Bell Helicopter has confirmed that certification for its developmental 525 Relentless has been delayed following a self-imposed grounding in the wake of the 2016 fatal crash of a flight-test aircraft.

However, the manufacturer is confident that its two remaining prototypes of the super-medium-class helicopter will be back in the air "in the next several months".

Two experimental test pilots died in the 6 July 2016 accident involving aircraft FTV-1 (N525TA) during high-speed trials in Texas.

Since then, its two sister aircraft have been grounded pending the outcome of a US National Transportation Safety Board (NTSB) accident investigation, with Bell not even performing ground runs of the GE Aviation CT7-powered helicopters in the interim.

Larry Thimmesch, vice-president of 525 sales and business development, admits that Bell's initial five-year certification window with the FAA expired in late 2016. This has now been extended for another two years, he says, taking it to "end-2018".

Despite the grounding, Bell has already modified the pair of rotorcraft to enable a swift resumption of flight testing once the crash report is published, says Thimmesch. However, he declines to detail the changes made, citing a need to wait for the NTSB's findings before releasing the information.

"The NTSB and Bell Helicopter have focused the investigation on a sequence of events for which corrective actions are being implemented.

"We are co-ordinating with the NTSB and [US Federal Aviation Administration] to resume flight testing in the next several months," he says.

Bell “remains very committed” to the Relentless programme, he says. “We are taking the process very seriously which will result in a safe, reliable and high performance helicopter when it comes to market,” he adds.

In addition, Bell has begun upgrade work on the two flight-test articles to bring them into a production-standard configuration as well as performing other ground-based test activities.

Another two 525s are in final assembly – aircraft four and five – which will enter flight testing summer and autumn, respectively, says Thimmesch.

As a result of the depleted prototype fleet, Bell has had to reallocate certification test points to the remaining helicopters, he adds. “It has been defined and we are now implementing that plan as we get ready to return to flight,” he says,

The three flight-test articles had accumulated around 200h prior to the suspension of flights.Bell holds 80 letters of intent for the 525.