A United Airlines Boeing 767 bound for Newark returned to London Heathrow on Thursday 15 October after the crew declared an emergency1. Open ADS-B data shows flight UA941 turning back at 32,000 ft over the Lancashire coast, north-west of Manchester, and landing at Heathrow about an hour and a half after take-off2. Airport fire crews met the aircraft1. United has not said what the problem was3. For crews, the case is a reminder of one of the less visible decisions in an early return from a long-haul departure: what to do about the weight of the fuel on board.

What happened to UA941

The aircraft was the Boeing 767-300ER registered N686UA, according to AirLive1. The US civil aircraft register lists N686UA as a 767-3CB built in 2003 and registered to United Airlines4. adsb.lol data shows it departing Heathrow's southern runway as UAL941 at 15:33 UTC (16:33 local time) and climbing north2.

At 16:01 UTC, level at 32,000 ft over the Lancashire coast, the aircraft began a wide turn back to the south2. It then descended to 15,000 ft and flew a full orbit over the West Midlands2. Its transponder showed squawk 7700 from 16:22 UTC, after which it tracked directly back towards London2. AirLive reported that the crew declared an emergency and requested an immediate return to Heathrow1.

The 767 touched down on runway 27R at 16:58 UTC (17:58 local time)2. AirLive reported that it was met by airport rescue and firefighting services after vacating the runway, that landings switched briefly to the other runway during a runway inspection, and that the aircraft then taxied to a remote stand on the south side of the airport1.

What the airline and the reports say

The number of passengers and crew on board has not been published, and no injuries have been reported. United had not announced the nature of the technical fault, London Now reported3, and The Mirror said it had asked the airline for more information5. AirLive reported that United was preparing a replacement departure, tentatively scheduled for about three hours later1. Whether the passengers have since left for Newark has not been confirmed.

London Now also reported that flight data provider AirNav Radar mentioned an "acrid burning smell", adding that "this has not been confirmed at this time"3. No airline or authority has confirmed a cause.

Jettison, hold or land heavy

According to AirLive, the crew initially planned to jettison about 20 tonnes of fuel to reduce landing weight, then revised that decision and flew straight back to Heathrow1. The Mirror and TheTravel also reported the abandoned plan, citing AirLive56. Whether the aircraft landed above its maximum landing weight has not been published.

The choice itself is a standard one. An aircraft that has to land soon after a long-haul departure may be well above its maximum design landing weight7. SKYbrary describes three options: burn fuel in a hold, jettison fuel, or land overweight7. Reducing weight takes time, which can let a situation deteriorate, while an overweight landing reduces performance margins7. Boeing's guidance notes that both overweight landing and fuel jettison are considered safe procedures, and that US rules allow the pilot in command to deviate from prescribed procedures in an emergency in the interest of safety8. It also notes that some 767s have a fuel jettison system installed even though the regulations do not require it8. After an overweight landing a special inspection is generally required, however smooth the touchdown7.

What squawk 7700 does

Code 7700 tells air traffic control that an aircraft is in distress or facing an urgent situation. The FAA's Aeronautical Information Manual says radar facilities are equipped so that the code normally triggers an alarm or special indicator at all control positions, and that pilots should then establish radio contact with ATC immediately9. The code says nothing about the cause: it is used for technical failures, smoke, medical emergencies and other problems alike. It is distinct from 7500, the hijack code, which featured this week in the Corendon XR456 and Air India AI2030 cases.