CHI96FA290

The pilot had just completed a fly-by down the runway, when the engine had a partial loss of power. Subsequently, the airplane entered a spin and impacted the ground in a steep descent. Control continuity was established. Airframe and engine inspection did not reveal any discrepancies. A witness reported that the airplane was flown down the runway at 90 to 100 mph, at a low power setting, followed by a pull up at the end of the runaway. Bendix Aircraft Carburetor Service Bulletin Number 74 reported that this type of maneuvering, along with an abrupt throttle movement may cause the engine to lose power or quit. The accident airplane's carburetor was not equipped with an accelerator pump. The temperature and dew point were reported to be 77 and 49 degrees, respectively. According to icing probability charts, conditions were conducive to carburetor icing. The carburetor heat control knob was found in the 'off' position.

History of flight

On August 9, 1996, at 1722 eastern daylight time (edt), an Aeronca Chief, N85825, was destroyed, after the airplane departed controlled flight, and collided with terrain near Charlotte, Michigan. The airplane had just completed a fly by down the runway at the Wend Airport, before the accident occurred. Witnesses reported that the engine seemed to have a loss of power, following the fly by. Both pilots on board the airplane were fatally injured. The 14 CFR Part 91 flight was operating in visual meteorological conditions, and no flight plan had been filed.

Wreckage and impact information

The airplane crashed in a wooded area. The damage to the fuselage area behind the engine mount, and to the wing leading edges was consistent with a near vertical impact. A portion of the airplane's propeller was found approximately 100 feet from the main wreckage. Before the airplane was moved control continuity was established by the Federal Aviation Administration. After control continuity was established and the wreckage area was document with photographs the airplane was moved to a aircraft salvage area in Charlotte, Michigan.

Medical and pathological information

Autopsies were performed on both pilots at the Sparrow Hospital, in Lansing, Michigan, on August 10, 1996. Toxicological testing was performed on both pilots by the Federal Aviation Administration in Oklahoma City, Oklahoma. The toxicological testing performed on the right seat pilot was negative for all tests conducted. The toxicological testing performed on the left seat pilot detected pseudoephedrine.

Tests and research

The airframe and engine were examined by the investigator in charge (IIC) and a representative from the engine manufacturer on September 19, 1996. This second review of the flight control system did not reveal any discrepancies. Documentation of the cockpit revealed the following. The magneto switch was consistent with the both position as found when tested with a volt ohm meter. The fuel valve push, pull knob was bent over, and was consistent with the control knob being aft during the impact. The engine area where the fuel valve was mounted was approximately one foot aft of its original position. The carburetor heat control knob was in, the throttle lever was not located.

The fuel transfer valve was found in the off position. Scratch marks were visible behind the fuel handle. The placard next to the transfer valve stipulated that the valve was to be in the on position only during level flight and descent.

No discrepancies were noted during the inspection of the engine. Both magneto's were damaged however, both produced spark when rotated. The spark plugs all appeared slightly sooted, with electrodes which appeared almost new. The spark plug ignition harness was damaged, and appeared almost new. No significant metal chips were found when the oil screen was inspected. All engine cylinders had compression when the crankshaft was rotated.

All intake and exhaust valves opened and closed during the rotation of the engine's crankshaft.

No foreign debris was found inside the carburetor. The carburetor float did not sink, or leak when tested in water. The finger screen in the carburetor was clean, and the needle seat appeared to be in good condition. The main jet for the carburetor was free from debris, and flowed freely when tested. Almost all carburetor parts appeared to have very few hours of time in service. This carburetor was not equipped with a accelerator pump.

The carburetor body, needle seat, needle and float were retained by the IIC for further testing. The overhaul specifications for the float level recommended an initial float level of 13/32 of an inch. When the carburetor was filled with fuel following the accident the float level measured approximately 3/4 of an inch. The float needle used in this carburetor was a steel float needle with a tip made of a rubber like material. The needle seat used in this carburetor was a sharp edged needle seat. Bendix Aircraft Carburetor Service Bulletin No. 71 states "DO NOT USE A SYNTHETIC-RUBBER TIPPED NEEDLE IN A SEAT HAVING A SHARP EDGE AT THE SEATING SURFACE. Use of the rounded seat with the rubber tipped needle will eliminate any possibility of the needle sticking in the seat or being cut by the seat."

The IIC allowed an employee of Aircraft Systems Inc., of Rockford, Illinois to inspect the carburetor from the accident aircraft. The employee measured the float needle travel at .052 inches of travel, the minimum travel was listed as .048 inches in the overhaul specifications.

When the employee inspected the float needle he believed that the needle was not built by the carburetor's manufacturer. The needle from the accident airplane had a stamped part number on it. When the employee showed the IIC a Bendix needle, the part number was hand etched in the needle. The surface finish of the Bendix needle was smoother than the accident airplane's needle. The needle tip on the Bendix needle seemed harder than the tip from the needle of the accident airplane. The needle tip on the Bendix needle seemed to have a different taper than the tip from the needle of the accident airplane.

The employee also stated that the needle tip from the accident aircraft may swell when placed in fuel. When the needle from the accident aircraft was tested in 100LL fuel for approximately 18 hours, no swelling was noted. The employee also gave the IIC a copy of a publication, which addressed the problems with float needle valves in Bendix carburetors. This publication is included as a supplement to this report.

The original carburetor parts retained by the IIC, and the additional parts required to make a complete carburetor were test run on a similar engine at Teledyne Continental Motors, in Mobile, Alabama on March 6, 1997. When the fuel line was attached to the carburetor the carburetor leaked a continuous stream of gasoline. During the actual engine runs the engine operated normally from idle RPM to the maximum RPM attained, which was approximately 2150 RPM. When rapid throttle advances from idle to maximum power were conducted the engine operated normally.

Bendix Aviation Corporation issued Aircraft Carburetor Service Bulletin No. 74 on February 24, 1952. This bulletin was issued " TO provide information designed to reduce the possibility of carburetor trouble, or apparent fuel metering inconsistencies, which could result in engine stoppage during flight idling operations." This bulletin which is included as a supplement to this report discusses the operational characteristics of this carburetor when propeller windmilling conditions, and rapid throttle conditions occur.

Additional information

The manager of the Wend Airport was interviewed by the Eaton County Sheriff's Department, on August 9, 1996. The manager reported that he estimated the airspeed of the fly by at 90 to 100 miles per hour. The manager also reported that "The engine was running, but it was not running at full power. I couldn't ... I was trying to imagine why he wasn't adding power. I understand him having the power cut back as he's coming down the runway and because he's gaining speed coming down as he's going down the runway, and then as he pulled up he still had that speed, but I believe ... He didn't have... He wasn't developing full horsepower." The manager also reported that the airplane entered a spin, after the loss of power.

The IIC contacted the owner of Safe Air Repair on October 3, 1996. The owner reported to the IIC that he had talked to the left seat pilot of the accident airplane the morning of the accident. The owner said that the left seat pilot reported the engine was not running correctly, and that the engine would begin to miss at higher power settings. The owner also reported that he had a new fuel line on order for the accident airc