A320 2 Eng Failure,No Fuel, Why Vapp is 150kt

[pacific900]

From the QRH, A320 dual engine failure, 1.21 . Min approach speed is 150kt , is that any reason behind since the min RAT speed is 140kt

[Rubber Dog]

The following, from the FCTM is obvious and I am sure you are aware.

"Following an all engine flame out, the flight deck indications change dramatically as the generators drop off line. The RAT is deployed to supply the emergency generator and pressurize the blue hydraulic circuit.
The approach speed must be at least min RAT speed (140knots) to keep the
emergency generator supplying the electrical network."

My own personal opinion is that I would probably like a margin of error on my speed to ensure the emergency generator stays on line and does not drop in and out on the cusp of its operating speed.

[Chris Scott]

Hi Rubber Dog,

See you are quoting from the Tech Manual. Just out of interest, what does the FCOM have to say on the subject of possible APU start nowadays?

Whereas it is probably inadvisable in the double-generator failure case (engine or engines still running), the battery depletion in the case of one failed APU start is unlikely to be an issue in the double-engine case.

Chris (A320 pilot, 1988-2001)

[Rubber Dog]

Hi Chris,

In the case of double engine failure (with fuel remaining of course) then the training guidance is as follows:
Once below FL250 , APU can be started . This restores
Electrics
Once below FL200 , APU can be used for Starter Assist
Relight
So, not an issue at all in this case.

And of course as you correctly mentioned, in the case of simultaneous engine gen failure, the probability of a successful APU GEN coupling is low. Therefore APU start attempts should be avoided as this will consequently reduce the flight time on batteries only ( about 3.5 minutes for one start attempt).

[Chris Scott]

Thanks Rubber Dog,

Should perhaps explain I’m presently away from my (out-of-date) manuals, and stuck with a pay-as-you-go, dial-up internet connection at 30Kb/s...

To return to pacific900’s thread subject: if you are lucky enough to get the APU started, you’ll retain Blue hydraulics from the RAT, and gain Yellow from the AC pump. Green will then also be available, courtesy of the PTU. All these will be of limited power, leading to big pressure falls under load. Nevertheless, is Normal Law retained for the approach and landing?

In the other case, where the APU fails to start, you are presumably stuck with Blue hydraulics only. if so, I guess we are talking about Alternate Law, slow THS, slow elevators, slow ailerons, limited spoilers, slow rudder, slats only (no flaps), and free-falling the L/G? With the new spec of RAT that you guys enjoy, does the flight-control law still downgrade from Alternate to Direct when the L/G is selected down?

Chris

[Microburst2002]

I thought that in a double GEN fail, APU GEN is what you need. Why wouldn't it couple successfully? Isn't it what it is for, in the first place?

However, if the AC buses are lost because the buses themselves fail, then there is absolutely no point in starting the APU. It will drain the batteries for nothing.

regarding the Yellow Elec, there are other threads on that subject. That pump is much much less powerful than the engine driven pump, and probably it cannot coup with everything. No procedure contemplates its use, so it is probably better not to use it, or if you decide to do it, at least switch the PTU OFF. With yellow and blue and gravity gear extension, we are more than happy...

Another question:

In the sim check, with both engines flamed out, I still had yellow and green pressure all the way till landing, due to the windmilling. Is this correct. Would that be that way in real life??

[Rubber Dog]

M2002, I see what you are saying but perhaps they don't differentiate between the two scenarios due to the likelihood of electrical damage done or the probability of it working. If both gens fail it must be something significant and probably bus affecting throughout.

The Y elec pump is rated at 18% of an EDP and as you say probably can't be relied on to cope with all of the strain. You will also get hydraulic power from the windmilling effects of the engines as you found out in your sim but once again cannot be relied on at lower airspeeds.

I certainly agree with your post.

Chris, I hope that covers your question too but remember the original post left us with no fuel therefore no APU start.

[Microburst2002]

I have been looking at the FCOM and when you lose one EDP and the PTU, the system is lost. A note says that you may use the Y elec if the affected system is the yellow.

So if I had AC to power the y elec, I would definetly use it after selecting PTU OFF.

In the sim, we had G and Y hidraulic power all the way till landing. But I don't think that is realistic.

