A320, vicinity London Heathrow UK, 2019

A320, vicinity London Heathrow UK, 2019


On 23 September 2019, the flight crew of an Airbus A320 on approach to London Heathrow detected strong acrid fumes on the flight deck and after donning oxygen masks completed the approach and landing, exited the runway and shut down on a taxiway. After removing their masks, one pilot became incapacitated and the other unwell and both were taken to hospital. The other occupants, all unaffected, were disembarked to buses. The very comprehensive investigation was unable to establish the origin of the fumes but did identify a number of circumstantial factors which corresponded to those identified in previous similar events.

Event Details
Event Type
Flight Conditions
Flight Details
Type of Flight
Public Transport (Passenger)
Flight Origin
Intended Destination
Take-off Commenced
Flight Airborne
Flight Completed
Phase of Flight
Location - Airport
Root Cause Not Determined, Monitored Approach
Non-Fire Fumes
Flight Crew Incapacitation
PAN declaration
Bleed Air
Damage or injury
Non-aircraft damage
Non-occupant Casualties
Occupant Injuries
Few occupants
Off Airport Landing
Causal Factor Group(s)
Aircraft Technical
Safety Recommendation(s)
None Made
Investigation Type


On 23 September 2019, an Airbus A320 (G-EUYB) being operated by British Airways on a scheduled international passenger flight from Zurich to London Heathrow was passing 4000 feet QNH in day VMC on an ILS approach to destination runway 27L when “strong acrid fumes were detected on the flight deck. Both pilots donned oxygen masksa ‘PAN’ was declared to ATC and after landing without further consequences, the aircraft was stopped once clear of the runway. The absence of any smell in the passenger cabin was confirmed but when the pilots removed their masks, the First Officer became incapacitated and after the engines had been shut down, both pilots were taken to hospital with “minor injury” recorded and all other unaffected occupants then disembarked by bus.


A Field Investigation was carried out by the UK AAIB with data available from the CVRFDR and a DAR, the latter providing additional data parameters to those available on the FDR.

It was noted that the 43 year-old Captain had a total of 12,700 hours flying experience which included 6,000 hours on type. Corresponding details in respect of the First Officer were not recorded. It was also noted that both pilots had previously been involved in separate fume events on the same aircraft type and reported that they had discussed these experiences during the previous evening whilst on their rostered night stop at Zurich. The investigated event occurred during their first flight the following morning.

What Happened

It was established that the aircraft had been parked at Zurich overnight after its evening arrival (operated by a different crew) and that, whilst parked, it had rained with the air temperature approximately 13°C. The departure of the aircraft from Zurich for the investigated flight was reported by the crew to have initially been uneventful. It was still raining and cloud was entered at approximately 1,000 feet agl and the subsequent climb was in cloud.

Soon after passing FL100, a “slight odour” became apparent on the flight deck. The Captain suspected a galley oven source but the First Officer described it as a “sweaty socks” smell which he stated was similar to smells he had previously noticed on the aircraft type but stronger. The Captain stated that he had been “concerned that they were preconditioned to detect fumes because of (both) their previous experience of fume events and their discussion the evening before”. He therefore suggested that they should wait 30 seconds before taking any action to see if the smell dissipated and after this interval, the smell had gone.

The crew reported that their previous experience “suggested that if the smell was going to reoccur it was most likely to occur when thrust was reduced for descent so, during the cruise, they discussed their actions if the smell returned and reviewed the SMOKE / FUMES / AVNCS SMOKE Checklist". They then briefed for a Monitored Approach with the Captain taking control for landing. In fact, the descent took place in clear skies and there was no recurrence of fumes. After a short time in a holding stack, the flight was radar vectored onto the runway 27L ILS LLZ at Heathrow but as the aircraft passed through 4,000 feet QNH fully established, both pilots “detected a sudden, very strong smell”. The Captain described it as having been a “manure smell [...] like a field which had just been muck spread” and commented that the smell had instantly hit him in the back of the throat. The First Officer described the smell as one of “strong sweaty socks” and reported having felt “itchy skin around his eyes and a scratchy throat”. There was no smoke or any obvious source of the smell.

Both pilots donned their oxygen masks in turn and, having requested and received an early transfer from APP to TWR, the Captain then declared a ‘PAN’ to TWR reporting flight deck fumes and requesting a priority landing. In response, TWR instructed the two aircraft ahead and the one behind to go around in sequence. The Captain decided to use autoland and advised TWR that after landing, they would exit to the parallel taxiway and then stop to await an inspection from the emergency services.

