Partial pilot incapacitation involving Piper PA-28, VH-TBB, 19 km south-east of Moree, New South Wales, on 23 September 2020

The occurrence

On 23 September 2020, the pilot of a Piper PA-28 aircraft, registered VH-TBB, was conducting a ferry flight from Moree to Scone, New South Wales. At about 1418 Eastern Standard Time,^[1] the pilot started the engine, carried out engine run-ups^[2] and then taxied the aircraft to the runway for departure. The pilot reported the engine was running for about 10 minutes while on the ground in Moree, with the side window, fresh air vents open and the heater off. Prior to take-off, the pilot closed the window and vents.

Shortly after take-off, the pilot began to feel unwell, with a warm feeling in their chest, dizziness, breathlessness, some confusion and disorientation. The pilot then conducted a visual scan of the flight instruments and observed a localised discolouration on the disposable carbon monoxide (CO) chemical spot detector. As the pilot looked closer at the detector, they noticed the spot rapidly getting darker. The pilot immediately turned the aircraft back toward the airport, reduced engine power, and opened all the fresh air vents and the side window.

The aircraft was landed safely at Moree and the engine shut down at 1435. The pilot was subsequently taken to hospital for a medical examination. While receiving supplemental oxygen, the pilot was starting to feel better after about 2 hours.

Context

Medical information

General health and fitness

The pilot was well rested and felt fit to fly on the day of the incident. They had completed two flights in a different aircraft prior to the incident, one of which was from Tamworth to Moree. The pilot felt normal throughout those flights and only became unwell during the incident flight.

The pilot was a non-smoker, was not taking any medication, and had no pre-existing medical condition that could have contributed to the incident.

Carboxyhaemoglobin

A blood sample taken from the pilot about 1.5 hours after engine shut down, indicated a carboxyhaemoglobin (COHb) level of 1 per cent. The pilot was administered oxygen at the hospital, starting about 15 minutes before the blood sample was taken. In total they were on supplemental oxygen for about 5 hours, until their oxygen saturation levels were at 100 per cent.

Carbon monoxide is an odourless, colourless and tasteless gas formed by the incomplete combustion of carbon-containing materials. When inhaled, it preferentially binds to haemoglobin, the oxygen carrying molecule in red blood cells. This creates COHb compounds and prevents oxygen from binding to the molecule and being transported, resulting in oxygen starvation.

As previously discussed in ATSB investigations AO-2020-026 and AO-2017-118, normal endogenous levels of COHb are generally within the range of 0.4-0.7 per cent. However, smokers, and those living in an urban area, may have higher than average levels of 1-6 per cent (Baselt, 2014).

After the source of CO has been removed, COHb will reduce to half its initial value within 4‑5 hours at sea level. This can decrease to 80 minutes with the administration of pure oxygen. Taking into account the CO half-life, the elapsed time since the symptoms and effects were first detected, and the pilot’s brief time on oxygen prior to the blood test, the ATSB estimated that the pilot’s COHb levels were likely between 1.5 to 2 per cent.

Although individuals’ reactions can vary, the physical symptoms and cognitive effects of CO poisoning tend to worsen with an increasing level of COHb (Lacefield et al., 1982). Typically, COHb levels of 10-20 per cent can result in symptoms of breathlessness, while levels of 30-40 per cent can result in mental changes, dizziness and confusion (Knobeloch & Jackson, 1999; Lacefield et al., 1982). While physical symptoms do not generally show at levels below 10 per cent, researchers have found that a person’s ability to perform complex tasks can be adversely affected at levels of 10 per cent or less (Baselt, 2014). Hawkins (1993) also noted that the effects of CO can begin to show with the deterioration of psychomotor function at COHb levels of about 3 per cent.

