Hydraulic line inspections that interrupted flight testing of the Northrop Grumman E-2D Advanced Hawkeye carrier-based airborne early warning aircraft were the result of concerns about vibration causing chafing of the lines, the US Navy has revealed.

The first E-2D was grounded for the inspections after completing its first two flights, on 3 and 6 August, and the discovery of chafing on a sample set led to a decision to inspect all hydraulic lines, delaying its return to flight status. The aircraft was ready for renewed testing on 16 September, but weather held up flying.

The inspection was required by a "flight operating limitation" established well before the start of flight testing, says the USN. This required inspection of a sample set of hydraulic lines selected as "leading indicators" of wear. When "several instances of chafing" were noticed, the decision was taken to inspect all lines.

This required opening panels, removing avionics boxes to access lines, and making adjustments where required. "The navy and Northrop Grumman underestimated the time it would take when we made the decision to inspect all lines," says the service's E-2 programme office.

During the inspection, 160 lines - about 17% of all hydraulic lines in the aircraft - were found to have some risk of chafing, but less than 1% required redesign and replacement, the navy says. The majority of issues were fixed by adjusting clearances and moving clamping points, and the changes will be rolled into production drawings and incorporated, beginning with the programme's third aircraft.

The risk of hydraulic line chafing "was not wholly unexpected", says the USN, as the E-2D has a new digitally designed airframe. The cost of inspection and correction has been small and is within the budget for assembly of the aircraft, it says.

Source: Flight International