Chiefs from two leading lessors have urged aircraft manufacturers to put their emphasis on their existing products rather than dilute their focus with the development of all-new aircraft.

Speaking during a conference session at Farnborough about the need for a new clean-sheet airliner like the Boeing New Mid-market Airplane (NMA) to deliver game-changing economics, Air Lease executive chairman Steven Udvar-Hazy said "the menu has got too large".

He argues: "For some airlines, it is very confusing; and what it leads to – when airlines have too many aircraft types and sub-types – is it significantly increases their cost of operations in terms of crews, infrastructure, spares, training, and ability to interchange aircraft."

Aengus Kelly, who heads the world's largest lessor AerCap, wants to see different priorities.

"The OEMs need to focus on what they're doing at the moment," he says. "There are just too many airplanes at the moment. If you look at Boeing, for example, we have a Max 7, Max 8, Max 9, Max 10, Max 200... It's too much customisation and too much reaction by the OEMs to what the other guy is doing. You've got to build one that you really feel your customer base needs and wants."

Boeing chief executive Dennis Muilenburg said during the show that the manufacturer plans to make a launch decision on the NMA in 2019 but remains committed to an entry-into-service date in 2025.