CAT III Single / One engine

[NoJoke]

When on one engine we get CAT III Single on the FMA. I am sure I have read that for CAT III Dual we need two engine generators but I can't find the reference. Is the the requirement for two engine generators to ensure the electrical split, and if so, what is the difference between two engine generators and lets say one engine gen plus the APU gen? Sorry should have mentioned the A320 family!

[Mike Tuck]

Yes I think I've that bit about eng gens. Will post when I find it.

M.T.

[mcdhu]

The electrical requirement for Cat 3 Dual is 2 independant electrical systems which the 2 eng gens give you, but with an eng gen and the apu gen, there is apparently a common relay somewhere in the system and thus that electrical configuration only qualifies for cat 3 single.

mcdhu

[mupepe]

I'm flying CAT III single engine with ERJ145 (HGS) and have 2 generators per engine.
I never been told about this requirement before !
this piece of info could maybe help you a bit !
mupepe

[CptRegionalJet]

On the CRJ -700/900 you can go CAT3 with HGS and one engine inop.
APU generator has the same capability as the engine driven one,so you are safe here.

Regards,
CptRegionalJet:ok:

[Admiral346]

Flying ERJ or CRJ CAT III by hand on the HGS has nothing to do whatsoever, one or two engines on, with the indication Cat III single/dual on an A320 !!!

The Airbus indication has to do with the Autopilot being fail/operational or fail/passive, as there is no autopilot doing CAT III for you on the ERJ CRJ the two don't compare...

Nic

[NoJoke]

The APU gen is the same as the Eng gens on the bus 90 KVA but I think it the problem that mchdu points out that brings us down to CAT III single. All the wiggley amps go into the same box and spoil the show. Still can find a reference.

[145qrh]

Pretty sure the Cat 3 Single limitation when eng out is more related to HYD redundancy.

Ie You need all 3 HYD systems to be be Cat 3 dual.

Thats why on 343/5 if you loose eng 1 or 4 you can be CAT 3 dual,(as you still have 3 sys) but eng 2 or 3 you only be Cat 3 single.

:)

[Mäx Reverse]

We don't use AI FCOMs rather customized company Manuals, so I don't know where it's hidden in the original FCOM.

But we have the following table in OM-B -> Normal Procedures -> Supplementary Procedures -> LVO -> Airborne Equipment required for CAT II/III Approach

and there it says 'Engine Generator - Number required for CAT III Dual: 2'

Rgds MAX

[mcdhu]

Max - the table you refer to is in the Airbus QRH 5.04, but no explanation apart from the title of that line which is 'Elecrical Supply Split'. I'll keep looking!

cheers,
mcdhu

[EMIT]

The original question, for A-320, states that the Autoland status is Cat3 Single for a single engine approach, then questions whether that has to do with one of the electrical sources being the APU generator instead of the generator of the broken engine.
No, the status is Cat3 Single because you have only 1 engine operating!

Cat3 Dual means the system is fail operational: no single failure (from that start situation) will preclude the system from completing the landing automatically.
Note: if that single failure occurs below Alert Height. If the failure happens above Alert height, you will see a reversion of Autoland status to Cat3 Single.

If you start the approach with already one engine out, then how can you expect the system to cope with a failure of the remaining engine? Thus when flying single engine, the system is not fail operational, but fail passive.
The misleading concept may be the point that in this Autoland status discussion, the failure of the second engine would be a “single failure”, because you have to look at the situation with which you started the approach, namely with one engine already out – from that initial situation, a “dual engine failure situation” is only one step away, so only “a single failure”.

Now, for a 2 engines operating situation, but with 1 engine driven generator u/s and replaced by the APU generator – indeed, there you will also see Cat3 Single, according to FCOM. The reason for that, not sure,like MCDHU stated earlier, there may be some common relay in the system.

[TyroPicard]

NoJoke
Do you really need to know the reason? Can you not just accept that the Autoland status of the a/c is as displayed and make operational decisions accordingly?
i.e keep it simple
TP

[Wireflyer]

My best guess is that the APU GEN has no constant speed drive like the two engine generators.

My company asked Airbus some time ago if they recommend to start the APU before commencing a low visibility approach (it has been a common practise by commanders who flew the MD before)
The answer was more or less: "as you like but there is no need for it and doing so doesn't improve your chances against a downgrading because the APU GEN has no constant speed drive."

W.

[NoJoke]

I suppose you could say that about anything. I apologise for having an enquiring mind. I see from your profile you are an SFI on the bus, good luck with your students. :cool:

[Checka-de]

In the QRH 5.04 a note at the bottom of the page states; 'Electrical Power Supply Split: this ensures that each FMGC is powered by an independent electrical source (AC and DC).'

