Explainer: What are the fuel switches at the centre of AI 171 crash probe?

Flight data shows both engine switches moved from 'RUN' to 'CUTOFF' seconds after takeoff, one after the other within a second, causing both engines to lose power

The preliminary report from investigators probing the Air India crash that killed 260 people showed that seconds after takeoff, the Boeing 787 plane's engine fuel control switches were switched off briefly, starving them of fuel.

Below are a few facts about the switches, their functions in the aircraft and their movements on the Air India flight.

What are fuel switches?

They are switches that regulate fuel flow into a plane's engines. They are used by pilots to start or shut down engines on the ground or to manually shut down or restart engines if an engine failure occurs during a flight.

Aviation experts say a pilot would not be able to accidentally move the fuel switches that feed the engines. But if moved, the effect would be immediate, cutting off engine power.

There are independent power systems and wiring for the fuel cutoff switches and the fuel valves controlled by those switches, according to US aviation safety expert John Cox.

Where are the fuel switches located?

The two fuel control switches on a 787, in Air India's case equipped with two GE engines, are located below the thrust levers.

The switches are spring-loaded to remain in position. To change one from run to cutoff, a pilot has to first pull the switch up and then move it from run to cutoff or vice versa.

There are two modes: 'CUTOFF' and 'RUN'.

What happened on the fatal Air India flight?

According to the flight recorder, a few seconds after takeoff, switches for both engines transitioned to 'CUTOFF' from 'RUN' one after another with a time gap of one second. As a result, the engines began to lose power.

One pilot was heard on the cockpit voice recorder asking the other why he cut off the fuel. "The other pilot responded that he did not do so," the report said.

It did not identify which remarks were made by the flight's captain and which by the first officer.

Seconds later, the switches flipped back to 'RUN', according to the preliminary report. Both fuel control switches were found in the 'RUN' position at the crash site, it said.

When fuel control switches are moved from 'CUTOFF' to 'RUN' while the aircraft is in flight, each engine's control system automatically manages a relight and thrust recovery sequence of ignition and fuel introduction, the report said.

"No sane pilot would ever turn those switches off in flight," especially as the plane is just starting to climb, US aviation safety expert John Nance said.