Eurocopter has voiced concerns about the safety of supplemental type certificates (STC), claiming three times more accidents are caused by STC’d hardware than by the original aircraft. The European helicopter manufacturer is concerned that some STCs cover such a wide array of major modifications that certifying a new version of the helicopter would be a more prudent course.

According to Michel Soulhiard, technique and maintenance senior manager at Eurocopter’s fleet safety directorate, on AS 350 Ecureuil singles, for example, “technical STC” is labeled a cause in 3.3 percent of accidents, while “technical engine” stands at 1.1 percent and “technical vehicle” accounts for zero percent.
These statistics come from a 92-accident study in the U.S. and Canada over a period of almost five years, ending in October 2008. However, the cause of 15.2 percent of those accidents has not yet been determined.

STCs are issued to applicants who do not have to request any authorization from the helicopter manufacturer. An STC allows an applicant to modify an aircraft from its original design. The STC “approves not only the modification but also how that modification affects the original design.”

The Eurocopter official challenges current awarding of STCs on the grounds that “STC holders cannot determine and foresee all possible interactions of the STC with the whole aircraft.” He alluded to vibrations, temperatures and power off-take.

In addition, Soulhiard expressed concern about possible incompatibility between two STCs or between an STC and an OEM option. He also notes that an STC-modified helicopter might be incompatible with OEM maintenance programs, and that there is a “lack of continuing airworthiness activity on some of these STC installations.”

Soulhiard is not condemning the entire STC process. In fact, he acknowledges that STC equipment is often necessary. “Some specific needs are not covered by OEM options.” Moreover, he concedes that STC hardware is often less expensive than the manufacturer option.

STCs most usually cover pieces of equipment such as cargo mirrors, cameras, avionic accessories, seats and so on. But Soulhiard cites one case where a single STC includes, for the AS 350, a new engine, increases in both internal and external load maximum weights, enlarged tail-rotor blades, completely new electrical system and a new instrument panel. He questions whether the modified helicopter should be considered an original AS 350 rather than a new model.

Soulhiard proposes improvements to the STC process. First, he suggests that a company risk-assessment procedure, such as the one Eurocopter employs, can provide pertinent information to national aviation authorities and customers in the event of an incident or accident due to a non-OEM STC.

He is urging the EASA to start discussions with the FAA and Transport Canada, the regulatory bodies of the two countries where the STC business has mostly developed, to share accident statistics. Soulhiard also suggests the agencies “analyze present weaknesses” such as compatibility issues and mandate that the STC holder ensure continued airworthiness.

Finally, Soulhiard wants the agencies to limit the scope of an STC.