Aircraft fuel tank and inerting system therefor
An inerting system for an aircraft fuel tank ( 10 ) uses an air separation device ( 12 ) to produce a supply of nitrogen-enriched air to replenish the fuel tank on consumption of fuel and to maintain the pressure difference across the walls of the tank ( 10 ) below design limits. The air separation device ( 12 ) provides substantially the whole of the mass of the gas required and so there is substantially no inward venting. This provides a substantially homogenous nitrogen content in the air in the tank ( 10 ).
1. An aircraft fuel tank system comprising:
at least one aircraft fuel tank;
an air separation device producing nitrogen-enriched air, and
a control device controlling said air separation device to supply nitrogen-enriched air into said at least one aircraft fuel tank during cruise conditions and to supply nitrogen-enriched air at a higher flow rate during descent, whereby the air separation device provides the whole of the mass of gas required to maintain the pressure of the fuel tank at or above the ambient pressure during descent.
2. The aircraft fuel tank system as claimed in claim 1 , wherein said air separation device in use provides nitrogen-enriched air having a high concentration of nitrogen at low mass flow rates, and a low concentration of nitrogen at high mass flow rates, wherein said high concentration is higher than said low concentration, and said high mass flow rate is higher than said low mass flow rate.
3. The aircraft fuel tank system as claimed in claim 2 , including a distribution network distributing the nitrogen-enriched air at a number of spaced locations in said at least one aircraft fuel tank, thereby in use to reduce variations in concentration of nitrogen within said tank.
4. The aircraft fuel tank system as claimed in claim 3 , wherein said air separation device comprises a Hollow Fibre Membrane.
5. The aircraft fuel tank system as claimed in claim 2 , wherein said air separation device comprises a Hollow Fibre Membrane.
6. The aircraft fuel tank system as claimed in claim 1 , including a distribution network distributing the nitrogen-enriched air at a number of spaced locations in said at least one aircraft fuel tank, thereby in use to reduce variations in concentration of nitrogen within said tank.
7. The aircraft fuel tank system as claimed in claim 6 , wherein said air separation device comprises a Hollow Fibre Membrane.
8. The aircraft fuel tank system as claimed in claim 1 , wherein said air separation device comprises a Hollow Fibre Membrane.
9. A method of inerting at least one aircraft fuel tank, comprising:
operating an air separation device during cruise conditions to deliver nitrogen-enriched air with a high concentration of nitrogen at a low mass flow rate into said aircraft fuel tank; and
operating said air separation device during descent conditions to deliver nitrogen-enriched air with a lower concentration of nitrogen and at a high mass flow rate, whereby the air-separation device provides the whole of the mass of gas required to maintain the pressure difference across the walls of the or each fuel tank at or above the ambient pressure during descent,
wherein said high concentration is higher than said low concentration, and said high mass flow rate is higher than said low mass flow rate.