A fully aerobatic oil system for an aircraft gas turbine engine includes two re-circulatory oil systems the pumps for which have inlets 19 and 31a disposed at locations in the engine such that when one inlet is starved of oil, due to the aircraft attitude changing, oil will be supplied to the other one. In a specific embodiment, the breather of the oil system 30 (FIG. 2) is positioned in the engine gearbox and provides the lowest pressure point in the system. The breather is of the type described in UK Pat. No. 1,508,212 which includes a rotating compartment filled with RETIMET (Regd. Trade Mark). The bearing chambers are pressurised with air from the engine and the air/oil mixture in each chamber is vented via a first series of vents 24 to the breather where the air and oil are separated. The breather is used as a pump to pump the separated oil back to the oil tank via a scavenge line 35 which includes a relief valve 38. A bleed line 36 takes some of the oil from line 35 to the main oil supply gallery 23 via a non-return check valve 37. In normal flight pump 20 supplies the gallery 23 from the tank 18 via pump inlet 19, the relief valve 38 being set to a pressure lower than that of relief valve 40 which controls the pump pressure. The check valve 37 thus remains shut. When the inlet 19 is starved of oil because the flight attitude of the aircraft changes, the pressure in gallery 23 drops to the extent that check valve 37 opens and the breather/pump takes over circulating oil from the gearbox and bearings around the second system.
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