When to slow to approach speed?

There is a discussion taking place on the Pilots Of America forum that is revolving around when to slow the aircraft to final approach speed on an instrument approach. Unfortunately, the Instrument Procedures Handbook, the Instrument Flying Handbook, and the Aeronautical Information Manual do not provide much guidance on this.

One way or the other, you need to be slowed down to your final approach speed and configuration by the FAF. You of course can do this far away from the airport, even before the IAF, and the earlier you get things established the more time you have to fine-tune your configuration.

But there is a place that is expressly designed for this purpose in case you're not configured earlier - the intermediate segment! This segment, which starts at the IF (Intermediate Fix) and ends at the FAF, for the most part exists primarily to help you get slowed down, configured and ready to descend at the FAF. Once past the FAF you should have all those changes already made (gear, flaps, etc. as appropriate) so that you can concentrate solely on flying the plane, staying on course, and watching your altitude.

This is encapsulated in the FAA Order for approach procedure designers, FAAO 8260.3B, which states:

Para 240: [The intermediate segment] is the segment in which aircraft configuration, speed, and positioning adjustments are made for entry into the final approach segment.

Further on:

Para 242d: Because the intermediate segment is used to prepare the aircraft speed and configuration for entry into the final approach segment, the gradient should be as flat as possible. The OPTIMUM descent gradient is 150 ft/mile. The MAXIMUM gradient is 318 ft/mile [or higher if there is a greater than 3 degree ILS]. Note: when the descent gradient exceeds 318 ft/mile, the procedure specialist should ensure a segment is provided prior to the intermediate segment to prepare the aircraft speed and configuration for entry into the final segment. The segment should be a minimum length of five miles and its descent gradient should not exceed 318 ft/mile.

So the intermediate segment should be flat to allow aircraft configuration, around 150 ft/mile, but no greater than 318 ft/mile (which is a 3 degree glidepath), with some exceptions if, say, there is a 3.2 degree glidepath.

Let's look at some examples.

To me, this is an example of the ideal intermediate segment, at Dayton, Ohio (KDAY) RNAV (GPS) RWY 18.

Notice that once you do all the initial maneuvering, you have a completely flat, level segment (GILPE to WALMA) in which to get your speed set. Why is this important? Most of us train for our instrument ratings in something like a Cessna 172, which isn't too hard to get to slow down even while you're descending - it has a lot of drag. But something faster and more slippery, maybe like a Mooney or even a jet, will have a harder time both going down AND slowing down. So this level segment is a good place to get that done. Then, when everything's configured and on speed, you just wait until the FAF, make your power reduction, and start on down. This is the easiest type of intermediate segment to get set up for and fly.

It doesn't always work that way, of course. Many things can cause the intermediate segment to require some kind of descent. It might be obstructions like antenna towers or buildings, it could be surrounding airspace, or many other things. So sometimes it's unavoidable. Here's the profile view of the Appleton, Wisconsin (KATW) VOR/DME RWY 3:

You cross the Oshkosh VOR (the IF) at 3000, then descend to 2700 by the FAF. This is only 300 feet of descent, and yet you have 10 miles to do it! So that's not bad at all - 30 feet per mile. Most pilots would probably just descend to 2700 in the first mile or so, then spend the rest of the intermediate segment getting configured.

But sometimes terrain drives this descent gradient up much higher. Here's the profile view of the Arcata-Eureka, California (KACV) ILS Y or LOC/DME RWY 32:

The FAF has a minimum altitude of 2100, but if you do the math you'll see that each previous fix (OMBEE, HURDU, KORBE) is almost exactly 318 feet per mile higher. This is okay and meets the criteria in the 8260.3B, but might make it hard to "go down and slow down". Fortunately, prior to KORBE (as can be seen on the plan view below), there are some segments that are a bit more level (like that long arc from HIDAK to JEBGA) where your final approach configuration can be established.

This is yet another example of why it pays to take a good look at the procedure well beforehand and plan your arrival. Don't want to get to KORBE and then realize you don't have a good place to slow down!

Happy flying!

Threshold Crossing Heights

I know, it's a really exciting title this time!

Flying with another instructor (always a scary thing) a few days ago, as we were coming in to land he said "doesn't it always looks like you're really low coming into this runway, even though the VASI says we're on glidepath?" I agreed that it certainly did seem like we were low, though sure enough we were right on glidepath.

Now, it's a possibility that the VASI was just misaligned, got knocked by a mower, who knows. But I started looking into it a little bit more. Turns out this runway has a lower-than-normal Threshold Crossing Height (TCH). From the A/FD:

That means the VASIs are set for a 3.0 degree glidepath, and the glidepath that they establish crosses the threshold of the runway 21 feet above it (since the VASI isn't physically located right at the end of the runway, but some distance down the runway). So, if you're coming down final perfectly on glidepath (aren't we all?), when you get to the very beginning of the runway you will be 21 feet above the ground. Actually, your eyeball will be 21 feet above the ground, so your wheels will obviously be somewhat lower. This is known as the Wheel Crossing Height (WCH), but isn't something you'll see published. If you're in a typical light training airplane that means your wheels are somewhere maybe 15 feet above the ground.

That's pretty low.

 

But it's a visual system, so you're expected to adjust as necessary. Just think - if you were in a larger airplane following this VASI glidepath, your wheels might touch the ground before the runway! That would be bad.

