B737NG refuelling question

[greekphotog]

First post here i am not an engineer so please bear with me...:O

Following incident occured at our airport:
Boeing 737-800 is beeing refuelled on stand via fuel dispenser.
Aircraft APU is off as power is supplied by fixed ground generator.
During refuelling there is a power shortage as the gpu goes inop.
Nearly the same time fuel starts to flow from the underwing fuel vent creatng a major spill on the apron.
According to the airline (engineer of flight deck crew...not sure as i was not present) ,the whole problem was caused by the gpu power shortage.

My question here is if the above is really true as its the first time i have heard about this kind of incident.Can a loss of electrical power during refuelling create the above effect? How does a 737NG "act" during this kind of situation?
Any reference material from Boeing would be deeply appreciated!

Thanks in advance

[d105]

737NG fuel systems are AC powered. Having the GPU cut out will cut that AC power and leave the aircraft on DC only.

[photons]

If GPU power was lost without the flightcrew being aware, then the 737NG will revert to DC power/standby and will deplete the battery dry within 30 minutes.
If the crew were aware of GPU loss, then there is a chance that the standby switch wasn't selected to battery or the flightcrew allowed the battery to deplete without powering the APU.

When AC power is lost the battery on standby converts DC to AC via static inverter to power the fuel cut off switch and fuel gauges. Basically GPU power loss will have no affect on re-fuelling as long as the standby switch was set to 'battery'.

Without any DC or AC power the fuel valves will close automatically, and will remain closed untill the red overide button is pressed continuously while refueling. A tool can be used to hold the spring in the valve to the 'open' position.
Basically the hose connected to the receptable will be pumping fuel into a closed fuel valve, this would normally be opened when AC power is available.

Also without DC or AC power there is no automatic fuel shut off, so there is no override to a over fuel situation in either tank.
Spillage could occur from both situations.

[FaHaD_7arB]

Fueling Has Nothing To Do With Ac
The Fueling Sys. Gets Its Power From Hot Battery Bus Once You Open The Refueling Station Door, And If You Connect The External Power,it Will Keep Getting Its Power From The Dc Bus
Through Refueling Power Control Relay Without Loosing Electrical Power To The Fueling Vlv's

[EW73]

Good on Ya!

I'm with FaHaD_7arB on this...

I don't recall having to switch to Standby Power (by any means) with no AC power available, to get the refueling system to work.

Cheers,
EW73

[greekphotog]

Thanks to all for your answers so far.

My guess is that there was at least one cockpit crew member on the flight deck to monitor the refuelling as this is not a home carrier plus an airline ground engineer.

What baffles us is how the loss of power during the refuelling can cause the venting of fuel from underwing...

Cheers
Greekphotog

[BOAC]

I seem to recall the requirement for AC was only to enable the wing gauges to function? Fuel valves were, I think, DC and bowser pressure operated. I never quite understood the way the NG 'selected quantity' function worked and I wonder if that would fail open with no AC - should still be a high-level float switch though?

Contributors note: there is NO suggestion that the battery went flat.

[photons]

REFUELING WITH BATTERY ONLY

When the APU is inoperative and no external power source is available,
refueling can be accomplished as follows:
Battery Switch...................................................... .........................................ON
Standby Power Switch ............................................................ ...................BAT
This procedure will operate the static inverter and supply the necessary AC
voltage to operate the entire fueling system normally, including the gauges
and fuel shut-off system. The only limitation during this type operation is
the battery life. No cooling limitation has been imposed on the inverter.
Therefore, this procedure would be the most desirable to use when no APU
or external power is available, since the entire fueling system will operate
normally.
* * * *
REFUELING WITH NO AC OR DC
POWER SOURCE AVAILABLE

With the APU inoperative, the aircraft battery depleted, and no external power
source available, refueling can still be accomplished.
Fueling Hose Nozzle ............................................................ .ATTACHED TO
THE REFUELING RECEPTACLE
Fueling Valves...................................................... .................OPEN FOR THE
TANKS TO BE REFUELED
Note: The No. 1, No. 2, and center tank refueling valves each have a red
override button that must be pressed and held while fuel is being
pumped into the tank. Releasing the override button allows the
spring in the valve to close the valve. A tool may be required to
hold the spring in the valve to the OPEN position.
Caution must be observed not to overfill a tank, since there is no
automatic fuel shut-off during manual operation. When the
desired amount of fuel has been pumped into the tanks, the
refueling valves for the respective tanks can be released. Tanks
No. 1 and No. 2 may also be refueled through filler ports over the
wing. It is not possible to refuel the center tank externally using
overwing ports.
* * * *

[bArt2]

Why would you have to refuel with the battery depleted, except for maintenance maybee. I would have the tech guy replace the battery first and then refuel :E.

[RAT 5]

Are you not getting off the thread's focus question. Why should the fuel spill out of the vents just because there was no AC power? I have to assume the battery was not depeleted, as 30mins of fuelling (but don't the NG's have 60mins batteries?) would be an enormous quantity of fuel. The valves are spring loaded shut unless the fueler was activating the red buttons and forgot to close them. Was this just a coincidence? But all this AC DC battery only techno babble is not answering th question. I'm curious too.

[BOAC]

You all need to re-read Post #1:ugh:

"During refuelling there is a power shortage as the gpu goes inop.
Nearly the same time fuel starts to flow from the underwing fuel vent creatng a major spill on the apron.

[NSEU]

Fuel tanks where full at the same time as the GPU went off line? Causing the shutoff valves to open?

The Maintenance Manual says that the fuelling valves will close automatically if power is lost.

Could it be a sticky fuelling valve/s?

Depowered, the float switches would not stop the fuelling. The last line of defence would be the mechanical float valves on the venting system. The reliability of these are not 100%.

Did fuel come out of both wingtips?