Pervasive fatigue cracking afflicts the U.S. fleet of antique aerial firefighting airplanes (see ASW, Dec. 9, 2002). However, even the purpose-built aerial bombers in Canada require attention. According to a Canadian structural engineer:

"Transport Canada (TC) tries hard to keep safe operations and requires higher load factors on new designs like the Canadair/Bombardier CL 215 and also attention to airframe fatigue. Despite this we had to issue an AD on the CL215 for wing cracks. Counting accelerometer readings we have taken indicate, as you might expect, a very severe fatigue environment in firefighting operations and I am surprised that the converted aircraft have lasted as long as they did/have!

"We also give attention to crop dusters which usually operate overweight and try to protect the structure with speed restrictions. Many crop dusters are designed to the FAR.23 6000lb standard and TC used to only allow 20% overweight hence many of ours restricted to 7200lb wt. However US allowed weights well in excess of this like 10,000lb so we had allowed the extra wt increases but as above, used the speed restrictions for airframe protection.

"Of course as a structures engineer I would like to see weights reduced for special operations like firefighting rather than increased but economics prevails. However we do owe the crews their safety."