The typical personal airplane lacks many creature comforts we’ve come to expect in private transportation. Perhaps foremost among them is air conditioning, but the cabin heat systems also can leave a lot to be desired, especially among piston singles. The primary reason is that the vast majority of these airplanes employ air-cooled engines, unlike the car we drove to the airport, which extracts heat from engine coolant to warm the passengers.
Instead, the piston single depends on fresh air flowing around exhaust system components to heat the cabin, which typically isn’t very efficient and potentially exposes the occupants to exhaust gases entering the cabin through cracks in mufflers, pipes and heat exchangers. That’s bad, since those gases include carbon monoxide (CO), which can displace the oxygen we need to breathe. In my experience, even idling on the ground with the tail into the wind while waiting to take off can result in exhaust gases entering the cabin and triggering a CO detector.
In winter or at high, cold altitudes, we often forget about the consequences of CO, an odorless, colorless gas that’s always present when hydrocarbons like aviation fuel are burned. Symptoms of CO poisoning include headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion, and in extreme cases can result in passing out or dying, any and all of which can contribute to a very bad day in an airplane. Here’s an example of what can happen, and how.
On November 9, 2018, at about 1715 Central time, a Piper PA-28-236 Dakota was substantially damaged when it collided with terrain near Guthrie Center, Iowa. The private pilot and three passengers, one of whom was a student pilot and co-owner of the airplane, were fatally injured. Visual conditions prevailed. Sunset occurred at 1703, and surface temperatures at nearby airports were around -7 degrees C, or about 19 degrees F.
Two witnesses observed the pilot make multiple attempts to start the engine, which resulted in loud popping and “backfire” sounds, before the engine started and ran. The takeoff and departure, which occurred at about 1620, were uneventful. At about 1700, ATC observed a radar target squawking the 7700 emergency code about 40 miles west of Des Moines, Iowa, and close to the town of Guthrie Center. Controllers established contact with the airplane on the Guthrie County Regional Airport (GCT) CTAF. The student pilot reported his certification status and that he was diverting because the pilot flying the airplane was having a heart attack. No other communication from the airplane was received directly by ATC.
However, pilots of two other aircraft on the frequency advised ATC they were able to communicate with the pilot, who reported that he was instead going to attempt to land at GCT. By 1730, the airplane had not landed at GCT or other airports, and an FAA Alert Notice was issued. The wreckage was located the following morning in an area of rolling hills and pastures with an elevation of 1200 feet MSL, six miles southwest of GCT.
Flight data recorded by an onboard GPS receiver indicate the airplane followed an almost direct track at 4000 feet MSL for about 40 minutes before reaching the Guthrie Center area. At 1701, the airplane made a 90-degree left turn in the general direction of GCT, followed by a counterclockwise (left-turning), three-mile-wide orbit around GCT at about 3500 feet MSL (1300 AGL). A series of maneuvers ensued, including turns, descents to as low as 400 feet AGL and climbs. Groundspeeds ranged between 83 and 174 knots. At 1713, the airplane turned southwest and descended to the last recorded location, at 2560 feet MSL, about 2½ miles northeast of the accident site.
The last time the cabin heat was likely used was on October 12, 2018. The pilot who flew that day did not smell exhaust fumes or experience any symptoms of what he considered to be CO poisoning.
The airplane’s most recent annual inspection was completed November 3, 2018, six days before the accident flight. The mechanic performing that inspection specifically recalled disassembling the heat exchanger assembly and exhaust mufflers. He stated the accident pilot helped him remove the heat exchanger shroud. He checked the mufflers and shroud for cracks, deformation and discoloration, and found none. The accident flight likely was the airplane’s first since the inspection.
Examination of the aft exhaust muffler revealed a crack in its outer skin extending 2.75 inches. Portions of material were missing at the crack opening, and it exhibited oxidation with dark-colored deposits on the surface, consistent with a preexisting crack. The inner surface of the heat exchanger shroud was coated in tan- and grey-colored particulate deposits. Similar deposits were also present on the inner surface of the cabin heat tube.
Autopsies performed on the pilot and student pilot found carboxyhemoglobin of 59.2 percent for the pilot and 45.3 percent for the student pilot/passenger. According to the NTSB, carboxyhemoglobin levels of 40 percent and above lead to “confusion, seizures, loss of consciousness and death.” There was no evidence the pilot suffered a heart attack; all aboard died from blunt-force trauma.
According to the other co-owner, the student pilot had flown the airplane solo on multiple occasions. The airplane was not equipped with any type of CO detector, nor was it required to be.
The NTSB determined the probable cause(s) of this accident to include: “Pilot incapacitation due to carbon monoxide poisoning as a result of an undetected crack in an engine exhaust muffler, which permitted entry of exhaust gasses into the cabin via the cabin heat system.”
According to the NTSB, “The crack and small perforations in the muffler wall were likely present at the time of the last inspection, which occurred shortly before the accident; however, due to damage from impact, it could not be determined if the extent of cracking was readily visible at the time of inspection. Also, the crack could have opened just before the accident flight due to an engine backfire that occurred during startup.”
Given the autopsy results, the evidence is very strong that CO overcame the accident pilot and the student, rendering them incapable of flying the airplane. Among other things, this accident should be a reminder of CO’s insidious nature.
Carbon Monoxide Detectors
According to the NTSB’s final report on this accident and in response to a 2004 NTSB safety recommendation, the FAA “concluded that the primary method to prevent CO contamination in the cabin is through proper inspection and maintenance of mufflers and exhaust system components and that CO detectors are a secondary method of preventing CO exposure. The FAA further stated that since a lack of a CO detector alone is not unsafe, installing a CO detector would not correct an unsafe condition.”
Extremely sensitive CO detectors suitable for use in personal aircraft can be purchased from the usual suppliers, and don’t require any FAA paperwork (although the FAA has developed a TSO standard for the devices). When it last looked at CO detectors, sister publication Aviation Consumer felt the CO Experts Model 2016 was the best of the units it reviewed. Since then, CO Experts has upgraded the 2016 to the PG-2017-5, which features the same low-level alerting capability and is now powered by two AAA batteries. The unit is marketed as the CO Experts Ultra by Aeromedix.com and is priced at $199.
Aircraft Profile: Piper PA-28-236 Dakota
Not accident aircraft
OEM Engine: Lycoming O-540-J3A5D
Empty Weight: 1608 lbs.
Maximum Gross Takeoff Weight: 3000 lbs.
Typical Cruise Speed: 143 KTAS
Standard Fuel Capacity: 72 gallons
Service Ceiling: 17,500 feet
Range: 650 NM
VS0: 56 KCAS
Jeb Burnside is the editor-in-chief of Aviation Safety magazine. He’s an airline transport pilot who owns a Beechcraft Debonair, plus the expensive half of an Aeronca 7CCM Champ.