According to NTSB Safety Alert 069, Pilots: Prevent Carbon Monoxide Poisoning, “Carbon monoxide (CO) is a colorless, odorless, tasteless gas by-product of internal combustion engines and is found in exhaust gases. Sufficiently high levels of CO in your bloodstream will lead to oxygen starvation and the onset of symptoms (such as headaches, drowsiness, nausea, or shortness of breath).”
It so happens that piston-powered airplanes are a frequent source of carbon monoxide poisoning. Because they are typically operated where the ambient air is thinner, there’s less oxygen content. And it also so happens that their heating systems are designed to capture residual heat from an engine’s exhaust, maximizing the chance of introducing CO into the cabin. Depending on their design, turbine-powered aircraft systems also can be a source of CO into the cabin.
Typically, CO poisoning in the cockpit results from faulty heating systems leaking exhaust into the cabin over time. An affected pilot likely will encounter the symptoms gradually. A savvy pilot will recognize the symptoms for what they are and take remedial actions. But as we’re about to see, that’s not the only way CO can bring down an airplane.
Background
On February 1, 2022, at about 1006 Eastern time, a Cessna 310R was destroyed when it collided with terrain near Danville, Virginia. The solo commercial pilot (male, 23) was fatally injured. The airplane was operated as a FAR Part 91 aerial surveying flight.
A second company pilot observed the accident airplane in the run-up pad for Runway 02, pointing 205 degrees for 8-10 minutes while the pilots completed pre-departure tasks. Winds at the time were observed to be from 060 degrees at seven knots, producing a quartering tailwind while on the run-up pad. The second company pilot departed first; the accident pilot departed several minutes later, at 1003.
According to ADS-B data, the accident airplane departed and turned southeast. Shortly after takeoff, the airplane’s climb rate decreased from 1200 fpm to about 500 fpm, and the airplane’s acceleration stopped. The airplane climbed to about 2625 feet msl by two minutes into the flight and began a shallow left turn at an airspeed of 136 knots. About 10 seconds after turning left, the airplane returned to wings-level and then rolled right at a rate of about three degrees per second while descending at a rate of more than 1000 fpm. The last estimated bank angle was greater than 60 degrees right at an altitude of 1175 feet msl. The airplane impacted a wooded area about four nm southeast of the departure airport. The accident site’s elevation was about 488 feet msl.
Investigation
According to the operator, the pilot had accrued 85 hours of flight experience in the same make and model as the accident airplane. The accident flight was his first solo aerial surveying flight for the company following several observation flights with the company’s owner. Maintenance records revealed the airplane’s engines and propellers had 18.6 hours of operation since they were overhauled.
The airplane was highly fragmented along the 382-foot-long debris path and oriented on a true heading of 246 degrees. There was a strong fuel odor but no evidence of fire. Neither the wings nor the fuselage was intact. The flap setting could not be determined. The landing gear were fractured from their mounts and located in various parts of the debris field. The landing gear actuator indicated the nose and main landing gear were in the retracted position at the time of impact. The pitch trim actuator indicated the elevator trim tab trailing edge was about 10 degrees tab up. Six propeller blades were recovered, all fractured from their mounts. All blades displayed impact damage, and some also had leading-edge gouging, chordwise abrasion, twisting and aft bending.
None of the cockpit instruments were intact. The throttle control quadrant was impact-damaged with the left throttle near idle, the left propeller near feather and the mixture set full rich for both engines. Flight control continuity could not be confirmed for the elevators, rudder, and ailerons due to impact damage. The rudder trim actuator indicated that the rudder trim tab was about 14 degrees right. The left fuel selector handle was found in the OFF position. The right fuel selector handle was found in the left main position. The left and right fuel selector valves were impact separated and had tumbled through the trees.
Examination of the engine cylinders using a lighted borescope revealed no anomalies to the pistons and valves. All magnetos sparked at all towers. All intact spark plugs displayed normal wear. Oil filters were opened and found free of debris. Examination of both engines revealed no preimpact anomalies or malfunctions that would have precluded normal operation. The airplane’s heater assembly was impact-damaged but did not exhibit any external fire or thermal damage. The welds and materials comprising the heater were intact and free of defects. The airplane was equipped with an adhesive, disposable “spot” carbon monoxide (CO) detector. In the presence of CO, the spot would turn gray/black, and the spot would return to normal color after it is exposed to fresh air.
Toxicological testing of the accident pilot’s blood revealed carboxyhemoglobin, a marker of CO exposure, was elevated at 31 percent. According to the NTSB, “carboxyhemoglobin is formed when CO binds to hemoglobin in blood, impairing the blood’s ability to deliver oxygen to body tissues…. Increasing levels of exposure may become impairing or incapacitating, causing more serious neurocognitive, cardiac, and/or vision problems, progressing to death above carboxyhemoglobin levels of about 50 percent…although symptoms are not simply predictable from carboxyhemoglobin levels.”
During the NTSB investigation, the operator was asked to duplicate the accident pilot’s actions in the same make/model of airplane. The goal was to determine the ability of CO to enter the cockpit from the engine exhaust during taxi and engine run-up in similar conditions. The following is a lightly edited excerpt from the research pilot’s report, as published by the NTSB:
“My startup and taxi time was ‘average’. I taxied to Runway 18 with the door cracked open and window open due to the heat that day. The heater was left off, including the fan. During the taxi, the electronic CO detector read ‘0’ ppm the whole time. As I approached the run-up area, I closed the door and window. Once I got to the run-up area I angled the airplane close to an east-northeast orientation to put the breeze off my right quartering tail based on the grass and other indicators. Almost immediately, I noticed the audible alarm on the CO detector going off. I looked over and could see the number on the CO detector rising through the 50-60 ppm range. It quickly rose above 100 ppm [within 17 seconds and] eventually climbed to around 150-160 ppm before finally coming back down. There were very minor exhaust odors present during the high readings.”
Probable Cause
The NTSB determined the probable cause(s) of this accident to include: “The pilot’s impairment due to exposure to carbon monoxide as a result of undetected engine exhaust penetration into the cockpit, resulting in the pilot’s failure to maintain a minimum controllable airspeed after partially securing an engine after takeoff.”
Since the investigation found no evidence of engine failure, “[it’s] possible that this impairment could have resulted in his perception of a left engine problem, and resulted in him partially securing it, as demonstrated by the postaccident positions of the engine controls,” the NTSB said. The performance analysis showed that the airplane banked about 10 degrees into the inoperative left engine at about 136 KIAS shortly before the airplane entered a steep, descending right turn.
According to the NTSB, “It is likely that during this left turn, the pilot allowed the airplane’s airspeed to decrease below a speed for which the airplane would have been controllable, which resulted in a loss of control and led to the airplane’s roll to the right and rapid descent toward the terrain.”
