A gas turbine engine for an aircraft, comprising: a high-pressure spool comprising an HP compressor and a first electric machine driven by an HP turbine; a low-pressure spool comprising an LP compressor and a second electric machine driven by an LP turbine; a combustor; and an engine controller; characterized in that the gas turbine engine further comprises an electrical energy storage unit and in that the engine controller is configured to: in response to receipt of a request for an increased electrical power supply by the engine (dP D) within a requested timeframe (dt), evaluate a current HP compressor surge margin (dR H /R H) and a current LP compressor surge margin (dR L /R L) 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 (dP D /dt), meet the requested electrical power demand using the electrical energy storage unit whilst increasing fuel flow to the combustor to accelerate the engine, and gradually increase the amount of electrical power supplied from one or more of the first and second electric machine whilst the engine accelerates. a high-pressure spool comprising an HP compressor and a first electric machine driven by an HP turbine; a low-pressure spool comprising an LP compressor and a second electric machine driven by an LP turbine; a combustor; and an engine controller; characterized in that the gas turbine engine further comprises an electrical energy storage unit and in that the engine controller is configured to: in response to receipt of a request for an increased electrical power supply by the engine (dP D) within a requested timeframe (dt), evaluate a current HP compressor surge margin (dR H /R H) and a current LP compressor surge margin (dR L /R L) 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 (dP D /dt), meet the requested electrical power demand using the electrical energy storage unit whilst increasing fuel flow to the combustor to accelerate the engine, and gradually increase the amount of electrical power supplied from one or more of the first and second electric machine whilst the engine accelerates. in response to receipt of a request for an increased electrical power supply by the engine (dP D) within a requested timeframe (dt), evaluate a current HP compressor surge margin (dR H /R H) and a current LP compressor surge margin (dR L /R L) 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 (dP D /dt), meet the requested electrical power demand using the electrical energy storage unit whilst increasing fuel flow to the combustor to accelerate the engine, and gradually increase the amount of electrical power supplied from one or more of the first and second electric machine whilst the engine accelerates.