverse at less than 60 KIAS or so is inadvisable.
At low speeds, idle reverse will remove any residual forward thrust, making braking and steering more effective on slippery surfaces like wet paint stripes.
OTOH, if you want to use more than idle reverse early in the landing roll, then just do it.
Reverse thrust is most effective at high forward speeds, so you should not try to rely on more reverse thrust as a last-ditch attempt to save a too-long/fast rollout. Also, unless you want to FOD your engines, using more than idle reverse at less than 60 KIAS or so is inadvisable.
At low speeds, idle reverse will remove any residual forward thrust, making braking and steering more effective on slippery surfaces like wet paint stripes.
OTOH, if you want to use more than idle reverse early in the landing roll, then just do it.
Centaurus, the technical logic of your argument appears sound, but choosing to have reverse available ‘just-in-case’ might be an ineffective tactical option.
As you say it takes time for the engine to spool-up, perhaps even longer for the mental decision to spool-up to recognise the need for more retardation.
From a landing safety perspective it might be more efficient (safety) to decide to use some higher level reverse as judged in the pre landing briefing based on the reported conditions, or even by SOP, and then during landing when assured of stopping, reverse can reduced to idle. Thus the suggested option of a ‘higher idle’ is moving in a safe direction.
Unfortunately many of the operational decisions are influenced by commercial efficiency, where safety aspects are deferred to late ‘tactical’ crew decisions exposing the crew to the frailties of human behaviour and performance.
Brakes may be cheaper than engine overhaul, but neither compares with the cost of an overrun.
As you say it takes time for the engine to spool-up, perhaps even longer for the mental decision to spool-up to recognise the need for more retardation.
From a landing safety perspective it might be more efficient (safety) to decide to use some higher level reverse as judged in the pre landing briefing based on the reported conditions, or even by SOP, and then during landing when assured of stopping, reverse can reduced to idle. Thus the suggested option of a ‘higher idle’ is moving in a safe direction.
Unfortunately many of the operational decisions are influenced by commercial efficiency, where safety aspects are deferred to late ‘tactical’ crew decisions exposing the crew to the frailties of human behaviour and performance.
Brakes may be cheaper than engine overhaul, but neither compares with the cost of an overrun.
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Centaurus
there will be probably no right answer for your question.
Personally I consider the company SOP and most of the time idle REV it is.
Only after having thought about and briefed the landing mass, any a/c unserviceabilities versus available landing distance and the prevailing weather conditions.
On a dry and even sometimes wet runway Idle REV usually is more than sufficient.
If the runway is wet&slippery or contaminated than that is a different story.
Even on a long dry runway with a too long flare I will insist on Full REV being used.
SOP's cannot cover all cases and I think experience and aerodrome familiarity come into play as well, even flight crew recency issues.
Anyhow I think you should think about, brief it and be ready to amend it when necessary, but I do not believe that an intermediate setting (35%) will be a very practical approach.
Personally I consider the company SOP and most of the time idle REV it is.
Only after having thought about and briefed the landing mass, any a/c unserviceabilities versus available landing distance and the prevailing weather conditions.
On a dry and even sometimes wet runway Idle REV usually is more than sufficient.
If the runway is wet&slippery or contaminated than that is a different story.
Even on a long dry runway with a too long flare I will insist on Full REV being used.
SOP's cannot cover all cases and I think experience and aerodrome familiarity come into play as well, even flight crew recency issues.
Anyhow I think you should think about, brief it and be ready to amend it when necessary, but I do not believe that an intermediate setting (35%) will be a very practical approach.
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check out:
Idle reverse versus full reverse thrust: the balance between safety, economics, and noise
Paper presented at the 24th annual European Aviation Safety Seminar (EASS)
Dublin, Ireland, February 29 – March 1, 2012
Idle reverse versus full reverse thrust: the balance between safety, economics, and noise
Paper presented at the 24th annual European Aviation Safety Seminar (EASS)
Dublin, Ireland, February 29 – March 1, 2012
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My take on our policy of minimum of idle reverse on every landing,certainly on later boeings is that, even with speedbrake not in the standard "armed" position, the ground spoilers will activate...They will not activate if the speedbrake lever were inadvertently left in the "down" position, with only idle thrust selected...
Also, in the case of a bounced landing, an armed speedbrake may disarm...so, another advantage of always using a minimum of idle reverse on every landing..well..on mine anyway...I think Miss Pamela Anderson would agree with me..sorry Pamela..ouch!
Also, in the case of a bounced landing, an armed speedbrake may disarm...so, another advantage of always using a minimum of idle reverse on every landing..well..on mine anyway...I think Miss Pamela Anderson would agree with me..sorry Pamela..ouch!
