Anti-skid testing
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Anti-skid testing
All the literature i have on anti-skid (manufacturers manuals etc)suggests that to be tested correctly, the anti-skid system must first be connected i.e. speed transducers etc in the correct sense.
My question then "is it possible for ANY anti-skid system to test correctly if it is not connected or has damaged or broken wiring?"
I am aware of a situation in Florence (Italy) recently, that a BAe 146 skidded to a hault and blew 1 or more tyres after landing. The brakes did not work as they normally do i.e. required more pressure than normal. Once applied heavily, the brakes locked up and there is considerable skid marks down the runway to suggest the tyres skidded from the first initial heavy application.
The previous night, engineering had changed the entire wheel assembly and allegedly forgot to connect the anti-skid. Even if this is the case, how did it test correctly the next morning by the flight deck crew (assuming it was in fact tested).
This is not the sort of surprise a pilot needs on a relatively short runway.
My question then "is it possible for ANY anti-skid system to test correctly if it is not connected or has damaged or broken wiring?"
I am aware of a situation in Florence (Italy) recently, that a BAe 146 skidded to a hault and blew 1 or more tyres after landing. The brakes did not work as they normally do i.e. required more pressure than normal. Once applied heavily, the brakes locked up and there is considerable skid marks down the runway to suggest the tyres skidded from the first initial heavy application.
The previous night, engineering had changed the entire wheel assembly and allegedly forgot to connect the anti-skid. Even if this is the case, how did it test correctly the next morning by the flight deck crew (assuming it was in fact tested).
This is not the sort of surprise a pilot needs on a relatively short runway.
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On the a/c I've flown, there is no way for the crew to do a functional test of the anti-skid system prior to t/o. All we can do is test for "braking" when the system is turned on. We have to rely on a warning light in the system to tell us something is wrong. If there is no light, we have to assume the system is operating properly.
The first time we find it isn't is when a tire blows.
It's not unusual to find a blown tire after work has been done on an anti-skid system.
When I do a test flight, the last landing is done with maximum anti-skid braking. If the system is not operating properly, the tire will blow then. Only after the a/c passes this functional test, will I turn the a/c over to the line crews for revenue service. If any work is done on the system after that, we have to rely on the guys with wrenches and test boxes.
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The first time we find it isn't is when a tire blows.
It's not unusual to find a blown tire after work has been done on an anti-skid system.
When I do a test flight, the last landing is done with maximum anti-skid braking. If the system is not operating properly, the tire will blow then. Only after the a/c passes this functional test, will I turn the a/c over to the line crews for revenue service. If any work is done on the system after that, we have to rely on the guys with wrenches and test boxes.
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With reference to Shakespeares comments relating to Anti-skid problems on a Bae-146 in Florence,The Anti-skid transducer is not affected during a normal straight forward wheel change as the transducer is located internally within the axle.
Is it at all possible that this pilot inadvertantly landed with the brakes selected to emergency yellow thus inhibiting the Anti-skid system and thus causing such a blow out?Normal SOP states for landing on Green Hydraulics but this situation points to an error of this nature as any failure of the Anti-skid system would give an Anti-skid "fault" or "inop" annunciator when the ground test was carried out on the pre-flight.
I hope this answer helps you to restore some faith in the machine that you are flying! The Anti-Skid system on the 146 is on the whole very reliable with very few defects throughout my career so far.
Having thought about this situation whilst writing, A dual adaptive Anti-Skid valve failure could result in such a failure or indeed if the computer sending the valve the signal did not do so!
I'll do some research for you.
Is it at all possible that this pilot inadvertantly landed with the brakes selected to emergency yellow thus inhibiting the Anti-skid system and thus causing such a blow out?Normal SOP states for landing on Green Hydraulics but this situation points to an error of this nature as any failure of the Anti-skid system would give an Anti-skid "fault" or "inop" annunciator when the ground test was carried out on the pre-flight.
I hope this answer helps you to restore some faith in the machine that you are flying! The Anti-Skid system on the 146 is on the whole very reliable with very few defects throughout my career so far.
Having thought about this situation whilst writing, A dual adaptive Anti-Skid valve failure could result in such a failure or indeed if the computer sending the valve the signal did not do so!
I'll do some research for you.
