a) Boeing 757

began flying in 1982 and final assembly of the series -- the 757-200, 757-300 and the 757 Freighter -- is done at the Boeing plant in Renton, Washington.

In July 2000 Boeing began converting 44 757-200 jetliners to freighter configuration for DHL Worldwide Express. Part of the conversion involved cutting into the fuselage and fitting a 20-by-22-foot cargo door, replacement of selected floor panels, strengthening of the main deck floor, and installation of a cargo handling system.

The converted 757 freighters are capable of carrying 60,000 pounds of cargo and have a range of more than 2,000 nautical miles.

Accidents involving 757:

b) Questions

How did the two planes get so close to each other in the first place, as they approached at right-angles on exactly the same altitude?

Why was only the Russian pilot apparently told to take avoiding action?

Why was the first instruction given to him only 50 seconds before impact - and what caused him to apparently miss or ignore it?

Tragically, within seconds of the Russian pilot heeding a second instruction to dive, a collision avoidance computer on the DHL plane suddenly told its pilot to do the same.

In other words, verbal instructions from air traffic controllers to the Russian crew, and computer instructions on the 757, put the planes on a deadly new collision course.

Attention in the inquiry may also focus on the handover of the flights to Swiss air traffic controllers. The Russian plane was handed over from German traffic controllers only five minutes before the crash, while the DHL plane had been handed over by Italian controllers just a few minutes before that.

The planes were already on a collision course - albeit about five minutes away - when the Germans completed the handover.

If the handover does turn out to be a factor in the crash, it will strengthen the argument for Europe's proposed Single Skies system, which would replace individual countries' airspace with an entire European Union zone.

And the fact that pilots have to change radio frequencies as they move across European air space could also come under the spotlight. Investigators will want to be sure that the Russian plane was listening to the right air traffic control frequency when controllers first tried to make contact.

Another area for investigation will be language. The instructions to the Tu-154 was given in English by a Swiss controller to a Russian crew - though so far there has been no suggestion that a misunderstanding occurred.

Why were the instructions given to the Tupolev passenger jet not coordinated with the DHL aircraft?

Whatever the final conclusions, it is likely that a sequence of events - rather than a single catastrophic mistake - was to blame for the tragedy.