Accident Overview
On February 24, 2012, at approximately 1315 mountain standard time, a Piper PA-28-161, registration N20GG, lost engine power while climbing to cruise altitude. The pilot executed a forced landing to a field near Rapid City, South Dakota. The flight instructor and a private pilot were uninjured. The airplane sustained substantial damage to the firewall. The aircraft was registered to an individual and operated by Skyways LTD under 14 CFR Part 91 as an instrument instructional flight. Instrument meteorological conditions prevailed, and an instrument flight rules flight plan had been filed. The flight departed Rapid City Regional Airport (RAP) about 1300, destined for Huron Regional Airport (HON).
Flight Details
During the pre-takeoff runup at RAP, the engine speed decreased about 75 rpm when carburetor heat was applied. The flight departed in visual flight rules conditions on a heading of 020 degrees, climbing to 3,700 feet mean sea level (MSL). Upon reaching 4,500 feet MSL, the aircraft entered a bank of clouds and continued climbing. At 6,500 feet MSL, the airplane seemed to quit climbing, and engine speed decreased about 50 rpm. The instructor reported using full power during the climb to 9,000 feet MSL, with an indicated airspeed of 95 knots and an outside air temperature of 20 degrees Fahrenheit between 4,000 and 6,500 feet.
Suspecting carburetor ice, the instructor turned on carburetor heat. The engine speed dropped about 900 rpm and ran very rough. The instructor leaned the mixture and turned off carburetor heat, allowing the engine to recover momentarily. However, the aircraft could maintain altitude but not climb. At 4,800 feet MSL, the aircraft was too low for radar vectors for the ILS 32 approach at RAP. During a turn toward RAP, the aircraft began losing altitude. The instructor again applied carburetor heat, and the engine reacted severely, resulting in a total loss of power. Descending to about 4,300 feet MSL with a clear view of the ground at 800-900 feet above ground level, the instructor performed a forced landing to a pasture. The airplane touched down on the incline of a hill, damaging the nosewheel and firewall.
The instructor noted that he had experienced carburetor icing in other airplanes and in this airplane during idle or descent, but never with climb power.
Post-Accident Examination
The airplane was recovered, and on February 26, 2012, Federal Aviation Administration (FAA) inspectors examined it. Damage included the engine firewall, engine mounts, bulkhead behind the nose landing gear, and wrinkling of the underside skin. Fuel samples contained no contaminants.
On March 5, 2012, an engine run was conducted with the aircraft tied down and cowling removed. A jammed throttle linkage (due to impact damage) was freed. The engine started within a few revolutions after priming and ran without anomalies at 1,000-1,300 rpm for a few minutes; higher rpm was avoided due to a bent propeller blade. The fuel strainer screen was unobstructed with only a few fine particles. The carburetor heat control was activated, revealing that the carburetor heat butterfly valve closed to only about 75 percent of full travel. The control cable swivel clamp and housing were secure with no evidence of impact movement. Maintenance records indicated that the carburetor heat butterfly valve, bushing, and control cable were worked on during a 100-hour inspection on September 8, 2011, at tachometer time 7,983.98 hours. The accident tachometer time was 8,058.91 hours.
The airplane was powered by a Lycoming O-360-A4M engine (serial number L-5025-36A) installed under supplemental type certificate (STC) SA1842NM, equipped with a Precision MA-4-4 carburetor. Engine time since overhaul was 2,384 hours.
Weather Conditions
The RAP automated surface observing system recorded at 1252: temperature 0 degrees Celsius, dew point -4 degrees Celsius. According to the FAA publication Winter Flying Tips (P-8740-24), conditions were present for light carburetor icing at that time. With an outside air temperature of 20 degrees Fahrenheit (-7 degrees Celsius) in 100 percent humidity, serious icing conditions at cruise or climb power were present.