Techy Q for 744 drivers...

[niallcooney]

Guys,

Just went out and bought PS1.3 (744 prof sim) which claims to be accurate in every respect. Much to my confusion, I noted that the APU can't be started in flight but can be started on the ground and THEN left running in flight. Goes against Boeing philosophy, don't you think? I mean, look at the 737, 757, 767, 777... as far as I know all off them can start their APU's inflight.

And why doesn't the 744 have a RAT?

4 engine flameout
- No RAT
- No APU inflight
-----------------------
= one 744 going SPLAT!

Do correct me if I'm wrong, but easy on the personal attacks. I'm just curious!

Nial

P.S: This is NOT an advertisement.

[18-Wheeler]

All those other planes you've mentioned are twins, and so need to be able to start the APU to satisfy the engine-out rules when over water.
The 747 is a four engine aeroplane and so is rightly considered to be quite safe from losing all four engines. Yes, it has happened before a couple of times but when you consider the number of flights that they do it's extremely rare indeed.
That's also why they don't have a RAT, they don't need one. But in the unlikely event that all four fail they will windmill over fast enough to provide enough hydraulic pressure to make the controls work. The battery will probably run for about twenty minutes os so, maybe longer, but if you haven't got the engines going again after that length of time you probably never will and so it's not going to be important any more.

FWIW, I'm a 747 Classic driver not a -400 driver, but the principles are the same. Having four engines allows you far greater flexiliblity as to where you can go with restriction.

[Cornish Jack]

Niall
While PS1.3 is not TOTALLY accurate in every respect, it certainly is re. the APU. No in-flight start, but you can start on the ground and run for take-off and initial climb providing AIR ONLY, for one pack, up to 15,000'. Must be shut down by 20,000' Why???
It is able to provide air for one pack during take-off and initial climb to offload the engines for hot-and-high or max weight departures. On the R-R powered 44s this gave (from memory) about 18 degs C advantage on the EGTs.
Why no in-flight continuous running or inflight start? Simply not necessary. The (automatic) electrical redundancy available with four totally separate 'gennies', any one of which could take care of the normal, basic, load requirements makes a FIFTH source a statistical non-starter. That fifth source, were it to be available, could not be connected into the system because neither APU 'genny' can be parallelled with an IDG.
Note, by the way, the other B7 variants ALL suffer from their own problem - ONLY TWO ENGINES !! :D They need all that extra stuff. ;) As to Boeing philosophy - (to paraphrase) "Mr Boeing, why do you always fly on four-engined airplanes?" "'Cos they ain't yet made a FIVE-engined airplane, son!"
Finally, as regards the four engine failure case, read Capt Eric Moody's account of how to deal with that (and more besides). :eek:

[niallcooney]

18W,

Much obliged for the easily understood response (I hate complicated explanations!)... makes perfect sense now that I think about it... ah well, hindsight is 20/20! So does that mean the 747 doesn't have to qualify for ETOPS (Engines Turned Off, Please Swim)?

Nial

[tinyrice]

The twins need the inflight APU for their ETOPS ( Engines Turn Or People Swim ) third power source certification. The cases where 747's have had multiple engine failures have usually involved volcanic ash ingestion. They were all recovered following engine start at lower altitudes, perhaps not all four, but enough to appease the Gods of flight. The probabilities of losing 4 engines and not being able to recover at least one, are probably similar to the RAT not working when it's needed, so why go through the motions and carry the complexity. Even windmilling, the engine hydraulic pumps wil provide adequate pressure for the flight controls, and doesn't the bugga carry a honking great battery for emergency power. Don't know about starting the APU on the ground and leaving it running after departure - sounds odd. Some current whale driver will comment I'm sure.

[Bellerophon]

Tinyrice

Don't know about starting the APU on the ground and leaving it running after departure - sounds odd. Some current whale driver will comment I'm sure.

Cornish Jack already has, correctly. As he says, it is used to supply air, to one pack, during heavy weight take offs.

Regards

Bellerophon

[tinyrice]

Holy crap...............! You're pullin' my chain. If the difference between getting off and not getting off, is the amount of air coming off the APU, then you're nuts. I'll watch that take-off from the terminal.

[mutt]

Actually tinyrice, he isnt pulling your chain, we have operated Classics for years with packs off for this reason, we have the option to do it on the 744, in these cases the APU is used to supply cabin air. We will soon reintroduce Pack Off takeoffs on the 777 in order to carry around 27 more passengers, but as the APU on this aircraft is not approved for use during takeoff, the passengers will have to melt ......

Cheers

Mutt.

[M.Mouse]

tinyrice

How very eloquent.

For somebody with an apparent occupation such as yours I am, suprised that you did not know that jet aircraft are often weight limited due to length of runway, low ambient air pressure, ambient temperature or lack of decent wind component. Using the APU to supply bleed air will increase the amount of power available from the engines and hence increase the max take off weight from a given runway with one or all the adverse factors mentioned above. The calculations take into account losing an engine at the most critical point enabling the aircraft to still get airborne and clear obstacles.

[tinyrice]

Ahhhhhhh the Classic - now you're talking!

[niallcooney]

Guys,

Incidentally does kind soul out there know where I could get some really good 744 system descriptions on the net? Might as well go the whole hog if I'm going to be flying this sim!

Nial

[HotDog]

Used to be able to start the APU in flight before a Boeing mod. removed the airscoop on the APU air inlet door. This reduced drag which outweighd the advantages an inflight APU start presented. I used to carry out APU inflight starts on a few occasions during approach when the allocated parking bay was very close to the turnoff from the runway.

[18-Wheeler]

FWIW, on the older 747's we often have trouble starting the APU after a long flight at altitude, because of cold soak.
So on approach the FE will open the APU door to start to get some heat into it, so it'll start on the taxi in.