"Diverse Departure" procedures

I've written about various types of departure procedures before - VCOAs here and the option for Part 91 operators to take off in "0/0" conditions here.

But I recently received some questions from a friend and reader based on a recent flight of his from Denton, Texas to McAlester, Oklahoma (MLC) and returning, which you can read about on his blog at pilottangocharlie.blogspot.com. After stopping at MLC, he got his clearance which consisted of the MLC VOR as the first fix. There were some real instrument conditions around, so this was a for-real instrument departure. But the only departure "procedure" that is published for MLC is a set of takeoff minimums. He realized this situation wasn't covered real well in his instrument training, and needed a little refresher on how this works.

I can sympathize! My instrument training was in southeast coastal Virginia, where the flat terrain makes Obstacle Departure Procedures (ODP's) purely an academic exercise for the most part. Add in that radar coverage was excellent and most IFR releases simply started with "Fly runway heading..." and the result was that ODPs were not covered very well during my IFR training (in fact, they may not have been covered at all). In my experience this is pretty common, which is unfortunate because every flight starts with a departure!

McAlester actually has a good example of a basic textual obstacle departure procedure (or lack thereof):

From Runway 2 it has a pretty typical set of takeoff minimums or a minimum climb gradient. This situation I covered in my "0/0" article, so I won't go into it here. But remember that while these takeoff minimums aren't required for Part 91 operations, they are a REALLY good idea.

From Runway 20 there are the same two options, plus a third new one - the option to reduce takeoff distance by 1900 feet. This allows the airplane to climb at a standard rate and still clear the nearby obstacle with an acceptable safety margin. Obviously you would have to carefully plan to make sure your airplane, on that day, given those weather conditions and loading, can be off the ground by then. Seems like a small additional amount of safety factor, and it is, but the reason for the shorter takeoff roll option is just because some obstacle just barely penetrated the clearance surface and this slight reduction resolves it.

This is a good time to note the "cross departure end of the runway at least 35 feet AGL" wording in various training and reference publications. This requirement has been removed from the TERPS - the procedure design standards - but is still referenced in many FAA publications, such as the Aeronautical Information Manual, para 5-2-8b1 and the Instrument Procedures Handbook, page 1-14. Both of these say substantially the same thing:

"...required obstacle clearance for all departures, including diverse, is based on the pilot crossing the departure end of the runway at least 35 feet above the departure end of runway elevation..."

Which, while a good idea from a safety perspective, is not technically accurate any longer. I believe the issue was one of planning - how do you determine whether you can cross the departure end 35 feet high? Many light aircraft performance charts give a 50-foot figure, but how do you extrapolate? So the standards were revised to the easier-to-determine method of just getting airborne by the end of the runway, unless otherwise specified.

Okay, so you took off, but now what?

In the MLC example, he was departing from runway 2, but the first fix in his clearance was the MLC VOR to the south - behind him. How to go about getting there?

In the absence of a departure procedure or specific ATC instructions, the short answer is "however you want" (within reason of course). There are only a couple of restrictions, both spelled out in the same AIM paragraph linked above:

1. You climb on runway heading to 400 AGL before turning.

2. You keep climbing at the standard rate (200 feet per nm) or as specified in the takeoff minimums up to your cleared altitude.

So in this case, the way to go would be to climb straight ahead to a comfortable altitude, then turn direct to the VOR and proceed on your cleared route. Depending on the ceiling and visibility, I might not turn all the way around at 400 feet, though it should be safe to do so - a little more altitude might be prudent in low IMC, plus it allows a little more time to get turned around and tracking direct to the VOR, which is still very close behind you.

This is what's known as a "diverse departure". "Diverse" in this sense meaning "any direction", as there are no restrictions placed on the pilot as far as routing goes. In non-mountainous areas of the country like McAlester, Oklahoma, the safety of a "200 feet per nm" climb gradient is evaluated out to 25 nm from the airport. In mountainous areas, it's 46 nm. This is almost always enough to get you on a published airway, above the OROCA, or into radar contact. And if you're wondering what the definition of "mountainous area" is, the FAA defines that as well in 14 CFR 95.

