Possible AirProx Incident What should I do ?
O.K. so I am flying along the 160 radial from LAM to the QE2 bridge en-route from North Weald to Biggin Hill in my C175 at 2200ft on the QNH.. .. .I am probably 4 miles North West of the bridge when all of a sudden the underside of a large white aircraft appears in the top of my screen having overtaken me from above and behind and I am looking at the back of this thing (probably with my mouth open and several unprintable words comming out) wondering if there is any point in trying to do anything seeing as it is now at least a mile away on what looked like a heading of 180.. .. .My passenger (a PPL with a few hundred hours) tells me (after issuing some of his own private words)to keep steady and try to hold everything straight because we are probably going to experience some wake vortex.. .. .The other aircraft crosses the Thames and banks right for what I guess is an approach to London City. My passeneger says he could clearly read the name of the airline along the side of the other aircraft as it passed us, and that it was a Dornier. . .. .It probably was not that close but it looked like 100ft! I guess it could have been much more because we did not experience any wake vortex.. .I can imagine that it may not be possible to see a small cessna in front and below because it could be below the windscreen if you are not peering out. Then again what I experienced may have been them taking avoiding action for all I know.. .. .I do know that it is not possible to see anything above and behind from a small high winged Cessna so I would never know until it were too late if something faster comes from behind.. .. .I was squawking 7000 and on the return flight I did confirm with Thames Radar that my Mode C tansponder was working correctly.. .The vis was very good.. .. .O.K. so here are my questioning thoughts.. .1. What is close ?. . How does one judge the distance accurately ?. . I am not sure how big this thing really was.. . If it was the same size as a 747 it could have been a mile away.. . If it is similar to a 146 it was close.. .. .2. What action should I take.. . Do I tell someone what it looked like from my point of view without knowing any actual facts, other than those above and the time and name on the aircraft ?. .. .3. If I am squawking 7000 does my signal get registered even if I am not working a ground radar station ?. .. .4. At what range would a small commercial airliner TCAS see my aircaft. (or would it at all ?). .. .I am not interested in assigning blame but merely in thinking about what I could and should do in any similar circumstance.. .Any ideas or thoughts would be appreciated.