![]() If you normally use a three-bar VASI you should be able to assume the ILS glide slope was aimed with you in mind. If you are flying a Boeing 747 into an airport that is designed for a Boeing 737 it could very well be an issue. The chart might say 50' and you may find yourself much higher because the chart was designed for somebody else. Wherever you go the threshold crossing height will be okay for you. If you are flying something much smaller than a Boeing 747 you are probably okay. So the math could be based on a displaced threshold. It gets worse: while most sources say these heights are defined where the landing surface begins or takeoff surface ends, some use the pavement itself. While the approach chart could very well say 50 feet threshold crossing height, the actual height may have been adjusted to accommodate the largest airplane that regularly frequents the airport in question. The wheels on landing, for example, will be below threshold crossing height. Threshold Crossing Height seems to mean the exact same thing in many manuals, but there are also sources that say it is the height from the end of the runway to the ILS glide slope antenna. ![]() During landing, for example, the landing gear will cross the runway threshold by screen crossing height. ![]() If you were going to rely on an RA to determine your minimums for the approach, you'd find the RA plummeting as you pass over the cliff at the end of Runway 23.Screen Height is defined in several places as an imaginary rectangle you fly through on takeoff or landing and that every thing in the airplane passes through. The problem is, the Yeager Airport is built essentially on a chopped-off hilltop sitting 300' above the valleys below. On the approach chart, it shows this is 250' above the runway's threshold. It's a CAT I ILS with minimums of 1,181' MSL. Let's say you're flying the ILS to Runway 23 at the Charleston, WV airport (KCRW). If you're flying a CAT II/III approach, you're flying to DH minimums using a radar altimeter. If you're flying a Cat I approach, which most of us are, you're flying to DA minimums. Whether it's a checkride, working with a student, or briefing an approach to another crew member, remember to use the correct phraseology. Minimums are always less than 200' above the threshold, in part, to ensure varying terrain/obstacle elevation below your aircraft doesn't interfere with your RA reading.īy the time you're less than 200' above the runway, you'll be flying over a flat surface just before the runway threshold, which is one requirement for an airport to certify a CAT II/III approach. That's why they're published as a CAT II/III approach, and not a standard CAT I approach.Īs you fly a CAT II/III approach, you'll reference your aircraft's RA reading to determine when you've reached DH. Most Cat I ILS approaches get you down to 200 feet above TDZE, but that's not always the case.ĬAT II/III approaches are flown to DH minimums less than 200' above the runway's TDZE. That means you'll be 200' above the touchdown zone elevation (TDZE) when you reach DA. You'll see in smaller numbers "200" published next to the minimums for the approach. As you're descending on the glideslope, when you reach 6,085', you're at DA. ![]() Looking at the ILS approach in the image below, the published minimums for the straight-in ILS 35R are "6085 - 1/2". When you fly a Category I ILS, which is what almost all general aviation pilots will fly, you fly to a DA. Standard Cat I ILS)ĭecision Altitude (DA) is an MSL altitude. For simplicity, we'll explain the concept using ILS approaches. The type of approach flown determines whether you'll use a DA or DH for your minimums. And if your airplane is certified for extremely low weather conditions, you might even fly a CAT II/III approach. In the lowest of weather conditions, you'll likely find yourself flying a CAT I ILS or RNAV LPV approach. ![]()
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