Klamath Falls, OR, USA
N6447M
MAULE M 7-235C
The commercial pilot reported that he did not note any drag or restrictions from the wheel brakes during taxi or takeoff. When the airplane touched down at the destination airport, it immediately began to veer left. The pilot was unable to overcome the uncommanded movement despite his application of right rudder and right brake, and the airplane nosed over and came to rest inverted. The pilot reported that as he was preparing the airplane for the flight, he removed the chocks and the airplane started to roll backwards. He assumed that his son had released the parking brake and replaced the downhill chocks the night before the accident when he finished washing the airplane. The pilot completed the preflight but did not follow the checklist to confirm that the parking brake was disengaged before departing on the flight. Postaccident, the brake knob was found in the ON (engaged) position. However, the pilot reported the absence of any drag during ground operations indicating that the parking brake may not have been engaged before taking off, which can occur if the toe brakes are not fully extended when the parking brake is set. Postaccident testing revealed that the left brake became stuck multiple times due to the interference of circumferential gouges and ridges on the left brake master cylinder piston rod and bushing, which likely resulted from normal operation of the parking brake. Because the parking brake knob was in the ON position during the flight, the left brake could have been set by depressing the toe brake. The rod damage would have then prevented the free travel required for the piston to release brake pressure and disengage the left brake, thereby causing the airplane to veer left after touching down. Although a visual inspection of the rod could have detected the wear, review of the maintenance logs since 2010 showed that the master cylinder had not been disassembled and inspected, nor was it required to be.
HISTORY OF FLIGHTOn August 24, 2017, about 0940 Pacific daylight time, a Maule M-7-235C airplane, N6447M, was substantially damaged 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, conducted under the provision of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a flight plan was not 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 the accident and then washed the airplane. The following morning the pilot prepared the airplane for a flight to LMT to purchase fuel in advance of 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 did not confirm the parking brake was OFF before he departed. 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 wind sock 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, which resulted in substantial damage to the engine firewall. The pilot reported that he does not normally engage the parking brake when the airplane is parked, but his son does and has had trouble engaging and disengaging the parking brake during previous attempts. He further stated that as the parking brake knob is similar in appearance to several other knobs on the instrument panel, he placed a pink ribbon on the parking brake knob, which had been removed during the last annual inspection and was not replaced. Brake System The airplane was equipped with two independent hydraulic wheel brakes actuated by two master cylinders on the left side and two slave cylinders on the right side. Depressing the brake pedal actuated a piston rod in the master cylinder, which applied hydraulic pressure to the brake pad. Releasing the pedal allowed the piston rod to be back-driven by a spring, which released brake pressure at the wheel. The parking brake system utilized a cable-actuated lever on each master cylinder to hold each brake master cylinder piston rod in place by 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. Examination According to statements taken from a mechanic and an airport representative, during the attempt to move the airplane from the active runway, the left main landing gear seized and that the right main landing gear was stiff, but could turn. The airport representative further remarked that he was one of the first people on scene, and had 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 the supervision of a Federal Aviation Administration (FAA) inspector. The mechanic and the FAA inspector set the parking brake a total of 30 times and discovered that the brake failed to disengage during 6 of the 30 cycles. During these tests they verified that the right main landing gear was immobile, but were unable to verify if the left main landing gear brake was functional, as it had seized during the accident. The mechanic then disassembled the left parking brake master cylinder, which revealed circumferential scoring that corresponded with the placement of the brake lever that sets and releases the parking brake. The inspector did not find any other unusual wear, improper rigging, or parts failures. Photographs submitted by the pilot's mechanic revealed similar circumferential damage to the right brake piston rod. On May 11, 2012, an airplane of similar make and model was substantially damaged when it veered off the runway and impacted a ditch during a landing attempt. According to the National Transportation Safety Board (NTSB) report, WPR12LA207, the parking brake system utilizes a lever that grips and holds the brake master cylinder piston rod in each brake cylinder to retain applied brake pressure. The brake system examination discovered multiple circumferential gouges and ridges on portions of one of the master cylinder piston rods. Further examination revealed an interference fit between the outside diameter of the damaged area and the bushing. If the brake pedal was depressed sufficiently in flight to drive the damaged portion of the rod into the bushing, the rod would no longer be free to slide through the bushing. This could prevent the rod from returning to release brake pressure as the spring strength was not sufficient to overcome the resistance of the interference fit. Measurements taken by the NTSB investigator-in-charge did not reveal an interference fit between the main landing gear piston rod and bushing. However, during multiple attempts to actuate the rod by hand, the bushing would catch one of the grooves or ridges of the rod and prevent it from returning to its original position despite the force carried by the rod spring. The airplane maintenance manual did not contain any specific inspection citations or component life limits for the brake master cylinder piston rod, and no Maule Service Letters or Service Bulletins concerning the brake master cylinder piston rod have been issued. Inspection Information According to paragraph (c) of 14 CFR Part 43.15, "Annual and 100-hour inspections. (1) Each person performing an annual or 100-hour inspection shall use a checklist while performing the inspection. The checklist may be of the person's own design, one provided by the manufacturer of the equipment being inspected or one obtained from another source. This checklist must include the scope and detail of the items contained in appendix D to this part." 14CFR Part 43, Appendix D states the "Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections" contained the following guidance to persons conducting these inspections: Paragraph d(7) "All systems—for improper installation, poor general condition, apparent and obvious defects, and insecurity of attachment. Paragraph e(9) "Brakes—for improper adjustment"
The pilot's failure to follow the pre-takeoff checklist and the failure of the left main landing gear brake due to a damaged brake master cylinder piston rod, which resulted in a stuck left brake and subsequent loss of control during the landing roll.
Source: NTSB Aviation Accident Database
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