The Royal Air Force has begun testing flying the first of it's fleet of Sentry E-3D airborne early warning aircraft repaired after being taken out of service in October due to electrical wiring problems.

If the tests go to plan the first aircraft could be returned to operational service in the second half of January. 

"E-3D Sentry test flying has already commenced, as part of a graduated return to flying for the fleet. This work continues and a return to the normal level of aircraft availability is forecast early in 2017," said an RAF spokesman.

Typically the RAF has four aircraft available for flying duties with two in  maintenance.

The six strong fleet of airborne early warning jets was deemed unserviceable earlier this year and effectively grounded after a routine inspection revealed major issues with some electrical wiring and cabin conditioning systems. 

Defense News exclusively reported the entire fleet had been grounded Nov 4. Later media reports said some underfloor wiring was arcing creating a potential fire risk. Fire blankets on the aircraft were also found to not be fireproof, said the reports.

Britain’s allies have had to step in and fill the temporary capability gap created by the aging aircraft being grounded while repairs are undertaken. In particular the British aircraft and its crews had been active providing support to anti-Daesh operations over Iraq and Syria.

The remedial work on the jets has been undertaken by an RAF-led team which also includes support providers Northrop Grumman and other industry suppliers.

Northrop Grumman’s UK arm recently signed a nine year extension to it’s support and availability contract for the aircraft.

The wiring replacement work has been undertaken at the Sentry’s temporary base at RAF Coningsby as the aircraft’s normal home, RAF Waddington, is closed to large jets while a new runway is laid.

The RAF Sentry fleet was due to be taken out of service in 2025, but the British announced in their Strategic Defence and Security Review last year that the aircraft's life was being extended ‎by a further 10 years to 2035. The extension‎ is expected to result in a major sustainment and update program on the aircraft starting around the end of the decade.

"As part of this requirement, a number of capability sustainment options are under consideration, the capability development plan will be confirmed in due course," said the RAF spokesman.