The map of the continental U.S. leads to some humorous observations, like Scottsbluff, NE being considered mountainous. I suppose they had to draw a line somewhere!

The great thing about takeoff minimums and departure procedures is that you can always (and should always) review them, on the ground, before even getting in the airplane. Once in flight you may have to land at an unplanned airport, but I haven't yet heard of the takeoff happening at a different airport!

TAA's and ILSes (and other acronyms!)

Be ready for a few acronyms!

The FAA has recently begun adding GPS routes into ILS procedures, which in my opinion is a great thing! This combination of GPS and ground-based navigation allows for more flexibility for routing and easier flyability among other benefits. Here's what I'm talking about, see this example at Statesville, NC (KSVH).

If you're not familiar with TAA's (Terminal Arrival Areas), they are on many RNAV (GPS) approaches already but are just now being added to ground-based approaches as well:

TAA's allow you to be cleared for the approach anywhere within the depicted area, usually within 30nm of the fix shown (in this case PEGTE). The minimum altitude you must be at depends on distance and what your course is to that fix. So, in this example, coming from the west the minimum altitude is 4300 until within 6 nm of PEGTE, then 3400. From the east it's simpler, just 3400 all the way to PEGTE. Also notice that any course to the fix from 195 clockwise to 015 is considered a "NoPT" segment, so you can skip the depicted Hold-in-lieu-of-Procedure-Turn (HILPT). From the western half of the TAA you are still required to execute the HILPT in the example.

Another benefit of the TAA is that they do not require a VOR-based route into the procedure. Notice that the IF, PEGTE, is not anywhere on the Low Enroute chart (though there are some crossing radials, it's not on an airway). The TAA gives you the flexibility to be on a published segment of the approach, flying direct to PEGTE from anywhere within 30nm.

Another example is that at Rock Hill, SC (KUZA).

This one has a complete "T" setup of IAFs and the TAA to match. Unlike at KSVH, this allow you to come from essentially any direction and avoid flying the HILPT. If you're coming from the northwest, you'd fly direct to GUCRE, then be on a NoPT segment after reaching GUCRE headed to CONEL. The same idea with TAGCU from the northeast. From the south, you'd fly to CONEL and then proceed straight-in after reaching it.

One caution - if like in many airplanes, you're displaying both your GPS and ILS guidance on the same CDI, make sure to switch the CDI source as you reach the IF.

Another great benefit of a TAA is that ATC can clear you for the approach from a long way away. In order for ATC to clear you for the approach, you are required to either be on radar vectors OR established on a published segment of that approach. Since being within the TAA is considered to be on a published segment of the approach, you might be 28nm from GUCRE and hear a very simple "N123, Cleared ILS Y runway 2 approach"!

Now, many of these TAA-to-ILS approaches probably also have an RNAV (GPS) approach to the same runway with LPV minimums. In that case, if you have a WAAS-equipped GPS receiver, you'd probably just fly the GPS approach. But sometimes the minimums aren't quite the same, or there isn't an LPV approach to that runway for various reasons, so the TAA-to-ILS might be a benefit even with a WAAS receiver. If you AREN'T WAAS equipped, then the TAA-to-ILS is great, because it doesn't take a WAAS receiver to fly it!

For more about TAA's themselves, please see BruceAir's great blog post about them here. Highly recommended reading!

I expect to see more and more of these published as time goes on. Let me know if you see a new one!

(Acronym count: 12. I guess that's not too bad.)

Feeder routes to procedure turns - don't make up your own!

There has been a very active discussion occurring on the Pilots of America web board the last couple of weeks regarding a Hold-in-Lieu-of-Procedure-Turn (hereafter HILPT) published on an approach chart in southern Oklahoma. Sadly, as is the case with many online forums, the discussion has degraded into name-calling, insults and other unproductive and uneducational matters. So I'll try to break it down here.

The approach in question is the Durant, OK (KDUA) VOR/DME RWY 35. 

Notice there is one published way for a non-GPS-equipped aircraft to enter this procedure without receiving radar vectors to final. That aircraft would start at the BYP VORTAC IAF, fly the BYP-321 radial until HANOM, turn right, then proceed inbound to the URH VOR/DME on the URH-184 radial. There is a HILPT published at HANOM, but since the route from BYP is labeled NoPT, you would not fly the HILPT coming from that direction.