Aircraft maintenance

Post-incident aircraft inspection

After the incident, and prior to any maintenance activity, engineers conducted ground runs and confirmed that CO was leaking into the cabin with a positive indication on a new, disposable CO chemical spot detector. The engineers then inspected the aircraft, focusing on the engine exhaust system and airframe. The inspection revealed 4-5 pinholes in the exhaust stack that were not covered properly and unserviceable scat (air duct) hoses. No breaches were found in the firewall or the aircraft belly, and the cabin heat valve was functioning correctly. The pinholes were repaired and the scat hoses were replaced. Ground runs were conducted after the repairs and the chemical spot detector showed no indication of CO in the cabin.

Most recent periodic inspection

The last periodic maintenance inspection was completed on 22 May 2020, about 4 months prior to the incident, where the exhaust system was visually inspected using a torch and mirror. The inspection of the exhaust system included the removal of the muffler shroud, security of the baffle cones and the cabin heat flexible hoses. The engineer reported that, exhaust stains on the engine and/or cowling can indicate the existence of an exhaust leak, which was something they looked for during inspections. However, in this case, there were no exhaust system defects recorded in the maintenance documentation.

The owner, who operated the aircraft a few days prior to the incident, reported never having observed a positive indication on the CO detector, nor had they felt unwell during or after a flight. However, they would sometimes smell exhaust fumes when they turned the heater on. The owner advised that they typically never left it on for very long, as CO was always in the back of their mind.

A study conducted by the United States Federal Aviation Administration (2009) noted that a small crack or imperfection can be difficult to see during a visual inspection, which is further impeded by densely packed engine compartments. Further, defects can form after the inspection from erosion and internal fatigue.

Carbon monoxide detector

The aircraft was fitted with an Aviation Supplies and Academics disposable CO chemical spot detector, attached to the instrument panel (Figure 1). The detector was only 4 months old, which was within the manufacturer’s 12-month replacement period. It consisted of an orange‑coloured circle (spot) in the middle of the card, which was designed to change colour to grey/black following a chemical reaction with CO in the immediate vicinity. The spot then returns to normal (orange) after it has been exposed to fresh air. The chemical reaction depends on the concentration of CO in the air and the time of exposure. This detector was designed to react to a minimum of 50 parts per million (ppm) of CO within 30 minutes, 100 ppm within 10 minutes and 200 ppm within 4 minutes.

The ATSB’s investigation and corresponding safety advisory notice for AO-2017-118, highlighted the limitations of these types of detectors. Although commonly used in general aviation, they are a passive device that rely on the pilot regularly monitoring the changing colour of the detector throughout the flight. Further, identifying a positive indication is also dependent on the detector being easily visible and accessible, in a well-lit position particularly when operating in a low  ambient light environment.  

Figure 1: Slightly discoloured carbon monoxide detector, after engine shutdown and the detector being exposed to fresh air

Source: Pilot

Similar occurrences

ATSB investigation (AO-2017-118)

On 31 December 2017, the pilot and five passengers of a de Havilland Canada DHC-2 floatplane, registered VH-NOO, were fatally injured when the aircraft collided with water in Jerusalem Bay, New South Wales. The occupant’s toxicology results identified that they had higher than normal levels of COHb in their blood. This was almost certainly due to elevated levels of CO in the aircraft cabin. The ATSB’s wreckage examination established that several pre-existing cracks in the exhaust collector ring very likely released exhaust gas into the engine/accessory bay. This then very likely entered the cabin through holes in the main firewall where three bolts were missing from the magneto access panels.

ATSB investigation (AO-2020-026)

On 22 December 2019, the crew of a Cessna 172R aircraft, registered VH-YXZ, were conducting aerial shark patrols. About 2 hours into the second flight of the day, the crew started to experience symptoms typically associated with CO poisoning, and subsequently observed a localised discolouration on the disposable CO chemical spot detector. Soon after, the aircraft was landed safely and the three crew were taken to hospital for assessment. While blood tests confirmed all crew had mildly elevated COHb levels, their physical symptoms and cognitive effects likely resulted from exposure to elevated CO levels in the aircraft cabin. The CO source within the aircraft could not be established. Further, the discrepancy between the low COHb levels and severity of experienced effects could not be resolved.

