Air New Zealand plans to return seven grounded Beech 1900Ds to service sometime next month, as it works to repair hairline cracks that were discovered in the tail structures of the aircraft.

The carrier grounded its entire fleet of 18 Beech 1900Ds on Aug. 7 after the cracking was found near the top of the vertical stabilizer during a routine inspection of one aircraft. Since then, 11 aircraft have been returned to service after being inspected and “were found to meet operational standards,” an airline spokeswoman says.

The remaining seven aircraft require repairs that “vary from aircraft to aircraft depending on the nature of the cracking,” the spokeswoman tells AviationWeek. “We expect all remedial work on these aircraft to be completed in September.”

Three of the seven aircraft were already offline for scheduled maintenance. The airline is working with manufacturer Hawker Beechcraft to determine what repairs are needed.

Air New Zealand regional subsidiary Eagle Air operates the Beech 1900Ds. Other aircraft from the Air New Zealand fleet have been used to replace the 1900Ds on their regular routes. While there were initially major schedule disruptions, Eagle is expected to be operating 93% of its usual seat capacity over the next few weeks.

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 around New Zealand.