N4787CDehavilland BEAVER DHC-2

On July 24, 2013, about 1140 Alaska daylight time, a float-equipped de Havilland DHC-2, N4787C, collided with trees following a loss of engine power near Lake Galea, about 15 miles northwest of Thorne Bay, Alaska, on Prince of Wales Island. 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 the provisions of 14 Code of Federal Regulations (CFR) Part 135 with a company visual flight rules flight plan filed. Visual meteorological conditions prevailed. The flight departed Ketchikan Harbor Seaplane Base (5KE), Ketchikan, Alaska, at 1100 and was destined for Shipley Bay, Alaska.

According to the pilot, the airplane was in cruise flight at an altitude of about 1,500 feet above mean sea level (msl), which he estimated was about 1,200 feet above ground level (agl) in the area over which he was flying, when he heard a loud "boom" and a series of loud and continuous "pop-pop-pop" noises. The pilot said that he reported to the company via the radio that he had lost an engine cylinder and was going to land. The pilot said that everything was shaking and that he did an immediate 180-degree turn to land on the lake that he had just overflown. The pilot stated that, as he turned the airplane on a base leg for the lake, he put in two pumps of flaps, and, about that time, the engine lost power completely. The pilot estimated that the amount of time that elapsed from when he first heard the loud "boom" to the time that the engine lost power completely was less than 1 minute. The pilot stated that, once the engine lost power completely, the airplane was soon colliding with trees. According to the operator, the airplane came to rest in a wooded area about 300 yards from the lake, sustaining substantial damage to the wing, fuselage, and empennage.

The pilot stated that he and two passengers were able to exit the airplane but the passenger in the right seat was unable to exit the airplane until more help arrived. The pilot located the airplane's 406-MHz emergency locator transmitter and flipped the switch to the "on" position to be sure that it was transmitting. According to the operator, the Rescue Coordination Center (RCC) telephoned the operator and provided coordinates for the downed airplane. The pilot also located the airplane's survival kit, and he and a passenger positioned a piece of wing wreckage in a marsh area to try to make the accident site visible to overflying aircraft. The pilot established cellular telephone contact with the operator, which had dispatched another company airplane to assist. The pilot said that he heard the other company airplane approaching and used a flare from the survival kit to signal his location. The other company airplane landed on Lake Galea, and company personnel hiked to the accident site to assist the pilot and passengers. A U.S. Coast Guard (USCG) helicopter from Air Station Sitka soon arrived. The USCG transported the pilot and all three passengers from the scene two at a time.

Aircraft recovery personnel who retrieved the wreckage and Federal Aviation Administration (FAA) inspectors who observed reported that engine cylinder damage was visible. According to the operator, the Pratt & Whitney R-985-AN-14B engine had accumulated 31 hours since major overhaul.

Subsequent disassembly examination of the engine under the authority of the NTSB revealed that the No. 1 cylinder head showed both vertical and horizontal cracks, and the top of the No. 1 piston was visible through the cracks. The No. 1 piston was positioned abnormally high within the cylinder barrel. The vertical crack in the No. 1 cylinder head extended over the top of and bisected the cylinder head, and a horizontal crack extended around the circumference, such that the bisected halves of the cylinder head could be lifted off by hand, exposing the piston top.

Internal damage to the engine precluded the removal of any cylinders using typical engine disassembly techniques. The No. 1 cylinder barrel and the No. 1 piston could not be removed. Removal of the Nos. 2, 3, 6, 7, 8, and 9 cylinders (accomplished by prying the cylinder barrels and applying force) and removal of their respective pistons allowed for a view inside the crankcase. The top (piston end) of the No. 1 linkrod was attached to the piston pin (established by feel), but the bottom portion (crankshaft end) of the linkrod and its respective bushing were missing. The linkpin for the No. 1 linkrod was attached to the crankshaft assembly.

The remaining visible linkrods (Nos. 2, 3, 6, 7, 8, and 9) all exhibited gouging damage. The No. 2 linkrod and No. 2 cylinder barrel showed pronounced deformation to the left (in the direction of engine rotation). The No. 3 linkrod also showed deformation to the left. The underside of the removed pistons (Nos. 2, 3, 6, 7, 8, and 9) all showed heavy gouging damage, and the No. 2 piston also was cracked on the bosses for the piston pin. A piece of a separated crankshaft counterbalance weight was found inside the crankcase with fragments of its attachment bolts present in the separated piece. The separated counterbalance weight piece showed heavy gouging damage. Metal fragments and heavy gouging damage were visible inside the crankcase. The oil sump contained metal debris, including fragments that appeared visually consistent with the color of bushing material, fragments that appeared visually consistent with the shape of piston ring pieces, and other metal debris.

A maintenance record for the airplane dated July 16, 2013, recorded the installation of engine SN JP206275 at an engine time since overhaul of 0.0 hours, an engine total time of 6,029.5 hours, and an airframe total time of 33,326.1 hours. The engine's authorized release certificate and airworthiness approval tag from the FAA-certificated repair station that performed the overhaul was dated June 6, 2013. A record dated May 28, 2013, documented that the engine's ground test run was 5 hours. A repair station record dated May 15, 2013, (which documented the engine's "inspection, reconditioning, and assembly in accordance with Pratt & Whitney Manual No. 123440") noted that the engine's linkrods were serviceable, and the linkpin bushings were replaced. A record dated January 23, 2013, documented the magnetic inspection of the linkrods. During a telephone interview, a representative from the facility that had performed the engine overhaul stated that the facility had experienced no changes in its linkpin bushing supplier and no differences in its techniques or procedures for performing engine overhauls with regard to the overhaul of the accident engine compared to others.