Airbus L/G Hyd Safety Valve

[DutchOne]

Hey,

A question for the astronauts among us. Please provide a sound explanation or a reference when answering.

Airbus had a safety valve that isolates the hydraulic supply to prevent the L/G operation. On the 330/340 this happens at 280kts. I think it's 260kts on the 320.

It does not mention what speed this is. IAS?, CAS?, EAS?, TAS?

What speed does it refer to?

[leamesis]

The speed is CAS

[DutchOne]

You have a reference?

[gAMbl3]

AMM 32-31-00

The safety valve is an electrically-operated, two-position valve that isolates the Green hydraulic supply to the L/G system. When the computed airspeed is more than 264 kts the safety valve closes (solenoid de-energized). The safety valve opens (solenoid energized) when the computed airspeed is less than 260 kts and the L/G control-lever is selected DOWN.

[vilas]

DutchOne & gAMbl3
One of the conditions for Flt Ctls to switch to alternate law (with no protection) is double ADR failure second not self identified Mach CAS disagree. May be that speed references are CAS.

[gAMbl3]

Isn't Computed Airspeed same as Calibrated Airspeed?

[vilas]

gAMbl3
Perhaps you are right. Since IAS has errors The computers and other systems may be using CAS instead of IAS.

[DutchOne]

Nowadays the IAS on the PFD is not the raw meassured AS. It is altered with internal logics by the ADCs to have a more accurated indication. So it is close to CAS.


If the speed mentioned for the safety valve is CAS, then it is close to the IAS. Then it is safe to say that when climbing through the higher FLs with a constant Mach. The IAS will end up well below the 280 or 260 kts.


Is this safety feature then not applicable up there?

[vilas]

DutchOne
If your cruising speed is less than 260KT then gear will come down if the lever is put down.

[gAMbl3]

Max altitude for landing gear down is 25,000 ft for 320 and 21,000 ft for 330.

[DutchOne]

Thanks gAMbl3,


But this is just a limitation to extend the gear. However it does not avoid it of being operated. Apparently nothing does!

[A-3TWENTY]

The interesting point here is:

It does not extend above 280 kts.

BUT ,

If you are at 37000 Ft, M 0.82 , with an indicated of , say 250 kts.....If you select it down , it comes down , because the computer only takes CAS in consideration. Not Mach.

PS :I know the max alt to extend the gear is 21000 ft, before someone comes to say it.

[gAMbl3]

Just out of curiosity, what factors decide Max altitude for landing gear down?

[DutchOne]

Prabably Density. TAS increases when climbing with an constant IAS. Above the 21000 it might damages the gear and doors. Remember it is only a limitation to extend. flights with gear down can go higher.