NTSB Factual Report

HISTORY OF FLIGHT

On April 18, 1993, at 0930 central daylight time (CDT), N8337L, a Cessna 172I, registered to Leo G. Stokes of Monticello, Minnesota, and piloted by a private pilot, was destroyed during a collision with the ground. Visual meteorological conditions prevailed at the time of the accident. The personal 14 CFR Part 91 flight was operating without a flight plan. The pilot and passenger were fatally injured. The flight originated from Monticello, Minnesota, at 0830 CDT.

A witness, driving an automobile northbound along highway U.S. Highway 53, observed the airplane descending vertically toward hilly terrain. The witness's observation of the airplane's flight path was momentarily obscured by the terrain. Shortly after losing visual contact with the airplane the witness observed black smoke.

A second witness stated he observed the airplane flying north followed by a turn to the south. He stated the airplane was flying straight and level after the turn's completion. Shortly after completing the turn, he said, the airplane suddenly pointed straight down with its engine making a louder noise. He said he observed grey smoke coming from the airplane after it started the vertical descent. The smoke, according to this witness, was continuous but not heavy. The witness was unable to state where the smoke was originating from.

A third witness reported hearing the "... noise of an engine. It was running wild it wasn't in control at all." The witness stated the airplane "...was intact..." when he first observed it. He stated the airplane burst into flames with "... pieces (that) flew everywhere. Then the plane hit the ground. Once it hit the ground I didn't see anything." He said the wings separated from the fuselage during the explosion.

A fourth witness, located approximately one mile north of the crash site, observed an airplane pass over him in level flight. He said the airplane's engine sounded normal. Shortly after observing the airplane he looked away, but said he heard an increase in RPM, followed by a loud bang.

PERSONNEL INFORMATION

The pilot obtained his private pilot's certificate on July 14, 1989. A third class medical certificate was issued to the pilot on January 1, 1993. His personal logbook showed a total time of 154.6 hours as of April 4, 1993. According to the logbook, his first flight in a Cessna 172 was on July 22, 1989. The logbook showed the pilot had flown the Cessna 172 type airplane exclusively since that date with one exception. The logbook showed the pilot had .6 of an hour of hooded instrument time. The last instrument time (.2 of an hour) flown by the pilot was on June 23, 1991. The logbook showed the pilot received a biennial flight review, on September 28, 1991.

WRECKAGE AND IMPACT INFORMATION

N8337l impacted a slopping, mowed, hay field. The soil was a mixture of sand and clay, recently moistened by rain. A ground scar approximately 21 feet long, varying in depth from three to five inches, aligned on a magnetic heading of 050 degrees, was observed with small fragments of green glass at its beginning. The ground scar lead to a tapered crater area which was approximately ten feet in diameter at the top and three feet deep.

The dirt on the sides of the crater had various sized pieces of burnt carpeting, green colored plexiglass, and insulation on, and below, the surface. The materials located below the side surfaces of the crater were at a depth of approximately one to three inches. The deepest part of the crater contained irregular shaped fragments of green plexiglass approximately 1/2 inch to one inch in perimeter size. Fragments of green plexiglass and tan colored plastic were observed interspersed with pieces of aluminum around the edge of the crater.

The airplane's engine and propeller were positioned approximately 31 feet from the crater on a magnetic heading of 075 degrees. The fuselage cabin area, tail cone, and empennage were located approximately 55 feet from the crater on a magnetic heading of 075 degrees. A section of N8337L's right wing was positioned approximately 34 feet from the crater on a magnetic heading of 095 degrees. The right wing strut was positioned 80 feet from the crater on a magnetic heading of 095 degrees. The left wing and the inboard section of the right wing were located 160 feet and 232 feet, respectively, from the crater on a magnetic heading of 095 degrees.

The fuselage was crushed and burnt from the firewall attach point aft to the leading edge of the dorsal fin. The fuselage and empennage assembly from the vertical stabilizer's leading edge was bent upward approximately 60 degrees relative to the fuselage centerline. The left and right cabin entrance doors were found adjacent to the impact crater. Both doors had their forward edges crushed aft. The right door was crushed further aft than the left.

