SEQUIM, WA, USA
N6912L
CESSNA 310K
THE PILOT TOOK OFF WITH APPROXIMATELY A 4-HOUR FUEL SUPPLY ON A PLANNED 1-HOUR FLIGHT. HE STATED THAT APPROXIMATELY 50 MINUTES INTO THE FLIGHT, WHILE OVER OPEN WATER AT AN ALTITUDE OF LESS THAN 1,000 FEET, THE RIGHT ENGINE DEVELOPED A 'STRONG' VIBRATION (CAUSE UNKNOWN). IN RESPONSE TO THE VIBRATION, THE PILOT SHUT DOWN THE RIGHT ENGINE. THE VIBRATION PERSISTED, ALTHOUGH LESS SEVERELY AFTER RIGHT ENGINE SHUTDOWN. ACCORDING TO THE PILOT, THE VIBRATION WAS SO 'STRONG' THAT HE WAS UNABLE TO ATTAIN SUFFICIENT POWER TO MAINTAIN LEVEL FLIGHT USING SINGLE-ENGINE OPERATION; CONSEQUENTLY, HE WAS UNABLE TO MAINTAIN LEVEL FLIGHT OR REACH A SUITABLE LANDING AREA. HE DITCHED THE AIRPLANE AT SEA ON 'GLASSY WATER.' THE AIRPLANE SANK IN ABOUT 150 FEET OF WATER AND WAS NOT RECOVERED.
On May 12, 1995, approximately 1130 PDT, a Cessna 310K, N6912L, ditched in the Strait of Juan de Fuca 6 nautical miles northeast of Sequim, WA. All three occupants, including the airline transport pilot, escaped the airplane and were rescued by boat. They suffered mild hypothermia from water exposure but were otherwise uninjured. The airplane sustained substantial damage in the ditching. It sank in approximately 150 feet of water and was not recovered. The flight was a local 14 CFR 91 flight originating from Fairchild International Airport, Port Angeles, WA. Visual meterological conditions prevailed and no flight plan had been filed. Three witnesses in a boat about one mile south of the ditching site observed the ditching and provided consistent accounts of the occurrence. They reported that the aircraft initially came into view from their right at a low altitude. The aircraft continued downward until entering the water in an apparently controlled fashion, in near wings level attitude at a shallow entry angle, on a generally westerly heading. One of these witnesses stated that he initially thought the aircraft was a seaplane making an intentional water landing. This witness reported that the wings broke off symmetrically at about mid-span after water entry and that the wing sections initially floated "vertically and symmetrically" on either side of the fuselage. Due to noise from the boat's engine, they were unable to note any sound from the airplane. However, they reported that prior to impact, the aircraft looked intact with no observable structural damage, fire or smoke. The U.S. Coast Guard situation report (SITREP) on the accident reported that a MAYDAY call was overheard on a marine VHF radio frequency at 1150 PDT. The report gave the position of the accident as 48 degrees 8.6 minutes North, 122 degrees 58 minutes West. The SITREP indicated that a Coast Guard helicopter scouting the area at 1228 PDT reported "small debris [and] no sheen at [the] crash site." The airplane's last refueling before the accident took place at Sandpoint, ID. A fuel slip obtained from the fixed base operator (FBO) at Sandpoint shows that the pilot purchased 56.7 gallons of 100 octane, low lead (100LL) fuel there; the FBO attendant who serviced the airplane stated to the NTSB investigator that she filled the main tanks but did not service the auxiliary tanks. However, aircraft servicing records supplied by the pilot indicate that all fuel tanks were full upon departure from Sandpoint. The pilot stated that the last flight before the accident flight, from Sandpoint to Port Angeles, was flown under instrument flight rules (IFR) at an altitude of 12,000 feet. The pilot indicated the route of flight as Sandpoint direct to 48 degrees North, 119 degrees West; then direct to 48 degrees North, 123 degrees West; then direct to Port Angeles. This routing is a distance of 281 nautical miles, approximating the great circle direct distance of 277 nautical miles from Sandpoint to Port Angeles. The pilot stated that he indicated an estimated time enroute (ETE) of 1 hour and 40 minutes at a true airspeed of 180 knots in his flight plan. He stated that he probably used 2,300 RPM for this flight; he could not recall manifold pressure. He further stated that to the best of his recollection, the main fuel tanks were selected for the entire flight, and that his fuel upon landing at Port Angeles was 110 gallons. He recorded a 1 hour and 40 minute flight for this leg in his pilot logbook, identical to his filed ETE. The pilot reported that the accident flight was to take off from Port Angeles, proceed west to Cape Flattery, back east to Port Townsend (30 nautical miles east of Port Angeles), then return to Port Angeles. The total distance of this flight as measured on the Seattle sectional aeronautical chart is approximately 163 nautical miles (52 nautical miles from Port Angeles to Cape Flattery, 81 nautical miles from Cape Flattery to Port Townsend, 30 nautical miles from Port Townsend to Port Angeles.) The pilot indicated that the accident flight departed Port Angeles at about 1040 PDT, that the airplane was below 3,000 feet above mean sea level (MSL) at all times on this flight, and that he cruised at 2,300 RPM and 22 inches of manifold pressure. The pilot stated