tigerpic:
The longranger fuel system has a fuel cell under each of the rear facing seats, which normally transfer fuel to the rear cell via jet pump. The only boost pumps are in the rear cell and supply the motive force for the jet pump.
The fwd cells should empty into the rear cell before the rear cell runs low.
Extract from UK AIR Accidents site.
Fuel is transferred from each forward cell into the main fuel tank by the action of a dedicated dual element ejector (jet) pump powered, respectively, by fuel under pressure from the output side of the left and right fuel boost pumps sited on the floor of the main tank. These fuel bleeds are passed through wire mesh filters/check valves upstream of the jet pumps and are sensed by flow switches. Loss of the fuel flow is indicated to the pilot by the respective boost pump warning light. Thus if the fuel pumps were to fail, or the supply in the main tank becomes exhausted, then these lights should illuminate. Fuel quantity in the main and additional tanks is measured using capacitive type sensors, their outputs being summed and presented as a total quantity on a single gauge on the instrument panel. As the forward tanks are interconnected, the single probe fitted to the left forward tank normally senses the quantity in both forward tanks.
Scenario:
If the jet pump becomes blocked, fuel will stop transferring to the rear. The fuel can then be used up in the rear cell when there is (apparently) ample fuel according to the qty indicator.
There are warning lights that will indicate if the motive flow is stopped, but have been ignored because there is still a pressure indication. The next indication will be a low fuel light but this can be ignored because the fuel qty indicator is reading the fwd cells quantity, which has stopped supplying the aft cell.
The next indication will be an engine out warning!
Moral of story: Always believe the low fuel light!
The fuel switch at the qty indicator will no doubt be there to assist in confirming if you have a jet pump problem.
The 206L fuel system has caught people out in the past and it is a good idea to understand it fully if you fly or fix them. The flight manual should have the info you need if you fly them.
Check this incident below. There is a diagram of the 206L1 fuel system given.
edited to edit some editing!
[ 09 December 2001: Message edited by: sprocket ]
206L 1 incident
[ 09 December 2001: Message edited by: sprocket ]