by Dave Klain
To explain the basic concepts of Non ILS instrument approaches
- A localizer is an electronic beam which transmits a specific signal and a VOR receiver can determine left/right (horizontal) reference to the beam. Think of it as a single radial off a VOR. It is typically (but not always) aligned with the runway centerline.
- A glideslope is an electronic beam which is vertically as opposed to horizontally oriented. As a result, a glideslope receiver can determine above/below (vertical) reference. A glideslope signal is typically aligned with a localizer beam.
- An ILS (Instrument Landing System) is a type of instrument approach consisting of a localizer, a glideslope and specific approach and runway lighting. It is one of many kinds of instrument approaches which enable an airplane to safely get from the enroute environment down to a position where it can see the runway and make a landing. There are three kinds of ILS approach with the CAT-III being the one with the lowest minimums...typically allowing the approach to be flown even with no visibility.
The nice thing about the ILS is it generally has much lower minimums (read how low you can descend without seeing the "runway environment") than other approaches...in most cases it's a decision height of 200 feet touchdown zone elevation. That said, if the ceiling is say...800' (don't forget, anything below 1000' ceiling and 3 miles visibility is considered Instrument Meteorological Conditions (IMC)) you might shoot a VOR, GPS or even an NDB approach. Let me walk you through various approaches in increasing order of precision:
- The least precise instrument approach is an NDB (Non-Directional Beacon) approach. These are much easier if the NDB is located on the airport, but sometimes the beacon is located off the airport. In either case, you typically fly to some designated fix, then fly towards the beacon or away from the beacon on a specific heading and descend down to your minimum descent altitude (MDA). You fly that direction for a specific amount of time based on your aircraft's speed. If you see the runway environment before the clock runs out, you land. If you don't, you go missed approach.
- A VOR Approach is just like an NDB approach but uses a VOR beacon as the primary source of navigation. The advantage of a VOR over an NDB is that a VOR has radials so you know what radial you are on from the VOR all the time making it much easier to stay on the specified track. Other than that, works the same...cross the final approach fix, start the clock and either see something or run out of time and go missed.
- It's very debatable which of the next two is more precise, but for argument's sake I'm listing GPS next...that said in many ways a GPS approach is easier to fly than the others! In a GPS approach your GPS receiver is programmed to give you guidance to a specific series of waypoints...basically the same thing as a flight plan on a GPS. You fly to the initial approach fix then fly to the next fix (generally a fix near the end of the runway in question) and descend down to MDA. You time for backup but with the GPS you know exactly when you reach the missed approach point. Again, you either see the runway environment and land or go missed. Note that approach-approved GPS receivers have functions in them that makes them more precise and likely to detect a problem than a normal GPS receiver...that's why you cannot legally shoot an approach with a non-TSO Approach approved GPS even though almost all of them have the approaches in their database. When flying a GPS approach, the GPS will drive your left/right needle just like a VOR. As you pass the final approach fix, the GPS makes that needle much more accurate than it is in enroute nav mode allowing it to give you very, very precise positional information...
- The localizer approach is simply that...a localizer beam. An ILS (we'll get to that later) actually has two separate beams transmitted on separate frequencies. The localizer beam gives you horizontal guidance, the glideslope gives you vertical guidance. Now a localizer approach might be an ILS with the glideslope out of service or they might just have the localizer installed. In this case you fly to a final approach fix, acquire the localizer beam and fly it towards the missed approach point. Since you don't have vertical guidance, you still just descend to MDA and fly until you reach the missed approach point. Since the localizer beam is a beam projected from the ground, just like a VOR beam, it gets more accurate as you get closer to the transmitter meaning one dot's needle deflection at 5 miles is much more distance than one dot's needle deflection at 1000 feet from the runway... There is NO difference between a localizer beam and the ILS localizer beam...they are the same gear and equally precise.