Last edited by Yaw String; 21st May 2012 at 15:35.
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brake temperature
With autobrake 1-2-3 on a runway with good braking action reverse thrust above idle does not give you any advantage in relation to landing distance.
As soon as reverse is starting to assist in the deceleration, the pressure on the wheel brakes is being reduced, as autobrake is looking for a certain deceleration rate. So in normal circumstances reverse thrust is only easing the use of the brakes, which keeps your brake temperature nicely under control - however your landing distance will be exactly the same with or without T/R.
Things are different in autobrake max, max manual braking or runway conditions where anti skid is going to operate: 2nd or 3rd detent is going to give you a nice reduction in landing distance (if you instantly use it, as it was already said before...)
As soon as reverse is starting to assist in the deceleration, the pressure on the wheel brakes is being reduced, as autobrake is looking for a certain deceleration rate. So in normal circumstances reverse thrust is only easing the use of the brakes, which keeps your brake temperature nicely under control - however your landing distance will be exactly the same with or without T/R.
Things are different in autobrake max, max manual braking or runway conditions where anti skid is going to operate: 2nd or 3rd detent is going to give you a nice reduction in landing distance (if you instantly use it, as it was already said before...)
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I cannot quite understand the 'aversion to'/issues with idle reverse
1) as FA10 says, the deceleration will be identical with and without reverse if autobrake is used, and
2) LDAs are checked for the reverse idle condition.
The only time 1) is not true is when braking action is compromised, when you would not be using reverse idle anyway.
As Dr F says , by the time you might realise you should have used it, even from a high idle you have troubles.
1) as FA10 says, the deceleration will be identical with and without reverse if autobrake is used, and
2) LDAs are checked for the reverse idle condition.
The only time 1) is not true is when braking action is compromised, when you would not be using reverse idle anyway.
As Dr F says , by the time you might realise you should have used it, even from a high idle you have troubles.
And there is no way of knowing what your min turnaround time is with idle reverse. Numbers only for no reverse and 2nd detent.
Idle reverse is a bad habit. It builds a 'muscle memory' and a mentality that has led flight crews to not use full reverse even when it is needed.
More than one runway overrun has been attributed to this.
Certainly the autobrakes will 'back off' to provide the requested deceleration rate when full reverse is used.
That is fine, unless you get the reverse in at higher speeds it is far less effective anyway. Once you have selected max rev you can brake as necessary.
This allows maximum use of all braking / deceleration devices as they are most effective.
Remember when this was standard ?
I have flown with more than one FO who was afraid to use max reverse even when required, concerned he would 'break something' or use an extra 100 pounds of fuel.
I use max reverse on every landing and brake as necessary, I know you will go on about carbon brake heating, so what, who cares about the brakes when you are off the end of the runway ?
More than one runway overrun has been attributed to this.
Certainly the autobrakes will 'back off' to provide the requested deceleration rate when full reverse is used.
That is fine, unless you get the reverse in at higher speeds it is far less effective anyway. Once you have selected max rev you can brake as necessary.
This allows maximum use of all braking / deceleration devices as they are most effective.
Remember when this was standard ?
I have flown with more than one FO who was afraid to use max reverse even when required, concerned he would 'break something' or use an extra 100 pounds of fuel.
I use max reverse on every landing and brake as necessary, I know you will go on about carbon brake heating, so what, who cares about the brakes when you are off the end of the runway ?
Last edited by stilton; 22nd May 2012 at 04:17.
More than one runway overrun has been attributed to this.
This allows maximum use of all braking / deceleration devices as they are most effective.
Last edited by Wizofoz; 22nd May 2012 at 08:12.
Really? Can you name one?
One over-run negates the savings of a thousand idle reverse landings.
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Other factors involved
Certainly other factors were involved in the QF overrun at Bangkok. Where to start? Contaminated runway, landed long and hot, lack of communication between LH & RH (Handling Pilot) seats, 25 Flap, Go Around countermanded without notification, one thrust lever left "stood up" and developing around 1.50 EPR during aborted Go Around before being retarded, Full Reverse not used, off the end at 88 Knots. From memory, less than ten observed actual Full Reverse landings between the three cockpit crew and few Full Flap landings. For mine using Reverse is like using Carby Heat. All symmetrical available, Full Reverse / Full Carby Heat or nothing.
Last edited by Old Fella; 23rd May 2012 at 04:14. Reason: Clearer statement
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The outspoken wisemen have had their say...instantly condemning those who do not subscribe to their views...so, name some more of my bad habits..well, apart from shaking Ms Anderson's breasts on that bounced landing!
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stilton
Idle reverse is a bad habit. It builds a 'muscle memory' and a mentality that has led flight crews to not use full reverse even when it is needed.
