Three Hydraulic Accumulators u/s - ok to dispatch!
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Three Hydraulic Accumulators u/s - ok to dispatch!
According to the A330 MEL you can dispatch with all 3 hydraulic accumulators u/s. I was pretty surprised to read this. I'd be uncomfortable to go with one u/s, but all three?
Would you accept that for dispatch?
I'm fascinated to know!
Would you accept that for dispatch?
I'm fascinated to know!
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Hi Algol,
Yes.
According to the A320 - it's the same - but the (Y) Brake accumulator must be working.
System Accumulators are fitted to absorb pressure change shocks to Hyd system. They are desirable but not essential.
On the other hand, a brake accumulator stores potential brake pressure for when you might need it, and therefore must be operative.
Would you accept that for dispatch?
According to the A320 - it's the same - but the (Y) Brake accumulator must be working.
System Accumulators are fitted to absorb pressure change shocks to Hyd system. They are desirable but not essential.
On the other hand, a brake accumulator stores potential brake pressure for when you might need it, and therefore must be operative.
Last edited by Goldenrivett; 15th Jan 2015 at 14:51. Reason: Typos
Both good answers.
The hydraulic pumps cannot respond instantly to a change of demand. Therefore, accumulators are fitted to maintain system pressure for the ?second or so before the pump's output is detected and increased.
An accumulator is a cylinder half full of hydraulic fluid under system pressure on one side and a spring of some sort - usually nitrogen gas I think - on the other side of a divider that can move up and down the cylinder. The gas is put into one side of the divider at a particular pressure so that when the hydraulic system is pressurised, fluid filling the cylinder on the other side of the divider forces the divider up the cylinder, until the gas pressure and the fluid pressure are the same. Now you have a small reservoir of fluid at system pressure, and if the system line pressure drops, this fluid is pushed out of the accumulator by the pressure of the gas to maintain the system fluid pressure. When the hydraulic pump output recovers, fluid is pushed back into the cylinder once more.
I hope that makes sense !
Therefore, as has been mentioned, you don't necessarily need accumulators for a hydraulic system to work, but they do make the system work much more smoothly, which is more gentle on the pumps, and also prevents nuisance warnings.
An exception is in the braking system, where an accumulator is often used to provide parking brake pressure, or emergency brake pressure.
The hydraulic pumps cannot respond instantly to a change of demand. Therefore, accumulators are fitted to maintain system pressure for the ?second or so before the pump's output is detected and increased.
An accumulator is a cylinder half full of hydraulic fluid under system pressure on one side and a spring of some sort - usually nitrogen gas I think - on the other side of a divider that can move up and down the cylinder. The gas is put into one side of the divider at a particular pressure so that when the hydraulic system is pressurised, fluid filling the cylinder on the other side of the divider forces the divider up the cylinder, until the gas pressure and the fluid pressure are the same. Now you have a small reservoir of fluid at system pressure, and if the system line pressure drops, this fluid is pushed out of the accumulator by the pressure of the gas to maintain the system fluid pressure. When the hydraulic pump output recovers, fluid is pushed back into the cylinder once more.
I hope that makes sense !
Therefore, as has been mentioned, you don't necessarily need accumulators for a hydraulic system to work, but they do make the system work much more smoothly, which is more gentle on the pumps, and also prevents nuisance warnings.
An exception is in the braking system, where an accumulator is often used to provide parking brake pressure, or emergency brake pressure.
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In addition to the above, or expanding on the points...
Hyd accumulators don't actually represent a lot of stored energy; you couldn't rely on them for very long to power things like flight controls, which have continuous demands. Even for brakes, often the guidance is to use the brakes, when on emergency/accumulator scenarios, in a single smooth application and not pump or cycle them, as you'll soon deplete the accumulator.
And additionally to that, and to the point in the OP - most hydraulic systems have multiple power sources (pumps or transfer units of some kind) - in fact, I'm tempted to say "all" but there will of course be counter examples for less-critical systems. What that means is that to lose hydraulic pressure in a system requires either a multiple pump failure scenario - which means it's really not your day, as it's pretty unlikely - or a system leak, which is the most likely way to lose a whole system. if there's a leak, all the accumulator does is help push the fluid out of the leak - when the system is dry, you have no system whether there was an accumulator or not.
Hyd accumulators don't actually represent a lot of stored energy; you couldn't rely on them for very long to power things like flight controls, which have continuous demands. Even for brakes, often the guidance is to use the brakes, when on emergency/accumulator scenarios, in a single smooth application and not pump or cycle them, as you'll soon deplete the accumulator.
And additionally to that, and to the point in the OP - most hydraulic systems have multiple power sources (pumps or transfer units of some kind) - in fact, I'm tempted to say "all" but there will of course be counter examples for less-critical systems. What that means is that to lose hydraulic pressure in a system requires either a multiple pump failure scenario - which means it's really not your day, as it's pretty unlikely - or a system leak, which is the most likely way to lose a whole system. if there's a leak, all the accumulator does is help push the fluid out of the leak - when the system is dry, you have no system whether there was an accumulator or not.