Well, then from what direction DO you fly the HILPT?

Seems like an easy answer, that you'd fly the HILPT if you were coming at the approach from the north - you'd fly to the URH VOR/DME, then outbound on the 184 radial to HANOM, execute the HILPT and proceed inbound. Except it's not quite that easy, for one main reason:

There is no published segment from URH to HANOM.

Sure, there's a published route from HANOM to URH, but that's not the same thing. A route from URH to HANOM would be properly identified by a thin line as a feeder route. Note that the indication "R-184" below the HILPT does not indicate a route, it is simply showing what radial the final approach course is on. A route would be indicated by an altitude, a course, and a distance. So what is missing here is a charted, evaluated, and published route from URH to HANOM. Note that courses on instrument approaches are one-way, not two-ways like on most airways, and aren't meant to be flown backwards. This is why each segment of an approach has a directional arrow.

This is especially confusing because the URH VOR/DME is right on an airway - V63 - so it would be a logical place to have a feeder. Would it be possible to fly that route, from URH to HANOM and turn around? Of course - any instrument pilot should be able to do it with no problem. But from the way this procedure is charted, that exact route has not been evaluated for obstacles, even though the route the other way has been (the intermediate and final segments).

Why does this matter? It seems from looking at it that flying from URH to HANOM at an altitude of, say 2500 should work just fine. The reason has mainly to do with the difference in size of the areas evaluated by TERPS for intermediate and final segments versus a feeder route; the area evaluated for a VOR final being much narrower than for a feeder route. (At the VOR, the final is 1nm each side of center, whereas the feeder is 4nm each side of center, not considering what are known as "secondary areas" (see my 3/30/14 blog post for more about secondary areas.)

Let's say you're approaching the VOR in such a way that you need to make a 90 degree turn to go outbound on the uncharted "feeder" route from URH to HANOM. An actual feeder route, being wider, allows for you crossing the VOR, then beginning your turn, with enough area to contain the turn radius. A final segment used in reverse would not have this, as the area is much smaller (and turns to line up on final are much more restricted in terms of heading change for this reason). Might not be a problem in a 172, but in something faster it could. What if there is an antenna tower or mountain off to the side of final?

Sometimes a picture is worth more than 1000 words. This is probably such a situation:

This diagram shows a notional view of the areas evaluated for this approach. Since the area from HANOM to URH has been evaluated as a final approach segment, it's pretty narrow. But if an aircraft inbound from the east crossed the VOR and made a turn to proceed outbound on the final approach course, it could easily exceed the boundaries of the evaluated area. At 150 knots, a standard-rate turn results in a turn radius of about 0.8 nm. The final approach area is only 1.0 nm wide at the VOR, so while it seems to fit, that's only in an ideal situation. Adding in a tailwind that will increase turn radius, a slightly delayed start of the turn, and imperfect pilot technique means you rapidly run out of safety margin. What's outside of that area? Could be an antenna tower, could be a mountain, or it could be level terrain as far as the eye can see. There's no telling, but the published altitude doesn't reflect that because it wasn't part of the evaluation.

Compare that to the case if a feeder route was published from URH to HANOM. The feeder route, being much wider for exactly this reason, easily accommodates the turn radius:

So, if we can't fly from URH to HANOM for the HILPT, and if the only published route from BYP to HANOM is a NoPT segment, what's the purpose of the HILPT in the first place? There is none. I speculate that the HILPT is charted correctly, but that a feeder was erroneously left off during publication. This has been brought to the FAA's attention, so it will be interesting to see their response.

An example of a similar procedure that has the feeder and the Procedure Turn (though not a HILPT) charted is the Springfield, OH VOR RWY 24. Notice the thin line from the SGH VOR labeled "300 to OHMEE, 055 deg, (6.4)". It has all the necessary data to serve as a feeder route and has been evaluated and charted. Thanks to a blog reader (and former student) for providing this example!

Instrument approach procedures exist to keep us all safe, but we can't "roll our own". If something doesn't seem right or there appears to be an error, we need to bring it up to the attention of the FAA - don't just assume!

