Use of autobrakes with full reverse. Who is fooling who?
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Use of autobrakes with full reverse. Who is fooling who?
Many 737 operators require autobrakes to be used for all landings as a matter of company policy. This is in addition to the commonsense need for autobrakes for operational reasons such as short runways, strong crosswinds and other situations recommended in the FCTM. One operator for example directs pilots to use auto brake 2 for all landings regardless of runway length.
Typical rate of deceleration figures for the 737-300 with autobrakes operating are given as:
Autobrake 1 1250 psi and 4 feet per second/per second.
Autobrake 2 1500psi and 5ft per second/per second.
Autobrake 3 2000psi and 7.2ft per second/per second.
Autobrake MAX 3000psi 12ft per second/per second below 80 knots and 14ft per second/ per second above 80 knots.
I have no guarantee the above numbers are correct (in fact I have a suspicion that autobrake MAX numbers are a bit out) - but near enough for the purpose of my question below, and that is:
Assume touch down on dry surface using Flap 40, max landing weight (737-300) and full reverse applied immediately. Assume autobrakes are not selected and manual braking not commenced yet. What would be the expected rate of deceleration caused by full reverse initially until down to (say) 80 knots? This includes auto-speed brakes operating.
For example, could the initial rate of deceleration from speed brakes and full reverse on touch down, be as high as the equivalent of (say) autobrake 2? I haven't got a clue which is why I ask the question.
The question arose in the classroom when discussing the autobrake systems where the amount of brake pressure applied is reduced automatically as reverse thrust comes in, in order to maintain a set deceleration rate.
Recently, the captain of a 737 decided to see how quickly the aircraft would pull up with full manual braking in conjunction with full reverse. It proved an unwise and costly exercise. As expected, the aircraft stopped in a remarkably short space. On arrival at the terminal the brakes were very hot and all tyres showed sign of heavy wear and tear including clearly defined flat spots.
Word quickly got around via the QAR and soon there was some spirited discussion over tea and bikkies with the captain pleading the anti-skid operation must have been defective to allow such obvious wear on the tyres. However a check of the anti-skid system showed no such problems and the offender was told he was a naughty boy.. Others would have taken a stronger view but there we are.. Imagine the cost of replacement tyres.
Where company policy mandates use of autobrakes at all times regardless of the circumstances, would it be correct to say that by using full reverse immediately on touch down, the initial rate of deceleration is so high that it far exceeds that of autobrakes 1,2, and even 3? Any use of autobrakes is bound to increase wear on the tyres since they are applied at high speed and thus high energy. So what is the logic of using the autobrakes on normal no sweat landings if the intention is to use full reverse on touch down and leaving manual braking until needed?
The FCTM clearly states the occasions when autobrakes are recommended. In many companies that seems carte blanche to use autobrakes for ALL landings, which is not what the manufacturer intended, I'm sure?
Typical rate of deceleration figures for the 737-300 with autobrakes operating are given as:
Autobrake 1 1250 psi and 4 feet per second/per second.
Autobrake 2 1500psi and 5ft per second/per second.
Autobrake 3 2000psi and 7.2ft per second/per second.
Autobrake MAX 3000psi 12ft per second/per second below 80 knots and 14ft per second/ per second above 80 knots.
I have no guarantee the above numbers are correct (in fact I have a suspicion that autobrake MAX numbers are a bit out) - but near enough for the purpose of my question below, and that is:
Assume touch down on dry surface using Flap 40, max landing weight (737-300) and full reverse applied immediately. Assume autobrakes are not selected and manual braking not commenced yet. What would be the expected rate of deceleration caused by full reverse initially until down to (say) 80 knots? This includes auto-speed brakes operating.
For example, could the initial rate of deceleration from speed brakes and full reverse on touch down, be as high as the equivalent of (say) autobrake 2? I haven't got a clue which is why I ask the question.
The question arose in the classroom when discussing the autobrake systems where the amount of brake pressure applied is reduced automatically as reverse thrust comes in, in order to maintain a set deceleration rate.
Recently, the captain of a 737 decided to see how quickly the aircraft would pull up with full manual braking in conjunction with full reverse. It proved an unwise and costly exercise. As expected, the aircraft stopped in a remarkably short space. On arrival at the terminal the brakes were very hot and all tyres showed sign of heavy wear and tear including clearly defined flat spots.
