Speculation on crash cause 'not helpful'

Deutsche Welle: We are hearing reports that one of the pilots on the Germanwings flight that crashed was locked out of the cockpit. Is being able to lock unwanted people out of the cockpit a feature on all aircraft, or just on Airbus?

John Walton: All commercial airliners will have some sort of reinforced cockpit door, so pilots can restrict flight deck access to authorized personnel.

Under what conditions is such a situation triggered?

In this case, the pilots' door is auto-locking. It is controlled by a three-way switch, which can be thought of as having three selections: "Neutral lock," "Positive lock" and "Unlock." "Neutral lock" is the standard lock situation. No one can unlock the door from the outside unless the pilots select "Unlock". If the pilots select "Positive lock," the door is locked for five minutes until they select "Unlock," at which point the door is, obviously, unlocked.

So even a pilot who is outside the cockpit will be unable to override that system and get into the cockpit.

If the door has been locked, that is correct.

Given the computer-controlled nature of modern flight, how much power does a pilot have to override computers and take control his or herself? There have been reports of an Airbus plane last year that was ordered to descend by its computers because the sensors on its tail were frozen, and the pilot only just managed to countermand the order just in time.

I think I know of the Airbus A321 incident to which you are referring. I am not sure it was as "just in time" as it might initially appear to outside observers. It's important to note that aircraft are controlled by pilots, not computers. Commands are transferred from the pilot to the aircraft's control surfaces by electronics and then hydraulics. Modern aircraft pilots are assisted by computers in terms of efficient and effective flight. This has been the case for many, many years; this is not a new thing. "Fly-by-wire" aircraft are decades old. In any case, the information the computers use comes from different sources: from sensors, from pilot input, and from information arriving into the aircraft from other sources, location data, weather and so on. In terms of referring to this incident, it would be inappropriate to comment on the causes of this crash at this stage or at any stage before the BEA makes its report.

What are the main mysteries surrounding this crash? What loose ends will the experts be looking to tie up?

At this stage, there are any number of possible reasons why this crash could have occurred in the way that it did. The purpose of the BEA, as with any air incident investigation organization, is to determine what those were. It is not helpful to speculate at this stage what might or might not have caused this particular incident.

What information can a flight data recorder provide to help in the investigation?

There are two types of flight recorder. One is the cockpit voice recorder, which records the audio that takes place in the cockpit. That is the one that was first found and that is the one that is being discussed in terms of what appears to have been recorded on that instrument. The other is the flight data recorder, which records a myriad of different parameters from the aircraft in terms of what the aircraft is doing, what its sensors are sensing, what its instruments are displaying. So between the two of them, they provide the principal record of what was going on within that flight.

John Walton is an international journalist working in the field of aviation, and contributes regularly to journals and other media oultets specializing in the topic. http://twitter.com/thatjohn