History of Flight
On August 24, 2017, about 0940 Pacific daylight time, a Maule M-7-235C airplane, N6447M, sustained substantial damage when it impacted the ground during a landing at Crater Lake-Klamath Regional Airport (LMT), Klamath Falls, Oregon. The commercial pilot was not injured. The airplane was registered to and operated by a private individual as a personal flight under 14 CFR Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight that departed a private strip about 0940.
According to the accident pilot, his son applied the parking brake the night before and then washed the airplane. The following morning, the pilot prepared for a flight to LMT to purchase fuel for a planned cross-country flight. The pilot reported that he assumed his son had disengaged the parking brake, as the airplane immediately began to roll backwards when he removed the chocks, and he did not confirm the parking brake was off before departure. He started the engine with the toe brakes engaged and was able to move the airplane uphill with minimal power. The pilot did not observe any indication that the wheels were dragging during the short taxi and takeoff from his 1,400-foot-long private grass strip, which had recently been cut. He joined the traffic pattern on the downwind leg for runway 32 and interpreted the windsock to read calm winds. The airplane touched down in a three-point attitude and immediately began a hard left turn. The pilot attempted to counter the uncommanded movement with right rudder and right brake, but the airplane nosed over and came to rest inverted, resulting in substantial damage to the engine firewall.
Examination
During the attempt to move the airplane from the active runway, the left main landing gear seized and the right main landing gear was stiff but could turn, according to statements from a mechanic and an airport representative. The airport representative discovered the parking brake in the engaged position. A firefighter and first responder reported that his team would not have manipulated the parking brake.
A postaccident examination of the brake system was completed by a certificated airframe and powerplant mechanic under FAA supervision. The mechanic and FAA inspector set the parking brake 30 times and found that the brake failed to disengage during 6 of the 30 cycles. During these tests, the right main landing gear was immobile; the left main landing gear brake could not be verified as it had seized during the accident. The left parking brake master cylinder was disassembled, revealing circumferential scoring corresponding to the placement of the brake lever that sets and releases the parking brake. No other unusual wear, improper rigging, or parts failures were found. Photographs submitted by the pilot's mechanic revealed similar circumferential damage to the right brake piston rod.
Brake System Findings
The airplane was equipped with two independent hydraulic wheel brakes actuated by master cylinders on the left side and slave cylinders on the right side. The parking brake system used a cable-actuated lever on each master cylinder to hold the piston rod in place, retaining applied brake pressure. The manufacturer reported that if the parking brake handle is pulled, the pilot can set the brake in flight by depressing the toe brakes.
A prior accident on May 11, 2012, involving a similar make and model (NTSB report WPR12LA207), documented that the parking brake system utilizes a lever that grips the master cylinder piston rod. That examination discovered circumferential gouges and ridges on one piston rod, and an interference fit between the damaged area and the bushing, which could prevent the rod from returning and releasing brake pressure. In the current accident, measurements did not reveal an interference fit, but during multiple attempts to actuate the rod by hand, the bushing would catch a groove or ridge and prevent the rod from returning despite the spring force.
The airplane maintenance manual contained no specific inspection citations or component life limits for the brake master cylinder piston rod, and no Maule Service Letters or Service Bulletins concerning the rod have been issued.
The pilot reported that he does not normally engage the parking brake when parked, but his son does and has had trouble engaging and disengaging it. The pilot had placed a pink ribbon on the parking brake knob to distinguish it, but the ribbon was removed during the last annual inspection and not replaced.