Inspect Airfoil Fans for Cracks

An incident occurred this week that serves as a good reminder that airfoil fans occasionally fly apart and that scheduled inspections are the best defense against equipment failure.

The fan in question was of the airfoil or "propeller" type. The fan blades were constructed from aluminum sheet metal and formed into a "wing" shape. The upper and lower surfaces and ends were joined by welding and the interior was hollow. Each of the six individual blades were impaled by and bolted to solid steel bars which in turn were symmetrically clamped as spokes into a center hub.

Close inspection showed notable cracks in several of the aluminum blades particularly adjacent to the attachment bolts. Cracks were noted on one blade that did not break plus three others that had become detached. The broken blades showed that the cracks started near the bolts and had been present for some time. Rather than showing fresh signs of  'ripped' metal, the initial cracks were dark colored as if they had been in this state for awhile.

An operational vibration cutout switch showed its worth by shutting the tower down before things got really out of hand.

So, if your tower has a prop fan-  regardless of whether it is sheet metal, a casting or even fiberglass- check it for cracks. Catching the problem early will reduce repair costs, equipment damage and down time. Plus it will keep you a lot safer. Include regular inspections- perhaps two each year- into your regular maintenance routine.