Operationally unnecessary use of autobrakes for landing
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When my company added 767s (with carbon brakes) to our 757 fleet it was immediately clear that manual application of the brakes without an unseemly jerk was just not possible however feather toed you were. And autobrake application was just as uncomfortable at any setting from 2 up.
After much experimentation I discovered that if you landed with AB1 selected there was no snatch and a delicate squeeze on the pedals would allow you to brake manually as smooth as silk.
Within a couple of weeks of this discovery the company banned the use of AB1. Really annoying!
After much experimentation I discovered that if you landed with AB1 selected there was no snatch and a delicate squeeze on the pedals would allow you to brake manually as smooth as silk.
Within a couple of weeks of this discovery the company banned the use of AB1. Really annoying!
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Its the extraordinary roughness of some pilots' braking that I find annoying, particularly the full-booted stomp to release an unnecessary auto brake, perhaps followed by complete relaxation of braking with another stomp of max effort at the turnoff.
One of the boons of auto brake is its smoothness. If you need it, deploy it, leave it to do its job and pickle it gently st 60kts or less. If you don't need it (can't brake manually like some, apparently?) leave it off.
Oh, and one other thing, the people who get half way down the runway on auto brake and then bury the neighbourhood in a fistful of reverse apparently unaware that it has no effect on braking distance...and then stomp on the brakes!
Pax comfort doesn't seem to feature in some peoples' books.
One of the boons of auto brake is its smoothness. If you need it, deploy it, leave it to do its job and pickle it gently st 60kts or less. If you don't need it (can't brake manually like some, apparently?) leave it off.
Oh, and one other thing, the people who get half way down the runway on auto brake and then bury the neighbourhood in a fistful of reverse apparently unaware that it has no effect on braking distance...and then stomp on the brakes!
Pax comfort doesn't seem to feature in some peoples' books.
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The full-booted stomp you describe is a system design.
To overcome and disconnect the auto brake a pressure greater than X psi must be applied to the toe brakes. This pressure on 737 is LESS than AB1.
When the full-booted stomp occurs you describe, it merely indicates that to disconnect the auto brake system, say 800psi was required by the pilot to be applied to the brakes but that the auto brake system was using (a lot) less than that pressure for the routine stopping of the airframe. So when the system disconnected, the additional brake force was immediately applied until the pilot was able to release his 800psi toes from the brakes.
Several techniques exist to different airframes to overcome this, the best one is to override the brakes when you know(!) in excess of 800 psi is applied by the brakes, e.g. when the reversers are stowed.
HENCE auto brake disconnect is the LAST item in Boeing's Landing Roll Out procedure, to avoid pilots inadvertently applying more brake pressure to disconnect the auto brake system than that used by the system itself.
To overcome and disconnect the auto brake a pressure greater than X psi must be applied to the toe brakes. This pressure on 737 is LESS than AB1.
When the full-booted stomp occurs you describe, it merely indicates that to disconnect the auto brake system, say 800psi was required by the pilot to be applied to the brakes but that the auto brake system was using (a lot) less than that pressure for the routine stopping of the airframe. So when the system disconnected, the additional brake force was immediately applied until the pilot was able to release his 800psi toes from the brakes.
Several techniques exist to different airframes to overcome this, the best one is to override the brakes when you know(!) in excess of 800 psi is applied by the brakes, e.g. when the reversers are stowed.
HENCE auto brake disconnect is the LAST item in Boeing's Landing Roll Out procedure, to avoid pilots inadvertently applying more brake pressure to disconnect the auto brake system than that used by the system itself.
At 60 knots, reduce reverse thrust to be at IDLE reverse when reaching taxi speed.
Approaching taxi speed, slowly move the reverse thrust levers to the full down position.
Prior to taxi speed, disarm the autobrake and continue manual braking as required.
Approaching taxi speed, slowly move the reverse thrust levers to the full down position.
Prior to taxi speed, disarm the autobrake and continue manual braking as required.
Autobrake Selector
1, 1250psi, 4ft/s2
2, 1500psi, 5ft/s2
3, 2000psi, 7.2ft/s2
Max, 3000psi 12ft/s2 (below 80kts), 14ft/s2 (above 80kts)
There is an "on ramp" period where autobrake pressure is applied over a period of time. Approximately 750psi is applied in 1.75 sec, then the pressures above are reached in another 1.25sec for autobrakes 1, 2, or 3 and approx. 1.0 sec for autobrake MAX.
To cancel the autobrake on the landing roll with toe brakes you must apply a brake pressure in excess of 800psi.
This is more difficult on the NG's because the feedback springs on the brake pedals are stiffer.
1, 1250psi, 4ft/s2
2, 1500psi, 5ft/s2
3, 2000psi, 7.2ft/s2
Max, 3000psi 12ft/s2 (below 80kts), 14ft/s2 (above 80kts)
There is an "on ramp" period where autobrake pressure is applied over a period of time. Approximately 750psi is applied in 1.75 sec, then the pressures above are reached in another 1.25sec for autobrakes 1, 2, or 3 and approx. 1.0 sec for autobrake MAX.
