METAL FATIGUE

METAL FATIGUE... (5699)

From: Rodger Herrington <rodger@mail.utexas.edu>
Subject: metal fatigue
I was just curious about a problem that I discovered in another aircraft type and if it could relate to the Swift. I have never heard of this problem in a Swift and I have tried to read all of the AD and accident reports that I could find. Has there ever been a problem, to your knowledge, of metal fatigue in a Swift, particularly the higher stressed areas like a spar? Visual inspections will not reveal the type of fatigue I'm curious about. Only an electron microscope, it seems, will discover it. The other aircraft was used for aerobatics and, I'm sure the situation would not apply to most Swifts, but I was still curious. I realize that a well maintained plane will last a very long time with no problems (My family has maintained the same plane for 18 years) and, obviously, many Swifts are still flying :) Thanks for any information! -- Rodger Herrington

Rodger,
I don't believe that (metal fatigue) has ever been found in a Swift wing spar. Any problems in my opinion are more related to corrosion and prior damage. There have been several cracks found in the horizontal stabilizer spar, usually at the inboard rivet of the upper front spar. I had a Swift with a not-too-smooth Beech-Roby prop. It was kind of scary to look back at the horizontal in flight. It shook. I mean constantly, so I removed the prop and had it overhauled, several lag bolts were broken in one blade. (which retain the wooden blade in its metal ferrule) Inspection revealed no cracks in the stabilizer spar. For similar reasons I limit practicing stalls, the stabilizer shakes then also. Incidently, the early (GC-1A) spars are .050 aluminum, the later ones are .065. I must also mention backing the airplane into the hangar by pushing on the stabilizer must be done with discretion. I feel the horizontal stabilizer is the most likely place where 50+ years of use and abuse will reveal itself first. -- Jim

(Editor's note: Don Bartholomew's opinion was also solicited on this question. His reply follows...)

From: Don Bartholomew <spectro@nanosecond.com>
Subject: metal fatigue
Here are some thoughts and ideas to your question. In my dealings with the Swift, I have seen a few problems caused by metal fatigue. These were isolated cases of skin cracking due to vibration, and one noteworthy one is the front horizontal spar cracking through the rivets in the inboard rib. Both of these conditions were enhanced by rivet holes which acted to concentrate the stress.

Fatigue is caused by two things: load reversals or cycles, a component being loaded or stressed and then unloaded, and the load or stress applied during these cycles. The more load applied, the fewer cycles are needed to cause fatigue. The less load applied, the more cycles are needed. Anything that causes a stress concentration (rivet holes, scratches, nicks, etc) will contribute to the fatigue. The small skin cracks were probably fatigue or work hardening due to vibration of an unsupported panel. The vibration causes a low load, high cycle condition. This can be from engine/prop vibration or vibrations from air blowing over a surface. As far as fatigue of a major structure component, the only problem I have seen is the horizontal spar crack. This would be a high load and fewer cycle condition. The loading here would be due to normal flying loads, gust loads, and even improper ground handling.

I am not familiar with any other areas of problems with the main structure. This is probably due to a number of things. First, the load applied during normal flight is fairly low compared to the load the plane is designed for. Second, most Swifts are fairly low time compared to other aircraft that do see fatigue problems. Acrobatics does put a load on the structure but the cycles are fairly low. For example, a loop will put about three G's and the two cycles on the structure during the initial pull up and the pull out at the bottom. Of more concern to me is flying fast in turbulence. You may see four or five G's and five or six cycles in a couple of seconds. The loads and cycles are cumulative. Each one adds to the total and there is a finite number before something will break. -- Don Bartholomew