Saturday, May 3, 2014

Basic approach construction part 2: Missed approaches and departures

I'm back! It has been a while since my last blog post, but in that time I've been moving from Dayton, Ohio to Oklahoma City, Oklahoma. Now that I'm all settled in, time to update the blog.

Like my last post on basic approach procedure development, here I want to discuss basic missed approach and departure procedure development. Though there are some different rules for each, some of the basic concepts are identical, so I'll lump them together at first. For this post, I'm only going to discuss the vertical, "climb" component of the obstacle clearance evaluation, not the horizontal or lateral areas. As before, there are many details I've had to leave out to make this a little more readable in a quick blog format.

The first general concept is that of a sloping surface. Unlike a (non-precision) approach where you level off at an MDA, a departure or missed approach involves a continuous climb from some starting point. Unless otherwise stated, you are expected to climb at a gradient of 200 feet per nautical mile. Note this is not a climb rate (like 200 feet per minute), but a climb gradient. At 90 knots (ground speed), 200 feet per nm requires 300 fpm. It is up to the pilot to ensure that the aircraft can maintain this climb gradient given the aircraft performance, density altitude, wind conditions, and other factors.

Let's consider departures for the moment. The worst case for obstacle clearance would be a takeoff roll so long that the aircraft finally leaves the ground right at the departure end of the runway. Therefore, this is where the climb is assumed to begin.

Obviously there needs to be some kind of obstacle clearance between the aircraft and the terrain or other obstructions. This OCS, or Obstacle Clearance Surface, also starts at the end of the runway and is also a sloping surface. However, to provide the aircraft with terrain and obstruction clearance, it rises at only 76% of the slope of the aircraft's climb gradient, or a standard 152 feet per nm (also known as a 40:1 surface, rising 1 foot for every 40 feet of distance).  This results in more obstacle clearance the further you fly.
If this OCS clears all obstacles along the path of the departure route, then great! However, let's assume there's something in the way a few miles out - an antenna, hotel, mountain, anything.

This obstacle penetrates this OCS. It doesn't matter if the aircraft would still clear it (as it would in the picture), if it penetrates the OCS then it is a factor and the procedure needs to be changed.

There are four options to avoid the obstacle:

1. Require a turn before reaching the obstacle.
2. Require a ceiling and visibility high enough to be able to see the obstacle.
3. Require the aircraft to be able to climb at a greater-than-standard gradient. This also results in a steeper OCS (since it's 76% of the climb rate).

4. Require the aircraft to leave the ground by a certain distance before the departure end of the runway.

There used to be an option to require the aircraft to be at least 35 feet AGL at the departure end of the runway, but that option no longer exists, and is replaced by option #4. However, some older procedures may still have it, as do some training references.

You'll see these options spelled out on the departure procedure. For Option 4, it will look something like this: "With standard takeoff minimums and a normal 200 ft/nm climb gradient, takeoff must occur no later than 1800 ft prior to DER (Departure End of Runway)." See the following example: 

This example is great since it shows both the increased climb gradient option or the reduced runway length option. Now, 216 ft/nm isn't that much more than 200 ft/nm. But some airports have much more than that! Can your Cessna 172 make it out of Steamboat Springs, under IFR and maintain this kind of climb gradient? Not likely!

Now we've talked mostly about departure procedures so far, and for good reason. For the most part, as far as the rising OCS goes, missed approach procedures are pretty similar, as you'd expect! In fact, in the last few years the FAA has even begun allowing climb gradient requirements to be placed on missed approaches. This can really help get minimums down lower.

Consider the following case - a runway has no real terrain on final, so as a result the MDA could be pretty low. But on the missed approach there is a hill or other obstacle in the way. Typically, to clear the obstacle the MDA will have to be raised, sometimes a lot!

But if instead the climb gradient could be increased allowing a steep climb at the missed approach point, maybe that lower MDA could be safe after all. Check out the following procedure at 65S - how steep you can climb determines how low you can go! On AeroNav charts, when climb gradients are established on a missed approach, the minimums are "asterisked" and then you need to refer to the notes section.

So if you can only maintain 200 ft/nm your MDA is 4480 MSL, 2150 above touchdown zone elevation. That's pretty high. If you can maintain 300 ft/nm you can get down to 3880 MSL (1550 HAT). Better yet, 400 ft/nm gets you down to 3260 MSL (930 HAT)! Know your airplane, know its performance capabilities, and above all make sure you figure this type of thing out way in advance, like before takeoff!

I think that's enough for now. Happy flying!

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