PilotEdge Forums

Hi All,

I'll post this in the training section as it seems most relevant, and it is a RW IFR training question.

Tonight I flew the KSMX RNAV (GPS) 30 approach. It was IMC but ceilings were pretty high - about 2000-2500 AGL so I broke out of the clouds and visually acquired the runway environment well before reaching the runway or even the FAF.

My question is: WHEN can I leave the approach track/Final Approach Course to line up with the runway? IF, as in this case, the final track does not directly align with the RWY? For circling, as I learned in Catalina many moons ago, there is a 1.2NM limit "officially" that one is supposed to stay within for category A/B aircraft. But, I haven't been able to figure out where to find the answer to this question as it is not circling. I would imagine that I could "call" it circling if I had to and be safe at 1.2NM deviating from the 316 track.

What I did - was line up (ie turn right and move laterally over about 1/4 mile) for about a 2 mile straight in final clear of clouds (halfway between RWY 30 and ANIBY). Is this legal? If not, why not? Does being "clear of clouds" and visually acquired the runway environment allow me to do this? If not, when exactly am I to line up on RWY heading (legally) - at the threshold?

Thanks much for sharing your wisdom! (Peter)

Todd

I tried checking Google and looking into the TERPS criteria for guidance on this question. I haven't yet checked the criteria involving RNAV approaches, but there was some information on VOR approaches, which is more or less the predecessor to GPS. Nothing I say here applies to the GPS non-precision approaches, but it should give you some ideas or a hint on how to search.

First, it's important to note that the obstacle clearance requirements for circling approaches are a lot stricter than straight-in (when available) because, as you noted, you need to be guaranteed clearance in a large area around the airport. It's not always practical to accept circling minima, since straight-in minima are usually better. The simplified version is that a straight-in approach can be published if the final approach segment differs by no more than 30 degrees with the runway and allows for a reasonable descent down to the field. You are essentially guaranteed primary obstacle clearance by at least 250 feet, out to a lateral distance of at least 2 miles from the course. This lateral distance does increase somewhat the farther away you are from the VOR. There's also a secondary obstacle clearance area that starts from the end of the primary clearance area and again ends at a spot dependent on the distance from the VOR. The protection tapers from 250 feet (beginning of zone) to 0 (end of zone).

So the bottom line is that you are expected to do some maneuvering to line up with the runway, and you are protected if you come off the final approach course a bit. You can probably do some trigonometry to see how far you can start lining up and still stay within the primary protection zone. As far as I'm aware (and I'm no expert), there aren't any hard rules on when to start lining up, though there are some regarding when you can descend from the MDA. No one seems to discuss this exact question either. This probably isn't the greatest answer, but for practical purposes, I would just use some judgement on when to line up; it doesn't seem all that reasonable to try lining up five miles away, for example. If you're otherwise ready to do that, I would assume that you see the field nearby and can maneuver visually, so you most likely are protected and would be able to see potential hazards in your flight path anyway.


The TERPS document: http://www.faa.gov/documentLibrary/medi ... 201-21.pdf

Disgraced,

Thank you for a wonderful answer. There is NO way I will ever be doing any trig in the cockpit

Everything you said is in line with what I was thinking - but with more precision. I know real world I would line up about 1.5-2miles out as I did. I was wondering what an FAA Examiner would say to me if I did that on a check ride.

It would seem the 100% "book" answer would be to hit minimums at the threshold and turn 16 degrees (or more) left to a nice long landing, but I don't think I could bear doing that even on a checkride! Its so much more common sense to line up your final at some indefinite distance inside of 2 miles...

I'll try to poke around some more for "the" answer

Todd

Not a problem!

I did a bit of arithmetic, and found that if there is a 30 degree misalignment between the final approach segment and runway orientation, you can start lining up roughly 3.5 miles from the threshold and still have primary obstacle protection. This isn't a hard number though, because the final approach segment may not take you directly to the threshold. It can simply intersect the extended runway centreline by a certain distance, as described in the TERPS document.

My feeling is that you shouldn't overthink it. The straight-in landing minimum assumes that you need to move over to line up with the runway. Just do what is reasonable for your aircraft type, approach speed, and weather conditions, and you should be fine. The thing I wouldn't do is wait until the threshold / MAP and then try to maneuver. It's too late to land straight-in and you're going to have to go around or fly the missed at that point.

My question is: WHEN can I leave the approach track/Final Approach Course to line up with the runway? IF, as in this case, the final track does not directly align with the RWY? For circling, as I learned in Catalina many moons ago, there is a 1.2NM limit "officially" that one is supposed to stay within for category A/B aircraft. But, I haven't been able to figure out where to find the answer to this question as it is not circling. I would imagine that I could "call" it circling if I had to and be safe at 1.2NM deviating from the 316 track.

The 1.2 (actually 1.3) doesn't apply in this case as it's not circling.

