RNAV Approach Quiz

From the “you learn something every day” file comes a fascinating Air Safety Foundation quiz on RNAV approaches.

For the non-pilots and/or non-instrument rated among us, RNAV is short for “random area navigation” and for the most part refers to satellite navigation — in other words, GPS. It’s not called GPS because there are other area navigation methods such as loran, omega, inertial navigation, and so on.

But they all do the same basic thing, which is to allow a pilot to fly from any random point in the world to any other point. Prior to RNAV, radio navigation consisted of flying from one ground-based station to another. A highway in the sky, if you will, but one firmly tied to the ground. These ground-based stations are housed in little buildings scattered around the country which transmit signals the aircraft’s navigation receiver can follow. The problem is, these buildings are not movable. They’re expensive to build, maintain, and monitor.

With RNAV, pilots can create virtual waypoints anywhere. RNAV systems therefore have more capability than the older ground-based navaids. If you’ve ever used a GPS, then you’re part of the RNAV revolution.

Of course, there has to be a down-side, right? Nothing is free in aviation, and so it is with RNAV. RNAV systems tend to be computerized and therefore more complex. They also tend to fly in the face of thing we’ve learned about IFR navigation. Curving approach paths, precision approaches without an ILS, etc.

For example, every instrument-rated pilot knows that in order to proceed below the published minimums for an Instrument Approach Procedure, three criteria must be met. In general terms, they are:

  1. The flight visibility must meet the published minimums for that procedure
  2. The aircraft must be in a position from which the pilot can make a normal landing using normal rates of descent
  3. The runway environment (pavement, lights, paint, etc) must be in sight

Aside from an esoteric 100′ rule dealing with a specific part of the approach lighting system, there are no exceptions. Or at least, that’s what I thought until the RNAV quiz taught me about “fly visual” segments.

“Fly visual” segments are typically seen on approaches to airports in mountainous areas. Treat them as red flags: If you see one, take some extra time and give the procedure a closer look.

There are a couple of reasons for this. First, as discussed in the main portion of the course, the visibility required for the approach is sometimes less than the length of the “fly visual” segment-meaning that the pilot can legally continue beyond the DA/MAP without the runway environment in sight, provided he/she has the required flight visibility. Obviously, this leaves a certain amount of room for interpretation. If you find yourself in such a situation, and there’s any doubt about whether to proceed (particularly if you’re not familiar with the local terrain and landmarks), it’s best to opt for the missed approach.

It’s also worth thinking about why the “fly visual” segment exists in the first place. Why did the designers of the approach essentially choose to “slide” the entire approach away from the airport by the distance of the visual segment? In many cases, the underlying reason is that terrain in the missed approach area would necessitate unreasonably high minimums if the MAP were in its normal position. By displacing the MAP a few miles, the designers can build a missed approach segment that doesn’t have terrain problems (a situation well illustrated by the NDB/DME or GPS-A approach at Hailey, Idaho).

Of course, the terrain is still out there, and the danger for pilots flying such procedures is that the unanticipated need to initiate a missed approach beyond the MAP can lead to obstruction conflicts (or, to put it more bluntly: a collision with a mountain).

The bottom line? For procedures like the one at Hailey, never continue the approach past the MAP unless there’s absolutely no doubt about the outcome.

Sounds like fun. Not! Imagine having 1/2 mile visibility and coming to the end of your RNAV highway in the sky, yet being permitted to continue flying visually without the having the airport in sight. TLAR (“that looks about right”) navigation at its best.

The scary thing about these approaches is that they occur in mountainous areas. By definition, these areas having high density altitudes in the summer and are prone to icing in the winter. A mountainous approach is one time I would want to start my missed approach segment earlier rather than later in order to assure adequate terrain clearance during the climb.

The RNAV Approach Quiz is free, and it was far more informative than I had anticipated. Normally I breeze through these things with nary a thought, but I really had to stop and think about some of the questions. And I must admit there were some things in there I didn’t know.

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