Question: I just started flying at the local flight school. They have two airplanes—a 1978 Cessna 172 and a 1975 Cessna 150. Both airplanes are parked on the ramp and have black rings around the rivets on top of the wings, which makes me wonder if this is corrosion and how bad it has to get before the aircraft are grounded. I asked my CFI, and he said it was nothing to worry about. Is this corrosion or something worse?
Answer: What you are describing is rivet fretting more commonly called a “smoking rivet.” It is caused by the rivet moving slightly in the rivet hole due to aircraft movement. Over time the rivet gets smaller and the hole wider, resulting in the rivet rattling or vibrating more, creating a black- or rust-colored ring around the rivet and sometimes a streak of black or rust coming off the rivet.
This is not the same as corrosion, according to the three airframe and powerplant (A&P) mechanics consulted on this question.
Please note that rivets are routinely inspected and retightened with an appropriate riveting tool if necessary. Other times the rivets are replaced.
According to the A&Ps, a single smoking rivet does not automatically mean the aircraft has a serious structural problem, but it is evidence that something is moving where ideally nothing should be moving—for example, a spar inside the wing.
Because the skin of the aircraft is between the rivet and spar, unless you are Superman with X-ray vision, you cannot see the spar moving ever so slightly out of position. However, a savvy and experienced mechanic or CFI might be able to identify an issue by running their hands along the wing and feeling something amiss.
The mechanics advise that if the pilot observes a row of smoking rivets, particularly on a structural joint, spar, bulkhead, or other highly loaded area, that deserves a closer inspection by a qualified maintenance technician.
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