Once the aircraft had stopped, the after landing checklist and the initial actions of the SMOKE / FUMES / AVNCS SMOKE checklist were completed. The First Officer then made initial contact with the attending emergency services whilst the Captain gave a ‘NITS’ brief on the situation to the Senior Cabin Crew Member (SCCM) over the interphone and was advised that there was no smell in the cabin and that the passengers were “not aware of anything unusual”. The Captain then spoke with RFFS and made a Passenger PA.

After the First Officer had briefly removed his oxygen mask to see if the fumes were still present, he advised that they were and the Captain decided to start the APU and shut down both engines and to open the flight deck windows. At this point, the First officer began to feel nauseous and then vomited out of his flight deck side window. Having originally intended that the aircraft would be towed to a parking stand, he then decided that the First Officer needed urgent medical attention and requested that steps be brought to the aircraft. The First Officer went to the forward cabin toilet and continued to vomit and the SCCM entered the flight deck to assist the Captain, reporting that on entering, he had detected what he described as “a chemical smell [...] a clean clinical smell” whilst noting that there was still no smell in the cabin.

After brief RFFS confusion as to where the access steps were to be positioned, they were brought to door 1R and fire service personnel, paramedics and a member of the aircraft operator’s engineering team came on board. No fumes were detected and following an assessment, both pilots were taken to hospital and the passengers and cabin crew, none of whom reported any ill effects, were subsequently disembarked using steps and boarded coaches for transport to the terminal. Both pilots were released from hospital later the same day.

The Cause

During comprehensive ground runs, the fumes which had prevailed in the flight deck during the final few minutes of the flight could not be reproduced nor could any lesser fumes occurrence be detected by use of a handheld air testing device. The aircraft was then handed back to the operator for further work utilising engineering Work Packages developed in response to previous fume events on its A320 family aircraft. When all these had been completed with no relevant findings, the aircraft was released to service, following which four further (and less serious) fumes events occurred over the succeeding three months for which no cause could be found.

However, it was concluded that the available evidence indicated that it is likely that the fumes in this event were “derivatives of contaminants entering the Environmental Control Systems (ECS)”. These may not necessarily have been comprised of only a single compound but potentially a combination of compounds which having reacted together had then become airborne in the bleed air supplied by the ECS concerned, in this case only by the system which supplies air to the flight deck since the cabin was unaffected. It was also concluded that the fumes in this event may have had “similar traits to hydrocarbon compounds combined with water vapour in low concentration which are liberated as water vapour condenses when it enters cooler conditions, for example as it passes into the flight deck or cabin via ducts”. It was also noted that this possibility is compatible with occurrences during sectors with prior exposure to high humidity or rain. This is supported by Airbus’ observation that in this type of event, “the fumes decrease, or in many cases disappear, when the humidity of the air in the cabin decreases at higher cabin altitudes”.

It was noted that, due to the failure to eliminate the regular occurrence of unexplained air conditioning-related fumes events on this aircraft type using enhanced maintenance procedures, Airbus has been involved in a significant effort to determine the possible causes of the problem. Analysis of data from operators’ events has led to the general conclusion that “the presence of fumes dissipates at higher altitudes” but whilst the prevalence of fume events has been reduced, they have continued to occur. One enhancement being developed which it is hoped will help to alleviate the problem is enhancements to the ECS filtration system which will be available as a retrofit to aircraft in service. In the meantime, Airbus has issued a SIL to provide background information, mitigations available and best practices on fume events.

Other Similar Events

During the first nine months of 2019, British Airways’ recorded over 500 fume events of varying levels of operational significance on its large fleet of A320 family aircraft. Some of these were identified as having occurred in similar operational circumstances to the event under investigation with similarly inconclusive maintenance investigations following and five of these were considered in detail during the Investigation. Whilst it was accepted that the available evidence in these and other events did not appear to show an obvious single precursor, it was noted that “several common traits” were reported by the crews involved to have been associated with such events:

