A United Airlines flight to Washington Dulles declared an emergency and was forced to return to Bradley International Airport in Connecticut shortly after takeoff because one of the flight deck windows was unlatched.

The pilots only discovered that the window wasn’t shut properly after the Boeing 737-900 had taken off. None of the 178 passengers and crew onboard the nine-year-old aircraft were injured after the pilots safely landed the plane less than 20 minutes later.

According to one report, United Airlines flight UA-1274 levelled off at 4,000 feet a short time after takeoff and declared an emergency because one of the flight deck windows had “popped open”.

The radio transmission from the pilots was difficult to hear because the sound of wind in the background but air traffic controllers were able to clear the plane for an immediate return to Bradley International.

In a statement, a spokesperson for United confirmed that UA-1274 had to return to the airport to “address an unlatched cockpit window”.

“The flight landed safely, and we reaccomodated our customers on another aircraft”.

In the end, the passengers reached Washington Dulles around six hours late.

There is a side window on either side of the Boeing 737 cockpit had can be opened from inside the cockpit and can be used for when pilots need to communicate directly with ground staff or to evacuate the aircraft in an emergency.

There are various cockpit window opening mechanisms depending on the make and model of aircraft, but on the Boeing 737, a simple lever and slide handle opens the cockpit window. To close the window, the pilots simply need to reverse the process and ‘lock’ the window in place.

There isn’t, however, any electronic indication to confirm that the window is closed and locked.

Interestingly, it is possible to open and close the flight deck window in flight (at lower altitudes), and Boeing has training materials for pilots to do exactly this. On this occasion, the aircraft was taken out of service for 12 hours, which might suggest that engineers wanted to make sure there wasn’t any damage to the aircraft or an underlying reason for the window to suddenly open.