B737NG Antiskid Functions
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B737NG Antiskid Functions
Hello everybody
IAW B737NG AMM, Antiskid/Autobrake Control Unit provides Touchdown Protection and Touchdown/Hydroplane Protection. Touchdown Protection releases brake pressure from wheel brakes 2 & 4 for some time after touchdown and Touchdown/Hydroplane protection releases brake pressure from wheel brakes 1 & 3 in case that the wheel speed decreases to 50 knots less than ground speed (source ADIRU). What is puzzling me is the selection of the wheel pairs that seems to be arbitrary. In my opinion a symmetrical wheel brake pairing pattern would make more sense. On the other hand I don't believe that some guys in Boeing's design department just chose randomly this configuration. Does anybody know something that is not described in AMM or Flight Manual?
Thanks in advance
IAW B737NG AMM, Antiskid/Autobrake Control Unit provides Touchdown Protection and Touchdown/Hydroplane Protection. Touchdown Protection releases brake pressure from wheel brakes 2 & 4 for some time after touchdown and Touchdown/Hydroplane protection releases brake pressure from wheel brakes 1 & 3 in case that the wheel speed decreases to 50 knots less than ground speed (source ADIRU). What is puzzling me is the selection of the wheel pairs that seems to be arbitrary. In my opinion a symmetrical wheel brake pairing pattern would make more sense. On the other hand I don't believe that some guys in Boeing's design department just chose randomly this configuration. Does anybody know something that is not described in AMM or Flight Manual?
Thanks in advance
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The combination of one inboard and one outboard wheel on each strut makes sense to me...
Because....? Same could be said for pairs of inboards or outboards.
Because....? Same could be said for pairs of inboards or outboards.
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Clarification on the antiskid wheel pairs issue
To my understanding having asymmetrical release of brakes during touchdown (wheels 2 &4) or touchdown/hydroplaning (wheels 1 &3) functions of antiskid system creates an effect similar to differential braking (to a lesser extent) that can veer the airplane to the direction of the outboard wheel that is not released. I'm an engineer not a pilot so I don't know if the result of releasing a pair of an outboard and an inboard wheel is negligible or not in real life. My concern is mainly theoretical. I suppose only pilots that have landed while stepping on the brakes without autobrake selected on can say if the effects of asymmetrical wheel brake release after touchdown function activation are noticeable. Perhaps there is no issue as most probably any of the two antiskid functions activates immediately after touchdown when the traction is minimal.
The combination of one inboard and one outboard wheel on each strut makes sense to me...
Because....? Same could be said for pairs of inboards or outboards.
Because....? Same could be said for pairs of inboards or outboards.
Or maybe the 'one inner one outer' configuration allows for cross-wind induced weight shift effects??
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Why would anti-skid not be fitted to every wheel fitted with a brake? Unlike a car I am thinking directional control ought not be the reason for anti-skid but rather stopping distance and tire wear. Has it got to do with failure modes?
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172,
ANTI-SKID is fitted to and operates independently for all 4 main wheels (axle pairs in alternate braking mode).
Touchdown protection prevents the A/C landing with the brakes already applied, and only operates on 1 wheel on each axle.
I'm having a read now to see if I can find any reasoning.
ANTI-SKID is fitted to and operates independently for all 4 main wheels (axle pairs in alternate braking mode).
Touchdown protection prevents the A/C landing with the brakes already applied, and only operates on 1 wheel on each axle.
I'm having a read now to see if I can find any reasoning.
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ANTI-SKID is fitted to and operates independently for all 4 main wheels (axle pairs in alternate braking mode).
Touchdown protection prevents the A/C landing with the brakes already applied, and only operates on 1 wheel on each axle.
I am sure there is a good explanation, and if you find something I'd be happy to learn. But so far this doesn't make much sense to me...
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Right, I think I have it.
TOUCHDOWN PROTECTION (wheels 2 & 4).
Prohibits wheel braking when the A/C in the air.
TOUCHDOWN/HYDROPLANE PROTECTION (wheels 1 & 3).
Senses a difference between Groundspeed (from the ADIRU) and wheel speed (from the anti-skid transducer) and releases the wheel when the speed difference is greater than 50kts.
Therefor all 4 wheels have touchdown protection, and wheels 1 and 3 only have hydroplane protection. (the touchdown protection being sensed in 2 different ways)
LOCKED WHEEL PROTECTION-
Compares the speeds of both inboard and both outboard wheels (i.e, 1 with 4 and 2 with 3) and releases the slower wheel when there is a 30% speed difference.
So i suppose the reason for the pairing is that locked wheel protection would be the most likely one to be activated, so this one operates symmetrically?
TOUCHDOWN PROTECTION (wheels 2 & 4).
Prohibits wheel braking when the A/C in the air.
TOUCHDOWN/HYDROPLANE PROTECTION (wheels 1 & 3).
