Accident Overview
On July 24, 2013, at approximately 1140 Alaska daylight time, a float-equipped de Havilland DHC-2, registration N4787C, collided with trees following a loss of engine power near Lake Galea, about 15 miles northwest of Thorne Bay on Prince of Wales Island, Alaska. The airline transport pilot and two passengers received serious injuries, one passenger was uninjured, and the airplane sustained substantial damage. The on-demand air taxi flight was operated by Promech Air, LLC, under 14 CFR Part 135 with a company visual flight rules flight plan filed. Visual meteorological conditions prevailed. The flight departed Ketchikan Harbor Seaplane Base at 1100 and was destined for Shipley Bay, Alaska.
Pilot Actions
According to the pilot, the airplane was in cruise flight at about 1,500 feet mean sea level (approximately 1,200 feet above ground level) when he heard a loud "boom" followed by a series of loud, continuous "pop-pop-pop" noises. The pilot radioed the company reporting a lost engine cylinder and his intention to land. He executed an immediate 180-degree turn toward a lake he had just overflown. During the base leg, he applied two pumps of flaps, and the engine then lost power completely. The pilot estimated about one minute elapsed from the initial boom to total power loss. The airplane soon struck trees. The operator reported that the aircraft came to rest in a wooded area about 300 yards from the lake, with substantial damage to the wing, fuselage, and empennage.
After the crash, the pilot and two passengers exited the aircraft; the right-seat passenger could not exit until additional help arrived. The pilot activated the 406-MHz emergency locator transmitter. The Rescue Coordination Center contacted the operator with coordinates. The pilot retrieved the survival kit, and he and a passenger placed a piece of wing wreckage in a marsh to increase visibility. The pilot contacted the operator via cell phone; another company airplane was dispatched. Upon hearing it approach, the pilot used a flare from the survival kit to signal. The company airplane landed on Lake Galea, and personnel hiked to the site. A U.S. Coast Guard helicopter from Air Station Sitka arrived and transported the pilot and passengers two at a time.
Engine Examination
Aircraft recovery personnel and FAA inspectors observed engine cylinder damage. The Pratt & Whitney R-985-AN-14B engine had accumulated 31 hours since major overhaul. Disassembly examination under NTSB authority revealed that the No. 1 cylinder head had vertical and horizontal cracks, exposing the top of the No. 1 piston, which was abnormally high. The cracks allowed the bisected halves of the cylinder head to be lifted off by hand. Internal damage prevented removal of the No. 1 cylinder barrel and piston using typical techniques. Removal of other cylinders (Nos. 2, 3, 6, 7, 8, and 9) by prying revealed that the top of the No. 1 linkrod was attached to the piston pin, but the bottom portion and its bushing were missing. The linkpin for No. 1 remained attached to the crankshaft.
All removed linkrods (Nos. 2, 3, 6, 7, 8, 9) showed gouging. The No. 2 linkrod and cylinder barrel were deformed toward the direction of engine rotation; the No. 3 linkrod also showed deformation. The undersides of removed pistons had heavy gouging; the No. 2 piston had cracked bosses for the piston pin. A separated crankshaft counterbalance weight piece with bolt fragments was found inside the crankcase, exhibiting heavy gouging. Metal fragments and gouging were visible inside the crankcase. The oil sump contained metal debris, including fragments consistent with bushing material, piston ring pieces, and other debris.
Maintenance History
A maintenance record from July 16, 2013, documented the installation of engine SN JP206275 at 0.0 hours since overhaul, engine total time 6,029.5 hours, and airframe total time 33,326.1 hours. The FAA-certificated repair station that performed the overhaul issued an authorized release certificate and airworthiness approval tag on June 6, 2013. A record dated May 28, 2013, noted a 5-hour ground test run. A May 15, 2013, record indicated inspection, reconditioning, and assembly per Pratt & Whitney Manual No. 123440, noting linkrods serviceable and linkpin bushings replaced. A January 23, 2013, record documented magnetic inspection of the linkrods. A representative from the overhaul facility stated no changes in supplier or procedures for this engine compared to others.