Monday, September 22, 2014

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!

Saturday, September 6, 2014

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!