Establishing final descent

4 Answers

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   John D Collins on Jan 19, 2016

   The descent angle and the TCH will be specified on a non precision straight in approach in most cases. The TCH varies and is not fixed at 50 feet, although this is a typical value,

   Your statement “which usually a conversion chart(ground speed and descent rate) comes with it.simply means if you maintain a specific descent rate according to your ground-speed you reach the 50ft THR with an uninterrupted and safe descent angle.” is wrong. At least with the FAA charts, there is not a speed-time table on RNAV approach charts. The speed time table is also not usually used when DME is required for flying the final approach course. The assumption that the path between the FAF and the threshold and below the MDA is clear is downright dangerous as it is not true. The advisory glidepath may be used above the MDA, but any conflicts between its guidance and the charted minimum altitudes, you must abandon the advisory GS and follow the approach chart.

   You are required to fly at or above the minimum altitudes as shown on the approach chart and any advisory glidepath is just that advisory.

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2. 
   

   
   

   Tyler Connell on Jan 19, 2016

   I think you’re overthinking this a bit.

   First, what approach are you referring to? It helps to talk about 1 or 2 approaches so that we can explain. Your question is worded weirdly and I think that is creating some confusion here.

   The advisory glidepath won’t be posted on an RNAV approach (like said above) but you can find them on other non-precision approach charts. Typically you can find them on VOR charts (that do not have a DME). Those are just posted so you know when to go missed on the approach that won’t have any sort of identifier otherwise to let you know you’re at the MAP …. like a VOR approach or NDB approach. RNAV gives you DME and a VOR-DME chart also gives you DME.

   The descent path that is given (3 degrees typically) is FROM the FAF. Don’t think about “if I’m flying faster I need to start early” because that’s not how it works. First of all you CANNOT descend below your published altitudes. If the minimum altitude is 5000 feet to the FAF, then you better maintain 5000 to the FAF and start your descent when passing over the FAF. That is giving you obstacle clearance from objects that you cannot see. Starting a descent early is very dangerous.

   Also, keep in mind if you plan on flying the descent angle, make sure you’re at the correct altitudes for step down fixes and MAP. If you get to your MAP and you’re a couple hundred feet high of your MDA from the descent angle, then you’ve just wasted your time shooting the approach. Personally, that’s not regulatory to fly that glidepath, it’s just a suggestion. I suggest flying a stable 500 feet per minute non precision approach that will get you at all your step down fixes at the proper altitudes, and your MAP at the proper altitudes. Maintaining that should set yourself up for a good stable approach. If the runway is not seen, go somewhere else or try again. Trying to maintain the glide path written in the profile view to cross the runway at the TCH doesn’t make sense if you don’t have the runway in sight to even get to 50′ AGL….because this is a non-precision approach we are talking about.

   Also, don’t overthink the approach descent. Your thought process is good but don’t be thinking of ways to “outsmart” the chart…just follow the chart. That being said, the time and distance table is a way to measure that you’re at your MAP…hence why it says FAF to MAP. You typically don’t need to use that and the glidepath angle, and try to maintain your minimum altitudes/step down altitudes. You’re over saturating the required thought process needed for a stable approach.

   Safe travels.

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   Skyfox on Jan 21, 2016

   The way I was trained to fly non-precision approaches is like this: once crossing the FAF, and when crossing any subsequent step-down fixes, descend somewhat rapidly (700-1000 fpm) to the next MDA. The previous answers are quite correct that at no time can you descend below any MDA, unless you actually have visual contact with the airport or runway environment and can continue the approach visually (refer to 91.175). By following that technique you ensure that (1) for each step going down you know you’re in protected airspace for that lower altitude, and (2) you don’t waste any unnecessary time up in the clouds when you could break out of them by getting down to that MDA. Once you’re down to that lowest MDA you have to maintain it until you either get the runway or approach lights in sight to continue the landing, or cross the MAP (or time runs out for a timed approach) and you have to go missed.

   While there is a climb & descent table in the approach plates for finding vertical speed based on ground speed, that would serve you best on a precision approach where you want to establish the correct descent speed to help follow the glide slope. And while you could use it on a non-precision approach to follow the glide slope down to the MAP, it wouldn’t be the most efficient use of your time and resources trying to follow an exact glide path on an approach in protected airspace when you have so many other things to keep track of.

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   Nibake on Jan 21, 2016

   I could be wrong here, but I think if you pitch for descent after crossing the FAF, you will start to descend pretty quickly. Pitching for descent and power for airspeed is the proper technique for instrument approaches. Also, based on your question I feel like what you are most interested in is a “Continuous Descent Final Approach.” Although I agree with what others have said here the FAA does strongly recommend the CDFA technique (I don’t.) but if you want to learn all about it here is an AC that will help you –

   http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%20120-108.pdf

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