United Airlines passengers were left terrified after their flight collided with another parked aircraft while backing up at a gate in San Francisco. 

The Denver-bound Flight 1871 struck another of the airline’s fleet at around 9.10pm on Monday, according to the Federal Aviation Administration. 

The plane hit the tail of Boston-bound Flight 796, which was parked while preparing for takeoff. 

After the aircraft was towed back to its gate, ‘passengers on both planes deplaned normally,’ United Airlines said in a statement. 

‘We arranged for different aircraft to take our customers to their destinations,’ the airline added. 

A image showed damage to the tail of one of the planes. 

One Boston-bound passenger on Reddit said they were, ‘in the middle of boarding when the plane shook.’

‘FAs told us to deplane and one of the pilots said we’d been hit,’ they wrote. ‘Seems like United ground crew is having a tough year.’

On Monday, Flight 1871 bound for Denver struck the tail of another of the airline’s fleet while pushing back from the gate around 9.10pm

The FAA is investigating the incident, which occurred in an area where traffic controllers do not communicate with the aircraft

Another, who claimed to have been on the Denver-bound flight, said: ‘It was a big shudder when we hit. We were all buckled up, [the other plane’s passengers] were boarding at the time. 

‘We were in D16, so they backed us out, down the taxiway, then pulled a big U turn to get us facing forward.

‘Near the end of the U-Turn apparently the tug connection broke, and instead of continuing to turn, the plane just kept going backwards at an angle and hit UA796 (SFO-BOS) which I believe was at E5.’

No injuries were reported, according to the FAA and the airline. 

The FAA is investigating the incident, which occurred in an area where traffic controllers do not communicate with the aircraft, it said. 

The crash was the first of two incidents involving United Airlines flights in as many days, after another of its fleet was forced to return to San Francisco on Tuesday after a burning smell was reported in the cabin. 

Flight 2664 bound for Baltimore, carrying 164 passengers, turned back around one hour after departing at 9.47am on Tuesday, the San Francisco Chronicle reported. 

One passenger on the flight said they were ‘in the middle of boarding when the plane shook’

The plane hit the tail of a Boston bound United Airlines plane, Flight 796, which was parked while preparing for takeoff at San Francisco International Airport 

In a statement to the outlet, the airline said the decision to return was made ‘to address a plastic burning smell in the cabin.’ 

Passengers deplaned safely and an alternative plane was arranged by the airline. 

The cause of the smell remains unclear, but no injuries were reported. 

Daily Mail has contacted the FAA for an update on the investigation.