747-400 AUX Hydraulics & Brakes Question

[Wil Neverbee]

As an airframe designer I spend my time at work calculating fatigue lives so these questions are rather outside my field, but something I’m interested in.

I have watched 747-400 cockpit videos showing engine start procedures and I’d like to understand the reasons for selecting the No.4 hydraulic system ac driven AUX pump prior to pushback.

At pushback the APU would be running which provides air and ac power. The normal brakes and parking brake require hydraulic pressure on SYS 4, so I can see why it is essential for pressure to be maintained on that system for pushback, engine start and taxi. My question is why is the AUX pump selected in preference to the air driven demand pump, which presumably could keep SYS 4 pressurised during this time? Maybe it is because the duct air pressure is not reliable during engine start? Does the duct pressure vary during start and does the order of engine starting come into this?

To help my understanding, maybe someone could tell me the rated flow rates of: the engine driven pumps, the ac and air driven demand pumps and the ac driven AUX pump on the 747-400.

I have done some research of my own (previous threads) and carefully read the report of an incident involving loss of brake pressure on an Iberia 747-300 after pushback at Madrid on 14th September 2002. The crew didn’t notice that the AUX pump had tripped out after an aborted start on engine No.4. Would there have been less liklihood of this collision happening had they been in a 747-400?

Many thanks,

Wil.

[Carnage Matey!]

During engine start you want as much duct pressure as possible to start the engines. Bleeding off air to drive air driven hydraulic pumps is undesirable during this phase. It's probably also overkill when all you need is the smaller output of the electrically driven aux pumps.

[Wil Neverbee]

CM, thanks for your answer, I was looking at the problem from the wrong direction!

Wil.

[SMOC]

The crew didn’t notice that the AUX pump had tripped out after an aborted start on engine No.4. Would there have been less liklihood of this collision happening had they been in a 747-400?

Shouldn't (never say never) happen on the -400 as the #4 HYD switch is in the AUX position but the pump is controlled by logic, so if the crew had an aborted start the AUX pump would be commanded to run again.

On the classic the AUX (or AC pump as it was called) is a magnetically latched switch which tripped to off when HYD pressure was generated by the engine driven pump during start, so after an aborted start the AC pump switch should be selected again, was a common fault practiced in the simulator.

[Spanner Turner]

Answers from SMOC and Carnage Matey are spot on, here's the answer to the other part to your question

To help my understanding, maybe someone could tell me the rated flow rates of: the engine driven pumps, the ac and air driven demand pumps and the ac driven AUX pump on the 747-400.

The four Engine Driven Pumps and the two Air Driven Demand
Pumps are physically identical and interchangeable. They are rated
differently due to variations in drive speed and flow capability.
Pump drive is supplied by the Engine Accessory Gearbox for the
EDP and by an air driven turbine for the ADP.

The EDP is flow rated at 37.5 gpm at 2850 psi when operating at
3750 rpm.

The ADP used in Sys 1 & 4 demand is flow rated at 32 gpm at 2850 psi when operating at 3200
rpm.

The AC Demand Pumps used in the hydraulic systems two and three are variable displacement type, rated at 6gpm
flow at 2,850 psi at 5,700 rpm.
Below 2,850 psi the pump is soft compensated so that pump delivery
increases to 12gpm at 1200 psi.


The ACMP used in 1 & 4 Auxillary position consists of;
• A Three Phase AC Motor
• A Centrifugal Booster Pump
• A Single Stage Hydraulic Pump.
The Hydraulic Pump is a pressure compensated, variable
displacement type. Pump output at 5800 rpm is 5.7 gpm at 2700 psi.

[Wil Neverbee]

It says at the top of the page that Tech Log is The very best in practical technical discussion on the web. Your replies (Carnage Matey, SMOC & Spanner Turner) have confirmed this.

Thanks,

Wil.