IFR departures - Visual Climb Over Airport?

"For Climb in Visual Conditions...." wait, aren't I IFR?

Ever see these words on an instrument departure procedure and wondered what they mean? There's a little-taught (and probably even lesser flown) type of instrument departure procedure called a VCOA - Visual Climb Over Airport. I know it wasn't covered at all in my instrument training - of course that was in coastal Virginia, so with the terrain being very flat there wasn't much in the way of actual departure procedures to fly anyway.

So what is this? It's an instrument departure, but involves a VISUAL climb to an altitude at which you can then proceed into IMC along your cleared route. Kind of the opposite of a visual approach in that regard, and typically would only be used in an area without radar coverage. When this is an option, you will see it in either the takeoff minimums/textual departure procedure listing at the front of the approach chart book, or in the takeoff minimums section of a graphic obstacle DP, and it looks something like this:

 

These VCOA procedures are only published when there is an obstacle greater than 3 sm from the airport that causes a required climb gradient of greater than 200 ft per nm to clear. Here's what the FAA's Instrument Procedure Handbook has to say about them (page 1-38):

Visual Climb Over Airport (VCOA)

A visual climb over airport (VCOA) is a departure option for an IFR aircraft, operating in VMC equal to or greater than the specified visibility and ceiling, to visually conduct climbing turns over the airport to the published "climb-to" altitude from which to proceed with the instrument portion of the departure. A VCOA is a departure option developed when obstacles farther than 3 SM from the airport require a CG of more than 200 FPNM.

These procedures are published in the Take-Off Minimums and (Obstacle) Departure Procedures section of the TPP. [Figure 1-36] Prior to departure, pilots are required to notify ATC when executing the VCOA.

Okay, so there's a specified ceiling and visibility requirement for this VCOA. The intent is for the pilot to take off, spiral up over the airport until reaching a certain altitude, and then it's safe to fly the cleared route even if entering IMC at that point, assuming the climb continues to an appropriate altitude in the clearance. In our example from California, you would make gradual, climbing turns up to 8300 MSL (3400 AGL) and then continue climbing on your cleared route.

At first glance, that almost seems a little silly, doesn't it? The required ceiling is 3500 AGL, and that's pretty solid VFR, so why not just depart VFR? However, the threat that the procedure is designed to avoid is really those times when the ceiling is high enough over the airport, but obstacles (like mountaintops) are still obscured by cloud. This is reflected in the design methods for these procedures.

Briefly, a "cylinder" of airspace is evaluated around the airport, with a radius determined by the elevation (higher elevations needing a greater turn radius due to increasing TAS). In our example, the radius used is 3.4 nm (source - FAAO 8260.3B, Vol 4, Chapter 4).

The highest obstacle in this cylinder is used to establish the "climb-to" altitude. If there are other obstacles outside the cylinder, a 40:1 slope is then evaluated to see if it clears the obstacles. If it does not, then the "climb-to" altitude is increased appropriately. Notice that the "climb-to" altitude also provides for a minimum of 250 feet of obstacle clearance, growing as you get further from the cylinder.

A VCOA can also have a "route" attached to it, like at Craig, Colorado (CAG), where you would climb up over the airport then proceed on a radial to the nearby VOR. This departure procedure also incorporates a "normal" departure if you can make the climb gradient (of 510 ft per nm off runway 7!) but the parts we're interested contain the words "for climb in visual conditions".

This is a pretty complicated textual departure procedure, so it definitely takes some review before takeoff! Note that once you get to the VOR, you're not done - you need to follow the "thence ..." instructions in the last paragraph, which can consist of a climb in a holding pattern depending on your route of flight.

Certainly if you need to execute a maneuver like this it's important to inform ATC when you get your clearance so everybody knows what you're doing and there are no surprises. But flying them is admittedly pretty rare, so let me know how it went if you have actually flown one!

RNAV (GPS) approaches - what happened to LNAV+V?

(8/18/14 update - looks like I was incorrect regarding the Garmin 430W/530W and LP+V! As of software revision 5.1, released in April 2014, these WAAS receivers now support LP+V. However, it looks like the Garmin 650/750 do not, at least as of software revision 5.0 which is the most current for them. Hopefully soon!)

