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-   -   Airbus A320 L/G safety valve logic (https://www.pprune.org/tech-log/507363-airbus-a320-l-g-safety-valve-logic.html)

Airmann 7th Feb 2013 12:43

Airbus A320 L/G safety valve logic
 
Bonjour,

Referencing DSC-32-10-10 p3&4

There is a statement under Normal Operation of Gear and Doors:


The green hydraulic system actuates all gear and doors. When the aircraft is flying faster than 260 kt, a safety valve automatically cuts off hydraulic supply to the landing gear system. Below 260 kt, the hydraulic supply remains cut off as long as the landing gear lever is up.
There is also a chart on the next page with the Boolean logic of the safety valve, which confuses me, it does not match at all with the statement. The logic chart shows the safety valved being closed? (or is it open) when:

L/G lever is DOWN

AND

ADR 1 or 3 <260kts OR
MLG on Ground

-------------

There are now 4 conditions mentioned, two in the text and two in the chart. The conditions mentioned in the text refer to a "safety valve" which cuts off hydraulic pressure when the aircraft is flying faster than 260kts.
Then it says that hydraulic pressure remains cut off as long as the L/G lever is up and the aircraft is <260kts. These two situations refer to every possible situation in the air. First it says when the speed is >260kts and then when the speed is <260kts? Is it correct to say that this is just badly worded and what the paragraph means is that in the air the L/G lever will not work above 260kts?

Then it gets more confusing with the chart as there are two other conditions that are completely different from the first two. These two are specific to when the L/G lever is down. In this case it mentions 1) that when the MLG is on the ground and L/G Lever is down (and then there is an arrow pointing to the safety valve), and I don't understand if its supposed to be closed or opened, but I'm assuming in this situation you want the safety valve closed and there should be no hydraulic power to stop an accidental retraction on ground.
But then there is another condition, 2) when the L/G gear lever is down and ADR 1 or 3 reports <260kts??? Why would you want the safety valve to stop the gear retraction at this point? this is exactly the situation you are going to be in after lift off (i.e. speed <260kts, LG lever Down), how would you raise the L/G if you didn't have power in this situation?

I'm very confused, I think I may be reading this whole thing incorrectly, or making a mistake in my assumptions.

Thanks in advance

gAMbl3 7th Feb 2013 14:01

ADR 1/3 less than 260 kts with L/G lever down ----> safety valve is open(lets hydraulic fluid pass through it) {take off condition}

ADR 1/3 less than 260 kts with L/G lever up ----> safety valve is still open as there is a 'Self Maintained' logic. {initial climb condition} As soon as ADR 1/3 more than 260 kts the safety valve will close (stop the hydraulic supply) {cruise}

ADR 1/3 more than 260 kts with L/G lever up ----> safety valve will remain close and when ADR 1/3 drops below 260 kts with L/G lever up it will still remain close. It will open only when L/G lever is selected down.{descent condition}



Why would you want the safety valve to stop the gear retraction at this point? this is exactly the situation you are going to be in after lift off (i.e. speed <260kts, LG lever Down), how would you raise the L/G if you didn't have power in this situation?
There is 'Self Maintained' logic for it.



when the MLG is on the ground and L/G Lever is down (and then there is an arrow pointing to the safety valve), and I don't understand if its supposed to be closed or opened, but I'm assuming in this situation you want the safety valve closed and there should be no hydraulic power to stop an accidental retraction on ground.
L/G lever is locked when main gear is compressed. No matter how much force you apply the lever won't go up on ground.

Airmann 7th Feb 2013 14:42

Ok thank you for your explanation. Still confused b the FCOM especially that chart. Could you please clarify what "Self Maintained" logic means, thanks


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