Well, we've navigated our way through the system and it's time for us to make our approach into the airport. There are many types of approaches we can execute into airports across the country and they're generally classified as Precision and Non-Precision approaches. I'll first start by telling you what the difference between precision and non-precision approaches is. Quite simply, a precision approach is one that provides an electronic glideslope. Since we don't receive any glideslope information from a VOR, NDB, or localizer, they are categorized as non-precision approaches. Current GPS approaches are also non-precision, but we'll discuss those in their own lesson. There are also other types of approaches which we won't discuss such as Precision Approach Radar and Surveillance Radar approaches. These are completely ATC-controlled, and consist of the pilot following directional commands from ATC. No special onboard instruments.
There are some rules which we must follow when making approaches, so let's look at those now. As we discussed in the "Arrivals" section, ATC will give us descent clearances until we're cleared for the approach. So, we must maintain whatever altitude we've been cleared to until we actually here ATC say "...cleared for the approach...". Once we hear that, we are free to navigate and descend at our discretion per the approach chart procedure. Once you are established on a published segment of the approach, you may descend according to minimum altitudes as charted. Keep in mind that ATC may instruct you to proceed along a portion of the approach course before they actually "clear" you for the approach. So, in real life, it's very important to be sure of what ATC has asked you to do. ATC instructions take precedence over charted procedure. Now back into the FARs for a look at 91.175-Takeoff and landing under IFR, specifically (c)operation below DH or MDA. We know that we're going to be descending to a certain altitude at which we hope to make visual contact with the runway and land.
Now, there are a few items we have to be aware of when making the decision to proceed below the MDA/DH and land. If you don't follow the rules, the FAA might be waiting for you on the ramp after your beautiful approach. The first rule says that we have to continually be in a position to make a normal landing. So, we can't pop out of the clouds right over the threshold at 400ft and decide we're going to chop and drop the airplane in. Second, the FLIGHT visibility must not be lower than the minimum visibility prescribed for the approach (located on the approach plate). Notice I say FLIGHT, not ground visibility. If the tower is reporting less than mimimuns and you've got at least the minimums from the air, you can go ahead and land. Just be prepared to justify it (I could see the threshold from the outer marker, etc...).
Lastly, you must have one of the following in sight(distinctly visible and identifiable) for the runway of intended landing: the threshold, threshold markings, threshold lights, runway end identifier lights(REILs), visual approach slope indicator (VASI), touchdown zone or markings, touchdown zone lights, runway markings, runway lights. There's also one last thing. If you have the approach lighting system in sight, you may descend to 100 above the touchdown zone elevation. However, if the approach lighting system is of the type ALSF-I/II (the kind with the red side row or terminating bars), you may descend to the runway if you have the red lights in sight. So, what all of this means if that, if you're making your approach and you pass over the McDonald's that you know is a mile from the runway, you still can't descend. Visual checkpoints don't count, only the items listed in the FARs. Believe me, it's tempting to do this. I flew the ILS 7 into Santa Barbara (SBA) once and the visibility was awful. I caught sight of the ground just as I was passing over K-Mart at about 350ft and I was thinking "Oh yeah, I'm almost there, no problem". But, I kept on cruising along and I didn't actually spot the runway area for about another 7-8 seconds right at the DH.
So, the moral of the story is, keep it safe. Don't stop flying the approach just because you're starting to catch glimpses of the ground. Take short glances up and then come back and keep flying the instruments. Once you've established visual contact positively with one of the proper items, then make your transition. It's an interesting part of the flight. You go from airline pilot to fighter pilot in just a couple of hundred feet. You want to be frozen on the intsruments until it's time to make the landing and then you've got to fly the plane like it was meant to be flown to get it on the runway in a normal manner. Now, of course, I'm not saying to fly wildly because the FARs say we have to be able to make a normal approach at all times. I'm just saying, do what you have to do. Popping out of the clouds at minimums is no time to be light on the controls.
As I said earlier, 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. 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 satellites (four), you can get vertical positioning as well as lateral positioning. So, there we have it. The ILS Approach will be the focus of this series, and we will cover non-precision approaches in an Advanced IFR series of lessons. Enjoy!
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