The silent killer

CO is a stealth killer : it is odourless, colourless, and tasteless. When inhaled, it is easily absorbed into the bloodstream where it attaches itself to haemoglobin and has a direct effect on the performance of those parts of human physiology that rely on oxygen for proper functioning. CO inhalation can lead to damage to the brain, heart, and nervous system[1]. The cumulative nature of CO means that low-level, long exposure can be as dangerous as a concentrated, short duration event. In both scenarios, the effects have the potential to be fatal. 

CO is produced by the incomplete combustion of fuel. Piston engines - by far the most common engine type used in small General Aviation[2] (GA) aircraft - produce particularly high concentrations of CO. Normally, these particles are directed away from the aircraft via the exhaust system. Problems can arise in the GA arena since many light aircraft heaters utilise air flowing over the exhaust manifold to provide cabin warmth but manifold cracks and poor sealing can result in CO entering the cabin with serious, potentially fatal consequences.

The catalyst for change

The Sala fatal air accident that occurred on 21 January 2019 in the English Channel, north of Guernsey, brought the issue of CO poisoning as a causal factor in GA air accidents to wider public attention. The Argentinian striker, Emilio Sala, was being flown from Nantes in France to Cardiff when the single-engine Piper PA-46-310P Malibu aircraft (registration mark N264DB) crashed at night, in bad weather. Mr Sala's body was subsequently recovered from the seabed. The pilot's body has never been found. Post-mortem tests on the passenger showed a blood carboxyhaemoglobin (COHb) level of 58%. The pathologist reported that at this level Mr Sala would almost certainly have been ‘deeply unconscious’ at the moment of impact.

The loss of N264DB, which occurred in international waters, was investigated by The UK Air Accidents Investigation Branch (AAIB). In its final report published on 13 March 2020[3], the AAIB concluded that the pilot had lost control of the aircraft in bad weather and was probably affected by CO poisoning. The originating source of CO in the cabin was likely to have been in a fault in the aircraft’s exhaust system. There was no record in the aircraft documentation of any type of CO detector unit having been fitted to the aircraft.

At the time of the AAIB's final report, in the UK since 2000 there had been three other air accidents, two of which were fatal, where CO poisoning was identified as the likely cause[4]. In its N264DB report, the AAIB flagged fifteen other CO events then recorded on the CAA occurrence reporting system. On 11 of those occasions, a cabin CO monitor had alerted the crew to the presence of CO. The occupants on another four flights experienced nausea and light-headedness. There were seven other reported occurrences of exhaust fumes in the cockpit where the aircraft was not fitted with a CO detection monitor. The actual number of CO instances in GA flights may be even higher: there is likely under-reporting since in GA aircraft that are not fitted with a CO visual/audio detector, those on board who are exposed to non-fatal levels of CO may attribute fatigue, dizziness, drowsiness, stomach pain or headache to causes other than CO poisoning and in not making the connection, fail to make a CO occurrence report to the CAA. 

The AAIB is empowered by the UK Secretary of State for Transport to make Safety Recommendations based on the findings of an investigation and the need for action to be taken to maintain and improve aviation safety. In its report into the loss of N264DB, the AAIB made five such recommendations. One directly addressed the issue of CO detectors in GA aircraft flights. The AAIB advocated for the carriage of CO detectors: “Routine maintenance is vital but cannot eliminate the risk of carbon monoxide leaks completely. Equipping aircraft with devices that provide warning of the presence of this odourless, colourless, and lethal gas, would enable pilots to take potentially lifesaving action. We are therefore calling for the regulators to make it mandatory for piston engine aircraft, such as the one involved in this accident, to carry an active CO warning device."  

Stakeholder views

In February 2024, by CAP 2975 (Consultation: Carbon Monoxide in Piston Engine Aircraft) the CAA sought stakeholder views on, "the challenges facing pilots in obtaining an active carbon monoxide (CO) detector, the importance of protecting passengers from CO, the role that maintenance plays in combatting CO, and whether active CO detectors ought to be mandatory for some operations". Responses received from the consultation, decisions taken as a result as well as identified next steps are set out in the CAA's Common Response Document CAP 3024, published in August 2024. The CAA advised of three decisions:

1. to introduce a limited mandate to have a functioning active carbon monoxide detectors in certain GA aircraft flight operations (see further below at CAA Safety Directive 2024001 V2).
2. not to introduce mandatory CO concentration checks in piston engine aircraft maintenance programmes beyond what is already specified by aircraft manufacturers and UK regulation[5].
3. to publish additional guidance on CO topics including (i) selecting an appropriate active CO detector (ii) where and how to securely position CO detector devices in aircraft (iii) how to respond to alerts, as well as (iv) guidance on exposure levels and thresholds for alarms.

