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Terrain clearance when using IAPs. How do these work in RC and RL?

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Hypothetical: Airport ABC has a single, towered East-West runway and is at sea level. Weather is 5 miles vis and overcast at 3000ft. You are approaching from the south. Approach descends you to 10000 and soon informs you to expect vectors to rwy 9. You request and are cleared for the full GPS. The chart shows that the IAF is a hold at 2000ft at a fix 10 miles out on the extended runway centerline. It also shows the MDA 25 miles of the airport is 5000' given your direction. Question: When you were granted the GPS clearance, responsibility for clearance over terrain is yours, correct?Question: AS I understand it, the MDA permits you to descend confidently to 5000 once within 25 of the airport. But you can't assume that there isn't a ridge or something between yourself and the 25 mile mark. Since you'll be descending in clouds and no visability, do you have only the sectional to consult to determine when to descend?Question: The MDA requires you to stay above 5000 unless you have the airport in sight, but because you are in clouds and further than 5 miles out (cutting diagonally toward the IAF) this will not happen. How do you get down to the IAF at 2000?Question: How do you know whether there is a 4000 (say) ridge between yourself at 5000 and the 2000 IAF? You need 10 miles to descend at 1000fpm/200kts. That's lots of room for terrain to hit on the descent to IAF. Have you been cleared to descend to the 2000 IAF even though MDA is 5000 and you can't see? Is this implied by the ATC clearance?Should you fly to the airport, turn 90 left, and then descend to the fix rather than cut diagonally to the fix. The advantage being that you could safely descend to 2000 if you were traversing the inbound course outbound, as you know the altitudes inbound are lower than 2000.I guess the issue is: I know that if I reach the IAF I will pop out of the clouds and for the rest of the route be assured terrain clearance all the way to the runway. But how do I descend to the IAF confidently, given the soup? Strictly by using the plates and sectional? Had you chosen vectors, vertical clearance would have been handled by ATC. So why would anyone choose GPS or any other IAP in these circumstances?Too much time on a long drive. Perhaps I'm over thinking...Thanks

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OK, let's see if I can clear up some of your questions. First, MDA is "Minimum Descent Altitude". Sounds like you may be confusing thatwith MSA or "Minimum Safe Altitude". They are completely different things: MDA is the lowest you may go on a non-precision approach (like a GPS approach) without having the "runway environment" in sight, and be in a position to land (or circle depending on what kind of approach you're doing).IAF is the "Initial Approach Fix". This is supposed to be the fix that transitions you from the enroute system to the approach environment.Now, terrain clearance is achieved in a radar environment by ATC and their MVAs (Minimum Vectoring Altitudes). If you're getting vectors to an approach segment, you won't be cleared below MVA wherever you are.Once Approach Control (or Center if you're in the boonies) cuts you loose, they'll usually say something like "Maintain at or above 3,500 until established on the localizer (or approach course or published approach segment). At that point, your approach plate is telling you how low you may go. As you cross the IAF you may be able to go lower, then there may be stepdown fixes (mostly on non-precision approaches) that let you, well, stepdown. When you reach FAF or "Final Approach Fix" you may descend to the MDA. You fly at the MDA on the Approach Course until one of two things happens: A)you see the airport and land; or :( you reach the MAP "Missed Approach Point" and you execute the Missed Approach Procedure.Alot of this may be cleared up by looking at an actual approach plate. Try www.airnav.com, click on "Airports" and type in your favorite field (US only). Click "Search" and when it comes up scroll down until you get to the listing of Instrument Approach Procedures.There you can download and print PDFs of all the current approaches for that airport.About the only other time that immediately comes to mind where the pilot is responsible for terrain clearance on an IFR flight plan is when launching from a non-towered field into IMC (Instrument Meteorological Conditions). Then, you'll sometimes get admonished that ATC is not responsible for terrain clearance.That's enough for now. I'm sure I've created more questions for you than I've answered, so keep askin'.Walt

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Thanks for the thorough reply Walt. It seems my understanding is not too far off. The key issues I was interested in were: I understand that the MDA restricts you (on a GPS non-precision) from descending below 5000 (in this example) without the runway environment in sight. Because the ceiling was overcast at 3000 and vis was 5 miles, you'll be in the clouds and unable to see the airport environment and thus be unable to descend to join the IAF at 2000, as specified on the plate (the rough real world example I was basing this on is KFAT, GPS runway 11L plate). In other words, how do you descend to 2000 to hit the IAF if your visibility restricts you from descending below 5000 and you're requesting a GPS approach and therefore are not covered by ATC vectoring. I'm probably not being clear.

