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On September 23, 1996, at 1700 Alaska daylight time, a float equipped Cessna 206G airplane, N7312C, registered to Stearns Air Alaska of Anchorage, Alaska, crashed during initial climb after takeoff from the Lake Hood Seaplane Base, Anchorage. The on demand air taxi flight, operating under 14 CFR Part 135, was departing Lake Hood for a sightseeing flight to Mt. McKinley. A company visual flight rules flight plan was in effect and visual meteorological conditions prevailed. The certificated commercial pilot and two passengers were fatally injured and two passengers were seriously injured. The airplane was destroyed by post impact fire.
During an interview with the NTSB investigator-in-charge, Anchorage Airport Safety Officer Marty Spinde, stated he watched the airplane takeoff. The takeoff run was normal and the airplane lifted off with, "plenty of room to spare." The airplane began to climb, and the officer stated that when the "pilot cleaned up the airplane" the airplane began to descend. He stated he could see the flaps retract.
According to a witness who watched the entire accident sequence, he stated that the engine was running, "OK", but the airplane's nose had a high angle of attack. The right float struck a high tension power line, flipped inverted, and crashed to the ground on Northern Lights Boulevard.
Another witness, Lt. Tibor of the Anchorage Airport Police, stated the engine was running normally at a high RPM. The airplane acted like it was overloaded and had a high "angle of attack." He saw a float strike the telephone pole wire support, flip upside down and crash onto Northern Lights Boulevard.
Mr. Roland Kenneson witnessed the airplane during the initial climb. He stated the airplane cleared a 60 foot high dirt pile located in the state maintenance yard. He stated the flaps were extended to 20 degrees and he saw the flaps retract to 10 degrees. The airplane then began to settle. He said the engine sounded "doggy" as if the propeller was in the cruise RPM setting. The engine sounded steady and he stated that it sounded like the airplane was heavy with an aft center of gravity. He estimated the highest altitude the airplane reached was approximately 100 feet above the ground, just over the tree tops. The airplane's speed seemed slow. He observed the airplane flying wings level with the nose of the airplane high.
INJURIES TO PERSONS
The pilot and two passengers were fatally injured and subjected to fire. Two passengers received serious thermal injuries.
DAMAGE TO AIRCRAFT
The airplane cockpit and cabin were involved in a post impact fire. The engine separated from the engine compartment and was found resting under the root of the right wing. The entire wreckage was resting inverted. One float was damaged by fire and the other float separated and was not damaged. The fire consumed the engine compartment, the instrument panel, cockpit, cabin, and portions of the empennage, part of the right float and associated struts.
The airplane struck a cross member on a telephone pole which supported two high tension power lines. One of the power lines was severed.
According to Federal Aviation Administration records and the Company records, the pilot had a total flight time of 5,000 hours. His last application for a medical certificate, dated 2/29/96, shows he had flown 450 hours in the previous six months. A review of the pilot records for the months of August and September show a monthly total flight time of 125.6 hours and 76.7 hours respectively.
The direction of Northern Lights Boulevard aligns with 256/076 degrees. The airplane struck a high tension power line utility pole, which was located on the south side of the road, and struck the road 105 feet past the utility pole on a magnetic bearing of 312 degrees. The float of the airplane struck the upper cross member on the utility pole and severed one power line. The other power line separated from the cross member and sagged down.
The nose of the airplane aligned with 120 degrees. The ditch in which the wreckage came to rest was approximately 4 to 5 feet below the surface of the road.
The calculated height of the utility pole, above the road's surface, was determined to be 53.5 feet. The utility pole was erected off the side of the road in soft terrain that was below the surface of the road.
Flight control continuity was established between the flight control surfaces and the cockpit yoke mechanism. The engine controls were not attached to the engine. The flaps were in the up position but were damaged by fire and impact.
The left float, which separated from the main wreckage and did not burn, had rolled upside down and the sealing balls in the float compartments were missing. The float was rolled right side up and each of the compartments was pumped out. No water was found.
The propeller remained attached to the engine but each of the blades was broken inside the hub assembly.
The fuel injector lines were checked prior to engine removal and all were found to be connected. The throttle cable linkage was found disconnected from the throttle arm at the accident site, although the castellated nut was in place on the end of the bolt which attaches it to the throttle arm. The castellated nut was fastened with a locking cotter pin.
MEDICAL AND PATHOLOGICAL INFORMATION
The autopsy report shows that the "anterior descending branch of the left coronary artery had extensive and severe arteriosclerotic change which virtually completely occluded the lumen approximately 0.4 cm from its bifurcation from the circumflex."
A statement submitted by Anne M. Stearns revealed that on September 20, 1996, the pilot had helped her and her husband, the owner of the operation, push back an airplane. The pilot complained the next morning of experiencing severe chest pains and he thought the pains were related to the activities of the previous evening which consisted of pushing back the airplane.
Unspecified levels of drugs, typically associated with over the counter cold medications, were discovered in toxicological samples taken from the pilot. See attached toxicological report.