But no mention of ENGINE or APU.

[Ipaq]

The information we had once from our tech. guys is that the APU generator shares the same circuitry to AC Bus 1 as the engine generator, therefore is not considered a completely seperate source of AC power, unlike the A330which is totally independant.

[Tree]

My best guess is that the APU GEN has no constant speed drive like the two engine generators.


APU's always run at the same speed (100%) unlike the engines which run at variable speeds therefore there is no need for a csd on an apu generator.

[TyroPicard]

Enquiring minds read on...

Many years ago some enquiring minds pulled a CB on a fully serviceable DC-10 to see if that piece of kit was involved in controlling N1 ... it was involved.. one of the engines oversped, and a fan blade detached, punctured the fuselage and killed a passenger who was quietly enjoying the view from his window seat.

Like I said, keep it simple.
TP

[Wireflyer]

APU's always run at the same speed (100%) unlike the engines which run at variable speeds therefore there is no need for a csd on an apu generator.

I'm aware of this - but still this is my best guess why you don't get CAT III DUAL if SE+APU GEN running.
The electrical supply of the relevant AC busses is assured as well, but something has to be different!

[NoJoke]

What? :rolleyes: Back to the thread ..... :oh:

[TyroPicard]

NoJoke..
I was merely pointing out that having an enquiring mind (without suitable guidance) can lead you into trouble...
Approximately on the thread...
A study of FCOM 3 Abnormals and QRH reveals that for CAT 3 DUAL you need 2 engines, 2 Engine GEN, and Electrical Split. If any one of those is INOP you lose CAT 3 DUAL capability. I don't teach the "why", just the "what to do next" bit, because life's too short...
Good luck with the search..
TP

[NoJoke]

Thanks for that. I think the same observations were made in previous posts. When I'm instructing I do try to provide 'suitable guidance' and I find that giving the guys bare bones information (especially to the young second officers) gives them nothing to latch onto. However, if I can provide some form of logic to make a peg to hang their hats on, I find the the information sometimes sticks.

I probably have less of my life left than you, but I keep trying ~ It's never a waste of time. "Make it so"

[matthewgamm]

"...for CAT 3 DUAL you need 2 engines, 2 Engine GEN, and Electrical Split"

Sir, what is Electrical Split?

[NoJoke]

Hi Matt, my definition would be two independant electrical circuits (Therefore they are split). This is why I am trying to find out what gives the commonality between the APU gen and the engine gens.

[kite]

MCDHU is correct.

The reason for the limitation is the electrical supply split. Cat III dual approaches require two independent electrical sources and thats why QRH 5.04 states an electrical supply split is required for CAT III dual. When configured with single engine and an APU generator, if a fault occurs in one electrical system the other bus ties to the failed side and provides power. Therefore they are not split. When both engines are operational on a CAT III B (dual) approach they remain split even following an electrical failure on one side.

One cautionary note: following an engine failure and an APU running, the System Display (SD) ELEC page appears to show the two sides split. This is just for representational purposes and if a failure occurred on one side the tie would connect the failed side to the remaining generator.

Does that help?

[TyroPicard]

kite
When both engines are operational on a CAT III B (dual) approach they remain split even following an electrical failure on one side.
er.. I don't think so. The Bus Ties have to be open for CAT 3 DUAL capability. If a GEN fails the Bus Ties close and the a/c downgrades to CAT 3 SNGL. (If the electrical system were to remain split half the ship would be electrically dead).
If the failure happens below Alert Height (100' RA) the FMA's continue to display CAT 3 DUAL and there are no operational consequences.

One cautionary note: following an engine failure and an APU running, the System Display (SD) ELEC page appears to show the two sides split. This is just for representational purposes and if a failure occurred on one side the tie would connect the failed side to the remaining generator.
It's not just representational, it's true! If you have any two GEN available AC BUS 1 and 2 are electrically split, with either one or two Bus Ties open. With only one GEN available both Bus Ties close, linking AC BUS 1 and 2.

TP

[JETZ Tech]

The reason for this is during the approach mode (only) the busses split into three and there is no closing of the bus ties. So in order to have Cat3 Dual you need two separate power supplies Gen1 and Gen2 powering each side to get Cat3 dual.The a/c maintains this condition until on the ground.
With the APU Gen whcih is alraedy running on a constant speed unit the power cannot be split to either bus separately as with for example a Boeing product.
Hope this is helpfull.

[kite]

TyroPicard, thanks for your input, but I disagree. The SD is very representational. The Single Engine + APU config, as you rightly point out, power each side, but the bus ties are still actively able and ready to transfer in case of GEN failure. This is the context of QRH 5.04 and is not considered split, even though on the SD page it looks like they're split.

