WPR18LA002

The private pilot departed for a long cross-country flight with the mixture leaned to the high field elevation. He then further leaned the mixture for lean-of-peak exhaust gas temperature as he climbed the airplane to 10,500 ft mean sea level (msl) for improved fuel efficiency. Thirty minutes into the flight, he descended the airplane to about 8,000 ft msl. The pilot did not enrich the mixture during the approximate 2,500-ft descent. Shortly after, the engine sputtered, followed by a loss of about 700 rpm. He verified the position of the throttle and enriched the fuel/air mixture with one full rotation of the control knob, but he never advanced the knob to the "full rich" position. The pilot then decided to perform a precautionary landing because he was concerned the airplane would not have the power to cross an approaching mountain range. He chose a dry river bed despite the availability of two nearby airports with dirt runways. During the landing, the airplane nosed over and came to rest inverted, which resulted in substantial damage to the rudder. Postaccident examination of the airframe and engine did not reveal any evidence of preimpact mechanical anomalies that would have precluded normal operation, and the engine achieved full and continuous power during a test run. The Pilot's Operating Handbook (POH) required a mixture adjustment during the descent for smooth operation, which the pilot did not do during the 2,500-ft descent. An engine manufacturer representative reported that the engine may suffer leaning misfires if the fuel/air mixture is not enriched during a descent. Therefore, it is likely that the engine sputtered and lost partial power due to the pilot's failure to properly enrich the fuel/air mixture during the descent in accordance with the POH. The pilot could have remediated this condition if he had advanced the mixture to the "full rich" position in accordance with the POH emergency procedures; however, instead the pilot made a minor mixture adjustment, which did not restore engine power.

Pilot's Operating Handbook (POH) The emergency procedures section of the POH includes the following guidance in the event of an engine failure in flight: Airspeed: 68 kts Fuel shutoff valve: ON Fuel selector valve: BOTH Auxiliary fuel pump switch: ON Mixture: RICH Ignition switch: BOTH The normal procedures section of the POH includes the following guidance for an inflight descent: Power – As desired Mixture – Adjust for smooth operation Altimeter – Set NAV/GPS switch – Set Fuel selector valve – BOTH Wing flaps – As desired Fuel/Air Mixture Adjustments A representative of the engine manufacturer reported that the engine may suffer leaning misfires if the fuel/air mixture is not enrichened during a descent. According to FAA records, the airplane was manufactured in 2002 and registered to L-Bird LLC on June 23, 2016. The airplane was powered by a Lycoming IO-360-L2A, fuel injected, 180 horsepower engine. Maintenance records indicated that the airplane's most recent 100 hour inspection was completed on August 1, 2017 at a total time of 1,770.4 flight hours. The engine's most recent 100 hour inspection was completed at the same time. According to the airplane's tachometer, at the time of the accident, the engine had accrued 1,810 hours of total time in service. On October 2, 2017, about 1230 Pacific daylight time, a Cessna 172S airplane, N5311H, was substantially damaged when it impacted a dry river bed near Tonopah, Nevada. The private pilot was not injured. The airplane was registered to L-Bird, LLC and operated by Tactical Aeronautics (a.k.a. TacAero) under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight departed from Winnemucca Municipal Airport (WMC), Winnemucca, Nevada at 0900, and was destined for St. George, Utah. According to the pilot, he refueled the accident airplane to capacity with 100 low-lead aviation grade gasoline, and then departed WMC in a flight with three other airplanes. The flight was uneventful until the engine began to "sputter" as the flight neared a large dry river bed in eastern Nevada. Almost instantaneously, the engine rpm decreased from 2,200 rpm to 1,500 rpm. At the time of the event, the pilot had BOTH fuel tanks selected on the fuel selector, and both magnetos were engaged. He enrichened the fuel/air mixture by turning the control one full rotation, but at no point did he advance the mixture control to the FULL RICH setting. The pilot then verified the throttle was in the FULL OPEN position and configured the airplane for a precautionary landing as he was concerned the airplane would not have the power to cross an approaching mountain range. He then completed a 180o turn over the riverbed to approach his selected landing zone with a headwind. After the airplane touched down, the pilot applied back pressure to the yoke to keep the nose landing gear from touching down for as long as possible. Oncethe nose gear settled down, the airplane stopped abruptly, nosed over and came to rest inverted. The airplane sustained substantial damage to the rudder during the nose-over. The debris field was composed of about 200 ft of main landing gear marks and a nose landing gear impact mark where the airplane came to rest. The pilot stated that he normally flies with the fuel/air mixture set to lean-of-peak