Word quickly got around via the QAR and soon there was some spirited discussion over tea and bikkies with the captain pleading the anti-skid operation must have been defective to allow such obvious wear on the tyres. However a check of the anti-skid system showed no such problems and the offender was told he was a naughty boy.. Others would have taken a stronger view but there we are.. Imagine the cost of replacement tyres.
Where company policy mandates use of autobrakes at all times regardless of the circumstances, would it be correct to say that by using full reverse immediately on touch down, the initial rate of deceleration is so high that it far exceeds that of autobrakes 1,2, and even 3? Any use of autobrakes is bound to increase wear on the tyres since they are applied at high speed and thus high energy. So what is the logic of using the autobrakes on normal no sweat landings if the intention is to use full reverse on touch down and leaving manual braking until needed?
The FCTM clearly states the occasions when autobrakes are recommended. In many companies that seems carte blanche to use autobrakes for ALL landings, which is not what the manufacturer intended, I'm sure?
Last edited by Tee Emm; 8th Oct 2011 at 10:41.
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What does full reverse do on a dry runway?
Reduce brake temp by using reverse deceleration effect to decrease applied automatic braking to a achieve the preset deceleration rate.
Only at and below ab 2 do you gain landing distance from applying full reverse,have a look at your qrh.
But full reverse also:
Increases noise at airport
Decrease engine life, higher engine temp,
Increase fuel burn
Increases airframe vibration
Scares the **** out of some passengers including kids and new flyers.
Unless you need full reverse dont use it,brakes are to cheaper to change than all of the above effects.
Calculate your landing distance and then decide on which a/b to apply.
Reverse is necessary for wet and contaminated runway or if runway limited/own performance limit.
Using full manual braking is just plain irresponsible if not required for emergency purposes.
Reduce brake temp by using reverse deceleration effect to decrease applied automatic braking to a achieve the preset deceleration rate.
Only at and below ab 2 do you gain landing distance from applying full reverse,have a look at your qrh.
But full reverse also:
Increases noise at airport
Decrease engine life, higher engine temp,
Increase fuel burn
Increases airframe vibration
Scares the **** out of some passengers including kids and new flyers.
Unless you need full reverse dont use it,brakes are to cheaper to change than all of the above effects.
Calculate your landing distance and then decide on which a/b to apply.
Reverse is necessary for wet and contaminated runway or if runway limited/own performance limit.
Using full manual braking is just plain irresponsible if not required for emergency purposes.
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Subjectively I would put full reverse at under the A/B2 mark overall, but the idea is simple:
If you use full reverse and a/b you SAVE brake wear but still get the benefits of early brake application, even left and right application, aid in crosswinds, a guaranteed deceleration for exit planning and a 'banker' in case reverse is either slow or does not function properly. Cannot see the problem.
Regarding max manual, I was once passengered into Ronaldsway in a 73NG by an anxious Captain who set A/B max, could not get it to disengage, pushed everyone's nose into the seat in front and stopped half-way down the runway. I was in 1C and practically doubled up against the partition. There was near panic in the cabin.
If you use full reverse and a/b you SAVE brake wear but still get the benefits of early brake application, even left and right application, aid in crosswinds, a guaranteed deceleration for exit planning and a 'banker' in case reverse is either slow or does not function properly. Cannot see the problem.
Regarding max manual, I was once passengered into Ronaldsway in a 73NG by an anxious Captain who set A/B max, could not get it to disengage, pushed everyone's nose into the seat in front and stopped half-way down the runway. I was in 1C and practically doubled up against the partition. There was near panic in the cabin.
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Good Question. Sorry I cannot really answer your concerns.
the manufacturer's recommendations are just that, Recommendations. If the company decides to implement some other way of doing things then they will be responsible if the deviation from the recommendation directly causes an incident/accident. the deviations operators make are therefore usually more conservative.
using autobrake for all landings is safer than not doing so as there is less chance of a pilot related error during the braking process. It's much easier to explain your actions during an inquiry if you can prove that you followed SOP (when it was safe to do so).
and by the way if the company is spending too much money on tyres they'll just cut our salary to pay for it so nothing to worry about.
the manufacturer's recommendations are just that, Recommendations. If the company decides to implement some other way of doing things then they will be responsible if the deviation from the recommendation directly causes an incident/accident. the deviations operators make are therefore usually more conservative.
using autobrake for all landings is safer than not doing so as there is less chance of a pilot related error during the braking process. It's much easier to explain your actions during an inquiry if you can prove that you followed SOP (when it was safe to do so).
and by the way if the company is spending too much money on tyres they'll just cut our salary to pay for it so nothing to worry about.