There isn't really any official easy to find guidance on when to start the turn. So we have to dig in a little bit.

In terms of protected airspace. RNAV approaches are not covered by the TERPS. RNAV approaches have FAA order 8260.58 governing their construction.

RNAV TERPS is a lot more complicated then regular TERPS in a lot of areas including this one. Note the protected airspace width for a LNAV RNAV approach is 0.6 NM on each side of the final.

How it done depends on the exact angular difference between the Final course and the runway course. In this case we have a 15 degree offset. So the RNAV TERPS says to do this:

For offset > 10 degrees and ≤ 20 degrees, the course must cross the runway
centerline extended at least 3000 ft prior to LTP (5200 ft maximum). (Offsets
> 15 degrees, CAT C/D minimum published visibility 1 SM, minimum HATh of
300 ft)

This of course makes no sense without any sort of context.

LTP is the landing threshold point, or the point on runway centerline at the beginning of the runway.

Or in other words when on the final approach course you must cross the runway centerline between 0.5 and ~0.9 NM from the runway. If that's the case you simply turn when aligned with the runway.

A quick look at the chart tells us that this approach does not meet that rule, so what gives...

There is another option:

Note: Where paragraphs 2.1.1a-d (PG Note: The first quote is paragraph 2.1.1c) cannot be attained or the final course does not
intersect the runway centerline or intersects the centerline more than 5200 ft from
LTP, and an operational advantage can be achieved, the final may be aligned to lie
laterally within 500 ft of the extended runway centerline at a point 3000 ft
outward from LTP.

For all intents the final cross the centerline at the LTP. So with a 15 degree offset at 3000ft (0.5 NM) we are 800' off the centerline when 3000' from the LTP. Hmm ok, that's weird...

So this approach isn't in compliance with the RNAV TERPS....

Well it ends up this approach was made in 2009, back where there was a different RNAV TERPS.

So let's look at the now outdated FAA order 8260.54A. Well the relevant part of this document is identical, once again meaning this approach isn't in compliance.

So now we go back into ancient history to order 8260.38A which was issued in 1995.

This order says the following about final approach alignment:

Straight-in. For a straight-in approach, the
alignment shall not exceed 15 degrees from the runway centerline
(RCL) extended. Optimum alignment is coincident with the RCL.
Where the alignment is 3 degrees or less from the RCL, the
optimum alignment is to the runway threshold. Where the
alignment exceeds 3 degrees from the RCL, the optimum alignment
is to a point 3,000 feet from runway threshold on the RCL. Where
operationally required, optional alignment is authorized to a
point between, and including, the runway threshold and a point
3,000 feet prior to the runway threshold on the RCL, provided
alignment is within 15 degrees of the RCL.

So there is your answer, this approach was made with now out of date criteria.

Note that in all cases no additional protection is given for the turn to the runway. You are only protected within 0.6 NM of the approach course.

TL;DR - The final approach course should intercept the runway centerline about 0.5 NM from the runway, turn at this point. Really old approaches (like this one) don't do that. Turn when you see the runway and won't hit anything.

I did a bit of arithmetic, and found that if there is a 30 degree misalignment between the final approach segment and runway orientation, you can start lining up roughly 3.5 miles from the threshold and still have primary obstacle protection. This isn't a hard number though, because the final approach segment may not take you directly to the threshold. It can simply intersect the extended runway centreline by a certain distance, as described in the TERPS document.

Be careful with this one. This assumes a 2 NM wide protected zone. For RNAV approaches you only get 0.6. Running this calculation for a 0.6NM protected area you end up with only being able to turn at a 2.4 NM mile final with a 15 degree offset and at only 1.2 NM with a 30 degree offset.

I actually noted at the beginning of my explanation that it was for the VOR guidelines, though it definitely wasn't clear in my follow-up. Thanks for the clarification!

Ahh yes, I think I missed that note.

Your explanation and calculations are 100% correct for VOR approaches.

Peter

I don't know which I find funnier - My completely unintentional ability to uncover FAA oddities, inanities, and irregularities or your ability to come up with fascinating answers to them.

If and when I take an instrument check ride, I would like to have your number on speed dial please

So, to recapitulate, completely legal to fly to the threshold and turn 16 (not 15) degrees to centerline. But pretty ugly.

2. NO legal restriction per se as to when you are "allowed" to align with runway centerline.

3. Using "circling mins" 1.3 (thanks) would give you something to lean on in an argument with an examiner.

4. I'm willing to bet that no examiner I might ever face would be as knowledgeable on the actual regs or how they have evolved. FYI 1995 was 7 years AFTER I got my PPL so it doesn't seem that ancient to me

Thanks

Todd

I stand corrected on the distance requirement, Ryan & PG are right, the distance from a fix on the approach needs to be furnished prior to the gate for radar arrivals.