National Transportation Safety Board investigation (CEN17LA101)

On 2 February 2017, shortly after take-off, the pilot of a Mooney M20C aircraft became incapacitated. The aircraft continued flying until running out of fuel and then collided with terrain, but the pilot survived. The pilot’s COHb level, taken 4.5 hours after the accident, was 13.8 per cent. However, given the half‑life of CO, the pilot’s level would have been at least 28 per cent at the time of the accident. A post-accident inspection of the aircraft identified several cracks in the exhaust muffler. In response to the experience, the pilot stated that:

Current technology has made portable CO detection very accurate and inexpensive. A high resolution detector would have not only prevented this accident flight, but may have alerted me to a compromise in my exhaust system many flight hours before the incident.

Safety analysis

Elevated levels of carbon monoxide

The post-incident aircraft ground runs produced elevated levels of carbon monoxide (CO) in the aircraft cabin, which was resolved once repairs to the exhaust stack and the scat hoses were complete. This indicated that the source of the CO was associated with the items repaired. No obvious breaches were identified that would have allowed CO to enter the cabin and therefore, it was not immediately clear how the exhaust gasses entered the cockpit on this occasion.

However, the observed physical symptoms and cognitive effects reported by the pilot were more likely associated with CO poisoning. This was further supported by the positive indications on the CO chemical spot detector and the blood test indicating a mildly elevated level of carboxyhaemoglobin (COHb). While the extent of the pilot’s symptoms for the recorded COHb level were inconsistent with the literature, research has shown that adverse effects on cognitive functions can occur at levels as low as 3 per cent and that individuals can react differently to COHb.

Findings

From the evidence available, the following findings are made with respect to the partial pilot incapacitation involving a Piper PA-28, registered VH-TBB that occurred 19 km south-east of Moree Airport, New South Wales, on 23 September 2020.

Contributing factors

• An exhaust leak likely exposed the pilot to elevated levels of carbon monoxide in the aircraft cabin, resulting in mild incapacitation.

Sources and submissions

Sources of information

The sources of information during the investigation included the:

• pilot
• aircraft owner
• repairer for VH-TBB
• maintenance organisation for VH-TBB.

References

Baselt, R.C. (2014). Disposition of Toxic Drugs and Chemicals in Man (10th ed.). Seal Beach: Biomedical Publications.

Hawkins, F. H. (1993). Human Factors in Flight (2nd ed.). Aldershot, England: Ashgate Publishing.

Hossein Cheraghi, S., Jorgensen, J., & Myose, R.Y. (2009). Detection and prevention of carbon monoxide exposure in general aviation aircraft (DOT/FAA/AR-09/49). Retrieved from Federal Aviation Administration website: http://www.tc.faa.gov/its/worldpac/techrpt/ar0949.pdf

Knobeloch, L & Jackson, R. (1999). Recognition of Chronic Carbon Monoxide Poisoning. Wisconsin Medical Journal, 98(6):26-9. Retrieved from https://www.researchgate.net/publication/12696822_Recognition_of_chronic_carbon_monoxide_poisoning

Lacefield, D. J., Roberts, P. A., & Grape, P. M. (1982). Carbon monoxide in-flight incapacitation: An occasional toxic problem in aviation (FAA-AM-82-15). Oklahoma City, Oklahoma: Federal Aviation Administration.

Submissions

Under section 26 of the Transport Safety Investigation Act 2003, the ATSB may provide a draft report, on a confidential basis, to any person whom the ATSB considers appropriate. That section allows a person receiving a draft report to make submissions to the ATSB about the draft report.

A draft of this report was provided to the following directly involved parties:

• pilot
• aircraft owner
• repairer for VH-TBB
• maintenance organisation for VH-TBB.

A submission was received from the pilot. The submission was reviewed and, where considered appropriate, the text of the report was amended accordingly.

__________

1. Eastern Standard Time (EST): Coordinated Universal Time (UTC) + 10 hours.
2. Run-up: a high-power run-up check is carried out in a piston-engine aircraft to check the aircraft’s ignition and other systems before commencing an initial take-off.