A320 Landing Gear

[lokeshm11]

It is possible to deploy the gear on the Airbus above the relevant limiting speed (260kts on the A320), with a ADIRS failure, electrical failure, hydraulic failure etc etc.???

Can the gear be extended above 260 kts if necessary? If yes, How?

[TopBunk]

I thought the gear limit was 250 kts?

Why would you want to deploy it above the limit anyway?

[Meikleour]

TopBunk: I think he is referring to the HYD deactivation speed.

[RAD_ALT_ALIVE]

ADR 1/3 determine if the current IAS is in excess of 260 kias. If either detects that the speed is higher than this threshold, then a safety valve closes that prevents the gear from being extended until the speed drops below 260 kias.

If ADR 1 + 3 fail, then the 'high speed' signal would not be able to be sent to the safety valve. Airbus have foreseen this and have designed the system to close the safety valve if ADR 1 + 3 fail (see PRO ABN 34 NAV ADR 1 + 3 FAULT). Gear must be extended by gravity and retraction is not available.

I can't find anything conclusive, but would guess that in this circumstance, the gear could be extended at any speed by using the gravity gear extension handle.

The same would then apply to a full HYD, or ELEC failure. It seems that the gravity gear extension system overrides everything.

[LEVEL600]

Agree,if I remeber it correctly,first turn on emer. gear extension handle opens safety valve.

[vilas]

LEVEL600
first turn on emer. gear extension handle opens safety valve.
If that were be the case then what is the purpose of gravity extension? Safety valve has nothing to do with emergency extension. Emergency handle cuts off the hydraulics by closing the cut off valve which is after the safety valve. It doesn't use hydraulics.

[LEVEL600]

My mistake,you are right, cut off and vent valves are operated by free fall, not safety valve which is pure electrical. I don't expect using hydraulic pressure for gravity extension,but forgot how is hydraulic fluid traped in actuators directed during free fall. Good recurrent for me :).

[Microburst2002]

The valve only prevents the system to be pressurized, and therefore prevents the hydraulically extension, but it doesn't prevent the gravity extension, which in my opinion would be possible at any speed.
In fact, I believe one of the effects of activating the gravity system is to open that valve, isn't it?

[LEVEL600]

No for second sentence,I used same bad logic:).Safety valve is not part of free fall sequence as Vilas said and can be both open or closed for gravity extension. Free fall is performed without hydraulic pressure, which is shut by cutoff valve, valve installed downstream of safety valve . Free movement of hydraulic fluid in actuators is achieved by two vent valves, one for nose and one for main legs,which allows fluid circulation and return to tank.