How is it possible to turn a plane's transponder off?

The Malaysian Prime Minister, Najib Razak, has said the communication systems of the Malaysia Airlines plane which vanished a week ago with 239 people on board were disabled deliberately.

Mr Razak said the shut-off was consistent with the actions of someone on board the airliner, and that the latest analysis of data showed that the Boeing 777 aircraft flew for nearly seven hours after its last communication.

David Gleave, an aviation safety investigator from Loughborough University, said whoever did this would need considerable knowledge and expertise.