(Part I of this blog begins here.)

We need a little bit of history before we continue.

The story is agreed by most to start 52 years ago, with the midair collision of two transport aircraft, a TWA Lockheed 1049A and a United Douglas DC-7 over the Grand Canyon on 30 June, 1956. 128 people died. Commercial flights in visual conditions were then conducted under visual flight rules (VFR), and the collision-avoidance mechanism is see-and-avoid. Pilots must keep a lookout for other traffic, and manoeuvre if necessary to avoid a collision. The Civil Aeronautics Board determined that the probable cause of the collision was that â¿¿the pilots did not see each other in time to avoidâ¿¿ it. The report contains discussion of the inadequacies of see-and-avoid.

Bob has written on this, which spurred the forerunner of TCAS:

â¿¿This tragedy spurred on an existing effort by the US airlines to create an aircraft anticollision system. The Air Transport Association (ATA) -- an association of US airlines -- chose Collins in September 1956 to create such a system, deeming Collins' proposal the most likely to succeed of all those submitted. Collins approach -- and the one that ATA desired -- was a "noncooperative" collision-avoidance system, meaning the system had to be able to detect all other aircraft, whether or not the other aircraft also had a collision-avoidance system aboard."

"Art Collins hoped to have a collision-avoidance system developed within two to four years, but as the company's engineers and scientists got deeper into making the theory of collision avoidance work in practice, it rapidly became clear that the technology just did not exist yet to make such a system. Collins decided to end the program less than six months after it began, and he directed the company to return all the money it had received from the airlines to develop the system. Publicly admitting failure and returning money was an unheard-of corporate practice (and still is); however, rather than hurt the Collins company's reputation, this move only served to enhance it among its customers.â¿¿

It is now common practice, standardised by the International Civil Aviation Organisation (ICAO), to propose safety recommendations in an accident report. There were none in the CAB report, but the immediate conclusion was clear: see-and-avoid has clear weaknesses. The result was that commercial operations were conducted in the future exclusively under Instrument Flight Rules (IFR) in which pilots were assigned a block of airspace in which to fly by an air traffic controller, who is responsible for keeping all other participating (i.e., IFR) aircraft out of that airspace while it is occupied.

Good enough as far as it goes, but since then commercial aircraft have gotten much faster, and in visual conditions there are all those other â¿¿non-participatingâ¿¿ aircraft around (i.e. VFR), including some small ones that are very difficult to see, indeed almost impossible at typical jet-aircraft speeds.

On 25 September 1978, a Pacific Southwest Airlines Boeing B-727 on approach to San Diego collided with a small single-engine Cessna C-172. All 135 on board the B-727 died, as well as the Cessna occupants and 13 people on the ground.

The reasonable administrative response was to try to keep the little guys away from the big guys. The critical phases are initial climbout, and final approach. Traffic Control Areas (TCA) were established for these areas of common airspace around major airports in the U.S., and the little guys were only let in under strictly controlled conditions (positive contact with a controller, who gave flight directions, and an on-board radar transponder with altitude-reporting, so that radar can tell the controller where you are and how high).

In the U.S., there is a tradition of allowing access to all. In Europe, authorities became much more strict about keeping â¿¿general aviationâ¿¿ away from major airports.

And this didn't work in the U.S. On 31 August 1986, an Aeromexico DC-9 hit a small Piper on approach to Los Angeles. The 64 people on board the DC-9 died. The Piper was in the TCA without permission, and his transponder was not operating, so he was all but invisible to radar.

The response this time was political. Congress mandated the installation in the U.S. of the latest version of on-board collision-avoidance system, known by then as TCAS and under development by MITRE Corporation and Honeywell. TCAS has been the law in the U.S. on commercial transports since 1991.

In the next ten years, there were two midair collisions involving commercial jet transports, one near Tripoli, Libya in December 1992 in which a B-727 collided with a Mig-23 jet fighter, and one near New Delhi, India, on 12 November 1996 in which an outbound Saudia Boeing B747 jumbo jet collided with an inbound Kazakhstan Air Lines Ilyushin Il-76 cargo jet.

Then we had ÿberlingen on 1 July 2002, and the Amazon collision on 29 September 2006, all with TCAS. A list of all midair collisions involving jet transports (with the exception of those in the former Soviet Union) may be found at airsafe.com .

(More to follow in the future)