Steamboat Springs, CO, USA
N223DH
POOL MURPHY MOOSE
The pilot reported that during the landing roll in the experimental kit-built airplane, the left wheel brake locked, and the airplane departed the left side of the runway. The left landing gear dug into the soft dirt and the airplane nosed over. The pilot/owner/builder disassembled both brake master cylinders and the parking brake valve without federal oversight and found no foreign material or operating problems with the master cylinder or parking valve. The variance of 0.009 inches in the dowel pin length documented by the pilot/owner/builder was explained by standard manufacturing procedures used to achieve near simultaneous operation of the valve between the left and right sides of the parking brake. Functional testing of the parking brake valve revealed no anomalies. Due to the disassembly of the parking brake valve and parking brake assembly without federal oversight, investigators were unable to determine their condition on the airplane at the time of the accident. Accordingly, the cause of the locked left brake could not be determined.
On May 12, 2012, approximately 1215 mountain daylight time, a Pool Murphy Moose experimental kit airplane, N223DH, was substantially damaged during the landing roll at Steamboat Springs Airport/Bob Adams Field (KSBS), Steamboat Springs, Colorado. The pilot and two passengers received minor injuries. The airplane was registered to and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which operated without a flight plan. The flight originated from Eagle County Regional Airport (KEGE), Eagle, Colorado, at 1100. According to the pilot, during the landing roll on runway 32, “the left wheel brake locked.” The airplane drifted off of the left side of the runway and the “left wheel dug into the soft dirt.” The airplane nosed over resulting in substantial damage to the vertical stabilizer and empennage assembly. The pilot/owner/builder “disassembled both brake master cylinders and the parking brake valve” without federal oversight. He stated that he “found no foreign material or any operating problems with the master cylinder or parking valve.” The left dowel pin for the parking brake valve measured “0.009 [inches] shorter than as specified on the MATCO engineering drawing.” The pilot postulated that with the shorter dowel “any small movement of the lever arm would cause the poppet to seat . . . and not allow the brake fluid back through the parking brake, master cylinder [sic], and into the reservoir. A small amount of flex in the parking brake cable or some type [of] vibration could have caused this small movement.” The parking brake valve, PVPV-1, was manufactured by MATCO Mfg, and was discontinued in August of 2008. According to the manufacturer, the information specified on their assembly drawing represented the part number of the dowel pin and not the engineering specifications for the component. During assembly, the dowel pins lengths would be modified as required to achieve near simultaneous opening and closing between the left and right sides, based on the position of the control arm. The manufacturer added that rarely were the two pins the same length. The National Transportation Safety Board (NTSB) received the parking brake valve, reassembled, on September 13, 2012. A Federal Aviation Administration inspector provided oversight for the examination and bench test of the parking brake valve. The parking brake valve was connected to the MATCO test bench and pressurized for functional testing. At 1,000 psi, both sides of the valve held pressure and both sides of the valve released approximately 30 degrees and similar pressure when the valve handle was positioned in a corresponding open position. Both of these results are within design and function parameters. The test was repeated at 500 psi. The left side of the valve released 5 degrees sooner than the right side; however, both were within the transition zone. The parking brake valve passed all seal and functional tests. According to the manufacturer, when the lever is moved to the closed position, fluid flows to the brake, but does not return. The lever can be positioned prior to pressure being applied or when pressure is applied. The manufacturer of the parking brake valve does not supply installation instructions to the customer; however, a drawing showing the operational range of the valve is provided at no cost to the customer. Due to the disassembly of the airplane without federal oversight, the operational range of the parking brake assembly on the accident airplane could not be determined. The parking brake valve was sent to the NTSB Materials Laboratory in Washington, D.C., for disassembly and further examination. No anomalies were noted.
A locked left brake for reasons that could not be determined because independent observations of the postaccident condition of the airplane could not be performed, and the pilot’s subsequent loss of control.
Source: NTSB Aviation Accident Database
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