The family-of-five killed in the New York City helicopter crash were believed to have been on vacation to celebrate the birthday of one of their young children.

Agustín Escobar, a Siemens executive from Spain, his wife Merce Camprubi Montal and their three children aged four, five and 11 were killed when their sightseeing chopper plummeted into the Hudson River at 3.15pm on Thursday.

The family were on vacation in New York to celebrate the birthday of one of their daughters, according to Spanish outlet Antena3.

They had just arrived from Barcelona and embarked on their first day touring the Big Apple, taking a scenic trip in the helicopter around the Statue of Liberty and up to the George Washington Bridge.

The tragic twist emerged as terrifying new footage appears to show the cause of the tragedy.

A rotor blade can be seen plummeting into the water, with aviation experts saying that this likely occurred because the main rotor blades separated from the aircraft and sliced the tail.

‘From the footage, it appears that the main rotor struck the body of the helicopter, cutting off the tail of the helicopter, which created an unrecoverable event,’ former military aviator and attorney Jim Brauchle of Motley Rice LLC said.

‘The two main causes of this phenomenon are mechanical failure or excessive maneuvering. Still, a full investigation is needed to understand why this tragedy occurred.

‘Having previously represented the families of tourists killed during a helicopter tour over the Hudson River, my heart goes out to the families at this catastrophic time.’

Michael Roth, 71, who owns New York Helicopter which provided the tour and the chopper, said the aircraft was running out of fuel before it crashed.

‘He [the pilot] called in that he was landing and that he needed fuel, and it should have taken him about three minutes to arrive, but 20 minutes later, he didn’t arrive,’ Roth told The Telegraph.

Roth said he was devastated by the crash and agreed with other experts that the video appears to show the main rotor blades had broken off.