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It is indeed possible that the deceleration level achieved by the reverse thrust + aerodynamic drag is higher than the target level of the autobrake. This happens typically at the lower autobrake selections (1 or 2) combined with low weights. The autobrake logical cannot adjust this as it only can control the brakes.
Boeing has written some interesting papers which show how reverse thrust contributes to the stopping performance under different conditions. It shows for instance that on dry runways brakes are much more effective in stopping an aircraft than reverse thrust. Check myboeing.com or ask your operator.
Boeing has written some interesting papers which show how reverse thrust contributes to the stopping performance under different conditions. It shows for instance that on dry runways brakes are much more effective in stopping an aircraft than reverse thrust. Check myboeing.com or ask your operator.
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Don't forget that Easy are mostly Airbus which have carbon brakes. They wear in a different way to steel...A lot of reverse on the 737 is to allow a fast turnaround with the brakes being cool enough for the subsequent departure, which never seems to be an issue on the Airbus...
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In the 744 full reverse will decelerate faster than Autobrakes 1 and cause multiple brake application/release cycles, so that combination is not recommended. Full reverse can be used with Autobrakes 2 or higher, or Autobrakes Off.
With a light airplane, I suspect Autobrakes 2 will also be outdone by full reverse, but I have no data to prove it.
With a light airplane, I suspect Autobrakes 2 will also be outdone by full reverse, but I have no data to prove it.
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HUD has as deacceleration scale on it.
Spoilers w/full reverser is approx. the same as autobrakes 2.75 at touchdown.
So initially with a/b's 2 they're not doing anything until the a/c slows and the deacceleration from the T/R's and spoilers decreases to '2' or less.
Spoilers w/full reverser is approx. the same as autobrakes 2.75 at touchdown.
So initially with a/b's 2 they're not doing anything until the a/c slows and the deacceleration from the T/R's and spoilers decreases to '2' or less.
Last edited by misd-agin; 10th Oct 2011 at 00:04. Reason: added "at touchdown"
You can debate about brake application / brake wear characteristics / idle reverse / full reverse / maintenance costs etc until the cows come home.
At the end of the day, as pilots, we would like to stop safely before the end of the runway. We don't pay for brake pads, fuel etc. I don't care what they cost. They are there to use to stop the aircraft. These things are cheap. Overruns seem to be one of those occurrences, like runway incursions and CFIT, happen way more than they should and are rather expensive.
Just ask Air France, Qantas, Garuda, that indian mob + so many I can't even remember them all.
At the end of the day, as pilots, we would like to stop safely before the end of the runway. We don't pay for brake pads, fuel etc. I don't care what they cost. They are there to use to stop the aircraft. These things are cheap. Overruns seem to be one of those occurrences, like runway incursions and CFIT, happen way more than they should and are rather expensive.
Just ask Air France, Qantas, Garuda, that indian mob + so many I can't even remember them all.
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OTOH, as a professional pilot I like to optimize my flight, and especially optimize the landing and rollout. At the end of a long day, I'd rather have the shortest, simplest taxi route possible without any warning lights. Knowing the system lets me do this better.
Occy, “… we would like to stop safely before the end of the runway”, exactly, and this should always be the first priority.
Intruder, the primary objective of professionalism is safety, then optimisation might be considered. “People are expected to be both efficient and thorough at the same time – or rather to be thorough, when with hindsight it was wrong to be efficient.” (E. Hollnagel page 6)
One of the Boeing documents (decurion #5) is at SmartCockpit - Airline training guides, Aviation, Operations, Safety - flight ops, flying technique, “landing on slippery runways”; this shows the contribution of bakes / rev for a range of runway conditons.
T.M. your last para (#1) covers the majority of the issues. Logic suggests that the crew should decide which would be the better option (brakes/rev) for each landing – maximizing efficiency – as per manufacturers’ recommendations.
