outlet 53 upstream of the primary air inlet 42 of the combustor 28 and so the conduit 50 receives air from the compressor either directly or indirectly via the recuperator 35 an outlet 54 of the conduit 50 communicates with a pair of annular manifolds 55 a 55 b which surround the combustor liner 38 downstream of the endwall 39 a second set of dilution holes 56 extend through the liner 38 within the manifolds 55 a 55 b downstream of the first set of dilution holes 44 the conduit 50 is located entirely outside of the combustor 28 i e outside of the internal space defined by the combustor casing 36 consequently the bypass arrangement 50 provides air from the compressor directly to a downstream end of the combustor liner 38 i e in the region of the second set of dilution holes 56 without extending through the combustion zones 46 48 the region of the combustor liner 38 within and downstream of the second set of combustor holes defines a non combustion zone 58 essentially no combustion takes place within the non combustion zone 58 since the fuel introduced by the injector 40 has largely been burnt by this point the bypass valve 52 modulates mass flow of air through the bypass conduit 50 thereby controlling the ratio of air flowing through.