A Concorde was operating a scheduled flight between London Heathrow Airport and New York John F. Kennedy International Airport on this day in aviation history in 1996. Onboard the aircraft was a crew of 9 and 55 passengers. After a normal take-off and climb the aircraft had reached its initial cruise level (FL 490) and was cruising at a speed of Mach 1.97.
Concorde in flight (Source: wallpapersafari.com © Unknown)
When the aircraft was roughly at a longitude of 18º west, the crew observed a rumble on the flight deck. At the same time, the master warning light for the fuel system came on as well as the #1 engine accumulator and fuel low-pressure lights. Furthermore, a fuel low-pressure indicator indicated open and the fuel scavenge pump started operating.
The crew then observed the #1 fuel tank quantity dropping quickly to 450 kilograms. The crew was under the impression they had a fuel leak. Using Oceanic contingency procedures the crew initiated a return to London at FL490 and made a "PAN PAN" call to Prestwick Oceanic Control. The aircraft was decelerated to a subsonic speed, and the #1 engine was shut down as a precaution.
A schematic representation of Concordes fuel system, (Source: www.heritageconcorde.com)
At the request of ATC, the crew set the transponder to squawk 7700 and were cleared to London via the Ockham VOR. A normal approach was flown and an overweight landing was carried out without further issues. After stopping clear of the taxiway the airport fire service inspected the aircraft confirming there was no external fuel leak. After the aircraft was parked, the fuel quantity of the different fuel tanks in the aircraft was checked and the departure fuel minus the burned fuel matched up with the fuel onboard. Further checks revealed that the "missing" fuel had transferred tp to the #6 fuel tank from the #1 fuel tank.
Some fuel transfer checks confirmed that, with the fuel system in flight configuration, fuel from the #1 tank would transfer to the #6 tank.
Schematic representation of an expansion coupling as used in Concorde fuel system (Source: AAIB © Unknown)
Fuel from the #1 tank was fed to the #1 collector tank (to feed engine #1) via a pipe, through the #6 tank. This pipe consisted of 4 sections of pipe and 5 couplings. One of these couplings had come apart, causing the fuel transfer. These couplings were expansion couplings, allowing for the aircraft expansion due to the heating of the airframe at supersonic speeds. Instead of (for example) lockwire, these couplings were locked with a spring. This was the first recorded failure of this type of coupling. The failed components were replaced, the system was tested and no further problems occurred, also on subsequent flights the system operated without faults. The Air Accident Investigation Branch investigated the incident and published an investigation report which is available by clicking here.