Gravity Feed of Fuel on Jetliners

[Vibes]

Hi All,

I would like to seek your perspective on the principle behind gravity feeding of fuel on jetliners?Why is there a particular ceiling for gravity feeding of fuel?What is the determining factor as to how high an aircraft should fly in order to start gravity feeding?I understand that the plane must be banked at a certain angle (usually above 2 or 3 degrees) on the lighter side in order to initiate gravity feed,however if it is the centre tank that is affected,how then do we gravity feed the fuel into the center tanks?

Thank you once again for shedding some light on the above inquiries.

Best Regards,
Vibes

[Intruder]

Gravity feed is certainly type-specific, so there can be no generalizations.

On the 744, the only possible gravity feed between tanks is the automatic feed of the reserve tanks to wing tanks 2 & 3, and the selectable feed of tanks 1 & 4 into 2 & 3 (done in non-normal situations only).

"Gravity feed" of fuel from the wing tanks to the engines is possible in some cases because the engine-driven fuel pumps will supply the engines if all boost pumps fail. However, at high altitudes and power settings there may not be enough pressure head on the fuel tanks from the vent system to reliably move fuel down the pipes to the engines. That will cause cavitation of the engine-driven pumps and likely flameout. High pitch angles with low fuel quantity will also uncover the boost pump inlets, interrupting fuel supply.