Senses a difference between Groundspeed (from the ADIRU) and wheel speed (from the anti-skid transducer) and releases the wheel when the speed difference is greater than 50kts.
Therefor all 4 wheels have touchdown protection, and wheels 1 and 3 only have hydroplane protection. (the touchdown protection being sensed in 2 different ways)
LOCKED WHEEL PROTECTION-
Compares the speeds of both inboard and both outboard wheels (i.e, 1 with 4 and 2 with 3) and releases the slower wheel when there is a 30% speed difference.
So i suppose the reason for the pairing is that locked wheel protection would be the most likely one to be activated, so this one operates symmetrically?
As somebody said, why not have all the protections on every braked wheel?
I wonder if it is because when the 73 was designed, such systems were very complicated and expensive, and/or perhaps they could only apply one function to each wheel, so they applied each different function to only half the wheels?
(I vaguely remember being taught about purely mechanical Anti skid systems on some early (British?) aircraft - the word 'Maxaret' is floating around the corners of my brain - but can't remember which aircraft makes).
Nowadays, with year on year improvements and reducing cost of electronics, every braked wheel can be protected against locking up.
I wonder if it is because when the 73 was designed, such systems were very complicated and expensive, and/or perhaps they could only apply one function to each wheel, so they applied each different function to only half the wheels?
(I vaguely remember being taught about purely mechanical Anti skid systems on some early (British?) aircraft - the word 'Maxaret' is floating around the corners of my brain - but can't remember which aircraft makes).
Nowadays, with year on year improvements and reducing cost of electronics, every braked wheel can be protected against locking up.
Last edited by Uplinker; 27th Jan 2017 at 08:53.
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Here's my 2p for the theory.. the 737 has rotation on main gear controlled by the shimmy dampers fitted to the end of the torque arm pivot pins. If you release both outer brakes, you would get rotation in opposite directions and therefore toe on the main gears and accelerated wear and stress on the legs. By braking the wheels on the same side of the leg each side, both will rotate the same way and this will allow lateral movement of the whole aircraft reducing stress on the main legs. It's only a few degrees but there you go.
I guess splitting the systems that way means that one of the two wheels compared by the antilock system is (if necessary) released by the hydroplaning protection. That way it spins up and provides a reference for the antilock system. Effectively, the antilock system extends the hydroplaning protection to the other wheel.
Presumably that makes hydroplaning protection on all four wheels unnecessary. So maybe limiting the authority of the hydroplaning system to two wheels is a failsafe measure (so a malfunction can't cause loss of all braking). Or maybe there would be undesirable interactions between the hydroplaning protection and antilock systems if both acted on all four wheels, though I can't put my finger on exactly how that would happen.
Actually, a much simpler answer is probably that the hydroplaning protection isn't intended to protect any particular wheels, but rather to prevent the situation where the antilock system doesn't activate because all four wheels are turning at the same reduced speed (or not at all). If the problem you set out to solve is that all four wheels may turn at the same speed, the last thing you'd do is design a system that acted on all four wheels in the same way.
Presumably that makes hydroplaning protection on all four wheels unnecessary. So maybe limiting the authority of the hydroplaning system to two wheels is a failsafe measure (so a malfunction can't cause loss of all braking). Or maybe there would be undesirable interactions between the hydroplaning protection and antilock systems if both acted on all four wheels, though I can't put my finger on exactly how that would happen.
Actually, a much simpler answer is probably that the hydroplaning protection isn't intended to protect any particular wheels, but rather to prevent the situation where the antilock system doesn't activate because all four wheels are turning at the same reduced speed (or not at all). If the problem you set out to solve is that all four wheels may turn at the same speed, the last thing you'd do is design a system that acted on all four wheels in the same way.
Last edited by Chu Chu; 28th Jan 2017 at 02:08. Reason: Added last paragraph.
Another reason for the 1 & 3, 2 & 4 configuration instead of 1 & 4, 2 & 3, might be because that way, all the pipes, brackets, wiring looms etc can be identical on each side of each leg, so you only need one set of components rather than mirror pairs. This helps keep the spares count down.
Since, say, the right hand wheel on each leg performs one function, then if it was a left main leg, that wheel would be inboard (#2), but if it was a right leg it would be outboard, (#4), which by happy chance would give some diversity of the runway conditions too, and possible reduction of stresses as RVF750 suggests.
If you were designing an aircraft from the wheels up, with modern electronics and technology, then you would apply all the protections available to every wheel brake. This might be why aircraft such as the A330 stop on a sixpence.
Since, say, the right hand wheel on each leg performs one function, then if it was a left main leg, that wheel would be inboard (#2), but if it was a right leg it would be outboard, (#4), which by happy chance would give some diversity of the runway conditions too, and possible reduction of stresses as RVF750 suggests.
If you were designing an aircraft from the wheels up, with modern electronics and technology, then you would apply all the protections available to every wheel brake. This might be why aircraft such as the A330 stop on a sixpence.
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Will touchdown protection function if the parking brake is set ?