There is a complex reconfiguration of electrical supply during a CAT III DUAL approach. JETZ Tech explains the nub of it. The autopilots and other essential systems for the approach must be completely independent and isolated to provide redundency. The bus ties powering these systems must be kept open. If there is a failure of one generator the autopilot and other essential systems for the approach on the failed side would be dead. The reason the failed half is not completely electrically dead is because of the unique bus configuration in CAT III Dual.

I know it could be argued why do we need to know this, but I fail to see why we as pilots shouldn't be told this. We know if it says CAT 3 SINGLE on the FMA not to do a CAT 3 DUAL, and we don't start pulling CBs willy nilly because we're told not to. My theory is that a shallow FCOM is less to do with some devilishly ingenious French philosophie but more to do with the hassle of translation. Like No Joke and many others here I'm not so good at sticking my head in the sand.

[Ipaq]

Guys; as I said before if you look at FCOM 1.24.10 page 2, the supply from the APU joins "downstream" of the gen line contactor for Gen 1 and as such is not an independent source of electrical power as a fault could exist between there and AC Bus 1, hence Cat 3 Single.
The A330 however, goes straight into AC1 without sharing any gen 1 circuit - and independent source of electrical power and therefore Cat 3 Dual when running with an engine generator!

[matthewgamm]

So, the only time you get CAT 3 SINGLE, is when you're single-engine or have lost one of the two engine GENs.
Don't you also lose CAT 3 DUAL capability when you lose N/W STRG, with the nose gear doors open?

I am not a bus pilot, but I think I read something about this.

[Ipaq]

Not necessarliy - there are many failures which will downgrade you to Cat 3 single, not just an engine failure - I couldn't even begin to list them all on here!
Some are monitored by the FMGS -some are crew monitored.

Yes - you are Cat 3 single with a n/w steering failure because automatic rollout is not permitted.

[Admiral346]

Ipaq, you are right.

CAT III dual means, that your systems are fail/operational, that is in case of a failure, you can continue after getting the click,click,click, and CAT III single means, you get the warning and have to abort. The system is fail/safe. The reasons for it are legion, Elec, Hydr, steerability, autopilot, IRS, just check the table...

Nic

[Scallywag]

According to Airbus, for A320.

The power supply split function that monitors the electrical segregation of the FMGC's does not consider the APU generator as an independent electrical source. As a consequence, when an engine generator is lost the capability is downgraded to CAT 3 Single.

The electrical power supply split function monitors......
-The two BUS TIE contactors
-The DC TIE contactor (between DC BUS 2 and DC BAT BUS)
When one of the above contactors is closed, the FMGC considers that the power supply split is lost (independently of the APU generator)

The A330 does not have this restriction and can be CAT 3 Dual capable even with one engine generator failure as APU is considered an independent electrical source.

Don't know why this isn't in the A320 fcom's.

Scally

[TyroPicard]

kite
There is a complex reconfiguration of electrical supply during a CAT III DUAL approach. JETZ Tech explains the nub of it. The autopilots and other essential systems for the approach must be completely independent and isolated to provide redundency. The bus ties powering these systems must be kept open. If there is a failure of one generator the autopilot and other essential systems for the approach on the failed side would be dead. The reason the failed half is not completely electrically dead is because of the unique bus configuration in CAT III Dual.

JETZtech's nub..
The reason for this is during the approach mode (only) the busses split into three and there is no closing of the bus ties. So in order to have Cat3 Dual you need two separate power supplies Gen1 and Gen2 powering each side to get Cat3 dual.The a/c maintains this condition until on the ground.

There seems to be a fair bit of confusion here. The electrical system does not behave differently just because CAT 3 DUAL is displayed on your FMA. In normal operation the AC BUS ties are both open; so is the DC TIE between DC BAT BUS and DC BUS 2. None of this changes on the approach, there is no "complex reconfiguration", no "unique bus configuration". The normal configuration ensures an electrical split (AC and DC) between FMGC 1 and 2.
In the event of a GEN failure the AC BUS ties close and the remaining GEN powers the whole system. You hear a triple click and see CAT 3 SNGL annunciated, because the FMGC's are no longer electrically split. You will also get an ECAM warning above 800 ft.

It is that simple, and the important reaction is operational rather than technical. Why delve deeply when you don't even understand the basics?
As to why the APU GEN is not considered an independent electrical source .. I haven't a clue! Airbus have decided, and because their decision works in a safe direction I am not going to question it. Not sticking head in sand, just practical aviation.
Hope that helps - but if not, print out your posts and mine and take them along to your friendly neighbourhood TRE and ask his/her opinion.
TP