All but four of Air New Zealand’s 18 Beechcraft 1900D turboprops remain grounded as the carrier seeks advice from the manufacturer about hairline cracks in the tail sections of the aircraft.

The cracks were first discovered near the top of the vertical stabilizer in one 1900D during routine maintenance on Aug. 7. The remainder of the fleet was grounded for inspection. The hairline cracking was found “to varying degrees” in 14 aircraft, an airline spokeswoman tells Aviation Week.

The four cleared aircraft have been returned to service with Air New Zealand regional subsidiary Eagle Air. The airline used other aircraft types to help move passengers booked on Eagle flights, but still canceled 90 flights on Aug. 8.

It is not yet known when the remaining 14 aircraft will be returned to service, the spokeswoman says. Air New Zealand says it is “consulting with the aircraft manufacturer [Hawker Beechcraft] to seek advice about any further maintenance work that may be required.”

Eagle Air has operated the 1900Ds since October 2001, and they have an average age of 10.5 years. The 19-seat aircraft operate to 20 destinations in New Zealand.