The upper section of the vertical stabilizer was crushed aft. The vertical stabilizer and rudder were bent approximately 30 degrees to left of the airplane's vertical plane. The lower hinge point of the rudder had separated from its fuselage mount. The horizontal stabilizer's left and right leading edges were bent down from the tip to approximately two feet inboard.

The right wing was fragmented, its components were confined to an area east of the impact crater and south of the fuselage/empennage remains. The wing root attach point had fire damage forward of the mid-chord point and around the wing fuel tank filler opening. The fuel cap was not found. The main wing spar, from the approximate wing tie down ring attach point to the tip, displayed aft bending of approximately 75 degrees. The wing skin sections had compression damage of varying magnitudes. The skin sections which were separated from their original positions displayed tension separations at the majority of their rivet holes. The wing strut was bent down approximately 80 degrees away from its central axis at the approximate 70 percent span position.

The left wing was separated from the fuselage at the root junction. The bottom of the wing had fire damage on the outer surface. The sooting extended from the root to an area behind the landing light. The skin on the bottom of the wing, below the fuel tank position, had separated from the main structure. The wing's leading edge was collapsed aft from the root attach point to approximately five feet outboard. The wing flap was in the full down position, the aileron was not attached. The wing's top surface, outboard of the wing flap/aileron juncture was wrinkled. The leading edge was collapsed aft.

The engine had separated from its fuselage at all four mount positions. The rockerbox covers were collapsed against the rocker arms. The exhaust and intake manifold systems had separated from the engine and were crushed. The voltage regulator did not have exterior, or interior, damage. The engine was rotated, the rocker arms for each cylinder moved with the propeller rotation. The cam shaft gear, at the aft end of the engine, moved during the rotation process. The top spark plugs to cylinder numbers one and three were removed and had a gray/tan color on the electrodes. Electrode contamination was not observed. The magnetos were separated from the engine and were destroyed. The vacuum pump's vanes were destroyed, the shaft was bent which prevented rotation.

Chordwise scaring was observed on both propeller blade front surfaces. One propeller blade was bent aft approximately 12 inches outboard from the hub. This blade had a 90 degree twist along its span. The second propeller blade was bent aft approximately 45 degrees at the mid-span point. This blade had a 90 degree twist along its span. The propeller spacer had separated into two sections and was dirt filled.

Trim tab and landing flap cable separations had broomed ends. Elevator control continuity was established between the elevator bellcrank assembly and control "U". The control "U" had crush damage and had been tension separated at four different positions. The control yoke sprocket shaft was separated at the universal joint between it and the yoke at both positions on the control "U". The forward elevator bellcrank and elevator push-pull rod were crushed and had separated from their mounting position. The elevator control cables were attached to the forward and rear bellcrank assemblies. Approximately 50 percent of the components which made up the control "U" and forward elevator control assembly were located in the burned out portion of the airplane. The Cessna 172 part manual illustration of this assembly is appended to this report.

Eight pieces of the windshield were found around the impact crater, as well as east-southeast of the crater. The remainder of the windshield was not found. Pieces of the side and rear windows were also found east- southeast of the impact crater. The windshield and window pieces were identified by measuring their thickness. Thicknesses were provided by Cessna Aircraft Company.

The windshield plexiglass pieces were matched to another Cessna 172 windshield. Their positions were along the door post and cowl combing forward of the instrument panel glare shield. See photographs for actual positions.

The felt seal which fits between the top rear, section of the windshield and the top fuselage metal skin (referred to by Cessna Aircraft as a "spar cap")was not found. The spar cap and structural carrythru had fire and abrasion damage on the section which would face into the cockpit. The forward end of the spar cap was bent upward and aft where the windshield top would normally be positioned. Illustrations of this section are appended to this report.

A smoke and fire damaged headliner was separated from the cabin carrythru. The forward end of the headliner is normally attached via triangular tangs which are pressed outward from a strip of aluminum that is attached to the carrythru. A section of headliner had tears at one end. The tears matched the spacing of the tangs previously identified. The headliner was oriented onto the fuselage cabin top utilizing the metal retaining rods.

