1 inch away from a very bad day-Cowl Horizontal Piano Pins

Gents,

I know you would never try this but I wanted to pass on a warning to all who use the horizontal hinges on your cowl .

Today I had my prop balanced. For those are not familiar with the procedure for prop balancing, it is fairly simple. Mount an accelerometer to the engine and a photo detector to detect when the prop blade passes (reflective tape is applied to one prop blade). You run the engine at cruise RPM and take a reading. Then you add weight to the "flywheel" and run it back up to cruise RPM. So it is an iterative process.

The A&P wanted to balance with the cowl on and fortunately I didn't need to remove the lower cowl (it is a bit of a pain on RV-7A with vertical induction), just the top cowl. My cowl is attached with all hinges (which except for near the cowl inlets (screws) and I have a medallion that covers the horizontal hinge pin access in the side of the cowl (2 screws).

So after removing the cowl and installing the accelerometer I thought that I really didn't need to secure the medallion covering the horizontal hinge pin access. So we continued and secured the photo detector and I started the engine. Not too long after a low idle I ran the engine up to 2450 RPM. The A&P started waving his hands and giving the cut engine hand signal. After the engine stopped I asked what was wrong. It turns out the the horizontal hinge pins were backing out. An inch more of backing out and I would have had a really bad day.

I thought I was a smart guy , just like all the other builders. I really didn't think the pins would back out like they did. I just wanted to make installing and removing the cowl a bit easier during the iterative process of prop balancing.

The moral I wish to pass on is make sure the cowl horizontal hinge pins are secured and cannot back out whenever the engine is run or you might really have a bad day. I think I dodged a bullet today .

Pmerems said:

I think I dodged a bullet today .

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Yep, I think you did for sure.

I have seen RV's parked at shows where the pins were inserted from the front (per plans) but NOT SECURED at all - just bare ends sticking out. That's a definite increase in risk to your prop, for sure.

On the topic of horizontal hinge pins, we had a neighbor take off after rushing to put the cowl back on his Glassair that used the same attachment method. He was on an IFR clearance - ceilings reported at 400', but entered the clouds at 100'....the top cowl departed the aircraft at 200' - he had missed all the hinge loops in his haste. Had to shoot an ILS to a neighboring airport with CHT's and Oil Temp shooting through the roof - no go-around, he didn't think the engine would keep running that long. Cowl nicked the prop on the way off - prop strike, top end damage from heat.....bad day.

Moral? Haste makes waste, and make sure the cowl pins are secure....

Paul

Ironflight said:

I have seen RV's parked at shows where the pins were inserted from the front (per plans) but NOT SECURED at all - just bare ends sticking out. That's a definite increase in risk to your prop, for sure.

On the topic of horizontal hinge pins, we had a neighbor take off after rushing to put the cowl back on his Glassair that used the same attachment method. He was on an IFR clearance - ceilings reported at 400', but entered the clouds at 100'....the top cowl departed the aircraft at 200' - he had missed all the hinge loops in his haste. Had to shoot an ILS to a neighboring airport with CHT's and Oil Temp shooting through the roof - no go-around, he didn't think the engine would keep running that long. Cowl nicked the prop on the way off - prop strike, top end damage from heat.....bad day.

Moral? Haste makes waste, and make sure the cowl pins are secure....

Paul

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Wow that is scary! I like my cowl pins, but there certainly room for operator errror...
Good topic all!