This from post #83 of the
2004 Post about the EC135 fuel system.
The fuel from the Main tank is fed by 2 transfer pumps Forward and Aft in the Main tank to the Supply tanks. This fuel is fed through flex lines which pass through the "overflow channels" to exit into the lower part of the Supply tanks. The "overflow channels" are approximately 50 mm in diameter and the transfer hoses maybe 15 mm. The fuel from the Supply tanks when the quantity is above 92 kg runs through the "overflow channels" back into the Main Tank until it is empty. Each transfer pump is capable of 6.6 litres per minute or about 315 kg/hour or more than 150% of cruise fuel flow. When the fuel level in the Main tank is below the level of where these pumps can pick up they should be selected OFF.
This is detected by the caution indication "F PUMP AFT" or "F PUMP FWD" and is completely normal per the FLM.
Failure of the FWD pump can result in an unusable quantity of as much as 59 kg if above 80 KIAS and is 3.6 kg if below which is effectively the same as normal unusable fuel in this cell.
Failure of the AFT pump can result in as much as 71 kg in the hover reducing to 7.5 kg above 80 KIAS increasing the unusable by 4 kgs.
With a little arithmetic the maximum increase in unusable at the failure of BOTH pumps could be as much as 215 kg in the level attitude. This would be the maximum that you couldn't get to by being a little bit cunning. This of course depends on when both pumps become inactive.
If the fuel is no longer being transferred to the Supply tanks by either gravity or pumps there is 92 kgs remaining of which 86.7 is usable. Due to the shape of the tanks Engine 2 will stop after approximately 23 minutes and then engine 1 after 27 minutes. The "LOW FUEL 1" and "LOW FUEL 2" Warning indications come on at the minimum of 24kg per cell. Hence the procedure "Land within 8 minutes."
So SOP is to switch off the Xfer pumps when advised by the caution that they have run dry. Let's assume that the pilot switched off the aft transfer pump in the cruise above 80Kt when it ran dry. That would be when the supply tanks were full and around 70 something Kg left in the main tank. Let's further assume that the a/c returned to the hover. After 15 minutes in the hover with that main tank fuel now unusable LOW FUEL 1 light comes on, followed by LOW FUEL 2 four minutes later. If those captions were missed then a further 8 minutes later Engine 1 quits, followed by engine 2 after a further four minutes.
Here's another quote from post #98 of the above thread:
The EC 135 sounds as if it has a similar fuel system to the BO105. In the case of the BO, the supply tanks are in front of the main, and they are of equal size so that if the transfer pumps fail both engines will quit within seconds of each other.
When checking out in the BO, we were told of a tall pilot who left his transfer pumps off and subsequently had a dual engine failure because the glareshield of that particular ship blocked his view of the "FUEL LOW" caption. We were told that you might have approx. 20 minutes to flameout in such a case. (I might add here that I remember thinking the requisite "Oh, that'll never happen to ME!") As a hedge against such a brain fart, I got into the habit of always without fail leaving the transfer pump switches on and NEVER shutting them off.
One day, I was shut down, showing another pilot the avionics suite in my ship. To save battery power I had turned the transfer pumps off. You guessed it, I forgot to turn them on at startup next time. I had been airborne for fifteen or twenty minutes, cruising along, fat dumb and happy when I began idly looking around the cabin for no specific reason. When my eyes spied the transfer pump switches, I about had a heart attack. YIKES! The "FUEL LOW" light was not on, and my supply tank quantity gage still showed full. What the...?