ERA20FA031

Pilot impairment due to exposure to carbon monoxide as a result of undetected corrosion and holes in the engine’s left exhaust muffler.

The 74 year-old private pilot departed in his airplane during day visual meteorological conditions for a local flight. The airplane climbed to about 2,500 ft mean sea level after departure and tracked in a southwesterly direction for about 20 minutes. The airplane then began a gradual descent, turned to the northeast, flew along the shoreline, and turned toward the airport. The airplane descended to around 250 ft agl over a golf course fairway less than 3 nm from the Class D airport without communicating with the air traffic control as required. The airplane then commenced a steep, climbing right turn to an altitude of about 4,000 ft msl, when the airplane then descended rapidly in a left circular pattern. A witness near the accident site reported seeing the airplane descend like it was going to land, then climb very high, pivot on its left wing, and descend straight down. Other witnesses reported that the pilot seemed to be performing aerobatics. According to family members and close friends, this was highly unusual behavior for this pilot whom they all considered to be a “very conservative pilot.” The pilot’s failure to communicate with air traffic control as required, his low-level flight over a fairway, and the erratic flight track and altitude changes that subsequently occurred in the absence of any meteorological conditions that could have restricted visibility are inconsistent with the pilot’s familiarity with the local airspace and his reported capabilities and are consistent with acute impairment. The cause of the pilot’s impairment could not be identified from available medical evidence. Coronary artery disease was present on autopsy but whether it contributed to impairment could not be determined. Due to the severity of the pilot’s injuries, there was no available autopsy information about his brain. A negative result of a qualitative carboxyhemoglobin test of cavity blood by diffusion was the only available toxicological information about carbon monoxide (CO) exposure. However, a limitation of diffusion testing is that it may produce a false negative result if blood is contaminated with other body fluids, as sometimes occurs in cavity blood; that possibility could not be evaluated from available toxicological information. However, the likelihood of CO impairment was also indicated after examination of the airplane, which revealed internal deterioration of the left muffler. The muffler metal was corroded and thin, and pin holes were observed on the sides of the muffler. The condition of the muffler would have allowed exhaust gasses, a common source of CO exposure, to leak into the cabin through the cabin heating system, which was found selected ON. CO poisoning can result from sufficient exposure, causing a wide variety of impairing effects, which may include serious neurocognitive and cardiac problems. The muffler’s corrosion and thinning of the muffler wall, along with the holes in the muffler, were likely present at the time of the last inspection, which occurred about 5 months before the accident; however, due to damage from impact, it could not be determined if the extent that the muffler was degraded was readily visible at the time of inspection. No other preimpact anomalies or malfunctions were identified during the examination that would have prevented normal operation. Based on available operational and physical evidence, it is likely that the pilot was impaired due to CO exposure via the corroded muffler.