Like my last article, this one comes from a question posed at a seminar at Oshkosh, where I was in the audience but was fortunately able to help answer the question.

The question pertained to RNAV (GPS) approaches with an "LP" line of minima, and was, in essence, "Why don't we get an advisory glideslope when conducting an LP approach, like we do when flying an LNAV approach?"

A little background (but brief, I don't want to get too much into types of RNAV minimums in this article).

The most basic line of RNAV (GPS) minimums is the "LNAV" line, meaning "Lateral Navigation". There is no vertical guidance, it's like a VOR approach, and can be flown without WAAS. GPS manufacturers thought that, if you had WAAS, it would be helpful to include an "advisory" glideslope to help make a nice, stabilized descent instead of the "dive and drive" that non-precision approaches typically resulted in. This was termed "LNAV+V" to show that an advisory glideslope was available and could be used for situational awareness.

However, this "advisory" glideslope was NOT evaluated by the FAA, and the pilot had to make sure to still comply with all pertinent altitude restrictions, to include leveling off at the MDA and going missed as appropriate. There was nothing depicted on the approach chart, because this capability was provided by the manufacturer, not the FAA.

Compare this to the LNAV/VNAV line of minimums. LNAV/VNAV minimums and glideslope ARE evaluated by the FAA for obstacle clearance and all the other factors, and they are flown to a DA like an ILS, meaning you don't have to level off at the DA, you just need to start your missed approach at that point.

When LNAV/VNAV minimums were published on the same chart as LNAV minimums, though, it could get a little confusing. For example, a Garmin 430W would annunciate the two, respectively, as "L/VNAV" and "LNAV+V". This is a little too easy to confuse if you don't look at it closely. Kind of reminds me of a cartoon of someone trying to fool the police with a license plate that is something like "8BB8B8" or "I1II1I1"!

Garmin 430W LNAV+V annunciation

This has actually been the source of some confusion throughout the pilot community. I've seen the questions - "My GPS says LNAV+V, does that mean I fly to the LNAV/VNAV minimums?" (no) or "If I'm flying LNAV+V, do I need to comply with the stepdown fix altitudes in final?" (yes).

Although more training could resolve this confusion, there is also a "human factors" issue - the two terms DO look alike and ARE confusing. I wish the manufacturers had used some other term to indicate the presence of an advisory glideslope, but that's the way it is.

Undoubtedly due to this confusion, when LPV and later, LP approaches started getting published the FAA originally disallowed manufacturers from providing an advisory glideslope with LP approaches. Can't say I blame them too much - I can imagine some confusion if the GPS started annunciating "LP+V" on the approach shown below. Is that a DA or MDA? Wait, there's no LPV. Do I have the right chart? What about that stepdown fix?

(Incidentally, that's the RNAV (GPS) RWY 22 at "Sporty's", I69.)

Since the FAA initially disallowed advisory glideslopes on LP approaches, manufacturers did not program them into their GPS receivers. However, in 2011, the FAA published AC 90-107 which changed that rule and allowed manufacturers to include an advisory glideslope with LP approaches (paragraph 6e(2)).

However, to enable this functionality, the manufacturers needed to develop and certify new software. I know that Garmin, for one, has not yet added it to either their new GTN 650/750 or as a software update to the GNS 430W/530W series.

This has created an unfortunate unintended consequence. If an approach has only LNAV minimums, the GPS will show an advisory glideslope as LNAV+V. But if it has LP and LNAV lines of minima, the GPS will by default annunciate "LP" and NOT provide the advisory glideslope. Since most IFR GPSes in use won't allow you to go in and select a "lower" line of minima to use, the advisory glideslope is for most practical purposes unavailable. In some ways, this means the "old" approaches are better than the "new" ones.

Take the above example at I69 - prior to the latest amendment, it had only LNAV minimums, and therefore had an LNAV+V advisory glideslope. Now, with both LP and LNAV minimums, it effectively doesn't!

Hopefully the manufacturers will be able to add this functionality at some point. Until then, just remember that there is no advisory glideslope on LP approaches. Be careful on those descents!

