If it's unverified it is useless information.
I disagree. An unverified Mode C is not helpful for official separation of aircraft—agreed. However, if I've got an aircraft I'm talking to at a particular altitude, and there's an unverified Mode C opposite direction (or crossing, etc) at the same altitude, I'm going to pass that traffic ("altitude indicates"). I've got 20 pounds that says that the vast majority of the aircraft drivers out there would like me to help them remain clear of that traffic if they don't see it.
PLEASE don't misunderstand, HD. I've got a great deal of respect for you (I've been at this for 23 years and, except for the fact that you’re in the UK, YOU might have trained the guy who trained the guy who trained me). I just see this as the difference between dogmatic and practical. Certainly we've all seen Mode Cs that were inaccurate. However, when I'm working somebody who is at the same altitude as an unverified Mode C, I'm pulling out all the stops. Especially since the VAST majority of Mode C readouts are pretty darned close. Now, I’m not saying that I depend upon an unverified Mode C to lull me into not calling traffic. What I’m talking about is the opposite… when the unverified Mode C is close to somebody I’m working, I’m all over it.
A brief anecdote… Once upon a time I had a friend who worked at the same facility as I who had something bad happen. This was the approach control at a very busy international airport. On the day in question there was lots of VFR traffic without Mode C showing up on his display. He was working arrivals in (what was then referred to as) the TCA. The non-Mode C traffic below the TCA (which was not talking to him, nor was it required to talk to him) was very, very heavy. He said they looked like ants swarming on an anthill. He assumed that the VFR aircraft were below the controlled airspace he was working and although he did call some VFR traffic for the IFR guys, he didn’t get them all. As he said, “when the elevator door opens, you expect there to be an elevator.” In other words, when there’s a solid mass of VFR aircraft being displayed, at some point you have to depend on them to stay where they are supposed to be. Such was not the case. One of those light VFR aircraft had blundered into the TCA and was struck by an arriving commercial aircraft, resulting in a great loss of life. If that VFR aircraft would have been Mode C-equipped, my friend would have pointed it out as traffic and this disaster might have been averted. Again, it’s not about seeing an unverified Mode C that appears to be 5,000 feet below your IFR traffic and ignoring it. It’s about the last, best chance to avoid metal-to-metal contact.
I am allowed to use a Mode C if it is accurate to within 299 feet. If the VFR in this anecdote had been showing an unverified altitude anywhere NEAR the IFR traffic (certainly anywhere near controlled airspace), this whole thing might have been a non-event. Hell, if he’d been showing any Mode C at all the traffic probably would have been called. In any case, my friend did what he was supposed to do, but it obviously wasn’t enough. That’s what he believes anyway.
The NTSB determined that the probably cause of the accident was the limitation of the air traffic control system to provide collision protection, through both air traffic control procedures and automated redundancy. Factors contributing to the accident were (1) the inadvertent and unauthorized entry of the [VFR aircraft] into the [TCA] and (2) the limitations of the “see and avoid” concept to ensure traffic separation under the conditions of the conflict. I’m just not clear on how an unverified Mode C would have been “useless” in this accident.
I’d only add that this facility was not the busiest in the US when it came to overall movements. However, it WAS the busiest when one divided the number of movements into the number of control positions ( this is to say, other TRACONs worked more aircraft, however, the number of aircraft per position was far greater at this facility). Even at this facility I was never too busy to call traffic and say, “altitude indicates…” Small price to pay given the alternative.
Dave