Interviews with friends and acquaintances of the pilot revealed that it was highly unusual and uncharacteristic behavior for the pilot to perform aerobatics. They considered him to be a very conservative pilot. According to a friend of the pilot who flew with him regularly, the pilot often flew toward Newport, Rhode Island, and then along the shoreline back toward West Island, Massachusetts, where the pilot would typically contact the EWB air traffic control tower when inbound for landing. According to the president of the EWB fixed based operator, the business staff monitored the tower frequency for incoming airplanes and did not hear the pilot make any radio transmissions, adding that the pilot “would never enter the airspace without calling.” Cabin Heat System According to the Cessna 150 Pilot Operating Handbook, “the temperature and volume of airflow into the cabin can be regulated to any degree by manipulation of the push-pull CABIN HT and CABIN AIR control knobs.” The left engine muffler was constructed with a shroud around the outside, which forms a heating chamber for the cabin heater air. The heated air from the muffler shroud is then routed to the cabin when the CABIN HT knob is pulled out. Maintenance and Muffler History A logbook entry indicated that the muffler was last replaced June 1, 2004, at tachometer time 4,321.5 hours, or 742.9 hours before the accident flight; however, the maintenance entry did not specify which muffler was replaced. The mechanic who performed the most recent annual inspection stated that he opened each muffler shroud to inspect the muffler, looking for stains, soot or cracks, which would indicate the need to change the muffler. He stated that he found no anomalies during the muffler inspections. The airplane was not equipped with any type of carbon monoxide (CO) detector. The inspection chart contained in the Model 150 Series (1977) Service Manual indicates that a general inspection of the exhaust system should be conducted every 50 hours and references a more detailed procedure later in the manual for an inspection every 100 hours. The 100-hour inspection information starts by stating, “Inspection of the exhaust system shall be thorough because the cabin heating system uses air heated by the heat exchangers of the exhaust system. Since exhaust systems of this type are subject to burning, cracking, and general deterioration from alternate thermal stresses and vibration, inspection is very important and should be accomplished every 100 hours of operation. The airplane was generally inspected on an annual basis, and not a 100-flight hour basis. A review of the maintenance records indicated that the annual inspections occurred at flight hour intervals ranging from 39 hours to 55 hours in the 10 years preceding the accident flight. Examination of the accident site and wreckage revealed that the airplane impacted a tree in a near vertical nose down attitude about 3.5 nautical miles and about 164° magnetic from EWB. The debris field extended about 240 ft from the tree on a heading of about 220°. The left wing with the strut attached was located about 10 ft from the tree, followed by the empennage, cabin and engine at 30 ft, the right wing at 40 ft, the carburetor at 87 ft, a ruptured fuel tank at 122 ft, and the main wheels and directional gyro at 240 ft. All major components of the airplane were present at the accident site. The fuselage was significantly fragmented. Both wings were impact fractured, bent on all surfaces, and exhibited leading edge damage consistent with tree impact. The empennage was impact fractured and bent. The instrument panel was largely fragmented. The engine controls were in the full forward position. The cabin heat control was in the full aft (ON) position. The engine was examined, and no pre-impact anomalies were noted that would have prevented normal production of power. The propeller remained attached to the engine crankshaft propeller flange. Examination of the propeller blades revealed aft bending and twisting along the length of both blades with leading-edge gouging and chordwise rotational scoring on the camber sides. Control continuity was established from the flight controls through overload fractures to the control surfaces. The left muffler outer casing was impact crushed exposing the end plate which exhibited an erosion hole and several other smaller pin holes. The cabin heat shroud was removed from the muffler, and additional evidence of internal deterioration was present. The muffler metal was corroded and thin, and pin holes were observed on the sides of the muffler. The right muffler was crushed and compacted, and the shroud, which provides heated air to the carburetor heat control, remained intact around the muffler. On November 4, 2019, at 1532 eastern standard time, a Cessna 150M, N714LK, was destroyed when it was involved in an accident near New Bedford, Massachusetts. The pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to an employee of the fixed based operator at the New Bedford Regional Airport (EWB), New Bedford, Massachusetts, where the owner/pilot based his airplane, the pilot was “cheerful and happy to be flying” the morning of the accident. He reported that the pilot spent about 10 to 15 minutes performing a preflight inspection of his airplane before departing. Automatic dependent surveillance-broadcast (ADS-B) data obtained from the Federal Aviation Administration (FAA) indicated that the airplane departed EWB around 1450 and climbed to about 2,500 ft mean sea level (msl) while tracking in a southwesterly direction for about 20 minutes. The airplane then began a gradual descent, turned to the northeast, and flew along the shoreline. It then proceeded to fly north toward EWB, descending to around 250 ft above ground level over a golf course fairway less than 3 nm from EWB. The airplane then commenced a climbing right turn at increasing pitch attitudes to an altitude of about 4,000 ft msl, when the airplane then descended rapidly in a left circular pattern. According to a witness who had exited his car in a parking lot near the accident site, he heard a sound "like a motor revving up high." He looked up and saw an airplane "swoop down like it was going to land,” climb very high, and then “pivot on its left wing” before “coming straight down.” He thought the airplane was “doing tricks.” Examination of the accident site and wreckage revealed that the airplane impacted a tree in a cemetery in a near vertical nose-down attitude. FAA Carbon Monoxide and Exhaust System Guidance On November 24, 1972, the FAA issued advisory circular (AC) 20-32B "Carbon Monoxide (CO) Contamination in Aircraft—Detection and Prevention." The AC provided information on the potential dangers of carbon monoxide contamination from faulty engine exhaust systems or cabin heat exchangers. It also discussed means of detection and procedures to follow when contamination is suspected. In October 2009, the FAA issued report DOT/FAA/AR-09/49, "Detection and Prevention of Carbon Monoxide Exposure in General Aviation Aircraft." The report documented research on detection and prevention of CO exposure in general aviation aircraft, with the objective of identifying exhaust system design issues related to CO exposure, evaluating inspection methods and maintenance practices with respect to CO generation, and the identification of protocols to quickly alert users to the presence of excessive CO in the cockpit and cabin. On March 17, 2010, the FAA published Special Airworthiness Information Bulletin (SAIB) CE-10-19 R1. It recommended that owners and operators of general aviation aircraft consider the information in the DOT/FAA/AR-09/49 report and use CO detectors while operating their aircraft. The SAIB also recommended a cabin CO