0/0 takeoffs for Part 91?

Was at EAA AirVenture (Oshkosh) this past week, and at one of the instrument seminars one of the attendees asked a question I thought would be interesting to others as well. It was involved, but the part I'm going to discuss boiled down to the following (paraphrased):

I am based at an airport with some high terrain nearby that drives up the MDA for the approaches to about 900 AGL. However, I know that I can depart the airport with 0/0 ceiling and visibility. Why is that?

The presenter wasn't able to answer the question (I think due mostly to not understanding what it was), but I was able to help.

First, some background.

As discussed in other blog posts of mine, the MDA (or other minimums) are often determined by nearby terrain. If you have high terrain within a few miles of the airport, on final, you will often have high MDAs for the approach.

This makes sense, and of course 14 CFR 91.175 tells us that if you need to make an instrument approach, you cannot go below the DA or MDA unless certain parts of the runway environment are in sight, as every instrument student learns (I hope).

However, for departure, instrument students are taught that you can legally take off in 0/0 conditions (zero visibility and ceiling at zero feet) and what's more, that's true!

91.175(f) states the standard minimums required for takeoff, such as "aircraft having two engines or less - 1 statute mile visibility", and also requires that aircraft comply with an obstacle clearance procedure.  HOWEVER, this subparagraph only applies to "persons operating an aircraft under part 121, 125, 129, or 135". In other words, NOT part 91 operators, which applies to many of us (including the person asking the question).

Also, from the Instrument Procedures Handbook, page 1-8:

Aircraft operating under 14 CFR Part 91 are not required to comply with established takeoff minimums. Legally, a zero/zero departure may be made, but it is never advisable.

So as a part 91 operator, I have to comply with high minimum altitudes on an approach due to high terrain, but yet I can depart the same airport with zero visibility? Why is that?

The simplest answer is that the takeoff minimums for part 91 are not determined by obstacles or even evaluated in any way. 0/0 is the rule only because it's not prohibited by 91.175. You can make up your own departure minimums if you will.

An examiner once told me to consider three things when thinking about doing something in an airplane - Is it legal? It is safe? Is it smart?

Taking off in 0/0 weather may be legal. It might even be debatably safe-ish, depending on aircraft performance and terrain. It is smart? I don't think so. Not many "outs" in the event of any kind of problem.

What should you do as a Part 91 operator? In my opinion, always comply with at least the established takeoff minimums for the runway.

Looking at a specific example, South County Airport in San Martin, California (E16) has the following departure minimums:

And then has an admittedly lengthy definition of the departure procedure to follow. But the standard takeoff minimums (the ones in 91.175 that do not necessarily apply to Part 91) can only be used if you can maintain a climb gradient of 324 feet per nautical mile (not per minute) to a certain altitude, depending on which runway you depart. Or if you can't meet that, it allows a departure in visual conditions (called a Visual Climb Over Airport, VCOA) with at least a ceiling of 1700 and a visibility of 2.5 miles. This is to get you high enough that once you enter the clouds you can keep climbing and have a good safe cushion over the nearby terrain. If you can't meet that, maybe you should wait a while until the weather improves.

But VCOAs are a good topic for another blog. Fly safe!

How are Class E surface area extensions determined?

Ever looked at a sectional chart and noticed those keyholes of dashed magenta lines sticking out from a Class D or E surface area? Known as Class E extensions, they designate where the Class E airspace extends all the way to the surface, instead of starting at 700 ft AGL or 1200 ft AGL like normal.

How do they get these shapes, and what are they for? Many people can answer the second question, correctly but vaguely, with a "to protect for instrument approaches." This is correct, but why do you see extensions at some airports and not others? And why are some big, some small, some fat, and some narrow?

Look at Ardmore, Oklahoma's Class E extensions (KADM):

If you look at the instrument approaches for this airport, you'll see approaches coming from the NE, SW, and SE. But the Class E surface area extensions only go out to the NW and SW - and the SW one is a lot larger than the NW one. Why?

It has to do with the way the airspace extensions are determined, which depends greatly on both the design of the approach from that direction and the terrain underlying that approach.

