Boeing has further obscured the timeline for introducing a 737 replacement by subtly pushing back a key reference point for the next-generation narrowbody project.

Official statements now refer to delivering a 737 replacement in the "latter half of the next decade", says the airframer. This subtly contrasts Boeing's previously updated position, which called for delivering a new aircraft "no earlier than 2015".

Boeing's message on the 737 replacement timing has continued to soften over the past year, even as new orders for the current generation of narrowbodies have galloped to record levels.

Almost 12 months ago, official statements by Boeing executives set the timing for a 737 replacement between 2012 and 2013. Boeing formed a project team led by Mike Cave, vice-president of aircraft programmes, in early 2006 called the 737 Replacement Study, or 737RS.

The goal of the project is to deliver a new airframe design that can deliver a minimum 15% improvement in fuel efficiency over the 737-800.

In recent months, the focus has shifted away from designing a new airframe as Boeing engineers have determined the technology is not yet available to support a firm configuration.

The 737RS team is instead continuing to focus on developing technologies that could support a next-generation single-aisle airframe that could deliver the minimum performance improvements.

Key technologies include new leap-propulsion efficiency, possibly including geared turbofan and open rotor designs, composite airframe, on-board systems and maintenance.