"Outer Tanks and Flapper Valves"????
According to my information there are no "flapper valves" in this fuel system. As for "outer" tanks, if you are referring to the Supply tanks they are immediately behind the Main tank and share the same hull form shape.
There are a total of 3 tanks, 1 Main tank, and 2 Supply tanks.
In a standard (i.e. not self sealing and prior to aircraft serial number 250) configuration the Main tank contains 565 litres or 452 kgs and the Supply tanks hold 115 litres or 92 kgs. The Supply tanks or divided lengthways by a fence and the RH one has a structural channel running through the bottom that reduces it capacity by 5 litres or 4 kg. In fact part of the Main tank capacity (72 kg ) is physically in the upper space of the Supply tanks and connected by "overflow" channels.
If the transfer pumps were not activated or failed, with simultaneous use the RH engine will stop first and the LH will stop 3-4 minutes later.
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."
The above is based on manufacturer's fuel flow data of 65% Q at SL ISA - give or take.
The Supply tanks are fitted with priming pumps which are only used for starting, the engines do not require boost pumps. Incidentally these pumps are identical to the transfer pumps.
The comment about a dual pump failure is interesting, as granted it has consequences but seems highly unlikely.
When operating at different speeds with low fuel quantity betwen hover and cruise the FUEL PUMP lights will come on and off which is normal and also depends on the location of the C of G.
Subsequent to CPDS Software Version V2100B there is the FUEL caption in the MISC indications if the fuel level in the SUPPLY tanks is below a certain value with fuel remaining in the MAIN tank. (read - Check Transfer pumps ON)
Letsby - Sorry about the bits left out.
Last edited by Giovanni Cento Nove; 4th Mar 2004 at 18:56.