[I-2021]

M2002 a lot depends on your RAT electrical standard. As you probably know with first generation RATs, lowering the landing gear would lead to stall the RAT, and therefore you would end up with flight on batteries only. For this reason, and also due to the fact that following a go around you would still be flying on batteries (no gear retraction), it was recommended to avoid APU starts, taking into consideration the fact that the APU eventually wouldn't couple with the generators. With new generations RATs this problem has been solved and you are only required to fly above 140 kts. Therefore if you deplete your battery, you still have the RAT with you basically until landing. Hence you can try to start the APU. For double engine failure it is slightly different since you are committed to land :O Regarding the use of APU in double eng failures, if you are "lucky" enough to loose first one engine, then the other, after the first engine fails you will be starting the APU anyway, therefore you will have it ready when you will loose the second one. What you want to do with it, it's up to you because the procedure does not contemplate its use. It becomes the so famous "safest course of action" :ok:

[Cough]

Maybe it has nothing to do with the RAT and more to do with enough windmilling of the engine cores to perhaps drive a little Y+G hyds?

Or maybe, whilst with the descent rate of a falling stone and only partial slats deployed, Airbus has calculated that its gonna take a little more than just Vls+5 to flare the beast? (OK, this argument falls at the first hurdle of why don't they add a bit more at the higher weights, its worthy of consideration though...)

Or maybe, as it is important that the RAT maintains its 140kt through the flare manoeuvre when you are going to loose lots of speed. If you start at 150 kt, you should still have the 140kt rat speed as you gently lower the aircraft onto the chosen surface.

[ReverseFlight]

I am informed that, with newer generation aircraft, delete the following references under ELEC EMER CONFIG:

"APPR: When L/G down: batteries only. Use manual pitch trim."

"GA: When L/G uplocked EMER ELEC PWR ... MAN ON."

Please speak up if your understanding differs from this.

[ReverseFlight]

Thanks, I presume QRH 1.05 reads as amended.

[Cough]

Correct.

Old QRH has resets.. new one blank.

[Chris Scott]

Rubber Dog, Quote:
Chris, ...remember the original post left us with no fuel therefore no APU start.

Whoops! :{

Good discussion, though.

Chris

PS (Historical note)
That problem of potential stall of the original RAT with the L/G down plus sideslip meant that the electrical system was still undergoing redesign when the first line-pilot courses were being conducted at Blagnac in January 1988. The A320 finally gained type-certification in the February.

[Microburst2002]

I understand that you consider not starting the APU in the "old" RAT models, in order to secure enough battery current during the time between gear is selected down till you land. If there is an unsuccessful APU start you would compromise supply during this phase. If you lose current during approach the only means of control would be the rudder (blue hyd) and maybe the THS if windmilling was enough.

But I don't understand why wouldn't the APU GEN supply the network when the ENG GENs are OFF. The distribution system through the BUS TIE contactors is automatic and the APU GEN would come on line normally, just the same as it does on the ground when you arrive to a "cold and dark" cockpit and turn the APU ON. If the APU starts successfully (which is what happens most of the times) then the APU GEN will give you precious electricity.

I have noticed that in my QRH (old, very old model) there is no mention to starting the APU. Only to using it. It only says "WHEN APU AVAIL BELOW FL200". There is no "APU .... START" line. Entirely left to Captain's judgement. Is it the same in the new models?

[Microburst2002]

I understand that you consider not starting the APU in the "old" RAT models, in order to secure enough battery current during the time between gear is selected down till you land. If there is an unsuccessful APU start you would compromise supply during this phase. If you lose current during approach the only means of control would be the rudder (blue hyd) and maybe the THS if windmilling was enough.

But I don't understand why wouldn't the APU GEN supply the network when the ENG GENs are OFF. The distribution system through the BUS TIE contactors is automatic and the APU GEN would come on line normally, just the same as it does on the ground when you arrive to a "cold and dark" cockpit and turn the APU ON. If the APU starts successfully (which is what happens most of the times) then the APU GEN will give you precious electricity.

I have noticed that in my QRH (old, very old model) there is no mention to starting the APU. Only to using it. It only says "WHEN APU AVAIL BELOW FL200". There is no "APU .... START" line. Entirely left to Captain's judgement. Is it the same in the new models?

[golfyankeesierra]

Min approach speed is 150kt , is that any reason behind since the min RAT speed is 140kt
Perhaps for a little extra hydraulic press from the windmilling engines?
(Just guessing, never flew A320)

[toby320]

I have noticed that in my QRH (old, very old model) there is no mention to starting the APU. Only to using it. It only says "WHEN APU AVAIL BELOW FL200". There is no "APU .... START" line. Entirely left to Captain's judgement. Is it the same in the new models?

Yes it's the same thing.

Toby:ok: