Boeing is using the prototype 737-900 to investigate a series of in-service issues cropping up on the Next Generation family as well as minor problems that appear to be restricted to the -900 series.

Tests are currently focused on a spoiler buffet effect that has been reported by 737-900 crews on touchdown. "We're just starting these tests," says Boeing, which describes the issue as "more of a nuisance".

The buffet appears to affect the tail area, says the manufacturer, which also adds that the issue "is specific to the 737-900 and is only noticeable when the spoilers are deployed on landing".

The heavily instrumented 737-900 prototype was originally scheduled to undertake another investigation into vibration and fatigue issues with the inlet connecting to the Honeywell 131-9(B) auxiliary power unit (APU).

These are now expected to begin in late August and will focus on measuring the frequencies in the tail compartment containing the APU, the exhaust and its inlet plenum. Boeing says operators have "found some cracking, but we did some borescoping and found nothing". Boeing is playing down the tests, but sources close to the tests say there could be "a real issue with delamination".

The 737-900 testbed has also been used to identify sources of excessive noise on the flightdeck, the effect of noise reduction modifications and lateral trim rigging rework.

Source: Flight International