Flying IFR, Lesson 7
Precision Approaches As I said in the last lesson, Precision Approaches have an electronic glideslope. We will be flying an ILS approach for our example which uses both a localizer and a glideslope. The localizer is just like a VOR except it only extends in one particular direction and it's four times as sensitive. So, you'll just have one radial extending out from the runway it serves which you'll be tracking inbound. The glideslope is also a single signal which extends upwards from the runway area at a preset angle(usually 3 degrees). It operates on a UHF frequency which is paired to the localizer frequency so we can receive both on a single frequency. The glideslope will provide us with precise descent guidance. Instead of the MDAs which non-precision approaches use, precision approaches have Decision Heights(DHs). Because we will receive descent guidance, we will arrive at the DH right at the point where we should be able to make normal landing. So, at that point, we immediately have to make a decision as to whether to land or whether to execute a missed approach. Hence the name Decision Height. In non-precision approaches, we leveled off at our MDA and continued on to the MAP. In the case of precision approaches however, we won't level off at the DH to make our decision, we will continue our normal descent all the way to the DH, make our decision, and then land or level off and start the missed approach. We may also fly ILS approaches without the glideslope. Some older aircraft do not have OBS insturments with glideslope indicators. Sometimes the glideslope may be out of service. In this case however, it become a non-precision approach since we no longer have the electronic glideslope. So, we would then follow the ``loc only" or ``gs out" procedure and minimums on the approach plate. The actual procedure for flying these approaches isn't so different from non-precision approaches except that we'll be following a needle down to the runway instead of descending to a minimum altitude at our discretion. Remember that our needles are going to be very sensitive, so you've got to have a steady hand and a lot of concentration. I flew the ILS into West Palm Beach for practice once at 160kts in a Seminole. That's faster than a lot of jets fly approaches. It was pretty hairy as I got down to the last 1000ft or so and I was totally focused on the needles. It was night time, but VFR, and I didn't see anything at all until I hit the DH and looked up. I was just practicing so I went missed and all I remember seeing was the tower as I zoomed by at 160kts. It's tough to keep your head in all the things you need to be aware of during an approach like this. Don't get tense and let it fall apart. A lot of people fixate on one needle and forget the other. There's a lot going on, don't get stuck on any one thing. There are other types of precision approaches, though none we can do in X-Plane. Some large airports and other which are located in the vicinity of military bases offer Precision Approach Radar. The controller can give you precise heading vectors and altitudes in order to bring you right down to the DH. GPS can allow for precision approaches too, though the FAA is just getting into implementing and approving it. If your receiver can aquire enough sattelites(four), you can get vertical positioning as well as lateral positioning. So, there we have it. The ILS will be the focus of our discussion here and then we will move on to DME arcs, which can apply to both precision and non-precision approaches. Enjoy!
©
1999 - AVSIM Online and Andrew Ayers |
