Crash Switches
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Crash Switches
Are aircraft Crash Switches still required to discharge fire suppression systems in the event of a sudden stop or excessive g ?
Two stories.
Whilst investigating the innerds of a RAAF Lincoln I came across a little device in the nose in which I could clearly see a shiny steel ball about 3 cm in diameter. I poked my finger to move the ball and was dismayed to hear a whole bunch of small bangs throughout the aircraft. All engine fire bottles had fired and perhaps some fueslage ones as well. I felt like P/O Prune as much effort was involved in replacing all of the bottles.
Then landing a Bristol Freighter 'back of the black stump' in Oz on a gravel runway I touched down on a small ridge of wind blown sand which threatened to remove the undercarriage. The crash switch triggered off the engine fire bottles. Was stuck in the middle of nowhere at a place called Giles waiting for some replacement bottles.
Does anyone know the horizontal and vertical g specifications for crash switches?
Two stories.
Whilst investigating the innerds of a RAAF Lincoln I came across a little device in the nose in which I could clearly see a shiny steel ball about 3 cm in diameter. I poked my finger to move the ball and was dismayed to hear a whole bunch of small bangs throughout the aircraft. All engine fire bottles had fired and perhaps some fueslage ones as well. I felt like P/O Prune as much effort was involved in replacing all of the bottles.
Then landing a Bristol Freighter 'back of the black stump' in Oz on a gravel runway I touched down on a small ridge of wind blown sand which threatened to remove the undercarriage. The crash switch triggered off the engine fire bottles. Was stuck in the middle of nowhere at a place called Giles waiting for some replacement bottles.
Does anyone know the horizontal and vertical g specifications for crash switches?
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Hi Milt,
I've not heard of those types of 'crash switches', but I do know of a similar system that fires off when any of the mercury switches are broken.
The Lockheed P3C Orion aircraft has several mercury switches fitted, one at each wing tip, a couple in the nose and a couple more in the dry-bay in the fuselage in the wing center section, that will fire off the emergency locator system when broken/crushed.
They have a unique little battery operated locator beacon fitted to the RHS of the vertical stabilizer which will 'spring' out (strong spring) into the airstream, water or onto the ground instantanously when triggered. It's even been shaped like a leaf (sort of), so it will flutter down to the ground if needed, or float on the water, transmitting for a very long time.
Just thought it rated a mention,
Cheers, FD
I've not heard of those types of 'crash switches', but I do know of a similar system that fires off when any of the mercury switches are broken.
The Lockheed P3C Orion aircraft has several mercury switches fitted, one at each wing tip, a couple in the nose and a couple more in the dry-bay in the fuselage in the wing center section, that will fire off the emergency locator system when broken/crushed.
They have a unique little battery operated locator beacon fitted to the RHS of the vertical stabilizer which will 'spring' out (strong spring) into the airstream, water or onto the ground instantanously when triggered. It's even been shaped like a leaf (sort of), so it will flutter down to the ground if needed, or float on the water, transmitting for a very long time.
Just thought it rated a mention,
Cheers, FD
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Originally Posted by Milt
Whilst investigating the innerds of a RAAF Lincoln I came across a little device in the nose in which I could clearly see a shiny steel ball about 3 cm in diameter. I poked my finger to move the ball and was dismayed to hear a whole bunch of small bangs throughout the aircraft. All engine fire bottles had fired and perhaps some fueslage ones as well. I felt like P/O Prune as much effort was involved in replacing all of the bottles.
(Were they BCF bottles ? Nice mess I bet !)These types of crash switches are also fitted to older fast jets eg Harrier GR3 from memory, and are also known as Inertia switches. Basically they consist of a spring detent holding a small mass. In the event of a crash ie high G the mass overcomes the spring detent and makes a set of contacts to complete the firing circuit to the fire bottle.
We used to test them by banging the switch with a hide face hammer (Having previously disconnected the fire bottle
) It took some banging to get them to trip, and you had to remember to reset them before reconnecting the bottles !!Don't know what the G loading required to trigger them was but well above the usual +9/-3 I would imagine, Becuase thats what the A/C was rated at in normal ops.
I'm not sure what the Lincoln G limits are (does it have a G meter ?) But I'm sure it's not as high as the Harriers so consequently the Inertia switches could well be of a lower rating and far easier to trigger !
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Further to the crash switches emptying all of the fire bottles I think the switch also isolated the battery buss at the battery.
Does that happen with your aircraft?
Does that happen with your aircraft?
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Crash Switches
The BAe ATP and HS748 are fitted with crash switches which sound to be of a similiar type. I think they were made by Kidde Graviner and the switches made when there was a horizontal deceleration of 3g or more. They were not sensitive to high vertical G otherwise a heavy landing would set them off. We used to test them by removing them and giving them a good shake - fire bottles disconnected hopefully!
Right then, this might help:
Crash Protection System
(a) In order to minimise fire risk and make the aircraft electrically dead (except for emergency services) in the event of a crash landing, 20 crash trip elements and 8 inertia switches are provided. The crash trip elements are beneath the forward fuselage and undercarriage fairings; the inertia switches are in the port and starboard nose equipment bays.
(b) The elements and switches are divided into four groups, each group operates a pair of crash service relays associated with an engine and is known as an engine channel. Each engine channel is sub-divided into two halves. The complete system, consisting of all four engine channels, is divided into independent halves, port and starboard. The port system consists of Nos 1 and 4 engine channels and the starboard system Nos 2 and 3 engine channels.
(c) Operation of the system.
(i) Operation of a single complete engine channel shuts down two alternators, discharges the associated fire extinguishers (first shot) and illuminates the amber warning light on panel 10P.
(ii) Operation of all engine channels disconnects the batteries from their normal busbars and connects then to the emergency service busbars only, shuts down all alternators, discharges all engine fire extinguishers (first shot) and illuminates four amber warning lights on panel 10P.
This is from the Pilots Notes of the Short Belfast.
Hopefully it will help answer most of your questions.
Crash Protection System
(a) In order to minimise fire risk and make the aircraft electrically dead (except for emergency services) in the event of a crash landing, 20 crash trip elements and 8 inertia switches are provided. The crash trip elements are beneath the forward fuselage and undercarriage fairings; the inertia switches are in the port and starboard nose equipment bays.
(b) The elements and switches are divided into four groups, each group operates a pair of crash service relays associated with an engine and is known as an engine channel. Each engine channel is sub-divided into two halves. The complete system, consisting of all four engine channels, is divided into independent halves, port and starboard. The port system consists of Nos 1 and 4 engine channels and the starboard system Nos 2 and 3 engine channels.
(c) Operation of the system.
(i) Operation of a single complete engine channel shuts down two alternators, discharges the associated fire extinguishers (first shot) and illuminates the amber warning light on panel 10P.
(ii) Operation of all engine channels disconnects the batteries from their normal busbars and connects then to the emergency service busbars only, shuts down all alternators, discharges all engine fire extinguishers (first shot) and illuminates four amber warning lights on panel 10P.
This is from the Pilots Notes of the Short Belfast.
Hopefully it will help answer most of your questions.
Can't remember the exact details, but the Canberra has a mix of crash switches and crash strips. The latter are under the belly in pairs, and upon ground contact they release a fire inhibitant into the belly fuel tank. The 4 crash switches only operate when subject to a longtitudinal deceleration (ie not when pulling 'g') and they operate engine fire bottles as well as isolate the aircraft battery and the generators. This should therefore isolate all of the electrical systems.
