Funny everybody always mentions the BA and KL encounters with vulcanic ashes but total silence on SQ. They encountered a similar engine out situation somewhere over Indonesia on a B742 and managed to get it safely down at Jakarta on not more than one and a bit despite zero forward visibility due to grit-blasting of cockpit windows, poor weather conditions (rain and moderate crosswind) and ILS u/s.

An other phenomenon that was attributed to volcanic eruptions was the crazing of the cabin windows. Thousands where having to be changed prematurity. There was a large study done in the 80's of the problem and was traced to the the acidity of the volcanic ash. Corrective action was to throughly wash windows of aircraft that flew through volcanic clouds.

A crazed cabin window is the least of concerns when flying through volcanic ash.

An older technique for a volcanic ash encounter involved use of the Overheat Test button on the window heat panel, to shatter the outer panes of the forward cockpit windows to restore some visbility after being damaged by volcanic ash.

That had no effect on the cabin windows of course...but cabin windows are far down the priority chain if a volcanic ash encounter has occured.

From the bleed air manifold, so you can still use the ADP even if the associated engine is not running, or from the APU.

Remember these ADP's are demand pumps and only cut in when the EDP can not supply enough pressure, normally in high work load situations, gear/flaps etc etc.


I presume you are talking about the AC demand/aux pumps. EDP's running or not (Engine Driven Hydraulic Pump) has no bearing on AC generation at all.

When selected to AUTO, yes. When selected to CONTINUOUS, no.

The batteries are already online, and in absence of generator power, external power, or APU generator power, are the sole source of electricity.

At this stage one is down to standby power, with few items powered off the standby bus, and a short time to enjoy it.

EDP's are engine-driven hydraulic pumps, and don't power anything with electricity, as they only pump hydraulic fluid.

AC motors, or the AC hydraulic pumps, are powered by electricity...but have nothing to do with the engine driven pumps. The electric hydraulic pumps, one on number 4 system and sometimes one on number 2, provide supplementary hydraulic pressure for ground operations, such as towing and powering body gear steering (hydraulic system No. 1) and for applying the parking brake (hyd system No. 4).

The cabin windows crazing didn't happen immeditely while flying throught the volcanic ash cloud. The window crazing shows up weeks or months later. If it was not addressed the outer panes would fail. So to prevent failures aircraft that were exposed to the ash cloud would have all the cabin windows changed. When a operator has to replace more than 100 windows at $300 per window it becomes a real concern.

In the air, engine bleed is the only source of air... unless you left the APU running after starting it on the ground (and were below a certain altitude... the APU runs out of "steam" at higher altitudes)

To get any electric powered hydraulic pumps to operate in the air, you need the engines to windmill fast enough so that the high speed rotor (N2 or N3) is above 50%. Engine electrical generators do not operate below this speed.

Electric pumps on #2 and #3 maybe be considered optional.. but there is a lot of aircraft flying around with this option. I wouldn't be surprised if the electric versions outnumber the non-electric ones.
Note that you still need main bus electrics to control the air-driven pumps, so, again, windmilling speed is a factor.

Rgds.
NSEU

Thanks for the clarifications!

Sorry if I sound confused, what about the primary source of power for the alternators to power the electric demands of the airplane? Does this come from the engine?

747 Electrical System (Classic)
 

In simple words -
The 747 have 4 electrical generators (1 per engine).
They produce alternating current 115V/400Hz and each are 60 KVA of power.
The APU is also equipped of 1 or (option) 2 same generators (for ground use).
The 747 Classic APU cannot be used in flight to produce electricity.
DC (28V) is produced by 4 transformers rectifiers which change the 115V current.
A small aircraft battery stores 24V DC for emergency use.
Another similar battery stores 24V DC to start the APU.
xxx
:)
Happy contrails

....although rated at 90kva due to better cooling :8
HEY! how did we get from backup hydraulics to window crazing and now electrics :confused::}

You're talking about the -400, then?

This isn't possible on the Classic, as the electricl pumps can't be powered in flight, nor can electricity be obtained from windmilling powerplants...it's hydraulics only with a minimum mindmilling speed of 160 KIAS, and no normal gear or flaps.

There is a lot of confusing information being posted here.

It is impossible to windmill an engine fast enough to run the generator or to supply sufficient bleed air to run a hydraulic demand pump. ONLY the engine-driven pump will work when the engine is windmilling, and ONLY above approx 160 KIAS.

After the APU is shut down, it cannot (-400) or shall not (Classic) be restarted while airborne, so it is not an option for an airborne hydraulic failure. The APU generators shall not be used airborne.

While the Classic has 4 air-driven demand pumps (1 per engine/system) in addition to the engine-driven pumps, the -400 is available in several optional configurations where 1 or 2 of the air-driven demand pumps is replaced by an electrical pump. The air-driven pumps require bleed air from at least 1 running engine connected to the bleed-air manifold, and the electrical pumps will run (as selected/required) as long as there is at least 1 engine running and its generator is powering the synch bus.

The electrical Aux pump on system 4 runs ONLY on the ground, and cannot be run airborne (except in some cases of failed air/ground switch logic). It is sized to charge the parking brake accumulator ONLY, and will not reliably run ANY other hydraulic system.

I am aware of only 1 case of a 2-system hydraulic failure on a 747, and no cases of a complete, 4-system failure. While the 747-8 may have a RAT, I don't know why anyone thinks it necessary for hydraulic power (though it would be nice for emergency electrical power).

Not necessarily. Some APU installations permit airborne operation, and some don't. Where the APU in flight operation is permitted, it's restricted to 15,000'.

Intruder,
Wasn't the JAL crash many years ago a result of all four hyd. systems being severed when the pressure bulkhead came apart?

Jal 123
 

JAL 123 was the flight - mid 1980s. Over 500 victims.
Loss of the 4 hydraulic systems, because faulty repair of aft pressure bulkhead.
No hydraulics = loss of all flight controls.
xxx
:{
Was not a happy contrails day...

It is possible that total hydraulic failure was named as the proximate cause of the JAL crash. However, that was, as you said, a catastrophic airframe failure that in turn caused the other failures. There was NOTHING that another hydraulic pump or RAT could have done in that situation.

Some 747 classics have a electric pump for number 1 system also, I think the old JAL classics had this and a few others.
Both the number 1 and number 4 electric pumps get there power from the ground handling bus and cannot be powered in flight.

While this is true, there remains one other safety feature that was fitted to only one type of first generation wide body jet transport, originally, from the factory (not added later).
Hydraulic fusing.
The type?
TriStar.
A superb example of aircraft systems redundancy...bar none.

Thought it would be the Tristar!

On my current type, the manuals state that Hyd power is available from a windmilling engine, but with no guidance as to what speed that is. The simulator gives us a warning of low Hyd press at around 130 kts if the PTU isn't operating. But a mate who had an engine failure for real one day said the windmilling engine kept of generating enough Hyd pressure down to about 40 knots. They didn't get a warning until they were turning off the runway!

L1011
 

Four hydraulic systems, RAT, spacious cockpit, superb autopilot and autoland system. Never had the pleasure of a -524 powered variant, but it must have been an improvement on the -22B. Still the only aircraft to use DLC to my knowledge and it was a pleasure to crew.