Here is a old NWA press release on the Nines. Note the 104,000 requirement. Delta does not plan to perform this maintenance. NWA estimated the aircraft would 100,000 cycles in 2010.

LONDON - A detailed structural safety analysis was conducted before Northwest Airlines [NWAC] decided to keep flying its DC-9 fleet another 15 years. The carrier operates a fleet of 173 DC-9s.

Northwest plans to fly these airplanes to the year 2010. Upon retirement, the planes will have accumulated 40 years of service.

Speaking at an aging airliner conference here, Brad Mueller, Northwest's manager of fleet planning, said the DC-9 was a perfect fit for routes it was flying. The question was whether the airplane's structure could be flown to 100,000 flight cycles or more.

"Douglas had tested the airplane to more than 200,000 cycles," Mueller explained. At the time Northwest decided to invest about $10 million per plane for hushkits, new interiors, and other upgrades, Mueller said Northwest's DC-9's had logged about 65,000-70,000 cycles.

The manufacturer's extended fatigue testing verified the durability of the design. "This was the single biggest concern," Mueller said. "If the study had not satisfied the question (of structural safety), we wouldn't have done it."

Northwest plans to fly its DC-9's to about 100,000 cycles, at which point they will be retired. "Major modifications are needed at 104,000 cycles," Mueller explained.