PREVENTING SURGE

A gas turbine engine 101 for an aircraft comprises a high-pressure (HP) spool comprising an HP compressor 105 and a first electric machine 117 driven by an HP turbine 107; a low-pressure (LP) spool comprising an LP compressor 104 and a second electric machine 119 driven by an LP turbine 108; a combustor 106; an electrical energy storage unit 305; and an engine controller 123 configured to: in response to receipt of a request for an increased electrical power supply by the engine (dP) within a requested timeframe (dt), evaluating a current HP compressor surge margin (dR/R) and a current LP compressor surge margin (dR/R) based on the operating points thereof; in response to evaluating that one or more of the HP compressor 105 and the LP compressor 104 has insufficient surge margin to facilitate a change of electrical power supply at a requested rate (dP/dt), meeting the requested electrical power demand using the electrical energy storage unit 305 whilst increasing fuel flow to the combustor 106 to accelerate the engine 101.