N3376BCessna 150F · 1966

On June 6, 2014, about 1315 mountain daylight time, a Cessna 150F, N3376B, was substantially damaged during an off-airport landing in Ronan, Montana. Both the pilot and his passenger received minor injuries. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no Federal Aviation Administration (FAA) flight plan was filed for the flight.

According to the pilot, the airplane was based at Polson airport (8S1), Polson, Montana. The pilot departed 8S1, and planned to travel to St Ignatius airport (52S) St. Ignatius, Montana, with an intermediate touch-and-go at Ronan airport (7S0), Ronan. The airplane departed with full fuel tanks, and flew directly to 7S0. Because there was another airplane operating at 7S0, the pilot executed one 360-degree turn for spacing, before conducting a straight in approach to land to the south at 7S0. The pilot reported that he completed the landing "routinely" with full carburetor heat and 10 degrees of flaps. After touchdown, he retracted the flaps, shut off the carburetor heat, lifted off, and began the climbout for 52S. When the airplane was about 300 to 400 feet above ground level (agl), the engine began to "slow down," despite the controls being set to full throttle, full rich mixture, and no carburetor heat. The engine continued to lose power, so the pilot turned left slightly to gauge whether the airplane could make it back to the runway. As the airplane entered the left turn, the climb performance decreased "substantially," and the airplane began to lose altitude quickly. The pilot then decided to perform an emergency landing in a nearby field. The pilot reported that due to the low altitude, he did not have time to perform any emergency procedures, other than to inform his passenger that he was going to land in the field, and advise the passenger to tighten his seatbelt. The pilot did not apply carburetor heat subsequent to the power decrease.

When the airplane was about 50 feet agl, the engine lost all "performance." During the forced landing in a field, the airplane bounced, and then struck a berm while traveling at a speed of about 35 knots, which caused the airplane to nose over. The airplane came to rest inverted, but there was no fire. The airplane was recovered by the owner, and retained by the NTSB for subsequent examination.

The 18-year-old pilot held a private pilot certificate with an airplane single-engine land rating. He reported that he had a total flight experience of 68 hours, including 2 hours in the accident airplane make and model. He successfully completed his private pilot practical examination on March 19, 2014. His most recent FAA third-class medical certificate was issued on June 20, 2013.

FAA information indicated that the airplane was manufactured in 1966, and was equipped with a Continental Motors O-200 series engine. The most recent annual inspection was completed on February 5, 2014. The maintenance records indicated that, at that time, the airplane had accumulated a total time in service of about 4,231 hours, and the engine had accumulated about 2,592 hours since major overhaul. The manufacturer's recommended overhaul interval was 1,800 hours or 12 years, whichever came first. Review of the maintenance records did not reveal any unusual entries.

The 1321 non-certified weather observation for Ronan included wind from the south-southeast at 2 knots, temperature 20 degrees C, dew point 10 degrees C, and a barometric pressure of 29.97 inches of mercury. The calculated relative humidity was 53 percent.

FAA information indicated that the airplane was manufactured in 1966, and was equipped with a Continental Motors O-200 series engine. The most recent annual inspection was completed on February 5, 2014. The maintenance records indicated that, at that time, the airplane had accumulated a total time in service of about 4,231 hours, and the engine had accumulated about 2,592 hours since major overhaul. The manufacturer's recommended overhaul interval was 1,800 hours or 12 years, whichever came first. Review of the maintenance records did not reveal any unusual entries.

The propeller, engine, cowling, and nose gear were all damaged and displaced aft by the impact. The fuselage was wrinkled in several locations aft of the main landing gear. The vertical fin and a portion of the rudder were also deformed. Photographs taken shortly after the accident, while the airplane was still inverted in the field, indicated that the flaps were in the retracted position. According to an FAA inspector, the owner informed him that the airplane had departed on the flight with full fuel tanks.

The airplane owner was advised by the NTSB that he could recover the airplane, but that only the FAA and/or an NTSB-approved mechanic were permitted to conduct any maintenance or investigative activities on the airplane. The owner agreed, and there was no evidence that the owner or any other persons tampered with the airplane after its recovery.

On June 17, 2014, an FAA inspector and an aircraft mechanic examined the airplane. That examination did not detect any indications of non-impact related distress or failures.

The lower engine mounts were fractured. The fuel line was fracture-separated from the fuel strainer, but the condition or contents of the fuel strainer were not otherwise noted. No airframe fuel system checks were conducted. The carburetor linkage and air box were deformed consistent with impact, which prevented movement of the throttle, mixture, and carburetor heat controls. The carburetor heat control was found lodged in the off (no carburetor heat) position. The throttle and mixture controls were found lodged in their full throttle and full rich positions, respectively. No additional checks of carburetor condition, integrity, or functionality were accomplished.

