A Delta Air Lines flight from London Heathrow Airport to Boston Logan International Airport was forced to make an emergency return on Sunday when fumes filled the cockpit. Flight 59 was carrying 259 passengers and operated by an Airbus A330-900 with registration N411DX, which features the special commemorative 'Team USA' external color scheme. It was able to make a safe return to the airport about an hour and a half after taking off.
This is the latest incident of a Delta A330-900 experiencing fumes or smoke in the cabin, with three other similar incidents reported last year on its next-generation A330 fleet. The issue may be related to the auxiliary power unit (APU) of the aircraft. Delta is currently replacing the APUs on its fleet of Airbus A320 family aircraft to address toxic fume leakages.
An Emergency Return To London Heathrow
According to Flightradar24, the aircraft took off from London Heathrow at approximately 10:00 local time for the seven-hour flight to Boston, and reached an initial cruising altitude of 36,000ft off the west coast of Cornwall. Travel website PYOK reports that at this point, fumes started to fill the cockpit, and the pilots elected to make an immediate return to London and requested assistance from air traffic controllers.
The pilots would have had to wear oxygen masks so that they were not overwhelmed by the fumes. They chose not to dump fuel but instead return directly, and the flight ultimately made a heavy landing on runway 27L at 11:33 local time, nearly an hour after the incident was initially reported. Upon arrival, the aircraft was surrounded by airport fire and rescue vehicles, and after the 'all clear' was given by airport fire officials, the passengers were disembarked and taken by bus back to the terminal.
In a statement issued by the airline, a spokesperson for Delta said:
"Delta flight 59 from London-Heathrow to Boston on October 5, 2025, landed safely after returning to LHR due to reports of smoke in the aircraft. The flight will operate tomorrow, and Delta's customer team is assisting customers with accommodations and rebooking. We apologize to our customers for the delay, but safety for our customers and crew will always be Delta's top priority."
Multiple Issues With Fumes Onboard Delta A330-900s
Avid readers of Simple Flying will know that this is far from an isolated incident, with reports of toxic fumes in the cockpit and/or cabin showing a sharp increase in recent years. The Wall Street Journal recently brought renewed attention to the issue of fumes from engine oil or hydraulic fluids entering cabins and cockpits through what's known as the bleed-air system. The Journal says that crew reports indicate "the frequency of such events has climbed dramatically in recent years", with symptoms ranging from odd smells to more serious neurologic and respiratory harm.
But Delta seems to be suffering from more than its fair share of these types of incidents. Despite its 39 A330-900s being some of the newest aircraft in its fleet, with an average age of just 2.9 years, it has had multiple reported incidents of fume-related diversions in the past couple of years.
Other Incidents Of Fume-Related Diversions Of Delta A330-900s
Date
Route
Details
September 12, 2024
New York JFK -- London Heathrow
Fumes were reported in the cabin, leading the crew to don oxygen masks and request medical assistance for one passenger. The flight stopped climbing at FL250 and returned to JFK for an overweight but safe landing on runway 13R about 50 minutes after departure. No injuries occurred.
August 7, 2024
Boston -- Paris
Fumes in the cabin caused illness among several crew members. The flight returned to BOS for a safe landing on runway 04R about 85 minutes after departure. The captain, first officer, and five cabin crew were hospitalized. Maintenance checks found no clear source of the fumes.
January 1, 2024
Rome -- Boston
Fumes were reported onboard at FL360. The crew declared an emergency and diverted to Shannon for a safe landing on runway 06. One passenger felt unwell and was examined by paramedics; the flight was canceled. The source of the fumes was not identified.
But the issue is not related to just the A330-900s either. This year alone there have been multiple fume-related incidents across the Delta fleet. These include:
February, Atlanta to Columbia: A Boeing 717 was forced to make an emergency landing back in Atlanta after thick, smoky fumes filled the cabin shortly after takeoff. Passengers and crew reported difficulties breathing. An investigation noted that firefighters saw a "tremendous amount of smoke" upon opening the flight deck door. February, Amsterdam to Detroit: Fumes and a strange odor affected several flight attendants on a Airbus A330-300, forcing the flight to divert to Dublin. Passengers reported a smell that made some crew members feel unwell. June, Atlanta to Orlando: A Boeing 757 flying one of Delta's 16 daily ATL-MCO flights was forced to divert to Jacksonville after smoke filled the cockpit. August, Orlando to Raleigh-Durham: An Embraer A175, operated by Republic Airways, reported fumes and a smoky odor and declared an emergency as a precaution. The aircraft landed safely at RDU. Could The A330-900 Fumes Be An APU Issue?
The fume incidents on the A330-900s, such as the one used for Flight 59 on Sunday, could be related to the APU units onboard. Delta is currently replacing the APUs on more than 300 Airbus A320 family aircraft to address an identified issue of toxic fume leakages. In its reporting, the Wall Street Journal says that Honeywell, which dominates the APU market for A320s and A330s, has had known issues with fume events related to load pressure seals in APUs "for over two decades".
Nearly all commercial jets 'bleed' fresh air into the aircraft from its engines, which is then fed through an air conditioner and into the cabin. But if an engine seal fails on either the main engines or the APU, oil and other potentially toxic particles can be released, blowing fumes into the cabin that crew and passengers typically describe as smelling 'metallic' or like dirty socks.
Far worse than a bad smell, the risks of these fumes to passengers and crew are very real. A prominent neurologist, Robert Kaniecki, says that the toxins in the fumes can cause brain injury and permanent damage to the peripheral nervous system. Over the past 20 years, Kaniecki has treated roughly a dozen pilots and more than 100 flight attendants for brain injuries linked to fume exposure on aircraft, which he describes as "akin to the traumatic injuries football players sustain after hard hits during play."