Let's assume for right now that the terrain surrounding Ardmore is completely flat - it's not, with variations up to a couple hundred feet nearby, but close enough for now. Ardmore's field elevation is 777 MSL, so let's round off and say that all the terrain around there is right at 800 MSL. Now, we know from our Private Pilot ground school that the shaded magenta means that Class E airspace starts at 700 feet AGL. So in order to protect the aircraft as it descends below 700 AGL, a Class E surface area is created - at the point that the airplane is expected to descend below 700 AGL. In addition, a 300 foot buffer is accounted for in there, so it's actually where the aircraft is expected to descend below 1000 AGL.

The methods for calculating this point are extremely detailed and laid out in FAAO 8260.19F, Chapter 5. If there's a cure for insomnia, this chapter is it! There are variations for procedure turns (before, after, and at the FAF), hold-in-lieu-of-procedure turns, precision vs non-precision final approaches, procedures without Final Approach Fixes (FAFs), ways to account for high terrain in segments before final, and more. But I'll briefly focus on two common situations - vertically guided and non-vertically-guided final approaches, with FAFs.

Vertically guided final (ILS, LPV, LNAV/VNAV):

This is conceptually pretty simple - to determine when the aircraft is going to descend below 1000 ft AGL after the FAF, you just use the established glideslope - the pilot should be following the glideslope, so that's what is used. If the FAF is 1500 feet above the ground and there glideslope is 3 degrees, then at 318 ft per nm descent the aircraft will cross 1000 feet above the ground at 500/318 = 1.57 nm "in" from the FAF. So that would establish where the Class E surface extension starts! 

Non-vertically-guided final (LOC, LP, LNAV, VOR, NDB, etc.):

This is more difficult, because there's no way to know how quickly a given pilot is going to descend down to the MDA. In order to simplify some already complex rules, the equation for this rate of descent doesn't account for the wide variations in charted descent angle, stepdown fixes, or even always realistic expectations. The pilot is assumed to descend at 300 feet per mile when within 7 nm of the runway threshold, or at 500 feet per mile if further than 7 nm from the runway threshold. So a FAF that is 9 miles from the runway will use a combination of both descent rates.

In both of these cases, the 1000-foot AGL point is determined using the highest terrain within the final segment. So if it's hilly, that will likely push the 1000-foot point out further from the airport. Also, if due to terrain the aircraft will descend below 1000 feet AGL before the FAF, there are a whole another set of calculations for that, and the airspace extension will likely be longer (and wider too).

So now we know where the aircraft is going to descend below 1000 feet, and how far out the Class E extension goes, but how wide is it? The width is determined by the size of the obstacle evaluation area. I'm not going to go into that here, but see my other blog post for an example of this calculation for a VOR.

Now back to Ardmore. There is no Class E extension to the southeast. The FAFs for the approaches from that direction are close enough to the existing Class D, the altitude is high enough, and the terrain low enough that the aircraft will not descend below 1000 feet AGL before entering the Class D. So no extension is needed.

The approach from the northwest, the RNAV (GPS ) RWY 13, has LP and LNAV minimums - so no vertically guided final.  The FAF is 2000 feet above the airport, and the final is 6.4 nm long. However, the terrain underneath this final is higher than the airport - a clue is the height of the antenna towers near the FAF - the terrain appears to be almost 1300 MSL in this area. So the aircraft will descend below 1000 AGL before reaching the Class D, and needs to be protected.

Lastly, the VOR approach from the southwest (VOR-B) has the VOR itself as the FAF. It is a long final, 8.8 nm, so the descent rate used for the calculation is the higher 500 feet/nm discussed above, until the aircraft is 7 nm from the runway. This means the aircraft gets to 1000 AGL quicker (further out), the extension needs to be longer, and it is.

Similar calculations are performed for the shaded magenta areas that indicate 700-foot floor Class E airspace. However, in this case they're trying to determine where the airplane crosses 1500 feet AGL (the normal 1200 foot Class E floor + 300 foot buffer). As a result, the areas extend further out from the airport. Also, often the areas merge with those of other airports so the airspace is shaped to encompass all of the other airspace, resulting in some unusual areas.

I think that's enough for now. Keep the questions coming!