Abstract: A method of operating an aircraft which includes a cabin for the aircrew which is pressurised in normal flight, and a deployment door which is opened in high altitude flight when the cabin will be depressurised, to enable a passenger and/or a load to be deployed from the depressurised cabin of the aircraft to the ground, the aircraft including an installed breathing gas system for providing a limited capacity supply of breathing gas to the aircrew in the cabin in the event of an emergency cabin de-compression, the installed breathing gas system including a source of breathing gas being substantially pure oxygen, and a line from the source to a face mask for delivering the breathing gas to an aircrew member, the method including prior to take-off on a mission according to a mission plan, determining whether the mission plan includes depressurising the cabin and opening the deployment door at high altitude, and where the cabin is planned to be depressurised at high altitude, prior to take-off on the mission, lo