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Thank you for your responses.
I am not in any way discounting pilot error in this particular case. However, as I said, the entire wheel assembly (wheels, axel etc) was changed the night before as a result of a set of cooked brakes getting that hot that they bent the axel! (Yes, believe it or not, that is correct.It is another story which I will not go into).
The engineers are saying BAe should have told them about a requirement to check certain things??????????????? after a assembly change like the one they performed.
I have never had any problems personally with the anti-skid on the 146. However I still do not understand how the system could test correctly (which is a first officer check on the first flight of the day)if the test was performed correctly, and the system was in actual fact not connected in the correct sense. Surely an anti-skid fault would be indicated in this situation!
Looking forward to more educated responses.
I am not in any way discounting pilot error in this particular case. However, as I said, the entire wheel assembly (wheels, axel etc) was changed the night before as a result of a set of cooked brakes getting that hot that they bent the axel! (Yes, believe it or not, that is correct.It is another story which I will not go into).
The engineers are saying BAe should have told them about a requirement to check certain things??????????????? after a assembly change like the one they performed.
I have never had any problems personally with the anti-skid on the 146. However I still do not understand how the system could test correctly (which is a first officer check on the first flight of the day)if the test was performed correctly, and the system was in actual fact not connected in the correct sense. Surely an anti-skid fault would be indicated in this situation!
Looking forward to more educated responses.
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shakespeare-
Just exactly what does the first officer do and what does he look for in the "taxi-out" check of the system?
What we do on our a/c (DC-8)is to arm the system, check for the absence of the warning light, and depress the brakes (lighly). Is your procedure much different?
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Just exactly what does the first officer do and what does he look for in the "taxi-out" check of the system?
What we do on our a/c (DC-8)is to arm the system, check for the absence of the warning light, and depress the brakes (lighly). Is your procedure much different?
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Quid.
The anti-skid is tested well before we start engines. There are 2 anti-skid test function buttons, 1 each for both the hydraulic systems.
The test button is pushed in and the anti-skid "fault" and "inop" annunciators both illuminate. After 4 seconds, the "inop" annunciator extinguishes after which we release the button. After a further 10 seconds, the "fault" should extinguish also. This confirms the system is servicable. If any part of the anti-skid system is u/s, one or both annunciators will remain illuminated.
As I said, this begs the question whether the system can be tested correctly if all the parameters are not measurable i.e not connected etc.
The anti-skid is tested well before we start engines. There are 2 anti-skid test function buttons, 1 each for both the hydraulic systems.
The test button is pushed in and the anti-skid "fault" and "inop" annunciators both illuminate. After 4 seconds, the "inop" annunciator extinguishes after which we release the button. After a further 10 seconds, the "fault" should extinguish also. This confirms the system is servicable. If any part of the anti-skid system is u/s, one or both annunciators will remain illuminated.
As I said, this begs the question whether the system can be tested correctly if all the parameters are not measurable i.e not connected etc.
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OK, then the anti-skid test is a test of the warning system and associated electronics. It isn't a "functional" test. (And I didn't think it would be with pax on board.) It is possible to have a malfunctioning system at the brake, but all indications are normal. I've seen this. We had one where all the indications were normal, but the brake pressure reducer didn't operate properly and was sending the full 3000 psi to the brake. Sure enough, blown tire.
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Back in the Sixties (?) a VC10 in Africa experienced heavy nosewheel vibration about 15 knots before V1. It was a night take off and some time previously a light aircraft had been stuck on the runway with a flat tyre. The owner changed the tyre and left the small car jack on the runway. The VC10 nosewheel ran over the jack - hence the vibration. The VC10 aborted with max braking.
Several of its tyres burst resulting in loss of 50% of total braking capability. It over-ran and ruptured a wing tank. Many were killed in the resulting conflagration.
The investigators discovered that several of the anti-skid systems had been wrongly wired up which caused some brakes to stay on when they should have momentarily released and vice verca with opposite sets of wheels.
Several of its tyres burst resulting in loss of 50% of total braking capability. It over-ran and ruptured a wing tank. Many were killed in the resulting conflagration.
The investigators discovered that several of the anti-skid systems had been wrongly wired up which caused some brakes to stay on when they should have momentarily released and vice verca with opposite sets of wheels.
