747 System flaw???

[rich49]

Hi,
I am doing a project on flight controls and part of it is on the hydraulic systems. I have looked at the 747-200 for an example.
I am curious as to why boeing have designed somthing in a paticular way, could somone tell me? If all four engines were to fail like on BA009, the only hydraulic system remaining is No.4, however this services the right inboard and right outboard aileron. The left inboard and outboard ailerons are serviced by hydraulic systems No. 1, 2 and 3. This means that the only ailerons left working after a total engine failure would be the ones on the right wing. Why did boeing not put one of the left ailerons and one of the right ailerons on No.4 instead of two right ones? Surly this would result in better handling??????
I am not sure if this is the same on the 400's.
Cheers
Richard

[OVERTALK]

Would not the windmilling output of all engine's pumps provide low but usable pressure at normal gliding speeds? It's not as if there'd be a lot of control movement. Any available Services would move slowly but still get there I'd imagine.

[RatherBeFlying]

There is a minimum IAS (approx. 250 kt.) to keep the engine driven pumps producing enough to run the flight controls. It helps to get a relight done as even if you have a runway conveniently placed, you may need some hydraulics to get stopped.

After BA009, the authorities published all sorts of new info on avoiding volcanic ash and volcanic ash is now included in the weather reports.

[QAVION]

" If all four engines were to fail like on BA009, the only hydraulic system remaining is No.4, ..."

Hi, Rich.
I'm not sure why you'd think that the #4 system would still be working and not the others? Windmilling will provide adequate hydraulic power to control surfaces (and, as I understand it, down to speeds a lot less than 250kts).

Depending on the hydraulic system (1,2,3,4) in question, hydraulic pumps can be powered by engines turning (mechanical input from the HP engine rotor), electric or pneumatic power. Main AC Bus electrics fail at around 50% N2 (or N3)...i.e just below idle... and, most likely, unless you were in a power dive, I doubt that the engines would spin fast enought to provide AC power to power the Electric Hydraulic pumps (Elec Hydraulic Pumps cannot be powered by Standby AC Elec power.... The batteries would go flat in seconds). Pneumatic-powered hydraulic pumps also need the engines to be turning, but the aircraft needs main bus elec power to control certain elements of the bleed system (I'm not 100% sure how the bleed system would behave without main AC power (certain valves...e.g. isolation valves... need certain Main AC busses to operate)).

Bleed and electric power can also be provided by the APU. However, I believe most, if not all, 747-200 airlines have modified the APU so that it can't be started in the air. I don't believe that any 747-400 has this air start facility (I did hear that one person figured out a way to start the APU by pulling certain circuit breakers in flight, but I never followed it up). If all engines have failed due to volcanic ash, I don't think the APU would fare much better. There is also a fire risk (only one extinguishing system on the APU) and the drain on the APU battery to consider, which, on the 747-400, powers flight instruments and a few other vital items. Several failed starts may leave you with a seriously depleted battery.


"....however this services the right inboard and right outboard aileron. The left inboard and outboard ailerons are serviced by hydraulic systems No. 1, 2 and 3."

If the #4 Hydraulic system was the only one available, the #5 spoiler on the left wing should still be available for lateral control, but I can't tell you how much turning effect this would give you. Also, don't forget that ailerons move up AND down. Up will lower the right wing, down will lower the right wing. Not quite as effective as ailerons on both wings for turning, but...

Another factor to consider is the effect the loss of AC power has on the Air-Ground system. Will certain flight controls behave differently if the Air-Ground System powers down? Speedbrake panel deflection is limited in flight.. If the aircraft Air-Ground system switched to ground mode onpower down, you might get a little more speedbrake deflection than you bargained for (if for some reason you needed to use the speedbrakes): Here's a question for the 747 pilots... If there is an emergency landing field which is too close for a normal idle glide, do you use your speedbrakes... or try to maintain altitude for as long as possible and dog leg towards the landing field without using your speedbrakes?

Anyway, hope this helps.

Rgds.
Q.

[boofhead]

The 747 classic can lose all hydraulics, like the one in Japan did, when the vertical stab was blown off. All fluid was pumped out of all systems when the lines were cut. Boeing offered, for a price, a fuse for the #4 system, to retain at least that system in the unlikely event of a similar failure. Not all airlines have bought it, so many classic 747s are still liable to the total failure of hydraulics. The 747-400 has the fuse, so it will not lose system #4 in the event of damage, provided the damage occurs after the fuse (which works by detecting unusually high flow rates and shutting off at that point, like an electric fuse does).
The classic also has a backup pitch control, using electrical power and the trailing edge flaps, but very few classic pilots I have worked with are aware of how it works. The Japan Airlines crew did not know, again an assumption on my part.

If the sim is to be believed, the loss of all engines will not result in hydraulic failure in the 747 and the 744; all four systems will continue to operate, even if the electrics and pneumatics are also lost, and since the flight controls are mechanically linked to the FCUs, the airplane can still be flown. I would expect the rates of control movement would be reduced as speed falls off, though.

The 777 is. like the Airbus, fly by wire, so it also needs electrical power. It has, I believe, some mechanical backup to one elevator and one aileron, but I don't know for sure. I think that the Airbus would be not so well off in the event of electrical failure.

[Flight Detent]

Hi all,
The Classic B747 will fly quite controllibly with all four engines windmilling, with all four hydraulic systems functioning, albeit at a lower output.
The book specifies a minimum IAS of 160 to maintain all this, but a slightly higher speed would be nice to try the re-starts!
It also mentions that gear and flaps should not be operated!

The re-starts would be powered by the battery, and the Captn. would lose his heading info during this time, on essential, although, on RR powered aircraft, heading info is not lost!

The B747 is a very well designed aircraft, proven day after day with it's reliability!

I believe the 'u/c indicating' (bottom LHS of the P12 panel), when that relay is de-energised, it will allow the APU to be started inflight - it will also stop the spoilers coming up automattically when landing, without the 'autospoilers' ennunciator light to warn you.

Cheers

[rich49]

Thanks for your replies,
it looks like a made a mistake then! The electrical hydraulic pump on No.4 System can only be driven by AC power and not off of the batteries then? Thanks for your help,
richard

[18-Wheeler]

Yep, AC power only, and on the ground only as well.

[Flight Detent]

Hi guys,
The electric #4 pump can ONLY be operated from the:
*** #1 external power side, or

*** #1 APU power side.

So, if you get an MEL that the #1 generator on the APU is inoperative, you cannot pushback using the #4 electric pump, you will have to use the #4 ADP!

Cheers

and, hi '18-Wheeler' - how are ya!

[QAVION]

"The electrical hydraulic pump on No.4 System can only be driven by AC power and not off of the batteries then?"

This is correct, but, as mentioned, the electrical hydraulic pump may be limited to ground use only anyway. Some 747-400's have electrical auxilliary hydraulic pumps on both the #1 and #4 systems. They have a lower output than your Engine pumps, Air-Driven Pumps and AC Motor pumps, but are useful for pushbacks and aircraft maintenance.

The only source of AC power with the engine generators out of commission is the battery-driven Static Inverter. This is a solid state device and wouldn't have the power to drive an electrically driven hydraulic pump.

Rgds.
Q.

[18-Wheeler]

FWIW there's a few Classic 747's also with an electric pump on #1 system. I've flown a couple of them, and from memory they're ex-JAL.

[SMOC]

Yep

ANA classics have electric pumps on #1 & 4 an well as the usual ADPs. Must be a Japanese thing.
I'll say they were very handy for maintenance.