Monday, May 19, 2014

Early turns on missed approaches; ILS MAPs

There you are flying down final (on any kind of approach) in real IMC, and something happens - the needle goes full deflection, you have an equipment problem, whatever - and you need to go missed. ATC isn't available, either due to RADAR or radio coverage, so you need to fly the published missed. Simple enough, we fly missed approaches all the time in training. You're on the ball and already had the missed all briefed and set up well before you reached the FAF. So it's a simple matter of flying it, right?

Many missed approaches have as a part of the instructions something like "climb to XXXX MSL then turn..." (in other words a delayed turn) because there's probably a good reason for the delay. Yes, it might be airspace, or traffic flow or ATC preference, which might not be your most significant concern when facing an emergency. Or it could very well be to avoid buildings, tower, or mountains, which obviously would be a very big concern! The problem is, when flying the procedure and looking at the chart, it may not be obvious which is the case. With the proliferation of tall antenna towers, you could have the same problem with antennas in Iowa that you'd have with mountains in Idaho.

So far so good. But where do you perform this turn? Regardless of the situation, you are expected to not perform any of the missed approach instructions until you reach the MAP. Of course, during training we almost always descend to the DA/MDA and decide to go missed when we're already at the MAP. Using the Burlington, Vermont ILS OR LOC/DME RWY 33 as an example:

Note the missed approach is "climb to 1200 then climbing left turn to 2800 direct  BTV VOR/DME". Let's say you're somewhere between JIDSO and KOTDE and find yourself well below glideslope and off course. You are clearly well behind the airplane. Maybe you're already down around 1700 before KOTDE. You (wisely) decide to go missed. Since the missed approach instructions say "climb to 1200 then..." and you're already above 1200, you can turn direct the VOR, right? No! Remember, all missed approach instructions presume you start the missed approach AT the MAP. There is some allowance for timing errors and such, but not this much.

From the Instrument Procedures Handbook, page 5-33, "When a missed approach is executed prior to reaching the MAP, the pilot is required to continue along the final approach course, at an altitude above the DA, DH, or MDA, until reaching the MAP before making any turns. If a turn is initiated prior to the MAP, obstacle clearance is not guaranteed. It is appropriate after passing the FAF, and recommended, where there aren't any climb restrictions, to begin a climb to the missed approach altitude without waiting to arrive at the MAP."

The Aeronautical Information Manual, section 5-4-21b and h, and the Instrument Flying Handbook page 10-21 have essentially the same information.

Looking at the previous example, you can see there is some terrain between you and the VOR if you were to make that left turn before KOTDE. How high is this terrain? I don't know, and you probably don't either. The one clue we do have is that the shading of the contours implies it's at least 1000 MSL, but less than 2000 MSL. That could put us in a pretty dangerous situation if we're starting at 1700!

So we need to wait until the MAP to turn. How do we know where that is? On a non-precision approach, of course, the MAP is often defined by crossing the VOR or NDB, a DME fix, an intersection, or lastly, timing from the FAF. However, the MAP on an ILS is defined as when you reach Decision Altitude on glideslope. If you are having some kind of situation (emergency or otherwise) requiring a missed approach, though, you don't want to keep descending. Much safer to get further away from the ground, so you level off and start to climb. Due to this climb, however, you can no longer identify the ILS MAP since you're well above DA.

Can you use the Localizer MAP? Probably the best idea if you're set up for it. In the BTV example, you've probably been using DME all along the procedure, so the best solution is to just climb straight ahead, wait until you reach the LOC MAP (0.2 DME past the VOR) then begin the turn.

If, however, the LOC MAP is identified by timing, did you start your timer at the FAF? Many people forget or neglect to do that when flying ILS procedures since it's normally not needed. If you didn't start your timer, then you might be out of luck as far as identifying the MAP goes.

Notice the next example, Grand Junction, Colorado, ILS OR LOC RWY 11. Part of the reason for the straight-ahead climb here is probably to gain some altitude first before turning toward the mountain that the VOR sits atop. If you turn before the MAP here, do you have enough distance to climb before you get to the rocks? (How's your high altitude climb performance? Any downdrafts to consider?) Let alone you're supposed to intercept the R-085 inbound, this could make for some confusing maneuvering.

But at least that one has a timing table. How about at Reno, Nevada, the ILS RWY 16R?

No timing table or even a LOC MAP, since it doesn't even have Localizer minimums (that's a separate chart). Now, the climb-to altitude is only a couple hundred feet above the DA (which is very high in itself), but you still wouldn't know when you were at the MAP and therefore when exactly to turn. In addition, since the DA is so high, the MAP is actually about 6 nm prior to the runway!

Any time you turn before the MAP you are stepping into uncharted (hah!) territory. Most of the time it would probably work out okay, but do you want to take that chance?

So what can you do? First, a good approach briefing is critical, and I recommend starting your clock at the FAF on every approach just as a habit. But if you didn't do that, then you're really in an emergency situation - you've lost situational awareness and don't know your true position. First, of course, climb! Then, ultimately, it's about making a "best guess" as to where the MAP is. If you have a portable GPS up and running, use that. If you can get a DME source, use that. Maybe there's a VOR on the field that, while not used on the procedure, would give you a TO/FROM flip when you cross over it and at least tell you where you are. If none of these are available, then consider climbing up to the MSA before turning - you know that will provide obstacle clearance for a 25nm radius. And in fact, it provides at least 1000 feet of obstacle clearance - an extra cushion when it's really needed.

Sometimes approach planning can take a lot more thought than normal, and identifying the MAP on an ILS can be one of those situations. As always, the best time to figure this out is on the ground before takeoff. Fly safe!

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!