5. Fuel System Flashcards
What tanks do the main and auxiliary fuel systems consist of and where are they located?
Main:
Integral outboard (wet wing), 4 rubber bladder tanks, and nacelle bladder. Fills at wingtip and flows downhill nacelle.
Aux:
One tank in wing centre section.
Where is fuel drawn from to deliver to engine?
Nacelle tanks
How does fuel transfer from outer wing to nacelle tank?
By gravity
What are the aux tanks constructed with?
Rubber bladder type
Why is there a 30-50 sec delay after start in automatic transfer system?
To allow all the fuel pressure to be used for engine starting.
Pilot should monitor that NO TRANSFER light is extinguished after this time.
How does the auxiliary transfer system stop when the aux tank is empty?
Will NO TRANSFER illuminate?
A float switch in the aux tank transmits a signal to close the motive flow valve. Normally 30-60 sec time delay to prevent cycling of the motive flow valve due to sloshing fuel.
NO TRANSFER will not illuminate as there is no fuel left to transfer
By what means is fuel transferred from the aux tank to the nacelle tank?
The engine driven or standby boost pump routes fuel through the normally closed motive flow valve, the jet pump and into the nacelle tank. Fuel moving through the jet pump Venturi creates suction.
What are the 3 functions of the standby boost pump?
- Backup for engine driven fuel pump.
- Pump AVGAS when flying above 20000ft.
- Pump fuel during crossfeed operation.
What prevents reverse flow from nacelle to outboard tanks when aux transfer system is in use?
A swing check valve in the gravity feed line
What happens if the motive flow valve/its circuitry fails in the aux transfer system?
Amber NO TRANSFER will illuminate. Valve can be manually energised by placing the AUXILIARY TRANSFER switch from AUTO to OVERRIDE.
Where are the firewall shutoff valves powered from?
No. 3 (left) and No. 4 (right) dual fed buses. Also from hot battery bus, so they can be operated regardless of battery switch position.
When they are closed fuel is cut off from engine
What is the primary function of the engine driven boost pump?
To deliver low pressure fuel to the engine high pressure fuel pump, thus preventing cavitation
What is the operating capacity of the engine driven boost pump?
1250pph at a pressure of 25 to 30psi
What pressure decrease will trigger a FUEL PRESS light?
If fuel pressure drops below 10 +/-1 psi
If a standby boost pump fails, can cross feed be accomplished?
Only from the side of the operative standby pump
Where is the low fuel pressure switch located and what are its functions?
Mounted on top of the firewall fuel filter.
If it senses low pressure it will close and cause the FUEL PRESS light to illuminate.
Also it sends a signal to aux transfer system advising if fuel pressure is available or not for tank transfer
Where are fuel flow transmitters located and what is the consequence of this?
Prior to BB-1401: located upstream of fuel heater and affected by FCU fuel purge flow.
BB-1401 onwards: downstream of FCU so only indicated fuel used for combustion (ZGD and ZMW)
Under what conditions will fuel automatically bypass the fuel heater?
If fuel temperature rises above 90 deg F
How does the purge system prevent fuel passing the dump valve while engine is running?
Fuel pressure holds the dump valve closed
How does the purge system prevent fuel or air from back flowing?
The are check valves on either side of the purge tank
What happens to purge tank fuel on shutdown?
Fuel is forced through the now open dump valve (no fuel pressure to hold it closed) by P3 air, forcing fuel into burner can. This is burnt and may result in a small rise in ITT/N1
For crossfeed operations, where must the STANDBY PUMP switches be?
OFF
What 4 things happens when the CROSSFEED switch is moved to left or right? What is a possible fifth thing?
- Green FUEL CROSSFEED annunciator illuminates.
- CROSSFEED vale opens.
- Standby boost pump on delivery side turns on.
- Motive flow valve on receiving side closes, stopping aux fuel transfer.
Possible 5. If there is fuel in receiving sides aux tank NO TRANSFER will illuminate (will not occur if no aux fuel available)
What does the green FUEL CROSSFEED annunciator indicate?
Crossfeed has been selected, but not that the crossfeed valve has moved.
This must be tested prior to engine start, by checking FUEL PRESSURE is extinguished when the CROSSFEED switch is moved LEFT or RIGHT.
How many fuel capacitance probes are in each wing?
8
1 in inboard box, 1 in nacelle, 2 in integral wet wing, 2 in onboard leading edge, 2 in aux tank
How are fuel tanks vented?
Each side has a recessed (unheated) and protruding (heated) vent under the wing, adjacent to nacelle.
An air inlet at wingtip vents the integral wet cell tank, aux tank and on BB-479 and subsequent, the nacelle tank.
How many fuel drains are there?
6
5 sump drains and a firewall filter drain in each wing.
(Leading edge tank, integral tank, firewall fuel filter, sump strainer, gravity feed line, aux tank.
Can CROSSFEED be used in normal operations?
No- authorised for single engine operation only
How long can an engine be operated with FUEL PRESSURE warning light illuminated?
10 hours
Then high pressure engine pump must be overhauled or replaced
What is the max fuel imbalance?
1000lb
Can the aux tanks be filled without full mains?
No
Fuel panel check tests continuity to which items?
Firewall valves, standby boost pumps and crossfeed valve
What 3 things do you need for crossfeed?
1) operating electric boost pump on feeding side.
2) open crossfeed line.
3) no opposing electric boost pump pressure on receiving side.
During crossfeed operations, will the feeding side fuel pressure light be on or off?
On.
The firewall shutoff valve will have been shut in securing failed engine, and the pressure switch is mounted on the fuel filter downstream of shutoff valve.
During crossfeed operations, how can we tell if feeding side boost pump operating?
Only way to tell is when fuel gauge begins with to decrease
