KLM B744 at Amsterdam on Aug 4th 2014, unreliable airspeed

A KLM Boeing 747-400, registration PH-BFF performing flight KL-643 from Amsterdam (Netherlands) to New York JFK,NY (USA), departed Amsterdam's runway 36L and was in the initial climb when the crew requested to level off at 6000 feet but to continue along the standard departure route. About three minutes later ATC queried whether the aircraft would be able to climb, the crew advised they had an indication problem with the airspeed but accepted a climb to FL240 and subsequently further to FL340. Enroute at FL340 about 10nm northeast of Blackpool,EN (UK) the crew decided to return to Amsterdam, descended to FL230 enroute to Amsterdam and landed safely back on Amsterdam's runway 27 about 2 hours after departure.

One of the air data computers was malfunctioning causing the first officer's speed indications to be intermittently unreliable. A bird strike during the takeoff run is being suspected.

The airline confirmed a post flight inspection revealed evidence of a bird strike.

A replacement Boeing 747-400 registration PH-BFW is estimated to reach New York with a delay of 4 hours.