It may be stated definitely that installation as a whole has shown little progress as compared with the development of either the aircraft structure or engine, and that to-day the installation of the engine in a machine presents a wide field for sound engineering design and improvement. On the other hand, failure in some part of the installation is the most prolific cause of forced landings. Statistics taken over a considerable period have shown that of the proportion of forced landings resulting from engine stoppage, seven out of eight are due, not to any actual defect in the engine itself, but to some trivial breakdown in the installation.

Apart, too, from the question of reliability and safety, installation development provides room for considerable economy, not only as regards its actual upkeep and repair, but also from its effect on the easy (or otherwise) removal of the engine from the aircraft for overhaul and replacement purposes.