Sunday, February 19, 2006

The Right Approach

One of the more common questions we get from flight deck visitors is "how do you find the airport?" It's a good question, but it doesn't have a short answer. We have many ways of finding the desired patch of terra firma to land on, with technology ranging from rather crude to bleeding edge. Here's a discussion for those of you not familiar with IFR flying.

The majority of the time we simply fly visual approaches. As it's name would seem to indicate, a visual approach is flown by visual reference to the airport...simply "eye-balling it." Good weather is required, particularly in mountainous areas. The key is planning your descent so that you arrive at pattern altitude just far enough from the airport to slow down and configure for landing. If you find yourself too high to make a normal descent, a visual approach allows you to maneuver (S-turns, 360's, etc) or reconfiguring the airplane for higher drag.

When clouds or poor visibility preclude a visual approach, we'll shoot an instrument approach instead. I'll get to the various types of instrument approaches in a minute, but there are some shared characteristics to mention. All instrument approaches are created, tested, approved, and published by the FAA. At most airlines, pilots use "Jepp Charts," but these are simply another representation of the FAA-published data for each approach. Every approach has a standard course that each airplane must fly, as well as "feeder routes" and instructions in case of a "missed approach." There is a minimum visibility value that must be met before airline operators can even begin the approach. Finally, and quite importantly, there is a published minimum altitude based on obstructions and terrain. You cannot descend below this altitude until the airport is in sight.

The most common instrument approach is an ILS (Instrument Landing System). An ILS uses ground-based antennas to transmit several signals that guide airplanes right to the end of the runway. The localizer signal provides lateral ("right or left") guidance. The glideslope signal provides vertical guidance, usually for a 3 degree glidepath. In the airplane, these signals are picked up by a receiver and displayed for the pilot in the form of needles. The pilot follows the ILS to the ground by following the needles. For example, if the localizer needle is to the right, the pilot needs to go right to get on course. If the glideslope needle is down, the pilot needs to steepen the descent to get on glidepath. Both localizer and glideslope are quite sensitive; for this reason, a ILS approach usually provides a minimum altitude of 200 feet above touchdown elevation.

At my airline, we actually have extra equipment installed that allows us to fly special ILS approaches that have minimum altitudes of 50 to 100 feet above touchdown elevation. These are known as Category II and III ILS approaches ("Cat II" or "Cat III"). Most airlines use autoland-capable autopilots to fly Cat II/III approaches; at my airline we hand-fly these approaches using heads-up displays like you'd find in a military fighter. Only large airports, and some fog-prone smaller airports, have published Cat II/III approaches. Airports that don't have them are often inaccessible during fog.

The basic enroute navigational aid in the United States is the VOR - Very High Frequency Omnidirectional Range, if you care. Besides it's role in enroute navigation, this navaid also is used for VOR approaches. These are tricker than ILS's because there is no vertical guidance, so you descend in steps as you cross certain points on the approach. You navigate laterally with the same needle you'd fly an ILS with, except that you need to select the course. The lack of a glideslope, as well as the fact that VORs aren't nearly as sensitive as localizers, means that VOR approaches usually have much higher minimums than ILS approaches. Some airports are served by only VOR approaches, but they are rare in the airline world. Most of the airports we fly to have other approaches, and VOR's are just a backup to those.

The dinosaur of navaids is the non-directional beacon, or NDB. The NDB approach is like the VOR approach in that it lacks a glideslope, so descents must be made in steps. NDBs aren't very precise, so NDB approaches often have quite high minimums.

A relatively new option is the FMS approach. In general aviation these are known as GPS approaches, but at the airlines they are usually flown with a Flight Management System that utilizes GPS signals. In my airplane, FMS approaches provide both lateral and vertical guidance, just like an ILS - you follow the needles the same way. Some FMS approaches are "standalone," meaning that they were specifically designed to be FMS approaches; "overlay approaches" are actually laid over an existing VOR or NDB approach. In this case, you still use the VOR or NDB minimums, but at least you have the added safety and convenience of a glidepath.

The newest up-and-coming thing is RNP Approaches. These are similar to FMS approaches, in that they are dependent on GPS for navigation, but are more precise and incorporate neat things like curved or zigzagged courses. This will allow for low minimum altitudes, even in high terrain where an ILS would be impossible. Our sister airline is already using RNP approaches, and I'm told that we'll be getting approval sometime within the next year.

So what approach do we use? Whichever is most convenient and provides acceptible minimums given current weather conditions. Usually if there is an ILS available, we'll use it. If an FMS approach is quicker and the weather isn't too bad, we'll use that. We tend to stay away from VOR or NDB approaches unless the weather is pretty good or no other approach is available; of course, most of these approaches now have FMS approaches overlaid, so they're a lot easier these days. Of course, no matter which approach we use, we can't go below the minimum altitude until we see the airport. So once the gadgetry "finds the airport," it's back to good ole fashioned eye-ballin' it to safely land.


Anonymous Uncle Billy said...

What!!!! Nothing about lights????
MALSR, MALSF, ALSF/SSALR, ODAL, PAPI, REIL, Green Threshold lights?

Boy, I need to whip you into shape

7:34 AM  
Blogger Sam said...

Heh, slightly out of the scope of a post for non-pilots...but if you're into that sorta thing, Aviatrix had a good series on the subject of lighting a month or two back.

11:50 PM  
Blogger Nate said...

Sweet picture, must be from the top of the IDS! Good Blog.

12:34 AM  

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