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11 Answers

Sensor FAF

Asked by: 1662 views Airspace, FAA Regulations

Reference (KGGE) NDB /GPS RWY 5.   I know FIVPA is a CNF, but is its only fuction to act as point to activate the GPS approach mode? Is FIVPA actually defined a “sensor FAF”? Also, any reason FIVPA is 4NM from the NDB? Since the GPS approach mode is supposed to scale the CDI to the proper approach mode parameters 2NM from the FAF, I guess the GPS approach mode would activate 6NM from the NDB in this case.

11 Answers



  1. John D Collins on Aug 23, 2013

    The PT has to be completed within 10 NM from the NDB. To accomplish this with a GPS, you would want to complete the PT outside of FIVPA by at least 2 NM or 6 NM from the NDB. That leaves at most 4 NM to contain the PT. If you started the PT right at FIVPA, you would might end up crowding yourself on the descent to the MDA, and if further than 4 NM from FIVPA you risk busting the 10 NM PT limit. Starting the PT somewhere around 7 NM from the NDB (3 NM from FIVPA) will put you about at 8 NM from the NDB by the time you are turned inbound to get established on the final approach course. Only after you are established on the inbound course may you start your descent to the MDA. From 1600 MSL to the MDA at 540 feet requires a descent of 1060 feet, but you want to complete the descent and be stabilized at the MDA while you look for the runway. A 3 degree descent from 540 feet to the runway puts a calculated VDP at 1.6 NM from the threshold or 2.2 NM from the NDB. Calculating a 3 degree descent angle from 1600 MSL to the MDA requires you to lose 318 feet per NM or another 3.3 NM. Adding the two distances together is 5.5 NM from the NDB, I would want to start down more than 5.5 NM from the NDB and probably add at least another 1 NM to to give me some time at the end of the descent to stabilize at the MDA and search for the runway. You would have to factor in winds for a specific approach, but the bottom line is that you would want to start down somewhere between 8 NM and 5.5 NM, so having the GPS transition to approach mode and to annunciate it at 6 NM from the NDB means that 4 NM from the NDB is a good place to put FIVPA.

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  2. Dan Chitty on Aug 24, 2013

    John,

    Great explanation and much appreciated.

    In summary, FIVPA is actually a “Sensor FAF”. Therefore, the GPS must be 2NM from FIVPA.
    to scale to approach mode of which is the reason you want to be at least 2NM from FIVPA to get turned around (PT).

    Is this correct?

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  3. Mark Kolber on Aug 24, 2013

    Dan, it seems like you’re trying your best to make it far more complicated than it is. Simplify. Simplify.

    This is a plain vanilla on-airport NDB approach that was probably there for 30 years before GPS was available. How would you fly it without GPS? The exact same considerations apply, with only one exception.

    With a relatively modern GPS with basic moving map functionality, it’s a complete no-brainer since the GPS will paint a location for the PT on-screen, like on the picture I linked to the other day: http://midlifeflight.us/posted/GNS530_FIVPA2.jpg

    FIVPA is there primarily to orient the unit. Not only to change modes but also for basic leg sequencing from outbound to PT to inbound to FAC. And perhaps to give the pilot some increased situational awareness.

    Related question: would you have as much difficulty with this approach if FIVPA were not in parenthesis? It shouldn’t make any difference.

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  4. Wes Beard on Aug 24, 2013

    Dan,
    From what I could find on the internet, it would appear that FIVPA is a sensor FAF. You are correct.

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  5. Dan Chitty on Aug 24, 2013

    Mark,

    I agree, I have taken this overlay concept and made it harder than it actually is. Reason is that I have never performed a overlay approach but want to be prepared. My confusion is about some of the terminology, namely “sensor FAF”, I found in the AIM and how “sensor FAF” relates to overlay approaches. As always, thank you for the feedback.

    Wes,

    Thank you for the feedback as well.

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  6. Mark Kolber on Aug 24, 2013

    Dan, just so you are aware, there are also :”sensor” fixes that are not associated with IAPs. They are also referred to a “computer navigation fixes” or “CNF.” Another importing thing to know about them is that they do not generally appear on ATC’s scopes since, as you’ll recall, they’re one and only function is for a GPS to use in orienting itself and providing navigation information to the pilot. So ATC won’t be giving you “direct FIPVA” any time soon. And if yo ask for “direct FIPVA,” there’s a good chance they won;t know what you’re talking about.

    So here’s your take-home quiz to test yourself on how well you understand this now.

    This is a link to the (YOSWO) CNF on the low en route charts http://skyvector.com/?ll=46.61511064349408,-110.61401367440102&chart=302&zoom=3&plan=F.K7.FIVPA

    What’s it for? (Sorry. No multiple choice!) 😀

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  7. Dan Chitty on Aug 24, 2013

    CNF ( YOSWO) is not at an existing point defined by means such as crossing radials or radial/DME, the CNF point is indicated by an “X “in this case to designate so.

    I hope this is correct. Thank you Mark.

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  8. Mark Kolber on Aug 24, 2013

    Actually, you =can= identify the locations very simply with VORs. Pilots have done so for a long time. So now there are two questions:

    1. How would you identify the vocation with VORs. real-world example: your clearance includes BIL V247 HLN. How would you navigate that without GPS?

    2. But the question is still, what is YOSWO for?

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  9. Dan Chitty on Aug 25, 2013

    1. How would you identify the vocation with VORs. real-world example: your clearance includes BIL V247 HLN. How would you navigate that without GPS?

    I will assume the airplane has dual VOR receivers: Fly outbound 287 radial from BIL using receiver 1. Have receiver 2 tuned to HLN radial 073. When CDI is centered for receiver 2, you are at YOSWO.

    2. But the question is still, what is YOSWO for?

    YOSWO is a CNF for a GPS to navigate to while on V247. It identifies the heading change on V247 between HLN and BIL.

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  10. Mark Kolber on Aug 25, 2013

    Bingo!

    And, coming full circle, the sensor FAF at FIVPA on the KGGE approach has the same overall purpose, in the FIVPA case, to identify the change between the intermediate segment of the approach and the final approach course.

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  11. Dan Chitty on Aug 25, 2013

    Thank you Mark.

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