- A special kind of localizer approach is the backcourse approach. Localizer beams are actually transmitted in two directions (way the antennas work). One way is obviously the localizer beam we just talked about. It also transmits a beam the opposite direction (off the other end of the runway). Works exactly the same except for one big difference. Since the beam is configured for the localizer in the other direction, you get reverse-sensing on the backcourse. What does this mean? If on a localizer the needle is to the left, you need to turn left/move left to get back on centerline. If on a backcourse the needle is to the left, you need to turn right/move right to get back on centerline. This frequently confuses pilots who lose situational awareness and blow the approach when they keep turning towards the needle and the needle just keeps moving away. This is also why the autopilot has a backcourse or bc button. That button tells the autopilot that it will have reversed sensing on the approach...
OK, all of the above are considered Non-Precision approaches. Let's shift gears and talk about Precision approaches and a couple of bonus kinds of instrument approaches most people have never heard of... Remember that a big difference between a non-precision and precision approach is that a non-precision has a Minimum Descent Altitude (MDA), a precision approach has a Decision Height (DH).
- First is the GPS WAAS approach. This is relatively new. It is just like a GPS approach but with Wide-Area Augmentation System equipment on the ground helping it. What WAAS gear does is help the GPS receiver locate itself more precisely (how it does that is too technical to get into here). Because of that, the GPS can be relied upon to not only give you horizontal guidance, but vertical guidance as well. You fly WAAS just like an ILS with both a localizer and glideslope needle...just it's the GPS unit driving the needles instead of a transmitter on the ground. Since you have vertical navigation, you now have a Decision Height instead of a Missed Approach point. You descend to DH. If you see the runway environment, you land...otherwise you go missed.
- Now comes everybody's favorite...the ILS or Instrument Landing System. An ILS is simply a localizer that also has a glideslope beam transmitter with it giving you vertical navigation as well. Fly down the beams to DH and land or go missed! See IntroILS for an in-depth look at the ILS.
OK, let's talk about two other kinds of instrument approaches that are available using a Ground Controlled Approach (GCA). This is a type of service provided by air-traffic controllers whereby they guide aircraft to a safe landing in adverse weather conditions based on radar images...some civilian airfields can do them, most can only do one of them. Most military airfields can do both:
- The ASR approach is a non-precision approach using the airport's airport surveillance radar. Basically the controller gives you vectors to a point off the extended centerline of the runway. He then has you turn to runway heading. As you deviate from the centerline, he tells you to turn left or right to keep you on course. Since it is non-precision, when you cross the final approach fix the controller will tell you to descend to MDA. When you reach the missed approach point you will either land or go missed.
- The PSR uses a Precision Surveillance Radar, also referred to as PAR or Precision Approach Radar. This is a radar that not only tells the controller where you are left/right of the runway centerline...it also projects a beam up like a glideslope and the controller can see if you are above or below that beam. Just like an ASR the controller gets you to the final approach fix and you start your descent. He tells you "left/right/up/down" and keeps you in the crosshairs of the two lines... the extended centerline and the extended "glideslope". Same thing, you reach a decision height (remember you have vertical nav guidance) and either see it or go missed. Flying a PSR or PAR is just like flying an ILS, an ASR is just like flying a localizer....just you can't see the needles! The controller effectively is looking at the needles for you and telling you where they are.
The great thing about an ASR and PSR approach is that you can fly them if everything in your plane breaks except the radio. These are frequently called no-gyro approaches and flown that way. You don't need a gyro since the controller can just tell you to turn left or right at a standard rate or half-standard rate turn to correct. These approaches have saved the bacon of more than one pilot who had a vacuum failure and lost their gyros and AIs...
Let's shift subjects and talk about what happens now when you get to that missed approach point. In most cases if you see the runway you go straight in and land. FAA TERPS rules say that if the course you come in on is more than 30 degrees off the extended runway centerline, you need a higher MDA so that you have time to circle around and set up to land on the runway. This is called a "Circling Minimum" and means just that. You break out, see the runway, and circle around (keeping the runway in sight the entire time) until you are lined up with the runway, then you land. Almost all approaches have a higher set of circling minimums that allow you to break out and circle to land on any runway at the airport. That's how you get to that runway that doesn't have any kind of approach associated to it but lines up with the wind! While circling minimums may be lower, many pilots will not fly a circling approach at anything less than about 800 feet. Much lower than that and you are setting yourself up to enter a spin and crash or run into something.