Why it's only four 737s Qantas is inspecting for fatigue cracking

Although the Qantas Group operates 21 Boeing 737-400 aircraft, it only needs to inspect four of those aircraft for fatigue cracks following an airworthiness directive issued after a Southwest 737-300 developed a five foot hole in-flight. (Note: no Qantas aircraft have been grounded.) Why does Qantas only have to inspect four aircraft?

The US Federal Aviation Administration airworthiness directive, which has also been issued by CASA, calls for inspections on 737-300/400/500 classic aircraft that meet specific criteria outlined in a Boeing service bulletin:

1. The airframe is line number 2553 through 3132, delivered between 1993 and 2000
   
2. The airframe has more than 30,000 cycles
   

Boeing has identified 175 such aircraft worldwide.

Five Qantas 737-400 aircraft have affected line numbers. Of those five, VH-TJU and -TJW have exceeded 30,000 cycles, according to Flightglobal's ACAS database. (Update: a Qantas spokeswoman says she believes only one aircraft has exceeded 30,000 cycles). Two additional aircraft, -TJX and -TJY are just below 30,000 cycles. The fifth, -TJZ, is in storage. Jetconnect aircraft are not affected.

Qantas aircraft with 30-35,000 cycles must be inspected within the FAA airworthiness directive's more lenient 20 day period. Affected aircraft with over 35,000 cycles must be inspected within 5 days. Those with fewer than 30,000 cycles must be inspected between the sooner of 20 days from the directive or before 30,000 cycles are accumulated.

Qantas' four 737-400 aircraft in service average approximately 5.2 cycles a day, according to calculations from ACAS data.

Qantas does not expect any service disruptions resulting from the inspections.