The inside of the fuselage's aft top section did not have fire damage. The headliner and plastic trim for the interior lighting and vents had separated from this section. Examination of the headliner, lighting assembly, and vents revealed tearing and instantaneous separation. A plastic strip which covers the headliner access zipper was deformed and had fire damage on a section of it. A section of headliner fabric was found wedged between the strip and the zipper area. The end of the fabric which is from the forward end of the cockpit was found under the strip. This piece of fabric did not have fire damage.

MEDICAL AND PATHOLOGICAL INFORMATION

The autopsy for the pilot and passenger states that their deaths were due to an "...aircraft crash." The autopsy reports for both occupants stated that their air passages did not have soot in them. Due to the general disintegration of the airplane occupants toxicological samples were not available. Samples of lung tissue were tested for toluene by the Federal Aviation Administration's Civil Aeromedical Institute with negative results.

FIRE

The post impact fire involved the left wing and fuselage-- engine compartment to the dorsal fin leading edge. The fire extended from the impact crater's east edge to a point approximately 400 feet east. Hay stubble, located in the area of the crash, was burnt. A portion of the dried vegetation under the left wing was not burnt. The majority of the fire was concentrated around the fuselage/empennage assembly.

ADDITIONAL DATA/INFORMATION

A partially burnt can of Alcor TCP fuel treatment was found amongst the main wreckage. The can's lower half was crushed upward. A 3/8 inch outside diameter plastic tube was found in the can, one end of the tube was melted. The can did not have outwardly distended sides. The can's threaded spout had a crease in it which did not match the crease observed in the lid. The can's interior had not been consumed by fire as much as the outside. The owner of the airplane stated he used this material in N8337L's engine to prevent sparkplug lead fouling.

The Alcor Company of San Antonio, Texas, was contacted regarding the aromatic qualities and volatility of the TCP substance. A company representative stated the product had a low volatility and had a noticeable scent. According to the material safety sheet provided by the company, the TCP product composition is approximately 81 percent toluene and 19 percent tricresyl phosphate. It has a vapor pressure of approximately 36mm. Toluene has a very noticeable scent. The box for the can, and the can containing Alcor's TCP product, does not contain a warning that the product should not be carried in an airplane. However, a copy of a warning label forwarded by the Alcor company states, "NOT RECOMMENDED TO BE CARRIED ABOARD AIRCRAFT." The company stated the copy of the label was what appeared on their cans of Alcor TCP. Label copies from the sample can and its box, are appended to this report.

A Material Safety Data Sheet from the United States Testing Company, Incorporated (USTC), showed that toluene vapors can flow along surfaces to distant ignition sources and flashback. Additionally, the flash point is stated as 40 degrees fahrenheit F. (closed cup). Above that temperature, according to the information, vapor-air mixtures are explosive. Toluene has a lower explosion limit of 1.2 percent atmospheric saturation (see Alcor's Material Safety Data Sheet appended to this report). The 1.2 figure, according to the USTC representative, shows that it is a highly explosive substance.

The Material Safety Data Sheet received from Mallinckrodt Specialty Chemicals Company of Paris, Kentucky, states, in part, "Toxic gases (e.g., carbon dioxide and monoxide) may be released in a fire involving toluene." During an interview with a company representative it was revealed that the fumes from a toluene based fluid such as TCP could explode during the lighting of a cigarette. She stated this occurred because the chemical's vapor has a low flash point. According to the representative, a spark caused by static electricity could ignite the vapors.

Tricresyl phosphate, the second ingredient in TCP, is an odorless liquid. The chemical has a 0.1mm vapor pressure. Its flash point is 437 degree F., its autoignition is 770 degrees F. According to information received from Fisher Scientific of Itasca, Illinois, tricresyl phosphate "May burn but does not ignite readily."

According to the owner of the airplane, the passenger in the airplane was a smoker. The pilot, according to the airplane's owner, would smoke on occasion. The wife of the pilot stated he was a non-smoker and did not allow smoking in his presence whenever he could. The owner of N8337L stated he kept a can of Alcor TCP in a cardboard box located in its baggage compartment.

The wreckage was released to Mr. Michael Colgan, Vice President of Operations for Colgan Aviation, LaCrosse, Wisconsin.

NTSB Probable Cause

AN INFLIGHT EXPLOSION OF A TOLUENE-BASED FLUID INSIDE THE AIRPLANE'S COCKPIT.