A320 G+B hydraulic failure

[7600pilot]

How is it better to put gear down at 200kts rather than after flaps 3 in the case of G+B hydraulic failure?, because how I see it is putting gear down after flaps 3 we will go into direct law after being trimmed by the THS, whereas configuring after 200kts with gear down means we will have to manually trim the aircraft each time for each flap.

[Jwscud]

There is no official Airbus source doc on this one but it’s due to the aircraft handling qualities in that configuration in Alternate law below 200 kts being determined by Airbus Flight Test to be sub-optimal and wishing to get the aircraft into Direct Law to avoid flying in alternate law in that speed/configuration regime.

[7600pilot]

Doesn’t make a lot of sense, but I guess since they tested it has to be the safest option. Thank you

[enzino]

I suppose because there is only one elevator remaining and you have more control in direct law.

[Mr Good Cat]

How is it better to put gear down at 200kts rather than after flaps 3 in the case of G+B hydraulic failure?, because how I see it is putting gear down after flaps 3 we will go into direct law after being trimmed by the THS, whereas configuring after 200kts with gear down means we will have to manually trim the aircraft each time for each flap.

"Extend the landing gear at 200 kt to revert sooner in direct law. This provides, below 200 kt , a better pitch control than in alternate law with one elevator lost and all slats lost."

The above is the Level 2 note pulled straight from the A320/321 FCOM.

[PENKO]

What exactly is the problem with manually trimming the aircraft from 200kts down to Vapp? None whatsoever for a half competent pilot!

I think you are confusing this scenario with the G+Y failure where there is actually a very good reason to be in trim before you lower the landing gear. With no G+Y pressure you loose the stabilizer trim so you want the side stick to be centered at Vapp before lowering the gear and activating direct law.

[vilas]

How is it better to put gear down at 200kts rather than after flaps 3 in the case of G+B hydraulic failure?, because how I see it is putting gear down after flaps 3 we will go into direct law after being trimmed by the THS, whereas configuring after 200kts with gear down means we will have to manually trim the aircraft each time for each flap.
In B+G elevator trim is available so trimming in direct law is not an issue unlike G+Y where stab is frozen at last position and you want elevators at Vapp. So you get to Vapp then put gear down. Here there's different problem.
Lowering gear creates lot of drag. With B+G only half elevator is available so it has to move double the amount to create required g. Also elevator movement is restricted to prevent damage from hitting stops. Higher speed creates more aerodynamic force makes half elevator more effective. Lower speed requires large stick movement in pitch which triggers stall warning.

[Sim25]

Also elevator movement is restricted to prevent damage from hitting stops.

Elevator travel is restricted with speed in single elevator operation: Simmilar to rudder travel limiting. Max authority is regained at 130knts. Doesn't matter if in direct law or alternate... Problem here are high torques at the rear of the aircraft...

[CaptainProp]

What exactly is the problem with manually trimming the aircraft from 200kts down to Vapp? None whatsoever for a half competent pilot!

I think you are confusing this scenario with the G+Y failure where there is actually a very good reason to be in trim before you lower the landing gear. With no G+Y pressure you loose the stabilizer trim so you want the side stick to be centered at Vapp before lowering the gear and activating direct law.

Yes, but as quoted already above the FCOM clearly states “Extend the landing gear at 200 kt to revert sooner in direct law. This provides, below 200 kt, a better pitch control than in alternate law with one elevator lost and all slats lost.” for G+Y SYS LO PR.
In this case you want to “revert sooner to direct law” for increased pitch stability.

In the case of G+Y, as you stated, you delay gear extension in order to keep auto-trim, through the elevators in ALT law, until you are fully configured (F3) and at Vapp and then put the gear down, revert to direct law and lose auto-trim.

Two different procedures for two different reasons.

CP

[PENKO]

CaptainProp, what are you trying to say? I don’t think we disagree..

[CaptainProp]

Ah that could very well be :} :ok: I guess my point was, there are good reasons for both of these procedures.

CP

[vilas]

I forgot to mention that in G+B the available half elevator being one side of longitudinal axis when applied imparts rotary motion. In alternate law side stick commands g which indirectly deflects the elevator it's difficult to accurately anticipate this effect. By moving to direct law pilot directly moves elevator so is in better position to judge and control this torsional effect. That's why it's recommended to move to direct law early. Actually even in G+Y also, flap3 is not important because you only have slat which doesn't have 3 position but getting speed to Vapp before gear is important because that positions the elevator for Vapp with stick neutral.

[7600pilot]

Thank you guys for the explanation. Makes much more sense now.

[CHAPARRAL]

Also a good reference is FCTM PR-AEP-HYD P1 to P4.

[RMC]

Interesting excercise in the sim. In reality the hydraulic system is the most reliable on the aircraft. Complete Failure of one system is incredibly rare failure of two at the same time is only likely to happen with a catastrophic/ uncontained engine failure. Out of interest, how many have lost one hyd system (and what caused it)?

[factanonverba]

Lost blue in the cruise with pump failure, lost all green fluid on undercarriage retraction through failure of a seal on undercarriage retraction system. So it does happen.

[Amadis of Gaul]

Interesting excercise in the sim. In reality the hydraulic system is the most reliable on the aircraft. Complete Failure of one system is incredibly rare failure of two at the same time is only likely to happen with a catastrophic/ uncontained engine failure. Out of interest, how many have lost one hyd system (and what caused it)?

I lost the blue system once, due to a leak that MX was unable to find. 10+ years on the A320 now.

[sangiovese.]

Blue system. Leak inside the pump itself and slowly lost contents. Turned it off and had sufficient for landing

[dixi188]

Earlier airbus, A300, similar systems. Had Yellow system Servo Jam warning, so selected Servos off, then had Green Hyd Lo and turned off both pumps. Landed with Blue for Flying Controls, gear Free Fall, still had flaps and slats, braking was on alternate but still with anti-skid, no steering so we stopped on the runway and got towed to stand. Cargo ops. so no big deal really.
Two hours later, hyd leak fixed, (filter seal), servo fault was some computer glitch that re-set, then we were on our way again.

[RMC]

My bad, should have said A320 onwards. Airbus had to massively improve its “Mean Time Between Failures” with FBW even the way the pipes were joined was revolutionised (Permaswage joints rather than nut and union). During manufacture we proof pressure tested all Airbus hydraulic systems to 5,200 PSIG and only once in ten years had a leak ( which coincidentally was what A of G has had in service) Looking at the experienced posters above can we agree an A320 and subs a single system is unlikely to fail more than once every ten years)?