Restarting a gas turbine engine

A multi-engine aircraft power and propulsion system comprising:

• first and second propulsive gas turbine engines, each comprising (i) combustion equipment, (ii) a plurality of spools, and (iii) first and second electric machines mechanically coupled with the plurality of spools and each comprising a first sub-machine and a second sub-machine;
• an electrical system connecting the electric machines of the first gas turbine engine and the electric machines of the second gas turbine engine, whereby electrical power generated by the electric machines of the first gas turbine engine can be transferred to the electric machines of the second gas turbine engine;
• an electrical energy storage system which can supply electrical power to the electric machines of each of the first and second gas turbine engines;
• and
• a control system configured to, responsive to determining a condition to the effect that a flame in the combustion equipment of the second gas turbine engine has been extinguished, control the first and second gas turbine engines and the electrical system during an in-flight engine restart procedure, the restart procedure comprising:
• supplying electrical power from the electrical energy storage system to the electric machines of the second gas turbine engine and operating one or more of the electric machines of the second gas turbine engine as one or more motors to limit a reduction in a speed of one or more of the spools of the second gas turbine engine following extinguishment of the flame in the combustion equipment of the second gas turbine engine;
• and
• restarting the second gas turbine engine by relighting the combustion equipment of the second gas turbine engine,
• wherein the electrical system comprises first and second dc distribution busses associated with each of the first and second gas turbine engines, the first and second dc distribution busses associated with the first gas turbine engine being different than the first and second dc distribution busses associated with the second gas turbine engine, and
• wherein for each of the first and second gas turbine engines, (i) the first sub-machine of each of the first and second electric machines is electrically connected to the corresponding first dc distribution bus and (ii) the second sub-machine of each of the first and second electric machines is electrically connected to the corresponding second dc distribution bus.