To cancel the autobrake on the landing roll with toe brakes you must apply a brake pressure in excess of 800psi.
This is more difficult on the NG's because the feedback springs on the brake pedals are stiffer.
Last edited by Skyjob; 20th Sep 2014 at 17:01. Reason: 737 autobrake data added
I was taught years ago that to achieve a smooth autobrake disconnect that you only need to push on one brake pedal, not both, and then the deceleration you get is only half that of using both pedals and with no sudden 'bump'.
Still gotta keep it straight though.
Works well on all the Boeings I've flown and the Airbus I presently fly.
Still gotta keep it straight though.
Works well on all the Boeings I've flown and the Airbus I presently fly.
On the 75 /67 you can achieve a very smooth Autobrake disconnect by just nudging the speed brake forward a tad.
Beats the 'lurch' that's so hard to avoid with applying manual brakes to disconnect.
Beats the 'lurch' that's so hard to avoid with applying manual brakes to disconnect.
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The lurch is easy to overcome, as posted above:
1) Ensure the auto brakes are applying pressure to the brakes by reducing reverse thrust to idle.
2) When reversers idle has been achieved, the brake pressure required to disconnect is less than that applied by auto brakes ensuring a smooth disconnect.
Note: failure to ensure pressure is applied to auto brakes results in the bump...
1) Ensure the auto brakes are applying pressure to the brakes by reducing reverse thrust to idle.
2) When reversers idle has been achieved, the brake pressure required to disconnect is less than that applied by auto brakes ensuring a smooth disconnect.
Note: failure to ensure pressure is applied to auto brakes results in the bump...
I have occasionally asked the PM to alter the autobrake setting on the rollout if it turns out to be too much/little for the planned exit. Seems to work fine. Otherwise I slowly apply increasing brake pressure until it disconnects, then slowly decrease/increase the pressure as required. That avoids most/all of the discontinuity in deceleration that disturbs some passengers...
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When my company added 767s (with carbon brakes) to our 757 fleet it was immediately clear that manual application of the brakes without an unseemly jerk was just not possible however feather toed you were. And autobrake application was just as uncomfortable at any setting from 2 up.
After much experimentation I discovered that if you landed with AB1 selected there was no snatch and a delicate squeeze on the pedals would allow you to brake manually as smooth as silk.
Within a couple of weeks of this discovery the company banned the use of AB1. Really annoying!
After much experimentation I discovered that if you landed with AB1 selected there was no snatch and a delicate squeeze on the pedals would allow you to brake manually as smooth as silk.
Within a couple of weeks of this discovery the company banned the use of AB1. Really annoying!
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What de-acceleration rate does the spoilers have? If it exceeds AB1 the A/B's don't engage until the de-acceleration rate slows to the AB1 rate.
Full thrust reversers and spoilers de-acceleration rate is approx. 2.75 at touchdown and decreases to approx. 2 around 100kts. That's how come, with AB2 selected, you don't feel them engage until below approx. 100kts if you're using full thrust reversers.
Someone mentioned keeping thrust reversers deployed until exiting the runway. The manufacturers have specific guidance for various engine/airframe combinations. It's often 'stowed by 60' or 'coming out around 60 to be stowed prior to taxi speed'. I'd go with the official recommendation.
Carbon brakes can be touchy but they can be smoothly manipulated. Early 767-200's were terrible and early 757's have that brake metering sponginess issue at low brake pressure.
The slower you are when you try to disengage the A/B's the bigger the lurch will be. Because of that I typically try to disconnect them around 100 kts(AB2 hasn't activated, AB3 is at approx. 1/3 rate- de-acceleration 2 is from spoilers/thrust reverser).
Full thrust reversers and spoilers de-acceleration rate is approx. 2.75 at touchdown and decreases to approx. 2 around 100kts. That's how come, with AB2 selected, you don't feel them engage until below approx. 100kts if you're using full thrust reversers.
Someone mentioned keeping thrust reversers deployed until exiting the runway. The manufacturers have specific guidance for various engine/airframe combinations. It's often 'stowed by 60' or 'coming out around 60 to be stowed prior to taxi speed'. I'd go with the official recommendation.
Carbon brakes can be touchy but they can be smoothly manipulated. Early 767-200's were terrible and early 757's have that brake metering sponginess issue at low brake pressure.
The slower you are when you try to disengage the A/B's the bigger the lurch will be. Because of that I typically try to disconnect them around 100 kts(AB2 hasn't activated, AB3 is at approx. 1/3 rate- de-acceleration 2 is from spoilers/thrust reverser).
Let's say you select AB3 and then smoothly disconnect with the toe brakes just after 100kts, or use AB1 and then manually brake, how does one determine the correct brake cooling schedule?
In one case you have disconnected in order to reduce the degree of brake application and in the other case you are using more braking than the system would otherwise give.
