Almost a quarter of the US Air Force’s of E-8C JSTARS aircraft has been grounded for inspection after a water leak caused $2.5 million in damages to the type's ground surveillance radar.

Four of the USAF’s 17 JSTARS recently delivered from depot maintenance at Northrop’s facility in Lake Charles, Louisiana are under investigation for possible flight safety issues at Robins Air Force Base, Louisiana, the USAF says. The service is assembling an independent review team to inspect and and validate quality assurance processes at Northrop’s depot, a USAF spokeswoman told FlightGlobal.

There are 17 JSTARS E-8 aircraft, including 16 E-8C and one E-8A trainer, with five aircraft in depot maintenance today. The service will continue flying the JSTARS mission while the four aircraft are under inspection, the USAF says.

The investigation follows a series of post-repair incidents traced to the Northrop depot, including a class A mishap damaging a mission radar. A class A mishap is associated with a $2 million or higher cost, a fatality or complete destruction of the aircraft. With a potential $2.5 million cost, the USAF has characterised the incident as a class A mishap. However, the service could determine the issue as a class B mishap as they continue to assess whether the radar is repairable.

Northrop’s APY-7 radar is housed in a 7.3m (24ft) fibreglass canoe under the aircraft’s fuselage. Drain holes in the canoe allow moisture to run out when condensation builds up as the jet flies from hot weather on the ground to colder temperatures above 30,000ft. During a flight from Louisiana to Georgia, the canoe did not drain, exposing the radar to moisture and damaging the system, sources told FlightGlobal.

Once programmed to be replaced by the the cancelled E-10A, the E-8C was never supposed to fly as long and hard as it has. Congress thwarted the USAF’s plans to retire five jets by 2011, so the service continued flying the modified commercial 707s.

But Northrop had followed the USAF’s retirement plan, scaling down its full-time depot workforce and hiring contract workers. In addition, the demand for repairs on the fleet has increased over the years, causing a backlog at the depot.

As a result of the backlog, government inspectors have found issues on the aircraft that were supposed to be fixed but were not addressed. While there are more issues discovered on the aircraft, the quality assurance rate for JSTARS remains at 98%, sources told FlightGlobal.

In a recent letter to the USAF’s commander of Air Force Life Cycle Management Center, the service’s head of Air Force Material Command expressed her concern that the accelerated maintenance rate could not keep up with quality assurance. Gen. Ellen Pawlikowski ordered additional quality inspections for the last four jets delivered to the operational unit.

As Northrop faced the backlog last year, the company and the government reassessed the depot workforce. The company is now in the process of rehiring full-time, rather than contract, employees.

Meanwhile, the legacy JSTARS fleet continues flying as a drawn-out replacement contract lags. This week, USAF acquisition leaders told reporters that a move by Congress to force a fixed-price contract would delay the recapitalization programme’s request for proposals by at least three months and could push back initial operational capability by a year.