ERA21FA232

The student pilot was on a multi-leg, solo cross-country flight in atmospheric conditions conducive to serious carburetor icing at descent power. Track data and interviews with the pilot’s instructor revealed that the airplane entered a gradual descent over a 5-minute span that included a 450° descending right turn. Shortly thereafter, the pilot called his instructor via cell phone and reported that the airplane’s engine was producing only partial power. The instructor stated that the pilot’s demeanor was calm, and as such, the discussion felt conversational and that there was time to discuss fuel state, engine control positions, landing at the nearest airport, or selecting a forced landing site. When asked, the pilot reported he was “40 to 50 miles” from his departure airport; however, track data revealed that the airplane was about 8 miles south of the departure airport. The instructor suggested the pilot make an “emergency landing in a field,” but the pilot reported that there were trees and mountains ahead of him before the sounds of impact were heard. The airplane was consumed by postcrash fire. Control continuity to all flight control surfaces was confirmed; examination of the engine revealed the accessories were destroyed by fire and that the core exhibited no preimpact anomalies. The fracture surfaces on a separated carburetor heat control revealed signatures consistent with overstress due to impact and high temperatures. Based on the lack of mechanical anomalies and the partial loss of engine power as reported by the pilot, it is likely that the engine lost partial power as the result of carburetor ice accumulation, which resulted in descent into terrain. It is likely that the pilot’s prompt application and use of carburetor heat in accordance with the airplane’s operating handbook would have restored engine power.

On May 25, 2021, about 0730 central daylight time, a Gulfstream American AA-5A, N26889, was destroyed when it was involved in an accident near Crossville, Tennessee. The student pilot was fatally injured. The airplane was operated as a Title Code of Federal Regulations Part 91 instructional flight. Track data obtained from the Federal Aviation Administration (FAA), an interview with the pilot’s flight instructor, and telephone records revealed that the airplane departed Crossville Memorial Airport (CSV), Crossville, Tennessee, on the second leg of a cross-country flight about 0715. The next planned stop was Cleveland Regional Jetport (RZR), Cleveland, Tennessee, about 50 miles south of CSV. The airplane climbed on a southerly track to 3,700 ft mean sea level (msl) before beginning a gradual descent about 0718. The airplane continued its descent on its southerly track until about 0723 and 3,000 ft msl, when the airplane entered a 450° descending right turn. Figure 1. – Overview of Accident Flight At 0726, about 2,300 ft msl, the pilot placed a phone call to his flight instructor. The instructor stated that the pilot reported that the engine was not making full power, producing about 1,700 rpm versus the normal cruise power setting of about 2,400 rpm. The instructor stated that the pilot remained calm during the conversation, and he assisted with troubleshooting, including asking about the fuel state, magneto switch position, and carburetor heat position. The pilot thought he was “40 to 50” miles from CSV. The instructor told him to land at the nearest airport, but the pilot reported that the airplane had slowed to 70 knots. The instructor then advised the pilot to perform an emergency landing to a field. The pilot reported that there were “trees and mountains.” Shortly thereafter, the instructor heard the sound of an impact and the connection was lost. Figure 2. Profile View of Accident Flight The airplane completed its course reversal about 8 miles south of CSV and traversed a large cultivated field before impacting rising terrain on a heavily wooded ridgeline. Figure 3. View of Flight Track Final Segment The pilot had begun flight lessons about 1 month before the accident. According to his instructor, the pilot had accrued 44 total hours of flight experience, all of which was in the accident airplane. The pilot was a conscientious student who flew an average of three times per week. The instructor stated that the pilot was enrolled in an online ground school and that they would discuss the lessons before each flight. The accident flight was the pilot’s first solo cross-country flight. The airplane’s most recent annual inspection was completed on December 11, 2020, at 5,221.84 total aircraft hours. Examination of the airplane at the accident site revealed that the wreckage path was about 1,800 ft elevation, oriented about 030° magnetic and was about 75 ft long. The initial impact point was in a tree about 50 ft tall, and pieces of angularly-cutwood were found along the wreckage path. The airplane was consumed by postcrash fire. Remnants of each wing and the main wing spars were found adjacent to main fuselage area. The tail section was impact damaged but remained largely intact. Control cable continuity was established from the control column and rudder pedals to the rudder and elevators. Continuity was established from the control column through breaks at each wing root to the ailerons. The cable breaks displayed features consistent with overload failure. The instrument panel and its contents were consumed by fire. The engine displayed significant fire damage, and the accessories, along with their associated wires, hoses, and fittings, were consumed by fire. Examination of the throttle control at the carburetor revealed that the ball-joint end of the throttle control cable was fractured at the carburetor arm. The carburetor end of the throttle control cable assembly with attached support bracket, carburetor arm with attached ball screw piece, and carburetor arm attachment nut and cotter pin were retained and forwarded to the NTSB Materials Laboratory in Washington, DC. Examination of the fracture surfaces revealed fracture features consistent with high temperature overstress fracture and ductile overstress fracture. The weather reported at Crossville Memorial Airport, Crossville, Tennessee, about 20 minutes after the accident included clear skies and calm wind. The temperature was 22°C and the dewpoint was 17°C. Review of the icing probability chart contained withing the FAA Special Airworthiness Information Bulletin CE-09-35 revealed the atmospheric conditions at the time of the accident were “conducive to serious icing at glide power.” According to the Gulfstream American Model AA-5A Cheetah Pilot’s Operating Handbook, Normal Procedures: Page 4-11, DESCENT, (3) Carburetor Heat – As required by engine power setting and weather conditions page 4-19, NOTE – If engine runs rough during cruise with carburetor heat on, it may be due to an over-rich condition. To correct for engine roughness in such a situation, lean mixture to smooth engine operation.” Section 3, Emergency Procedures, page 3-16: ROUGH ENGINE OPERATION OR LOSS OF POWER - Carburetor Icing – An unexplained drop in RPM and engine roughness may result from the formation of carburetor ice. To clear the ice, apply full throttle (do not exceed red line) and pull the carburetor heat knob full out until the engine runs smoothly. Then remove carburetor heat and readjust the throttle. If conditions require the continued use of carburetor heat in cruise flight use the minimum amount of heat necessary to prevent ice from forming and lean the mixture for smooth engine operation. FAA Special Airworthiness Information Bulletin (SAIB) CE-09-35, Carburetor Icing Prevention Pilots should be aware that carburetor icing doesn’t just occur in freezing conditions, it can occur at temperatures well above freezing temperatures when there isvisible moisture or high humidity. Icing can occur in the carburetor at temperatures above freezing because vaporization of fuel, combined with the expansion of air as it flows through the carburetor, (Venturi Effect) causes sudden cooling, sometimes by a significant amount within a fraction of a second. To recognize carburetor icing, the warning signs are: A drop in rpm in fixed pitch propeller airplanes. … The pilot should respond to carburetor icing by applying full carburetor heat immediately. The engine may run rough initially for short time while ice melts.