However, safety evidence suggests that pilots don’t always understand the circumstances, and in many instances critical information on the runway condition is sparse or incorrect – at the expense of thoroughness (safety). Thus many companies mandate the use of autobrake (changing from the manufacturers’ recommendation), but some, without providing supportive guidance, e.g. A/B setting for reported runway conditions, matching A/B setting with % of landing wt, need for reverse.
Without guidance or even with it and poor assessment, many landings can be made with inadequate landing distance safety margin, and I suspect that many pilots have relied on reverse to save embarrassment.
Some company attitudes towards the use of autobrake, and weak or misapplied knowledge, might be contributing to the many overrun accidents.
Intruder, the primary objective of professionalism is safety, then optimisation might be considered. “People are expected to be both efficient and thorough at the same time – or rather to be thorough, when with hindsight it was wrong to be efficient.” (E. Hollnagel page 6)
One of the Boeing documents (decurion #5) is at SmartCockpit - Airline training guides, Aviation, Operations, Safety - flight ops, flying technique, “landing on slippery runways”; this shows the contribution of bakes / rev for a range of runway conditons.
T.M. your last para (#1) covers the majority of the issues. Logic suggests that the crew should decide which would be the better option (brakes/rev) for each landing – maximizing efficiency – as per manufacturers’ recommendations.
However, safety evidence suggests that pilots don’t always understand the circumstances, and in many instances critical information on the runway condition is sparse or incorrect – at the expense of thoroughness (safety). Thus many companies mandate the use of autobrake (changing from the manufacturers’ recommendation), but some, without providing supportive guidance, e.g. A/B setting for reported runway conditions, matching A/B setting with % of landing wt, need for reverse.
Without guidance or even with it and poor assessment, many landings can be made with inadequate landing distance safety margin, and I suspect that many pilots have relied on reverse to save embarrassment.
Some company attitudes towards the use of autobrake, and weak or misapplied knowledge, might be contributing to the many overrun accidents.
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SP - we do not need to get hooked up on contam/slippery here - the question was about 'routine' use. Certainly on a 737, and I assume an AB, using the published LDA's to select an A/B setting for 'professionalism' as Intruder and I wish is very simple and makes for a comfortable landing roll without what Mrs B used to describe as 'braking to avoid the dogs' which one so often sees/feels when no thought has been given to exit planning and the exit always seems to leap out and surprise these pilots at the last minute. Limiting LDR/A's do not need to be in this discussion.
Limiting LDR/A's do not need to be in this discussion.
BOAC, “… contam/slippery …”, “Limiting LDR/A's do not need to be in this discussion.”
I would agree, but a significant issue in accidents appears to stem from the inability (reluctance, omission) of crews to change ‘habit’ from otherwise normal operations.
This might be due to poor assessment of the situation, or mistakenly assessing a potentially difficult situation as benign. These problems might be aided by inappropriate SOPs, a complacent attitude to braking – the use of standard autobrake settings without correlation with the conditions.
Thus an operator’s choice in operation, even if commercially driven, must consider how they will aid crews in judging when and how to change operational priorities to safety.
Of course no operator would admit the need to do that (safety is always #1); also, perhaps many operators might not appreciate that with fallible humans the crew might not change when required – 'we have SOPs requiring a check', 'they will change to a new setting', but what triggers the new SOP and why would the crew do it.
Accidents originate from routine operations. The source of success in routine operations is also the source of failure; all I ask is that operators recognising this.
I would agree, but a significant issue in accidents appears to stem from the inability (reluctance, omission) of crews to change ‘habit’ from otherwise normal operations.
This might be due to poor assessment of the situation, or mistakenly assessing a potentially difficult situation as benign. These problems might be aided by inappropriate SOPs, a complacent attitude to braking – the use of standard autobrake settings without correlation with the conditions.
Thus an operator’s choice in operation, even if commercially driven, must consider how they will aid crews in judging when and how to change operational priorities to safety.
Of course no operator would admit the need to do that (safety is always #1); also, perhaps many operators might not appreciate that with fallible humans the crew might not change when required – 'we have SOPs requiring a check', 'they will change to a new setting', but what triggers the new SOP and why would the crew do it.
