Aviation In Canada

Sunday, August 28, 2005

Misused IF's

A long time ago, in an FIR far, far away, IFs were established. These Intermediate Fixes were placed on the final approach courses of many an approach, typically somewhere around 10NM final, but with some variation. The idea was to allow aircraft with RNAV a certain flexibility, at least when approaching from some directions, to conduct their own navigation to final without the need for DME arcs, procedure turns or ATC intervention in the form of vectors, if ATC were even able to see the aircraft on final on radar. The theory was good, but there seemed to be a lack of direction and some problems started to occur. Well, they weren't problems all that often, but certainly the potential was there. The problems were small, insidious little things that kind of crept up.

These IFs were great when the airplane was coming pretty much straight in already, or even off to the side in what would amount to a base leg, an angle of 90° or so from the FAC. But what about when the angle exceeded that? Many aircraft systems would reject the turn to final in such cases. Especially when the FAC was pretty much a 180° turn from the course inbound to the IF. That didn't stop aircraft from asking for direct to the IF, though. Pilots are pretty clever, and the nav gear became more sophisticated, and this combination had pilots doing some things which were a little unexpected. Many would "project a waypoint" off to the side of the IF, then navigate to the IF and join the FAC from there. Problem is, they'd ask for a clearance to the IF, receive and acknowledge it, leading those crotchety old ATC types to think they would navigate to the IF instead of an imaginary, temporary and unnamed point. The best part is, the ATC would think they could depend on the aircraft's nav gear to take it on a nice, straight track to the IF, since it had the ability, only to find out the aircraft was aiming often around 5NM beside the IF instead of at it. In a terminal environment, ATC could have another aircraft on a vector approaching the fix, planning on the course for direct to the IF for the "self sufficient one", when he only needs 3NM radar separation between them. The unannounced course could have the airplane on the wrong side of his traffic. A little communication could go a long way to alleviate expectations for this one, though.

Another problem occured when the pilots received a clearance for a straight-in approach via an IF. Some pilots, especially those flying into uncontrolled airports, often decided after acknowledging such a clearance to disregard the IF and point directly at the Final Approach Fix, hoping to save a minute or two of flight in the process. Hey, with gas prices in this FIR, anyone can appreciate that. Unfortunately, those darn ATC types would be taken by surprise again, expecting to have that minute to use when issuing a clearance to an aircraft who wanted to take off. Now, the window of validity for the departure was compromised, as was the lateral separation expected when the departure got airborne since the arriving aircraft might be as much as 5NM or so closer to the airport than he should have been when approaching on a base leg style track when he makes the turn to final.

And the last one was a little different, since it often didn't compromise separation. Some pilots, keen on using the advanced nav gear and wanting to set up an FMS to do its thing, would ask for clearances to these IFs from hundreds of miles away. This often meant that those controllers had to ask "down the line", or so to speak: One enroute controller had to ask another, who had to ask another, who often had to relay the request to the controller working the airport in question, who had to send the approval or denial back up the line to the original controller. Many controllers, not wanting to make a pilot wait that long, would often clear the pilot to the requested fix and either tell the next guy on the hand off, or actually forget to tell them. Sometimes the fix requested wasn't associated with the runway in use, which meant the pilot would set up his computers for an approach he might not get. There was even the rare pilot that would get upset about this, and demand an inactive runway saying that it had already been approved for him, since he received a clearance 30 minutes ago or other such arguments. Another tough one to deal with.

Well, the problems, small as they were perceived by some, continued on a daily basis without either side being aware of the other's concerns, since both groups had bigger fish to fry. Unfortunately, for those reading this story, no such thing has happened in this little far away FIR for us to learn from, so we'll have to come up with our own little solutions. Maybe we can solve these issues before they lead to a problem, perhaps even before those in the little imaginary FIR where this story originated.