Blind FastenersThe danger in assumingBy Emily Refermat
In April of 1997 a New York helicopter operated by the Colgate-Palmolive Company crashed into the East River, killing one passenger and seriously injuring another. The cause was a vertical fin fatigue failure — accelerated by the improper use of blind rivets when a replacement yaw SAS mount support (originally using solid rivets) was installed. The failure left the helicopter without directional control.
Although the design of the vertical fin was susceptible to fatigue cracking, it was incorrect and conflicting information, as well as a faulty assumption, which ultimately left the mechanics of this helicopter in front of the National Transportation Safety Board.
According to the NTSB report, the mechanics working on the Colgate-Palmolive helicopter were misinformed about a specific blind rivet mentioned in AC65-9A the Airframe and Powerplant General Handbook. AC65-9 says that a bulbed CherryLock Rivet with "its proven fatigue strength makes it the only blind rivet interchangeable structurally with solid rivets."
The mechanics then assumed that a newer version of this blind rivet could be used. The NTSB found the AC65-9 statement about blind rivet to be incorrect when a representative from Eurocopter — Germany presented evidence that the blind rivets had only about half the fatigue strength as solid rivets in cyclic applications.
The mechanics also mentioned AC 43.13 -1A, but said the NAS (National Aerospace Standard) numbers were out of date. The current AC 43.13-1B says "Blind rivets in the MS20600 through MS20603 series rivets and the mechanically locked stem NAS 1398, 1399, 1738, and 1739 rivets sometimes may be substituted for solid rivets. They should not be used where the looseness or failure of a few rivets will impair the airworthiness of the aircraft. Design allowables for blind rivets are specified in MIL-HDBK-5. Specific structural applications are outlined in MS33522. Nonstructural applications for such blind rivets as MS20604 and MS20605 are contained in MS33557."
There were no specific repair instructions of the yaw SAS mount support in the manufacturer's instructions or about replacing solid rivets with blind rivets. And the ACs presented conflicting information.
When in doubt, talk to the manufacturer
Steve Karno, director of marketing for Textron Aerospace Fasteners, says Textron states in its manuals and warning labels (which are available to mechanics) that blind rivets and solid rivets are not interchangeable for all applications. And while the company's manuals do provide important information on mechanical stress and related properties, the best authority for a particular fastener application is the original equipment manufacturer. “It is always prudent to follow the manufacturer’s instructions because they are the most knowledgeable concerning what rivet is required for a particular application,” says Karno. “We strongly recommend that if the manufacturer has not specified the exact type of rivet, which should be a rare occurrence, that mechanics obtain the information from the most knowledgeable source — the manufacturer.”
Although it was the CherryLock blind rivet in this incident, it's important to double-check any solid to blind rivet conversion. Err on the side of caution. The moral is don't assume you can use blind rivets, but check with the manufacturer.
NTSB Identification: NYC97FA076
Textron Fastening Systems