Pilots Make Emergency Landing While Wearing Oxygen Masks

— -- Pilots on a U.S.-bound British Airways flight carrying 221 passengers were forced to don oxygen masks and make an emergency landing at London's Heathrow Airport after they realized airflow to the cockpit was restricted, an official report has revealed.

On March 6, the captain and his two co-pilots began experiencing nausea, headache, light-headedness, and "a constant urge to take deep breaths and difficulty maintaining concentration" at around 34,000 feet, according to a report unveiled Thursday by the British Air Accident Investigation Branch (AAIB).

When efforts to reboot the Boeing 777’s air conditioning and recirculation systems failed, the pilots became concerned that the flight controls could overheat, forcing them to open the cockpit door -- normally closed for safety post-9/11 -- to improve airflow, the report states.

"Curtains were drawn to prevent the passengers from seeing that the cockpit door was open," according to the report, and and one of the co-pilots “remained on the flight deck for added security."

After about 15 to 20 minutes, the cockpit door was closed and all three pilots eventually put on oxygen masks.

Though the pressurization in the passenger cabin remained normal, they decided to return to their departure airport. Jettisoning around 28,0000 kilograms of fuel over the Irish Sea, they landed without incident at Heathrow.

"The safety of our customers and crew is always our priority and actions have been taken to help prevent this situation from recurring," British Airways said in a statement to ABC News. "Our highly trained pilots took the decision to put on oxygen masks and return the aircraft to Heathrow after they noticed a fault, where the plane landed normally. Our cabin crew looked after customers and we arranged an alternative aircraft so that they could continue their journey."

A subsequent investigation revealed that an accumulation of wire, bubble wrap, and insulation material was blocking the air duct servicing the cockpit, according to AAIB, which also noted that the "brittle appearance" of the debris suggested it had been building up in the duct "for some time."

In the month preceding the March 6 incident, the report noted, pilots had twice reported inadequate airflow and high temperatures in the cockpit.

The first report was "rectified" by "cleaning restrictors," the second by adjusting the flight deck temperature sensors, according to the AAIB.

"Pilot reports of low flight deck airflow existed but were sufficiently rare that it could be concluded that such problems were not endemic," the AAIB report concluded.

However, it said, "there were no routine maintenance tasks to check flight deck airflow" in the Boeing 777.

ABC News' Jon Williams contributed to this report.