A gas turbine engine 10 is provided with rigid raft assemblies 290, which may be electrical harness raft assemblies 290 comprising electrical conductors embedded in a rigid composite material. The rigid raft assemblies 290 can be provided with engine dressings, such as pipework and ECUs to produce an electrical raft assembly. In order to assemble or dress the gas turbine engine 10, the rigid raft assemblies can be pre-prepared to incorporate at least a part of at least one gas turbine engine system/component before being installed on the gas turbine engine 10. During maintenance, whole raft assemblies can be removed and replaced with corresponding, pre-prepared assemblies. This can save considerable time during engine build and maintenance.