In one case you have disconnected in order to reduce the degree of brake application and in the other case you are using more braking than the system would otherwise give.
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Mikehotel152
You cannot unless BTM is installed, as then any way you brake is irrespective as it is measured by temperature monitors which must not be read until 10-15 minutes after landing, after which brake release can occur for the determined time.
Most short haul operators do not have a BTMS and schedule turnarounds which would/could be adversely affected should any brake release requirement be established for XX minutes after 10-15 minutes on block as the turn around time does not allow for this.
The above mentioned operators rely on aircrew to plan ahead and apply correct braking technique reducing brake temperature rises by selection of proper amount of reverse thrust, deployed at the correct time and for the correct duration.
At least, that is thought in training and management, as on the line many pilots have no clue about brake cooling as the matter is never adequately explained, thus who could blame them if a higher ranking (training) crew member tells them "you do not need to use reverse thrust, it has no effect on landing"...
These pilots come onto the line not knowing any better and are a safety hazard for those crew flying AFTER them on the aircraft's next rotation as they leave the aircraft after their (e.g. 4 short sectors of) flying with all landings performed IDLE reverse and AB3, never releasing brakes to cool on stand in their turnarounds...
This threat can only be assessed by the next pilot by feeling the brakes or heat radiating from them. Remember cool to the touch requirement before subsequent departure? If you cannot touch the brake discs, they are probably still too hot between the discs internally at the friction layer.
Alternatively if the side wall of the tire is warm to the touch it must mean that the NOx inside the tire has risen enough to heat a layer of rubber through its entire depth. Caution should then be exercised as rubber is NOT a conductive material, thus these temperatures of NOx must have been significant to heat a layer of rubber.
Let's say you select AB3 and then smoothly disconnect with the toe brakes just after 100kts, or use AB1 and then manually brake, how does one determine the correct brake cooling schedule?
Most short haul operators do not have a BTMS and schedule turnarounds which would/could be adversely affected should any brake release requirement be established for XX minutes after 10-15 minutes on block as the turn around time does not allow for this.
The above mentioned operators rely on aircrew to plan ahead and apply correct braking technique reducing brake temperature rises by selection of proper amount of reverse thrust, deployed at the correct time and for the correct duration.
At least, that is thought in training and management, as on the line many pilots have no clue about brake cooling as the matter is never adequately explained, thus who could blame them if a higher ranking (training) crew member tells them "you do not need to use reverse thrust, it has no effect on landing"...
These pilots come onto the line not knowing any better and are a safety hazard for those crew flying AFTER them on the aircraft's next rotation as they leave the aircraft after their (e.g. 4 short sectors of) flying with all landings performed IDLE reverse and AB3, never releasing brakes to cool on stand in their turnarounds...
This threat can only be assessed by the next pilot by feeling the brakes or heat radiating from them. Remember cool to the touch requirement before subsequent departure? If you cannot touch the brake discs, they are probably still too hot between the discs internally at the friction layer.
Alternatively if the side wall of the tire is warm to the touch it must mean that the NOx inside the tire has risen enough to heat a layer of rubber through its entire depth. Caution should then be exercised as rubber is NOT a conductive material, thus these temperatures of NOx must have been significant to heat a layer of rubber.
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I don't believe that you'll find much NOx inside an aircraft tire, or you shouldn't anyway, if it's been serviced correctly. Aircraft tires are serviced with Nitrogen Gas, which is abbreviated N2, NOx stands for Nitrogen oxides which are combustion products.
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Manually disconnect the autobrakes on this type using brake pedals for every landing at various different speeds and have never experienced any sort of unexpected lurch. Perhaps they got the design right on this type.
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There is a very interesting video on how carbon brakes work.
And as a320 fctm states, the pilot has to reduce the number of brake applications in order to reduce brake wear. The wear is not dependent upon the pressure or time brakes are used.
Therefore, I believe autobraking system generally reduces wear, since you are using the brakes just once. And if you vacate at the last taxiway, you will probably use your brakes many times until your parking stand, thus increasing your wear.
Maybe those are the reasons why operational procedures recommend routine use of the autobrakes.
And as a320 fctm states, the pilot has to reduce the number of brake applications in order to reduce brake wear. The wear is not dependent upon the pressure or time brakes are used.
Therefore, I believe autobraking system generally reduces wear, since you are using the brakes just once. And if you vacate at the last taxiway, you will probably use your brakes many times until your parking stand, thus increasing your wear.
Maybe those are the reasons why operational procedures recommend routine use of the autobrakes.
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Actually, after looking a the video, we know that only 2% of brake wear is during the taxi-in because the brakes are hot. Therefore, while you are correct in stating that reduced brake events reduce brake wear, the brake wear is almost down to zero on the taxi-in. As stated in the video, approximately 20% of wear is during the landing(on average of course). And the most interesting part is that almost 80% of brake wear is during the so-called cold taxi when taxiing for takeoff.
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And the most interesting part is that almost 80% of brake wear is during the so-called cold taxi when taxiing for takeoff.
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