TESTS AND RESEARCH
The engine was removed from the accident and externally examined at Sea Air Inc., under the supervision of the NTSB IIC. The left and right magnetos were removed and bench checked. They checked normally. All had blue and even sparks.
The spider and fuel injector line assembly was removed and flow checked. The flow streams were even/equal and straight. The fuel screens were clean.
The flap jack screw was measured and found to have 1 inch of threads showing. According to Cessna Aircraft Company, this equates to having 5 degrees of flap extended.
The elevator trim screw was located and it showed 1.5 inches of threads. Cessna Aircraft stated that this equates to a neutral trim position.
The rest of the engine was shipped to Teledyne Continental Motors in Mobile, Alabama. The engine was disassembled under the supervision of the NTSB IIC, and no mechanical anomalies with the engine, other than the throttle arm, were discovered.
The throttle arm assembly connected to the throttle plate was removed and examined by a metallurgist at Continental. The brass arm, Continantal part number 632555-23, had a section of metal missing from around the throttle cable mounting hole. The missing metal was sufficiently large enough to allow the passage of the throttle cable linkage attachment bolt. The brass around the hole in the throttle arm was necked down and was thinner than the main brass section of the throttle arm. The steel bushing remained attached to the throttle cable and the steel bushing would not spin on the mounting assembly. According to a representative of Teledyne Continental Motors (TCM), if the throttle cable linkage becomes disconnected from the throttle arm, the pilot has no method by which to control the power output of the engine, and the loss of the linkage may result in no change to the engine power, a partial reduction, or a reduction to idle speed, power setting. He said there is no "fail-safe" mechanism to automatically increase the throttle setting to maximum if the cable fails or becomes disconnected. He also said he is aware of the throttle arm having worn through in the past, and that he believes this is because the linkage retaining bolts are fastened with excessive torque by maintenance personnel after an engine change, or other work, which requires the disassembly of the throttle linkage.
TCM has issued Service Bulletin SB95-2, dated April 21, 1995, which addresses the inspection and maintenance of engine control cables and linkage. Cessna Aircraft Corporation also addresses the maintenance and inspection procedures of movable parts and linkages in their Cessna 206 Maintenance Manual (excerpts attached).
Further examination of the throttle arm showed that the steel bushing, which is pressed into the throttle arm mounting hole and is designed to prevent wear on the brass portion of the throttle arm, was a different thickness than the throttle arm. This throttle arm shaft (bolt), which is mounted through the steel bushing and onto the brass arm, was tightened sufficiently to compress the brass throttle arm to the same thickness as the steel bushing insert. The throttle arm, and its associated components, were submitted to the NTSB Metallurgical Laboratory in Washington, D.C., for analysis. A copy of the metallurgical report is attached.
A review of Service Difficulty Reports (SDR) submitted to the FAA regarding problems with the throttle arm, disclosed 14 reports in the preceding 10 years of excessively worn, or worn through, throttle arms. In at least six of these events which precipitated the issuance of an SDR, the submitter noted the engine lost power, and the pilot made an emergency landing. One SDR contains the comment: "Steel bushing has worn away about 90 percent of the throttle arm. The bolt was correctly installed and properly torque[d]. This area has been subject to close inspection at each 100 hour inspection since earlier reports of similar problems (Cessna SEB 95-15), A.C. 43-16. Submitter suggests this damage progressed rapidly since the last 100 hour inspection."
Discussions between NTSB investigators and aviation engine/accessory overhaul shop personnel, revealed that it is "common" for them to see throttle arms attached to the Continental IO-520 engine which are severely worn at the linkage attachment hole, and occasionally, those which have worn completely through.
According to the Operator, the pilot had sufficient fuel on board the airplane to fly from Lake Hood to Mt. McKinley and return, plus a 30 minute reserve. The straight line distance between Lake Hood and the Kahiltna Glacier near Mt. McKinley is 120 nautical miles. According to the Cessna 206 pilot information manual the average cruise speed at 75% power is 130 knots. The manual also states the engine consumes 16 gallons of fuel per hour. The owner did not see the pilot fuel the airplane and there is no fuel slip for the refueling. The fuel could not be measured due to the destruction of the airplane. The following information has been derived from the operator, airplane manuals, medical records, and personal interviews.
Airplane speed = 130 knots Absolute minimum distance to fly, round trip------- = 240 Derived flight time----------------------------------------- = 1 hour and 50 minutes Fuel reserve required---------------------------------------= 30 minutes Absolute minimum total fuel required-------------------= 2 hours and 20 minutes Fuel consumption-------------------------------------------= 16 gallons per hour Minimum fuel required------------------------------------= 37.5 gallons Weight of fuel per gallon----------------------------------= 6 pounds per gallon Minimum total fuel weight--------------------------------= 225 pounds
Airplane empty weight---- =2292.7 Fuel weight------------------= 225 pounds pilot weight------------------= 185 pounds passenger 1-------------------= 150 pounds sub total = 1175 passenger 2-------------------= 230 pounds passenger 3-------------------= 175 pounds passenger 4 ------------------= 210 pounds
Total weight----------------- = 3467.7 pounds Certificated Gross Weight = 3600 pounds