Aviation In Canada

Thursday, September 15, 2005

Projecting Approach Fixes

I received a question about aircraft using RNAV to project fixes for use on approaches. I have to say that from an ATC point of view, there are some possible pitfalls for this procedure. Let me explain.

In years past, just before the publishing of Intermediate Fixes (IF) on final approach courses, some operators with RNAV (most notably certain Air Canada aircraft) were asking for "direct to the centerline fix". I'm not sure if these were projected in the cockpit or calculated on the ground by system engineers and added to the waypoint databases on the aircraft. Either way, the idea was the same as the use of the IFs now: to allow, where possible, the use of the aircraft's RNAV and FMS to fly the airplane to final approach without, or with as little as possible, ATC intervention. Some controllers disliked it for various reasons, some controllers made use of it where they could, but eventually, it was determined as an inappropriate procedure and we (ATC) were directed not to allow it.

The reason for this is that the fixes were not published. This meant it left some ambiguity as to what altitude was safe for a given area, where the fixes actually were (Air Canada's, I believe, were all placed at 10NM, but some operators got keen on this and projected 8NM gates, and one did a 12NM gate), and some aircraft seemed more capable of others on "smart turns" which turn before the fix while others flew over it before turning, allowing a window of opportunity for separation problems or unexpected flight paths.

With IFs published now, things along those lines are considered reasonable. Now, some pilots are asking for clearances for "direct FIXXX" which is an IF, but fail to mention that they plan to project a fix off to the side for base leg when approach angles to the IF and final approach course are great. The projection of a fix falls again under the same issues that were raised in the past (obstacle clearance, non-standard positioning, etc), but the bigger part to me is the fact that pilots doing this are rarely ever mentioning that's what they plan to do. They ask for direct FIXXX and fly to a projected waypoint somewhere off to the side. In a terminal area, the controller might be expecting the plane to track nicely directly to the fix requested (why wouldn't he? -- the plane is obviously capable of a direct track since he asked for it), but instead makes an unexpected turn. If the fix is, say, 5NM from the centerline and an aircraft is put on what is expected to be a parallel vector, this could lead to a converging traffic situation since the controller may be using as little as 3NM, effectively aiming the vectored aircraft on the wrong side of where the RNAV aircraft is unexpectedly going.

Until fixes on base leg are published, there is a level of ambiguity that must be resolved somehow, and the only way to resolve it is by communication. My guess is that someone will eventually have a hair-raising incident with this sort of practice, from the ATC side or the pilot side, and the brakes will be put to it anyway. It may lead to published fixes, and perhaps that's what is desired and needed. I'm sure we'll run out of five-letter fixes before they get published, though. Perhaps some sort of naming convention including numbers can be published for this?