Accidents originate from routine operations. The source of success in routine operations is also the source of failure; all I ask is that operators recognising this.
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Our company policy is to use brakes over reverse. Apparentely for our company the engine cost is more than the brake wear cost. Company policy for the use of autobrakes = pilots discretion including OFF. Of course the FCTM recommendations when A/B should be used are in there also
We regularely land at airports with runways more than 3000 meters. I usually put the A/B OFF and use idle reverse. The airplane stops just fine in about 2500 meters.
Dry runways: A/B settings with idle reverse except if required for brake cooling.
Wet runways: A/B setting with up to full reverse.
Contaminated: A/B setting with max reverse.
As alreay mentioned by de facto you can look up in the QRH when use of reverse has initially a higher decel rate than the brakes. In the section normal landing distances you have a column "reverse thrust adj"
We regularely land at airports with runways more than 3000 meters. I usually put the A/B OFF and use idle reverse. The airplane stops just fine in about 2500 meters.
Dry runways: A/B settings with idle reverse except if required for brake cooling.
Wet runways: A/B setting with up to full reverse.
Contaminated: A/B setting with max reverse.
As alreay mentioned by de facto you can look up in the QRH when use of reverse has initially a higher decel rate than the brakes. In the section normal landing distances you have a column "reverse thrust adj"
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At the end of the day if the companies accountants have driven an sop because they have worked out their contract for brakes is cheaper than the cost of the engine wear then that is what you will do. Other than that you plan your landing roll to get you off the runway in a controlled, comfortable and expeditious manner.
Other factors affect the decision and as professionals that what we get paid to evaluate, for example anyone working in and out of London Gatwick the worlds busiest single runway international airport then you will be expected to be on the runway for the min time possible. Companies are given target speeds to try to achieve at various RET's and this is monitored and graded by the airport. My operation is on classic 737's and aiming for a 35 min turn around causes issues with autobrake 2 anywhere near max landing therefore to mitigate the brake cooling we use reverse as a matter of course.
In summary you do the calculations and you plan the landing roll like anything else, this is the only and only way to prevent runway excursions.
Other factors affect the decision and as professionals that what we get paid to evaluate, for example anyone working in and out of London Gatwick the worlds busiest single runway international airport then you will be expected to be on the runway for the min time possible. Companies are given target speeds to try to achieve at various RET's and this is monitored and graded by the airport. My operation is on classic 737's and aiming for a 35 min turn around causes issues with autobrake 2 anywhere near max landing therefore to mitigate the brake cooling we use reverse as a matter of course.
In summary you do the calculations and you plan the landing roll like anything else, this is the only and only way to prevent runway excursions.
Some time ago an Asian operator visited a simulator training provider requesting recurrent training. That operator had a SOP requiring autobrake 2 for all landings regardless of runway length. The recurrent training session called for an all flaps up landing on a landing length limiting dry runway. In other words, challenging for a flapless approach and of course requiring precise airspeed control (185 knots Vref) and accurate touch down point.
Guess what! The captain selected Autobrake 2 because that was their SOP said. His first officer, ever the obedient (culture) second in command, stared impassively at the autobrake selector and decided future promotion over-rode safety issues and so he said nothing.
The speed was accurate and so was the touch-down point but the captain was too slow to select reverse and autobrake 2 did the rest resulting in the 737 going off the end at 80 knots still in full reverse and AB 2 doing its best. At no point did the captain over-ride the autobrakes in order to increase the deceleration by using full manual brakes.
The second in command watched impassively; but true to company SOP did call out "60 knots" as the mud at the far end was blown all over the wings and passenger windows as the 737 went over the simulated cliff into the sea...
Guess what! The captain selected Autobrake 2 because that was their SOP said. His first officer, ever the obedient (culture) second in command, stared impassively at the autobrake selector and decided future promotion over-rode safety issues and so he said nothing.
The speed was accurate and so was the touch-down point but the captain was too slow to select reverse and autobrake 2 did the rest resulting in the 737 going off the end at 80 knots still in full reverse and AB 2 doing its best. At no point did the captain over-ride the autobrakes in order to increase the deceleration by using full manual brakes.
The second in command watched impassively; but true to company SOP did call out "60 knots" as the mud at the far end was blown all over the wings and passenger windows as the 737 went over the simulated cliff into the sea...
