scope:

Description of Driving Event:

A failure occurred at the Transonic Dynamics Tunnel (TDT) when a blade separated from a rotor (i. e., an airplane propeller). The incident did not result in damage to the TDT, but significant damage occurred to the Wing and Rotor Aeroelastic Testing System (WRATS). This is a reusable testbed placed inside the TDT test section that allows for testing different rotor models. The rotor model being tested at the time was not NASA owned; it had been designed and manufactured by an industry customer.

A thorough investigation concluded that the most likely cause was using the wrong material to fabricate the cuffs of the rotor model. A cuff is the mechanism that attaches a blade to the rotor hub. The model designer specified aluminum 7075-T7351 as the cuff material, but the part was actually fabricated using aluminum 6061. The stress analysis performed prior to testing indicated that 7075 had the proper material properties for the cuff's intended ! task. However, an examination of the properties of 6061 showed that a cuff manufactured from 6061 was, "marginal at best for the task intended."

It was clear that the material change was not properly analyzed and that some of the people who needed to be informed about the change had not been informed. The customer stated after the incident, "that the material change was approved, but that information was not incorporated into the original [stress] analysis."