Sikorsky S-61

The airframe is aluminium: sheet metal over a riveted frame, the way most aircraft have been built since the 1940s. The design is held under 2 type certificates. The FAA has issued 61 airworthiness directives (orders that owners must comply with) that name this model, most of them about rotorcraft systems. Operators and repair shops have filed 325 failure reports with the FAA since 1995, 12 of them in the last ten years: about 3.2 a year for every 100 aircraft on the register; the system most often named is rotorcraft systems. 134 approved modifications exist for it, the most by Croman Corporation.

Sources: the FAA's type certificate data sheets, airworthiness directives, supplemental type certificates, special conditions and special airworthiness information bulletins (Dynamic Regulatory System), and the FAA's service difficulty reports. A failure report is what an operator or repair station chose to file; the counts show reporting as much as reliability, and larger and commercial fleets report more. How the systems are grouped.