An American Airlines Boeing 737-800 returned to Chicago O'Hare shortly after takeoff on Friday 16 October after flight attendants saw smoke coming from a trash bin in the galley. The crew declared a mayday and landed safely. According to the airline, no fire extinguisher was used1. No injuries were reported, and the Federal Aviation Administration (FAA) is investigating12.

What happened

The aircraft was operating flight AA618 from Chicago to Dallas/Fort Worth, a scheduled flight of about two hours13. According to The Aviation Herald, it departed from runway 22L, and the crew stopped the climb at 5,000 ft, reporting smoke from a trash bin and a possible fire4. In the radio call quoted by Paddle Your Own Kanoo, one of the pilots says: "Mayday, Mayday, Mayday… American 618, fire in the cabin." The crew asked for an immediate return to O'Hare3. The FAA said the flight returned to Chicago at around 12:30 local time1.

The 737 landed on runway 28C, where airport rescue and firefighting crews were waiting43. Responders found no fire4. The aircraft then taxied to the gate under its own power, followed by the fire trucks. On the ground frequency, the pilots explained that the flight attendants had seen smoke coming from a trash can in the rear galley3. Early reports had placed the smoke in a lavatory bin3. American Airlines told NBC Chicago that the smoke came from a trash can in the galley1.

The aircraft

The aircraft is Boeing 737-823 N974AN, delivered to American Airlines in October 2001 and now about 25 years old2. It was taken out of service for the rest of Friday3. According to Simple Flying, a replacement aircraft left O'Hare at 16:04 local time and reached Dallas/Fort Worth a little over three hours behind schedule2. What caused the smoke has not been disclosed3.

MAYDAY or PAN-PAN

Pilots have two international emergency signals. MAYDAY, preferably said three times, signals distress. PAN-PAN signals an urgency condition. Distress traffic has absolute priority over all other communications and imposes radio silence on the frequency; urgency traffic has priority over everything except distress5. The FAA's Aeronautical Information Manual notes that pilots do not hesitate to declare an emergency when faced with distress conditions such as fire, mechanical failure or structural damage, and urges crews to ask for help early: "Delay has caused accidents and cost lives"6.

Smoke in the cabin: why landing fast is the rule

Smoke in the cabin is one of the most serious situations a crew can face, because the source is not always visible. In its advisory circular on in-flight fires, the FAA notes that fires spreading into the overhead area can become uncontrollable in as little as 8 to 10 minutes, and that a crew facing a hidden fire may have only 15 to 20 minutes to get the aircraft on the ground. Its advice is to begin planning for an emergency landing as soon as possible after the first indication of fire. The same circular notes that the vast majority of cabin events are controlled and do not require an emergency landing7.

For the flight crew, the FAA says the first step of any smoke, fire or fumes checklist should be to don oxygen masks with the regulator set to 100 percent, and smoke goggles where there is smoke. The crew then plan an immediate descent and landing at the nearest suitable airport and notify ATC and the cabin crew, while the cabin crew locate the source and fight the fire7. On the flight deck these procedures sit in the Quick Reference Handbook (QRH).

Trash bins on airliners are designed for exactly this scenario. US certification rules require every receptacle for flammable waste to be fully enclosed and able to contain fires likely to occur in it in normal use. Lavatory waste bins also need a built-in extinguisher that discharges automatically if a fire starts inside8.

Landing heavy soon after takeoff

An aircraft that returns soon after departure still carries most of its trip fuel and can be above its maximum landing weight. The Boeing 737 has no fuel jettison system9. The crew can burn off fuel in a hold or land above the maximum landing weight. With smoke or fire on board, getting down quickly takes priority: an overweight landing is permitted in the interest of safety and is followed by a special maintenance inspection109. Boeing has noted that there are no accidents on record attributed to overweight landings9. Whether N974AN landed above its maximum landing weight has not been disclosed.

What training teaches you about this

Distress and urgency phraseology is part of every pilot's radiotelephony training. Smoke and fire on board come back in type rating courses and recurrent simulator training. The FAA expects operators to train crews to take immediate and aggressive action against smoke, fumes and fire, to don oxygen masks quickly, to recognise signs of a hidden fire and to plan an immediate descent and landing at the nearest suitable airport7. Coordination with the cabin crew is central, because, as on AA618, they are often the first to see the smoke73.

What is not yet confirmed

The cause of the smoke is not known. Neither American Airlines nor the FAA has named one, and the FAA is investigating1. The number of people on board has not been released by American Airlines or the FAA. Reports differ on the time airborne: about 10 minutes according to The Aviation Herald, 26 minutes according to Simple Flying42.