Monday, June 16, 2014

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!

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