A gas turbine engine for an aircraft comprises a high-pressure (HP) spool comprising an HP compressor and a first electric machine driven by an HP turbine; a low-pressure (LP) spool comprising an LP compressor and a second electric machine driven by an LP turbine; a combustor; an electrical energy storage unit; and an engine controller configured to: in response to receipt of a request for an increased electrical power supply by the engine (dPD) within a requested timeframe (dt), evaluating a current HP compressor surge margin (dRH/RH) and a current LP compressor surge margin (dRL/RL) based on the operating points thereof; in response to evaluating that one or more of the HP compressor and the LP compressor has insufficient surge margin to facilitate a change of electrical power supply at a requested rate (dPD/dt), meeting the requested electrical power demand using the electrical energy storage unit whilst increasing fuel flow to the combustor to accelerate the engine.