CAA Safety Directive:

While catastrophic structural failures or dramatic weather encounters often dominate aviation safety discussions, one of the most persistent and insidious threats to UK General Aviation pilots is completely invisible.

Carbon monoxide (CO) poisoning has unfortunately been cited as a factor in multiple fatal General Aviation accidents. Although these tragic events are relatively rare, the presence of CO remains a persistent background threat for anyone flying piston-engine aircraft.

The UK Civil Aviation Authority (CAA) highlights critical prevention and detection strategies in Safety Sense Leaflet 34. This Red category briefing breaks down the science of the threat, the vital maintenance checks required, and the regulatory mandate demanding active detection in our cockpits.

Understanding the Enemy:

Carbon monoxide is an exceptionally dangerous gas because it is a colourless and odourless gas that is impossible for humans to detect unaided. It is formed by the incomplete combustion of carbon-based fuel and is produced by virtually all internal combustion engines. To put the risk into perspective, most piston engine aircraft exhaust gas typically contains about 5% CO, so the risk of poisoning is significant.

​When inhaled, CO is toxic to humans as it inhibits the ability of red blood cells to carry oxygen around the body. Consequently, tissue and organs do not receive the oxygen they need to function, resulting in damage to the brain, heart, and nervous system. For pilots, this hazard is compounded by the environment: susceptibility to CO poisoning increases with altitude due to the reduced oxygen density of the air, making those who fly particularly vulnerable.

​The symptoms are notoriously difficult to self-diagnose in flight. Mild poisoning may not present any symptoms at all, or just a slight headache. Because the physiological effects of CO poisoning are cumulative and take time to disperse, even relatively low CO concentrations over several hours can be enough to cause impaired judgement and degrade pilot performance. Moderate to severe exposure can rapidly lead to bad headaches, nausea, dizziness, blurred vision, and eventually unconsciousness.

How Does CO Enter the Cabin?

In normal operation, CO is safely directed away from the aircraft via the exhaust. However, CO can find its way into the cabin via two main pathways: internal migration and external ingestion.

​Internal Migration: Many light aircraft utilise a heat exchanger that takes advantage of the hot air flowing through the exhaust system and uses it to heat air that is directed to the cabin. In the event of a failure in the exhaust manifold (e.g., cracks or worn seals), exhaust fumes can escape and enter the cabin via the heater vents. Unsurprisingly, research indicates that CO poisoning incidents are more prevalent in colder months when cabin heater use is high. CO can also enter via poor sealing of the bulkhead between the engine compartment and the cabin.

​External Ingestion: CO ingestion into the cabin can also occur externally; there is usually a stream of exhaust gas flowing down the outside of the fuselage that could enter the cabin via poorly fitting doors, windows, panels, or vents. In an unpressurised aircraft, the interior air pressure is typically slightly lower than outside, which can have the effect of sucking flowing fumes inside through external gaps.

The Mandate for Detection:

Historically, CO detectors that simply change colour in the presence of CO have been used by GA pilots, but their lack of any attention-getting capability and short life span make them problematic. The CAA recommends that GA pilots flying piston engine aircraft have an electronic CO detector capable of providing audible and/or visible alerts that actively engage the pilot's attention; these are known as 'active CO detectors'.

​In a major regulatory shift, Safety Directive SD-2024/001 dictates that from January 2025, operators of piston engine aircraft are required to have a functioning active carbon monoxide detector on board when operating with passengers who do not hold a recognised pilot qualification.

The CAA allows both aviation standard and commercial-off-the-shelf (COTS) active CO detectors as an acceptable means of compliance. Aviation standard devices are approved for aircraft use (e.g., EASA ETSO-2C48a) and can be permanently installed. Conversely, COTS devices designed for domestic use are also acceptable; findings from a CAA 12-month study suggest that they can function reasonably well at typical GA altitudes up to 5,000 ft.

Practical Advice: Fitting and Position of Device:

• Aviation standard active detectors can be installed in UK-registered aircraft as 'standard changes' under the provisions of CS-STAN, removing the need for a formal modification application. These must be installed by a suitably qualified individual.

  
  

• ​No airworthiness approval is required for COTS active CO detectors that are independently powered.

• ​Pilots must ensure an aural CO warning is audible even when wearing noise-cancelling headsets, but not so loud as to create a distraction.

• D​evices should not be located near fresh air vents as this could affect their ability to detect CO.

• ​Ensure the device is securely held in place for the flying being undertaken. By keeping the device in the pilot's line of sight, alerts are more likely to be noticed. Do not keep your detector inside a flight bag or compartment where it will not be able to sample the cabin air.

Maintenance and Response:

The best way to prevent CO poisoning is to avoid exposure by adhering to a thorough and regular maintenance programme. Exhaust components should be periodically inspected for any signs of damage or corrosion.

A smell of smoke/fumes in the cabin and/or a large drop in engine rpm when applying carburettor heat are often associated with an exhaust system requiring attention.

Furthermore, for aircraft maintained under the UK Part-ML Minimum Inspection Programme, there is a specific CO concentration check requirement, and measurements exceeding 50 ppm should prompt further investigation.

If your active CO alarm triggers in flight, you must act decisively. Keep flying the aircraft; the alert will likely come as a surprise and could be a distraction.

Turn off the cabin heat supply and maximise fresh air entry into the cabin, normally by using fresh air vents rather than opening a window.

Make a PAN or MAYDAY call if appropriate (e.g., experiencing symptoms). Land as soon as possible; do not wait for things to get worse, and consider that returning to the departure airfield may be closer than your destination.

Seek medical attention on the ground if experiencing symptoms, ensure the problem is rectified before further flight, and report the occurrence to the CAA via the MOR scheme.