  • The incidents took place both on the short haul European inter-city flights and with regional flights of less than two hours duration.
  • The aircraft often arrived in, and stayed for varying periods of time, usually overnight, in damp humid environments with drizzle or rain present.
  • In many cases the pilots described a faint smell during climb out on departure.
  • The flights usually continued normally with no signs of any fumes during the cruise phase.
  • When events occurred, the pilots described the fumes and odours reappearing on descent and in a number of cases at about 4,000 ft AGL in stable flight.
  • The presence of fumes can have a rapid and adverse effect on flight crew.
  • The crews described similar smells and odours, such as sweaty socks, manure and farmyard smells, which were unpleasant and distinctive.
  • The fumes described in these events were invisible.
  • FDR recordings did not show any correlation between the engine settings or other system selections or settings and the point in the flight at which the fumes appeared.
  • In most cases the cabin crew were unaware, until they were told, of any fumes or odours; the problem seemed to be confined to the flight deck.
  • When the fumes were detected by the cabin crew, they appear to linger in the galley areas.
  • In those cases, the passengers were usually unaware and unaffected.
  • In the more severe cases, the fumes prompted the pilots to declare a PAN and go onto oxygen.
  • The generation of fumes appeared to be transient and dissipated very rapidly, in some cases before landing and in others very shortly after landing.
  • In most cases when other individuals were given access to the flight deck after landing, they could not detect any smell or odour.
  • In the small number of cases where those entering the flight deck were initially able to detect something, it did not have any adverse effect on them and seemed to go away very quickly.
  • There were no detectable traces of condensate, solid compound or dust present in the flight deck afterwards.
  • The use of the smoke and fumes abnormal and emergency procedures appeared to alleviate, but not completely eradicate, the effects in the main cabin.
  • The use of aircraft washing fluids, detergents and anti-icing fluids. In most cases washing or anti-icing operations had not been carried out prior to the flights in which the events occurred.

It was also clear that other operators of this type were continuing to experience similar fume events but also that such events were not confined only to the aircraft of the A320 family.

Susceptibility to Fume Exposure

Although no sample of the fumes experienced was available and no deductions of their general or exact type were established during hospitalisation of the two pilots, it was observed that the potential consequences of exposure to fumes depends not only on the nature of such fumes but on the individual concerned.

The following general remarks on this matter were included in the Official Report:

The human olfactory system is extremely sensitive and complex and can detect minute concentrations of airborne compounds; as few as four molecules can give a recognisable smell. The ability of a person to interpret a smell requires a cognitive process and therefore the perceived intensity and the effect of a smell will vary between individuals.

When exposed to an unusual or stressful situation a person will experience an unconscious ‘stress’ response based on several factors, including: the context of the event, any prior anticipation of the situation, the perceived level of danger the situation presents and previous experiences of a similar situation. This reaction can produce physiological effects which differ markedly from person to person. For example, the act of cutting up an onion often causes an extreme reaction resulting in excessive tear production, stinging and watering of the eyes. In a kitchen environment where cutting up an onion is quite normal, the affected individual can quickly and easily understand what is happening and, although uncomfortable, it is not a cause for concern. However, if the same adverse stimuli, watering and stinging of the eyes, occurs in a situation that cannot be explained by the context of the environment, a person will experience an increased level of stress and associated physiological response.

It was also noted that the consequence of variable individual responses to particular fume events was - as in the case under investigation - that in any such flight deck event, both the perception of its seriousness and the extent of any immediate consequences from exposure generally varies between two pilots exposed to the same fume event. In this context, evidence from this and many similar previous investigations was that prompt use of an oxygen mask was an important early action for both exposed pilots.

The formally documented Conclusion of the Investigation was as follows:

While it has not been possible to positively identify the compound that was responsible for the fumes and odours experienced in G-EUYB, or any of the other recent events, a number of common factors have been identified:

  • The majority of events occurred after the aircraft had been parked or operated in precipitation.
  • The fumes become apparent during the later stages of the descent, sometimes preceded by a minor event during the climb phase.
  • The generation of fumes appears to be transient; they dissipate rapidly and leave no detectable trace.
  • No link between changes to engine power or changes in other system settings and the generation of fumes was identified.

Safety Action taken in response to this and other recent similar A320 family events was noted as having included the following:

  • Airbus has:
    • Initiated ‘Project FRESH’ to investigate and regularly inform operators of fume event occurrences.
    • Published an In-Service Information paper (Ref ISI setting out all the known aspects of fumes and smoke events and includes the details of a filter and sensor product research and development programme.
  • British Airways has:
    • Developed the post-smoke and fume events maintenance procedure.
    • Taken action to ensure that the correct APU start up bleed air selection and shut down procedures are used.
    • Carried out a fleet-wide check to confirm that oxygen masks were correctly stowed and issued a Quality Alert Bulletin to all engineering staff to remind them of the importance of stowing the masks in accordance with the AMM.

The Final Report of the Investigation was published on 30 July 2020. No Safety Recommendations were made.

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