exhaust gas temperature, and on the day of the accident, he leaned the mixture prior to departing from WMC, which had a field elevation of 4,308 ft mean sea level (msl) and then further leaned the mixture in flight as he climbed the airplane. His altitude varied throughout the flight and the partial loss of power occurred shortly after he descended from 10,500 ft msl to about 8,000 ft msl. The pilot, who was the Director of Maintenance for the airplane operator, stated that the loss of rpm may have been due to the presence of carbon in the spark plugs. He added that he regularly removes carbon from their fleet airplane's spark plugs as the students improperly lean the fuel/air mixture during flight operations, and the airplanes often fail magneto checks during their engine run-ups. The 1253 recorded weather observation at Yelland Field, located about 60 nm northeast of the accident site, included wind from 250° at 5 knots, visibility 10 statute miles, few clouds at 8,000 feet, temperature 08° C, dew point -11° C, and an altimeter setting of 29.91 inches of mercury. The pilot, age 54, held a private pilot certificate with a rating for single-engine land. The pilot's most recent second-class medical certificate was issued on August 15, 2016, which included the limitation, "must have available glasses for near vision." According to the pilot, his flight time included 145 hours of total flight time in all aircraft of which 9 hours had been accumulated in the accident airplane make and model. The airplane came to rest inverted in a dry river bed located about 13 nm southwest of Currant Ranch Airport (9U7), Currant, Nevada, and 20nm south of Duckwater Airport (01U), Duckwater, Nevada. A postaccident examination of the airplane and engine was completed on January 9, 2018, by a representative of Textron Aviation under the supervision of the National Transportation Safety Board Investigator-in-Charge. The engine mounts were unremarkable, and the engine case was intact. Loose sand and debris from the accident site were found on the battery case and engine case. Continuity of the mixture and throttle controls were confirmed from the cockpit to their respective arms on the fuel control unit, and both controls could be operated unencumbered from with the cabin. The air filter was removed from the air induction system and did not exhibit any contamination or debris. Each of the engine's four Champion 38S series fine wire top spark plugs were gray in appearance with some carbon deposits, consistent with normal wear when compared to the Champion Aerospace AviationService Manual. A thumb compression test revealed intake and exhaust pressure for each cylinder. Each ignition harness lead displayed normal wear, with the exception of the cylinder no. 1 ignition lead, which exhibited some foreign debris. Continuity was verified from the magneto switch to each of the engine's 8 ignition harness leads, which displayed spark when the propeller was turned. Fuel System A trace amount of fuel that resembled 100 low-lead aviation grade gasoline was drained through the fuel strainer. The fuel selector moved normally through its detents from the LEFT, RIGHT, and BOTH positions. Flight Control System Flight control continuity was traced from the cockpit to the elevator, rudder, and ailerons. The wing flap actuator was not extended, consistent with a flap retracted position. Engine Test The airplane tail was secured to a forklift and several gallons of 100 low lead aviation grade gasoline were plumbed into the aft port of the airplane's left tank at the wing root. Additionally, the original propeller displayed aft bending and was replaced with a test propeller. The auxiliary fuel pump was activated, and positive fuel flow was verified. The following engine start procedure was used: Throttle: open ¼ inch Mixture: Full lean Aircraft Master: ON Ignition Switch: "START" Mixture: Full rich when engine starts The engine momentarily ran rough at idle before power was increased to 1,800 rpm for a run-up engine test. At the engine run-up power setting, the right and left magnetos were individually tested, and both showed a drop of about 100 rpm. The engine power was then reduced to idle performance, about 500 rpm, without any roughness before it was increased to full power. The engine held 2,300 rpm with the throttle in the FULL OPEN position and did not exhibit any roughness or hesitation. During the test, the fuel flow, amperage, oil temperature, and oil pressure all displayed normal measurements. Post-Engine Run Examination The bottom spark plugs exhibited normal wear when compared to the Champion Aerospace Aviation Safety Manual. The bottom spark plug to cylinder no. 2 displayed evidence of lead deposits on the cone adjacent to the ground electrode, and the bottom plug to cylinders no. 3 exhibited bridging between the cone and the ground electrode. However, neither plug displayed any bridging or debris between the ground electrode and post. The fuel injectors were not obstructed. The rocker arm movement for each cylinder was smooth and unremarkable when the propeller was rotated. The fuel strainer was disassembled and contained trace amounts of debris, but was not obstructed. The fuel lines from the right and left wing roots to the engine driven fuel pump were free of debris.