NTSB Factual Report

On October 26, 2008, about 1000 eastern daylight time, an experimental amateur-built Vans RV-8A, N88FG, was substantially damaged when it was struck by an experimental amateur-built RV-6A, N914ES, while standing at Cambridge-Dorchester Airport (CGE), Cambridge, Maryland. The certificated commercial pilot of the RV-8A, and the certificated private pilot of the RV-6A, were not injured. Visual meteorological conditions prevailed at the time of the accident, and neither airplane was operating on a flight plan. The personal flights, which both departed from Eastern West Virginia Regional Airport (MRB), Martinsburg, West Virginia, were conducted under the provisions of Title 14 Code of Federal Regulations Part 91.

Both pilots provided written statements that recounted a similar series of events. According to the pilots, after landing at CGE, both airplanes taxied to the parking area. The RV-8A parked first, and the pilot of the RV-6A planned to park his airplane immediately to the right of the RV-8A. As the RV-6A approached the stationary RV-8A from the right and perpendicular to the RV-8A, the RV-6A pilot applied right brake in order to turn the airplane into the intended parking spot. The brake "failed totally," and the pilot then pumped the brake several times, with no response. The pilot then applied left brake to avoid hitting the RV-8A, but the right wing of the RV-6A struck the right wing of the other airplane. The RV-6A turned to the right, and the propeller of the RV-6A struck the right horizontal stabilizer of the RV-8A, substantially damaging it.

A Federal Aviation Administration (FAA) inspector examined both airplanes, and noted the presence of brake fluid both on the exterior of the right brake and on the ground around the right wheel of the RV-6A. According to the inspector, the right wing tips of both aircraft were damaged as well as the horizontal stabilizer of the parked airplanes.

According to the pilot/builder of the RV-6A, a post-accident disassembly of the right brake revealed that the o-ring between the main brake housing (cylinder assembly) and the piston had "deteriorated."

The RV-6A's most recent conditional inspection was completed on September 23, 2008, and the airplane had accumulated 718 total hours of operation on that date.

The National Transportation Safety Board subsequently retained the o-ring, and shipped it to the brake assembly manufacturer for further testing under the supervision of FAA inspectors. An acceptance test was performed utilizing a new brake assembly, with the accident o-ring installed. Once the o-ring was in place, pressure was applied using a test stand, and hydraulic fluid immediately leaked from the brake assembly piston area. The accident o-ring was removed and a new o-ring installed to verify the condition of the test brake assembly. The brake assembly passed the inspection with no leaks noted using the new o-ring.

The manufacturer subsequently performed an analysis of the o-ring for material type, hardness, and dimensions. A Fourier Transform Infrared Spectroscope (FTIR) analysis showed that both the accident o-ring and a new (control) o-ring were composed of a nitrile rubber compound, conforming to the o-ring material requirements set by the manufacturer's engineering drawing; however, the FTIR spectrum of the accident o-ring indicated the presence of an unknown substance on its outside diameter surfaces.

Dimensional measurements showed that the accident o-ring was smaller in size that the control o-ring, with a compression set of about 8 percent on contacting surfaces in radial dimensions. Hardness testing revealed that the accident o-ring exhibited a greater hardness than the control o-ring. The testing concluded that the abnormalities observed were consistent with the accident o-ring having experienced excessive heat.

NTSB Probable Cause

The failure of the RV-6A's right brake assembly o-ring due to excessive heat.