Surprising ... Prince William was pictured looking for his seat on the Washington bound plane. Picture: @willypumarol/Twitter Source: Twitter

The Duke of Cambridge was on his way to a meeting with US president Barack Obama when he was snapped looking for his seat after boarding the plane in New York's LaGuardia Airport.

The UK royal sparked excitement among passengers on board. One of them, Sam Kim, wrote on Instagram: "Our flight was delayed and a caravan of black suburbans pulled up to the plane.

"Then the most unexpected guest stepped onto our DC shuttle flight this morning."

NY sightseers ... Prince William and Kate watch an NBA basketball game between the Brooklyn Nets and the Cleveland Cavaliers in New York. Picture: Frank Franklin II/AP Source: AP

Prince William is on his first official visit to the US with the Duchess of Cambridge. The pair have been enjoying sights in New York including a trip to watch an NBA basketball game between the Brooklyn Nets and the Cleveland Cavaliers.

This is not the first time the British second-in-line to the throne has been spotted on a commercial flight.  While this time he travelled First Class, earlier this year he was spotted boarding a flight in Memphis and sitting in economy.    

After his trip to Wasthington Prince William returned to New York for a visit to the National September 11 Memorial and Museum on Tuesday morning.

The visit to the site, where 67 British citizens were among those who died, the most of any foreign country, is came on the last day of the Duke and Duchess of Cambridge's trip.

The visit to New York City and Washington was the first time either of them has visited those cities.

A handful of onlookers braved heavy rain and wind to await a glimpse of the royals on the memorial plaza.

"I love them so much," said Kristina Crossan, a 24-year-old teacher. Her mother, Marianne Crossan, called the weather "a small price to pay".

Ariana Lesko, a 19-year-old student at the Fashion Institute of Technology, said Kate "was just a regular girl" before marrying William. "She's just so down to earth and I think it gives the rest of us normal girls some hope."