Jane's Defence aviation security expert Chris Yates explains the problem.

Q: How frequently are safety warnings like this issued?

A: Fairly regularly.

Q: Then is this problem with the fuel pumps particularly significant?

A: It is potentially a major problem, because it affects the fuel tank.

Q: What is wrong with the fuel pumps?

A: The problem is that in the manufacture of these pumps the wiring has been placed too close to moving parts.

Q: What could happen?

A: The fear is those moving parts will in some way chap the wiring, which could cause an explosion.

Q: What is being done to protect passengers?

A: The Federal Aviation Administration (FAA) wants to find out in the next four days where these devices are, if they are on aircraft and if so, how many they are on.

Q: How widespread could it be?

A: It looks very much as though we are going to have a global issue here.

Q: Are all Boeings at risk?

A: Right across the board - Boeing 737s, 747s and 757s.

Q: How many are in the United States?

A: The FAA has called for 1,400 or so aircraft to be inspected in the United States.

Q: Is that the majority?

A: I have seen suggestions that there are slightly more than that flying elsewhere in the world. There are an awful lot of operators out there.

Q: What will happen if the faulty fuel pumps are located?

A: They will eventually be removed - but it will not be possible to do it immediately.

Q: What will happen while they are awaiting removal?

A: Every pilot needs to be told that if they have that particular part they must maintain sufficient fuel in the tanks to keep the pumps submerged.

Q: How will that help?

A: If the wiring does become frayed and chapped and there is a short-circuit, it would not cause an explosion because it would be immersed in liquid.