Valid point, but if you do a hands on drill and actually simulate what you are intending to do, then you retrain yourself. I always do it, before lining up a touch drill for your unlikely RTO and inflight for your landing
More than one runway overrun has been attributed to this.
exactly it was a contributing factor, but usually they did not fully deploy them after they were either hot&high and overflared
Certainly the autobrakes will 'back off' to provide the requested deceleration rate when full reverse is used.
That is fine, unless you get the reverse in at higher speeds it is far less effective anyway. Once you have selected max rev you can brake as necessary.
This allows maximum use of all braking / deceleration devices as they are most effective.
Remember when this was standard ?
I have flown with more than one FO who was afraid to use max reverse even when required, concerned he would 'break something' or use an extra 100 pounds of fuel.
Well that is a training issue as far as I am concerned. During their line training the trainers will surely have covered an important topic like that. Breaking something and fuel saving are most certainly not mentioned in a training curriculum as reasons for not using a thrust reverser. If your FO mentions those reasons on the day why not talk about it and let him have a go
I use max reverse on every landing and brake as necessary, I know you will go on about carbon brake heating, so what, who cares about the brakes when you are off the end of the runway ?
I calculate my landing distance that gives me a feel for how much extra distance is available and also what exit point is possible (sometimes ATC throws a vacate via....at you with the landing clearnce). If there is a lot of extra runway available I follow my company"s SOP, if not I use full reverse. But I mentioned it before there are many ways to accomplish the task safely. The same principles are valid for reduced thrust take offs, we don't do full rated thrust TO all the time do we?
Valid point, but if you do a hands on drill and actually simulate what you are intending to do, then you retrain yourself. I always do it, before lining up a touch drill for your unlikely RTO and inflight for your landing
More than one runway overrun has been attributed to this.
exactly it was a contributing factor, but usually they did not fully deploy them after they were either hot&high and overflared
Certainly the autobrakes will 'back off' to provide the requested deceleration rate when full reverse is used.
That is fine, unless you get the reverse in at higher speeds it is far less effective anyway. Once you have selected max rev you can brake as necessary.
This allows maximum use of all braking / deceleration devices as they are most effective.
Remember when this was standard ?
I have flown with more than one FO who was afraid to use max reverse even when required, concerned he would 'break something' or use an extra 100 pounds of fuel.
Well that is a training issue as far as I am concerned. During their line training the trainers will surely have covered an important topic like that. Breaking something and fuel saving are most certainly not mentioned in a training curriculum as reasons for not using a thrust reverser. If your FO mentions those reasons on the day why not talk about it and let him have a go
I use max reverse on every landing and brake as necessary, I know you will go on about carbon brake heating, so what, who cares about the brakes when you are off the end of the runway ?
I calculate my landing distance that gives me a feel for how much extra distance is available and also what exit point is possible (sometimes ATC throws a vacate via....at you with the landing clearnce). If there is a lot of extra runway available I follow my company"s SOP, if not I use full reverse. But I mentioned it before there are many ways to accomplish the task safely. The same principles are valid for reduced thrust take offs, we don't do full rated thrust TO all the time do we?
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I have flown with more than one FO who was afraid to use max reverse even when required, concerned he would 'break something' or use an extra 100 pounds of fuel.
That extra 100# is the premium you pay for the insurance policy against an overrun.
It's only IF you use maximum braking that reverse has any effect on your stopping distance.
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I'm pleased it is not only me challenging this 'fooey'. 'I always use max reverse' - 
Very popular on a 10000ft runway with an exit point at the far end, I guess.
It's called airmanship. You plan the deceleraton method to suit the LDA. I have in my time, paying lip service to the QAR, opened the reversers at idle and then cancelled them to avoid having to apply power to reach the turn-off point on a long runway. According to some here I should have screeched to a juddering halt with TOGA reverse, and then blocked the runway for 2 minutes while I taxy the other 6000'. Yes, as wiz observes, I should also obviously have been using max autobrake as well Doh! Little wonder we are seeing the accidents and incidents that are occurring. Where are these people learning to aviate?
Very popular on a 10000ft runway with an exit point at the far end, I guess.
It's called airmanship. You plan the deceleraton method to suit the LDA. I have in my time, paying lip service to the QAR, opened the reversers at idle and then cancelled them to avoid having to apply power to reach the turn-off point on a long runway. According to some here I should have screeched to a juddering halt with TOGA reverse, and then blocked the runway for 2 minutes while I taxy the other 6000'. Yes, as wiz observes, I should also obviously have been using max autobrake as well Doh! Little wonder we are seeing the accidents and incidents that are occurring. Where are these people learning to aviate?
Last edited by BOAC; 22nd May 2012 at 15:45.