Cold compression checks of the cylinders revealed that three cylinders were unremarkable, but that the initial check on the No. 3 cylinder resulted in a reading of 18/80, with significant leakage past both the intake and exhaust valves. The mechanic staked both valves, and then obtained a normal reading. The reasons for the initial low pressure reading, or the success of the staking, with its implication that the valves were initially not completely sealed on their seats, were not determined, or investigated further, by the inspector or the mechanic.

All eight spark plugs were removed and examined; their electrodes exhibited varying states of wear, plus carbon or oil deposits. The electrodes of the four spark plugs from the Nos. 1 and 2 (aft) cylinders were noticeably cleaner and lighter in color than those from the Nos. 3 and 4 (forward) cylinders, which was consistent with the aft cylinders operating at a leaner mixture than the forward cylinders. However, all electrodes were consistent with the range of appearances associated with normal service.

The mechanic reported that the No. 2 top spark plug "looked like it was not firing." The appearance of that spark plug was very clean, consistent with the Champion "Aviation Check-a-Plug" normal condition. A lack of empirical industry data and guidance regarding the appearance of non-firing spark plugs prevented a conclusion based solely on appearance. A non-firing spark plug would manifest itself as noticeable engine roughness during an engine run up magneto check, and the pilot did not report any such roughness.

The magneto switch tested satisfactorily at all selection positions. The magneto to engine timing for both magnetos was found to be 25 degrees before top dead center (BTDC). The Continental Motors specified setting was 24 degrees BTDC, with a tolerance of plus or minus 1 degree. Impact damage precluded functional testing of the magnetos.

The rocker covers were removed, and all visible valve train components were observed to be intact, and to operate properly with engine rotation. The interiors of all cylinders were examined, and were unremarkable. No contact marks from any valves were observed on the piston heads.

The damage to the carburetor airbox, combined with the inability to test run the engine, prevented the determination of the pre-impact functionality and performance of the carburetor heat system.

Airplane Manufacturer's Procedural Guidance

According to the airplane manufacturer's published operating procedures, the before takeoff procedures included verification of proper operation of the carburetor heat, and a magneto operation check. Proper carburetor heat operation would be indicated by an rpm drop when the carburetor heat was applied. Proper magneto operation would be indicated by the engine continuing to run smoothly on either magneto, plus no more than a 75 rpm differential between operation on either magneto. The manufacturer specified that carburetor heat was to be "cold" (in the off position) for takeoff. The before landing procedures specified that "full" carburetor heat was to be applied "before closing throttle." The guidance specified that in the event of an uncommanded rpm/power reduction in flight, full carburetor heat was to be applied.

The pilot stated that engine start, taxi out, and engine run up were all normal. The carburetor heat operated normally, with an observed rpm decrease. The engine ran smoothly on each magneto, with the respective rpm decreases within limits.

Carburetor Icing

FAA Advisory Circular AC 20-113, regarding induction system icing, discussed two components of carburetor icing; throttle ice and fuel vaporization ice. The AC stated that "Throttle ice is usually formed at or near a partially closed throttle, typical of an off-idle or cruise power setting, " and "occurs when water vapor... freezes because of the cooling restriction caused by the carburetor venturi and the throttle butterfly valve." The AC stated that fuel vaporization ice "usually occurs in conjunction with throttle icing. It is most prevalent with conventional float type carburetors." The AC stated that "vaporization icing may occur at temperatures from 32 degrees F to as high as 100 degrees F, with a relative humidity of 50 percent or above."

Carburetor icing prevention procedures advanced by the AC included maintenance checks, and preflight operational checks by the pilot. The AC stated that pilots should conduct takeoffs without carburetor heat, "unless extreme intake icing conditions are present." The AC noted that carburetor icing "is more likely" when relative humidity is above 80 percent.

Siting of the ambient conditions on one FAA carburetor icing envelope chart indicated that the conditions were near the center of the "Serious Icing (glide power)" region, and near the lower edge of the region labeled "Carburetor Icing Possible" on another FAA carburetor icing envelope chart.

Water Contamination of Fuel

Water contamination of fuel can adversely affect engine operation, and can result in a partial or complete loss of power. Over time, if water is present in the fuel, it will precipitate out, and settle at the lowest point(s) in the fuel tank or system. The airplane was equipped with fuel tank sump drains to permit the pilot to check the fuel for water contamination, and remove it from the tanks. The airplane was also equipped with a fuel strainer which, within limits, separates the water from the fuel and thereby prevents water from being introduced into the carburetor.

The pilot stated that prior to the flight he drained the sumps and strainer, and did not detect any water in the fuel.