Gravity fuel feeding A320
Join Date: Jul 2010
Location: Mumbai
Age: 34
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Engines can be started and take off can be performed with fuel pumps off. Hence the FWCs were modified so that they can monitor fuel pumps (along with hyd pumps and system pressure, generators and IDG disconnection) and trigger a caution when Take off config test is performed.
As this thread has a high count of numbers from the book, but is short on reasons why, may I throw in some information I gleaned 'hands on' from testing fuel systems both for a fuel gauging manufacturer and an airframe manufacturer.
- JET A1 when viewed in a transparent container and subjected to altitude conditions which replicated a typical take off and climb did not significantly start degassing (releasing air bubbles) until over 15000ft. As the altitude exceeded 20000ft, the degassing started to resemble a shaken bottle of sparking mineral water being opened.
- Significant degassing continued for about 20 to 30 minutes.
- Only fresh fuel de-gasses significantly. Reabsorption of air takes several days, if not weeks, and subsequent altitude cycles produced a lot less degassing than the first cycle.
- JET B with a higher degree of volatile content would de-gass more, thus being more likely to create vapour locks at altitude.
- If the aircraft is fitted with a trim tank in the tailplane (horizontal stabilizer), and the valves at both ends of the transfer pipe are shut, then the sudden degassing of the fuel in the pipe when they are opened at altitude can prevent any gravity fuel transfer to the wing/centre tanks.
- Cavitation can occur at outlets from tanks into feed pipes when doing gravity transfer which encourages degassing. This was particularly bad on one trim tank, requiring the outlet to be redesigned to achieve any useful flow.
Last edited by Mechta; 5th May 2014 at 20:28.
