N96145Cessna 172Q · 1983

On January 16, 2020, about 2000 mountain standard time, a Cessna 172Q airplane, N96145, was substantially damaged when it was involved in an accident near Big Timber, Montana. The instructor and student pilot were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.

According to the flight instructor, after an uneventful flight, the student pilot configured the airplane to land on runway 24 at Big Timber Airport (6S0), Big Timber, Montana. While on approach, the student pilot wanted to perform a go around because the airplane was too high for a normal approach to landing, but the instructor had the student perform an approach over the runway instead. The instructor noted that his student’s initial climb actions were consistent with the airplane checklist. The student pilot added that the attempted climbout was in dark night conditions. They leveled the airplane off at 200 ft agl and flew the full length of the runway before continuing the climb at the departure end of the runway at an airspeed of 80 knots (kts).

As the airplane passed about 250 ft agl in a climb to their desired altitude of about 500 ft agl, the airplane lost power, made a vibration-induced sound, and experienced a slow drop in airspeed from the initial climb speed of 80 kts. In a subsequent statement, the instructor reported that the engine lost all power after the airplane established a climb; the student pilot could not recall if the engine lost all power. As the airplane slowed to 70 kts, the instructor took the controls from the student and began a left turn towards the airport. According to the student pilot, the engine began to make a sound that resembled an engine knock after the instructor started the left turn, but he did not observe any vibrations or engine roughness. The instructor verified the throttle control was at full power and noted that the noise was continuous.

According to the instructor, the tachometer indicated 2,100 rpm when the airplane reached 65 kts, which was 200 rpm less than what the instructor expected during climbout. According to the student pilot, the instructor appeared to have difficulty capturing and maintaining the selected airspeed of 65 kts. The instructor pitched the airplane for an airspeed of 50 kts and made a right turn moments before the airplane impacted the ground.

According to a diagram furnished by the instructor, the airplane impacted the ground about 425 yards northwest of runway 24 and slid about 125 yards before it came to rest. The instructor located an approximate 1-ft-long section of blade in the debris path. The propeller blade tip was not recovered. The airplane flipped over inverted the following day due to weather.

Photographs of the accident site revealed substantial damage to the airplane’s fuselage.

Airframe and Engine Exam

Postaccident examination of the fuel system components, which included the fuel filter, gascolator, fuel lines, and fuel selector valve, revealed that they were free of obstructions, corrosion, or visible leaks. Examination of the carburetor did not reveal any anomalies. Continuity of the throttle, mixture, and carburetor heat cables were verified from the cockpit to the engine, and flight control authority was continuous from the cockpit to each flight control surface.

Rotational continuity of the crankshaft and valvetrain were observed as the propeller was rotated by hand. All rocker arms and push rods actuated in equal ranges of operation while the propeller was rotated. The magnetos functioned normally when actuated by hand and the ignition harness each produced spark as the magnetos were rotated; however, the no. 1 top and bottom leads did not display sparks that were consistent with the magneto click. The spark plugs each displayed varying amounts of wear and were placed in a spark plug tester for an impulse test. During the impulse test, the cylinder no. 1 bottom spark plug failed to display spark despite having been cleaned prior to testing; the other spark plugs produced sparks. A borescope inspection of the no. 1 cylinder revealed trace amounts of a liquid that resembled water; the other cylinders did not reveal any signs of catastrophic internal failure, heat damage, or scoring. The air induction system could not be properly evaluated because the heat box and air filter were not recovered after the accident.

The propeller impacted frozen ground during the accident and one of the two propeller blades separated in two pieces, but only one of the pieces was recovered. Together, the separated blade and recovered piece exhibited bending and tip curling.

Materials Laboratory Examination

Examination of the fractured blade and its recovered fragment by the National Transportation Safety Board Materials Laboratory revealed signatures consistent with overstress. Small cracks were observed at each bolt hole on the forward spinner bulkhead, and the cracks displayed fretting consistent with contact under vibration loads between the forward spinner face and the washers under the bolt heads. Additionally, the fracture surface of each crack exhibited ratchet marks, which were consistent with fatigue. The spinner itself was intact and did not exhibit any indication of distortion due to fatigue cracking. The Materials Laboratory’s examination of the other propeller blade revealed that it was intact and remained attached to the hub but was bent aft about midspan and displayed evidence of tip curling, chordwise scoring, and leading-edge gouges.