Accident Overview
On March 5, 2014, about 1930 Pacific standard time, a Zenair CH2T, N285AM, operated by Sierra Academy of Aeronautics, made an off-airport forced landing near Woodland, California. The flight was conducted under 14 CFR Part 91 as an instrument flight rules (IFR) cross-country instructional flight. Visual meteorological conditions prevailed, and an IFR flight plan had been filed. The flight instructor and student pilot were not injured; the airplane sustained substantial damage. Postcrash examination determined that both wing spars had been damaged, and the incident was upgraded to an accident on April 2, 2014.
Flight History
The flight departed Merced (MER) with a planned destination of Yolo County Airport (DWA) in Woodland. The purpose was IFR training. After one leg including an instrument approach to Willows, the flight continued toward Woodland. During the en route portion, the flight instructor reported intermittent engine rpm drops of about 100 revolutions per minute (rpm) over 10–15 minutes while cruising at 4,000 feet mean sea level. Application of carburetor heat, magneto check, and mixture/throttle adjustments had no effect, and the anomaly ceased after 15 minutes.
Approximately 12 miles from the GHEER intersection on the RNAV 16 approach to Yolo County Airport, the airplane entered a stratus cloud. Inside the clouds, the student reduced power and pitched down to increase descent rate. The engine rpm dropped to 800–1,000 rpm, and the propeller sounded windmilling. The flight instructor took control and attempted troubleshooting using emergency procedures, including carburetor heat, fuel selector, mixture, fuel pump, fuel gauge, throttle pumping, magneto check, starter check, and engine instruments. The attempts were unsuccessful. The airplane exited the cloud layer between 1,500 and 2,500 feet. The instructor declared a Mayday.
The crew prepared for an off-airport forced landing. During the descent, the airplane shook violently at about 60 knots indicated airspeed for approximately 300 feet of descent, then stopped 300 feet above ground level (agl). At about 200 feet agl, landing lights revealed a power line. The instructor chose to fly under it, as climbing would stall the airplane. The aircraft touched down smoothly; the instructor held the nose wheel off as long as possible. The wings struck trees during the rollout.
After coming to rest, both occupants exited and confirmed fuel tanks undamaged, no fuel smell, and the airplane secure. They later re-entered to use the radio but found the avionics master switch still on. Unable to make contact, the instructor used a cell phone to call air traffic control with the GPS location. A rescue helicopter spotted them about 1.5 hours later and directed emergency crews.
Postaccident Examination
An FAA inspector examined the wreckage at the operator's facility. The left and right fuel tanks contained 5 and 6 gallons of fuel, respectively. The gascolator contained a few ounces. The fuel selector valve was in the OFF position. Oil level was 5.5 quarts. Throttle, mixture, and carburetor heat cables were intact and operated properly. One propeller blade was bent; the other was undamaged. The engine could not be run; a cold compression check showed all cylinders testing at or above 68/80 psi.
Carburetor Ice Conditions
At the time of the accident, ambient temperature was 61°F and dew point 55°F, resulting in 82% relative humidity—conditions in the area of serious icing at cruise power according to FAA guidance. The FAA's Handbook of Aeronautical Knowledge discusses that when the throttle is closed during flight, the engine cools rapidly, increasing susceptibility to carburetor icing. Application of carburetor heat may cause a further power reduction and engine roughness as melted ice passes through, symptoms that can last 30 seconds to several minutes. The handbook recommends periodically opening the throttle for a few seconds to keep the engine warm.
Airplane Flight Manual
The pre-landing checklist in the airplane's flight manual specified that carburetor heat should be ON during approach. The manual did not contain guidance on periodically adjusting power during extended low-throttle operation.