Vibration-Induced Aluminum Fatigue Fracture

Something to Add to TSIO-550 Condition Inspection

by barry knotts

Figure 1

At my last condition inspection I found a fractured aluminum intercooler mounting tab. I believe the fracture resulted from vibration-induced fatigue. The fracture location suggested a high likelihood of future failures, and I recommend close inspection of the bracket on each condition inspection.

The “third” intercooler for the Continental TSIO-550 in the Lancair IV-P installation mounts to the front engine baffling on the left side. An aluminum tab welded to the bottom, outboard edge of the intercooler provides support for both the intercooler and engine baffling. The tab is attached to the valve cover for #6 cylinder with a steel bracket. It is this tab, identified by the red circle in Figure 1, which fractured. The raw metal edges of the fracture appear to have ground against each other due to engine vibration (See Figure 2).

Figure 2

I’m not a structural engineer or materials scientist, but it seems to me that this aluminum tab on the intercooler is subjected to both engine vibration transmitted through the bracket attached to the #6 valve cover and to the air loads of the blast air coming in through the left cowl cooling inlet. In fact, it is also subjected to the propeller blade air impulses passed through the cooling inlet.

Figure 3

The fractured occurred at the very edge of the tab, making the damage difficult to see. I located the broken tab as I investigated  the cause of excessive movement between the intercooler and front engine baffling. I had a new, heavier tab welded to the intercooler, and then attached the new tab to the old bracket on the engine through a vibration damper fabricated from ¼ inch thick hard rubber (see Figure 3). 

I don’t know if the “fix” will, in fact, eliminate the fatigue fracture problem, so I plan to keep a close eye on it; I will report back on the results. In the meantime, I recommend close inspection of this tab, support bracket and surrounding baffling at each condition inspection.

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