the following regarding operation of the aircraft fuel system on the accident flight: "...I believe I [changed] the fuel selectors to the aux[iliary] tanks after reaching the intended cruising altitude and I also believe I changed them back to the main tanks before approaching the Port Townsend area, all of which would be consistent with my usual practice...." The pilot described the accident sequence in the narrative of his pilot accident report as follows: "After leaving Port Townsend on a westerly heading, a descent was made to below 1,000 feet over open water of the Strait of Juan de Fuca, on a heading to pass to the north of Protection Island. Sometime around 1130 PDT [50 minutes after the flight's reported takeoff time] the right engine developed a strong vibration (the cause unknown) and was shut down. Almost immediately thereafter the left engine developed some vibration and sufficient power loss that level flight on a single engine was not maintained. The aircraft was ditched in a controlled manner...one to two miles northwest of Protection Island." In follow-up questioning, the pilot stated that the left engine was still running at water entry. Based on estimated fuel and occupant loading, the aircraft gross weight at the time of the accident was approximately 4,600 pounds. Using this gross weight and the atmospheric conditions at the ditching site, the Cessna 310 owner's manual indicates a single-engine service ceiling of 8,450 feet and a single-engine climb rate of approximately 480 feet per minute at sea level. The pilot stated in follow-up questioning that he feathered the right propeller upon shutting down the right engine, and that he had not yet begun to configure for landing at Port Angeles. He did state, however, that after right engine shutdown he was uncomfortable with using full power on the left engine since full power made the vibration on that engine more severe. He was also unable to recall whether he attained the aircraft's best single- engine rate-of-climb speed (Vyse) between right engine shutdown and water impact. The airplane was retrofitted with Continental IO-520-E engines by Colemill, Inc. Data in the Cessna 310 owner's manual, along with Colemill-produced IO-520-E fuel consumption information and statements from the pilot, indicate that fuel required to fly the stated flight from Sandpoint to Port Angeles is approximately 42 gallons, and that the accident flight would require about 28 gallons to fly as planned, a total fuel requirement of approximately 70 gallons. Cessna's manufacture records indicate that the accident airplane was manufactured with a fuel capacity of 131 gallons consisting of two main tanks of 50 gallon capacity each, and two auxiliary tanks of 15.5 gallon capacity each. Cessna 310 fuel system design is such that during operation on auxiliary tanks, excess fuel supplied by the engine-driven fuel pumps, not needed by the engines, is returned to the main rather than auxiliary tanks. Using fuel pump flow rate data supplied by Teledyne Continental Motors, the estimated range from full to empty auxiliary tanks under cruise conditions outlined in the pilot's statements is approximately 144 nautical miles. Distance along the flight path from the pilot's reported level-off point (where he said he selected auxiliary tanks) to Port Townsend (by which point he believed he returned to main tank operation) is 129 nautical miles. Distance along the flight path from the reported level-off point to the ditching site is 138 nautical miles. The airplane had a total time of 5,008 hours at the time of the accident according to the pilot's report. The airplane's last annual inspection was on October 1, 1994, 7 1/2 months and 60 flight hours before the accident. Further examination of the airplane's maintenance history revealed that anti-corrosion work had been required on the airplane before being certified as airworthy on the last annual inspection. According to aircraft logs and work orders, this work included: replacement of the left outboard main gear door and hinge; replacement of hinges on the right inboard and left outboard flaps; replacement of "many" missing rivets on both left and right engine augmentor tubes; and cleaning and treatment of various corrosion spots on the left elevator and around the wing root fairings on the belly. The right rear lower inboard spar cap and two areas of aft lower wing skin were also replaced on October 17, 1994, due to corrosion identified on these components in an area behind the inboard exhaust. The passengers on the accident flight were the pilot's wife and daughter. Repeated attempts to obtain information regarding the accident from them were unsuccessful.
FAILURE OF AN UNDETERMINED AIRCRAFT COMPONENT, WHICH RESULTED IN A SEVERE VIBRATION, AND SUBSEQUENT DITCHING AT SEA. A FACTOR RELATING TO THE ACCIDENT WAS: THE PILOT'S LOW CRUISING ALTITUDE (OVER WATER) OF LESS THAN 1,000 FEET, WHICH LIMITED HIS ABILITY TO SUCCESSFULLY RECOVER FROM THE SITUATION.
Source: NTSB Aviation Accident Database
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