A gas turbine engine (40) having an active combustion control system (54) including a sensor element (44) positioned proximate the combustion chamber (46) as part of a sensor assembly (84) including a spring-action bellows (74). The sensor assembly is installed through an opening (69) in the casing (48) of the engine to make contact with the combustor wall (62). The bellows is compressed from its resting position by tightening mounting bolts (70) against a mounting flange (66) of the sensor assembly. The spring action of the bellows ensures contact between the sensing assembly and the combustor wall in spite of differential thermal growth between the casing and the combustor. The sensor element may be replaced by simply removing the mounting bolts without further disassembly of the engine.