(article) Beware Symmetry

[Wilfred]

(article) Beware Symmetry

 

I was with a student teaching the preflight inspection on the six-passenger, retractable-gear airplane he'd just purchased. Looking in the left-wing wheel well I pointed out a broken spring on the landing gear uplock mechanism. It was a small, thin, wiry spring, partially obstructed (as is normal) by a fabric cover.

 

I declared the airplane rejected for flight (I don't want to have to write about by own gear-up landing, especially if it results from a previously noted mechanical cause). But with a lot of new ground-time on our hands we continued the "teaching" preflight. Wouldn't you know, the corresponding spring in the right wheel well was broken also.

 

It looked the same on both sides of the airplane... enough to make me wonder if I had misdiagnosed the problem.

 

Symmetry

 

Admit it, you know what I'm talking about. You get a little pain on one side of your body, but if you feel it on the other side too you declare it "normal" and move on. In our airplane example, my student and I saw something we thought was wrong on one side of the airplane, but comparing the "mirror image" on the other side revealed the same picture. It would have been very easy to declare the airplane airworthy because the discrepancy was symmetric. No doubt this is why it wasn't found by the pilots who had flown the plane before us, or the mechanic that had inspected the airplane before my student bought it (although he/she should have known better). There was no "right" configuration to compare it two and, in airplanes, two wrongs do not make it right.

 

Aero-tip of the day: Check the airplane against informed expectations -- not just against the airplane itself.