The airport we were flying into was a small airport. Runways at larger airports designed for larger aircraft, by necessity have higher TCH (and therefore WCH).

The same concept exists in instrument procedures, only the TCH isn't really from the pilot's eye but from the antenna on the aircraft. This is captured in the design standards for instrument approaches, and the VASIs (or PAPIs, or other visual glidepath indicators) are often set to match the approach. The FAA determines the TCH by basing it on the type of aircraft that generally lands on that runway, breaking it down by "Height Groups", with Group 1 being small aircraft like the one we were in, and Group 4 being Boeing 747-size aircraft.

The optimum TCH provides a 30-foot WCH, but the range is 20 feet to 50 feet. In order to accommodate the largest range of aircraft, somewhere around 50-55 feet is a pretty common TCH. All this information is out of FAAO 8260.3b, Volume 3, paragraph 2-3 and Table 2-2.

Often on instrument approaches you will see the following comment, or similar, in the profile view:

"VGSI and ILS glidepath not coincident"

This simply means that the TCH and/or glidepath for the VASI and that for the approach procedure (like the ILS glideslope) do not coincide - they aren't equal and therefore don't overlap. Ideally they would, but sometimes for siting or design reasons they can't. The note is there so that when you're flying a perfect approach, on glideslope, and pop out below the clouds, you don't get startled by a different indication on the VASIs. The criteria for when they put the note on? If there's a difference of more than 0.2 degrees between glidepaths and/or 3 feet between TCHs. Both of these values are now published on the chart, though that's a recent change within the last few years so not all procedures will have the VGSI information. (Source: FAAO 8260.19F, para 8-6-6n)

So back to the original situation with me and another instructor - is the low TCH of the VASI the reason we thought we were low? Could be. If you're used to flying into runways with a 55-foot TCH, being 34 feet lower on short final is likely going to be pretty noticeable!

Additional geek content for extra credit:

The glideslope angle and TCH form a right triangle, so with some basic trigonometry, you can calculate how far down the runway the glidepath will intersect the pavement with a simple formula:

GPI (Ground Point of Intercept) = TCH/(tan (Glidepath Angle))

So, a 3.0 degree glidepath with a 21 ft TCH gives a GPI of 401 ft down the runway. This is a simplified calculation not accounting for runway crown and slope, but is close enough for visual work.

If you wanted to figure out where your wheels would touch the ground, use the WCH for your airplane. For the one we were in, let's say it's a 6-foot difference, so 21 ft - 6 ft = 15 ft, meaning our wheels would touch down at 286 ft down the runway (assuming we didn't flare at all), which is really really close to the beginning! A standard GPI with a 50-foot TCH is 954 ft down the runway, and a 55-foot TCH makes it 1049 ft. Of course, that's right about where those 1000-foot markers are. This, I believe, is not a coincidence!

Why do antenna towers top out at about 2000 feet?

I thought this was interesting, so I'm passing it on - that's the purpose of a blog anyway, isn't it?

As you look at an aviation sectional chart, you will see more and more tall antenna towers. I know of one northwest of Dallas that I once came across while flying - the top was in the clouds but I was in perfectly legal VFR. That's tall! It's this one on the chart, topping out at 1999 ft AGL, 2859 MSL.

As you look around the chart, you see a lot of towers that are 1999 or 2000 feet tall, and some that are a little bit taller - 2008 feet for example. Or this one at 2063 northwest of Fargo, North Dakota (it's also the tallest structure in the U.S., and the third tallest in the world).

But you don't see any that are taller than that. You don't see any towers 2300 feet tall, or 2500 feet tall, or even taller. Why not?

Aside from the engineering challenges and massive expense of a tower that tall, there's another reason - regulatory. Both the FCC and the FAA have established a "rebuttable presumption" against structures over 2,000 feet tall. This means that unless proven otherwise by the proponent, the FCC considers antennas taller than this to be inconsistent with public interest, and the FAA will presume anything taller to be a hazard to air navigation.

From the FCC's part of the CFRs - 47 CFR 1.61, the note at the end:

Applications for antenna towers higher than 2,000 feet above ground will be presumed to be inconsistent with the public interest, and the applicant will have a burden of overcoming that strong presumption.

http://www.ecfr.gov/cgi-bin/text-idx?rgn=div6&node=47:1.0.1.1.2.1#47:1.0.1.1.2.1.151.44

It states that this has been their view since 1965.

The FAA has similar wording in 14 CFR 77.7(d). (Some references still say 77.17(c), this was changed in 2011.)

If you propose construction or alteration to an existing structure that exceeds 2,000 ft. in height above ground level (AGL), the FAA presumes it to be a hazard to air navigation that results in an inefficient use of airspace. You must include details explaining both why the proposal would not constitute a hazard to air navigation and why it would not cause an inefficient use of airspace.

http://www.ecfr.gov/cgi-bin/text-idx?rgn=div5&node=14:2.0.1.2.9#14:2.0.1.2.9.2.1.2

One more note about that 2063-foot tall tower in North Dakota - it was constructed in 1963, before the FCC's establishment of their current policy. Might there be a connection?

More information at the FCC's web site - see the last two paragraphs at:

http://transition.fcc.gov/mb/policy/dtv/lighting.html

(Notice the outdated reference to 14 CFR 77.17(c).)

Look out for those towers!