Short Mayo Composite 

Experimental piggy-back long-range seaplane/flying boat combination, produced by the Irish-based British Short Brothers aviation company, to provide a reliable long-range air transport service to North America and, potentially, to other distant places in the British Empire and the Commonwealth.

The idea was based around the concept of aircraft having the capacity of maintain flight with a greater payload than that possible during takeoff, and so, it involved a small, long-range seaplane, the

Short S.20 Mercury, on top of a larger carrier aircraft, the Short S.21 Maia, using the combined power of both to bring the smaller aircraft to operational height, at which time the two aircraft would separate, the carrier aircraft returning to base while the other flew on to its destination. 

The complexity of the whole operation, coupled with the arrival of bigger, longer-ranged boats, waned interest in the project, and the start of WW2, in which Maia was destroyed in a German air raid, killed it for good.