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Again, it seems you are confusing MDA with MSA.Let's check this KFAT RNAV (GPS) approach to Rwy 11LThe MSA is 7700 ft from the RWY11L GPS fix.You can arrive at the IF (MORLA) at 2000 ft in this way:- if coming from BEREN you should have been cleared to, say 3000 or so, and from BEREN to MORLA you can safely descend to 2000 (see the "2000" on the leg ?)- if coming from FRA VOR, you should have been cleared to 6000 or similar and then you may safely descend not lower than 5000 from FRA to KECPO then you may descend to 2000 inbound MORLA-if coming from FRAME or BLEAR you may have been cleared to 3000 or similar and maintain safely 2000 ft till HOJRI- if coming from SIBOC, you may descend to 4400 till KEPCO then descend to 2000 ft to the IFIn this case (but in all cases too) the MSA is given BUT also trasnitions from the enroute structure or STAR environment are given with the altitudes that provide safe terrain clearance in all the segments.Obviously, an higher MSA in RC will cause the RC Controller to prevent your further descent when given vectors (and this may be a problem in mountainous terrain with high MSAs that will result in porr altitude handling of RC with respect to a smooth approach capture: you shoul enable NOTAMs in arrival to avoid this), but if you ask a full published approach, you may descend accordingly to your plates.Hope this helps !

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Thanks guys. This is great and I appreciate your patience. Sorry for obscuring things: I DID in fact use MDA where I meant MSA. And I will do some more reading. In a nutshell my question was if the MSA (!) is 5000 and the IAF you want is given as 2000 and you are in overcast down to 3000, can you descend from 5000 to 2000 with confidence that nothing in the intervening 10 miles it will take to get down will be in your way?

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Strictly by how you have asked the question, I would have to answerNO. If I'm not talking to ATC, I would absolutely not descend below what ATC last cleared me for (let alone MSA) until and unless I'm on a published approach segment. Not only is terrain/obstruction clearance a concern, but if I'm IMC, I also want to make sure I'm not gonna hit another airplane.Walt

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Thanks. That is what I thought. The reason it came up was if you ask for the full IAP in RC, ATC clears you and quits talking to you apart from traffic until final.

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When using an IAP you are expected to follow a published transition and approach. These include altitude minimums and/or restrictions. Traffic alert calls should not be abandoned when following a transition and IAP I would hope until the IAF.I highly suggest a TCAS be implemented on your panel.Many but not all times the last specified waypoint altitude in a STAR falls within the 25 nm MSA altitude and the IAF is within that area. In those cases the IAP from the IAF specifies the minum altitudes to be flown to intersect the LOC with its timed or DME altitude intersect scheduled final descent or the GS capture.

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One other point that I didn't see in responses above. MSA, MVA, MDA, IAF, and FAF on approach charts are all related to altitude above sea level - MSL, as read on correctly set altimeter. Cloud heights in weather reports are related to ground level - AGL. Approaching Denver, for example, a METAR or ATIS reporting "3000 overcast" is indicating clouds at 3000 AGL, but this would be 8000+ MSL, and descending on an instrument approach, you'd expect to be in the clear below 8000 on the altimeter.Doug

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Ceiling given in AGL certainly helps visualize the breakout point and for those transitioning to VFR in the clear know at what point (or if) they can be allowed to do this since the rules are expressed in AGL.On US plates, minimums/decision heights are expressed expressed in MSL and in () AGL. I'm not sure about other publications.BTW, Doug, good training articles in Computer Pilot on VFR and IFR PPL. You are the same Doug, right?

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Yes, that's me writing for Computer Pilot magazine for the past five years.Doug HortonChicago Aurora KARR

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