Mandatory action to restore an acceptable level of safety

CAA Safety Directive 2024001 V2, (Active Carbon Monoxide Detectors for Piston Engine Aircraft Operations) published on 30 August 2024 contains an Operational Directive requiring mandatory action that is necessary, "to restore an acceptable level of safety". It is issued in accordance with Article 248 of the Air Navigation Order 2016. The Safety Directive comes into effect from 1 January 2025 and remains in force until further notice. 

• The Safety Directive is of application to pilots of piston engine aircraft with certain, limited carve-outs for (a) single-seat aircraft, (b) aircraft with an open cockpit/cabin, (c) aircraft performing aerobatic manoeuvres, (d) those with piston engines located above/behind cabins (eg helicopters and gyroplanes) and (e) aircraft with only wing-mounted piston engines. The carve-out for each of (d) and (e) does not apply if cabin heat is also provided via an exhaust heat exchanger or a combustion heater.
• Not all operations of affected aircraft fall within the scope of the Safety Directive : operations in which all occupants hold a recognised pilot qualification are not required to comply although the CAA "strongly recommends" that all pilots of piston engine aircraft at risk of CO fly with an active CO detector, regardless of who is on board.
• The CAA paid heed to those responding to the consultation who had expressed concern as to the potential cost implication for installation of compliant CO detection equipment. To facilitate compliance, both aviation standard and off the shelf units are recognised as acceptable. Although not specifically approved for aviation use, findings from a CAA 12-month study (CAP2560: Carbon Monoxide Detector Trial Summary Report) suggest that domestic CO detection units can function reasonably at typical recreational GA altitudes of up to 5000 ft. 

CO does not discriminate

The carve out for affected aircraft flight operations in which all persons on board (including all passengers) are qualified pilots seems a little curious. The Safety Directive states in express terms that it prioritises the protection of passengers and is mindful that those without a pilot qualification may not be aware of the dangers of CO in GA flight operations. However, the mere holding of a pilot qualification does not afford any individual enhanced CO detection ability or immunity from succumbing to the effects of CO exposure. The decision arguably sits at odds with the CAA's statement in CAP 3204 that "CO does not discriminate based on who is in the aircraft…" and observation that CO is of an "insidious nature."

A basic internet search for domestic CO detectors identifies a plethora of options. Prices are highly competitive with numerous models falling in the £20 to £30 range. Some units cost as little as £15. There are compact units - as small as 6.5 cm x 6.5 cm - and weighting just 60gms (about the third of the weight of an average smart phone). Smaller units are designed to be highly portable, featuring a clip to attach to clothing, making them ideal for many GA cabins in which space is limited. As the CAA has observed[6] " it has never been easier for pilots to find a device that suits their needs and budget". 

The ultimate GA stocking filler 2024

When one considers (i) that CO detector units are potentially life-saving and (ii) the peril they guard against is a silent killer, a mandate to install or to carry a unit on all GA flight operations of affected piston engine aircraft, regardless of whether all on board are qualified pilots or not, seems a no-brainer. In our homes, we install CO detector units as a matter of course. The installation of functioning CO detector units has been a legal requirement in the private housing rental sector in the UK since 2015[7]. 

Safety Directive 2024/001 (V2) comes into force on 1 January 2025. As the Autumn evenings draw in and our minds turn towards Christmas, what better stocking-filler is there for GA pilots of piston-engine aircraft this year than a CO detector unit.

[1] Taken from Section 1 Factual Information, AAIB AAR 1/2020 Piper PA-46-310P Malibu, N264DB.

[2] General Aviation (GA) is defined by the International Civil Aviation Organisation (ICAO) as "all civil aviation operations other than scheduled air services and non-scheduled air transport operations for remuneration or hire."

[3] AAIB AAR 1/2020 Piper PA-46-310P Malibu, N264DB.

[4] G-BGEW; accident occurred on 20 September 2009. AAIB reference EW/C2009/09/20. G-ARIE; accident occurred on 12 May 2001. AAIB reference EW/C2001/05/03.

[5] Reg (EU) No.1321/2014 Annex Vb (Part-ML), Minimum Inspection Programme (MIP)

[6] CAA CAP 2560.

[7] The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022