Tuesday, March 1, 2016

Procedure turns - when can you descend?

This blog is a tie-in with the Stuck Mic AvCast episode 115, available here!

On the podcast we talked about the procedure turn (PT) and hold-in-lieu-of-procedure-turn (HILPT), and specifically, WHEN can you descend when executing the maneuver?

This type of question comes up often in instrument rating checkrides, job interviews, and of course in real flying and it's important to know the proper point to begin your descent in all phases of flight. So let's get right to specifics!

The first procedure discussed was the Pocatello, Idaho VOR RWY 3:

This procedure has a fairly standard layout, with one exception which we will get to in a bit. But first, some definitions!

Let's say we are starting from somewhere east of the field, cleared direct to the PIH VOR, maintain 8000, and cleared for the approach. When can we descend? There is a "7200" minimum altitude depicted on the left of the profile view, so at some point we know we can descend to that altitude - but where to start?

From the FAA's Instrument Procedures Handbook, Chapter 4:

"The altitude prescribed for the procedure turn is a minimum altitude until the aircraft is established on the inbound course."


"Descent to the PT completion altitude from the PT fix altitude (when one has been published or assigned by ATC) must not begin until crossing over the PT fix or abeam and proceeding outbound."

The result of this is that we remain at 8000 (since that's what was assigned by ATC) until crossing the PIH VOR/DME. As we turn to that outbound course of 235, we can then begin descending to our procedure turn completion altitude of 7200. We do not have to wait until we're turned around inbound - in fact on some procedures, depending on the amount of altitude we need to lose, that might cause problems in itself! The descent gradients established within a procedure turn are based on the expectation that we will begin descending when crossing the PT fix - the longer we wait, the steeper and steeper we will have to descend to make the FAF altitude (or MDA if there is no FAF).

Once we are turned around and established on the inbound course, THEN we can continue our descent to the FAF altitude - 5600 in this case. From there on in, the procedure is flown like any other.

I skipped over something for a bit - that "PT Fix altitude" of 7800 in this case. Not all procedures with a PT have these. This is an established minimum altitude we must maintain until crossing the fix outbound. In our example, ATC cleared us to the fix at 8000 - so we're above the PT Fix Altitude and there is no problem. But maybe we're flying along an airway, say V21 southwest-bound:

Note the MEA along V21 is only 7000. If we don't plan ahead and are flying right at the MEA, we may find ourselves having to CLIMB to 7800 to meet the PT Fix Altitude! If there is no PT Fix Altitude (as is usually the case), then the PT Completion Altitude is the minimum for entry as well (of course - you wouldn't climb in a PT).

The method of indicating the PT Fix Altitude above is the current charting standard. However, you will still see approaches that have the altitude shown as in this example from Livingston, MT (LVM):

The next example we discussed on the show was the Twin Falls, ID (TWF) ILS OR LOC RWY 26:

This procedure incorporates a holding pattern in lieu of a procedure turn, often called a hold-in-lieu or a HILPT. Just like a procedure turn, the holding pattern is established as a means of turning ourselves around. The only expectation is that we perform the holding pattern entry (using any method, such as the three "standard" holding pattern entries). Then, when we are established on the inbound course we continue on with the procedure. ATC does NOT expect us to perform multiple circuits of the holding pattern, and if we need to do so (in order to get established or maybe to lose altitude) we are required to inform ATC prior to doing so. Again, from the Instrument Procedures Handbook, chapter 4:

"If pilots elect to make additional circuits to lose excessive altitude or to become better established on course, it is their responsibility to so advise ATC upon receipt of their approach clearance."

This procedure has a slight bit of an unusual twist, in that once we are established on the inbound course of 258, we can descend an extra 100 feet, down to 5900 for glideslope intercept.

The last procedure we discussed on the show was the Asheville, NC (AVL) ILS OR LOC RWY 35:

Here's the question - if we are inbound from the SUG VORTAC on the established feeder, where can we descend and to what altitude? The answer is that if we are cleared for the approach from over the SUG VORTAC, we can descend to 6200 while flying that feeder route to the BRA NDB. Once crossing the NDB (which is the HILPT Fix) we can further descend to 5200 while outbound on the holding pattern entry (which, using one of the three standard entries would be a parallel entry). We would stay at 5200 as we turned inbound and all the way back to the NDB. Once crossing the NDB we would begin our descent to 4000 for glideslope intercept.

Those were the three procedures we talked about on the show, but there are a couple more examples of unusual situations that I want to mention.

Some PTs even have a MAXIMUM altitude established at the PT Fix, like Twin Falls, ID (TWF) again, this time the VOR RWY 26:

Notice that we must cross the TWF VORTAC to begin our PT at no higher than 10,000! Maximum altitudes are rarely used on procedures but here one is. Often they are at the request of ATC, but when it comes to PTs they can also be used to limit the size of the evaluated area. For a given Indicated Airspeed, True Airspeed increases with altitude, and therefore turn radius does as well, so PTs above 10,000 have a larger area for obstacle evaluation than those at lower altitudes.

This procedure also has a stepdown fix along the inbound course at 3 DME (XULXU). Just like with any stepdown fix in final, if you can't identify it you have to use the higher set of minimums. In this case, if you find that once you get established inbound you're already inside 3 DME, then you can begin further descent right away. 

One last example, the Kremmling, CO (20V) VOR/DME-A (notice how all of the fun examples are in mountainous states?):

This one actually has a 15NM PT distance limitation, to give pilots more distance to deal with the high altitudes and descents involved. There are some 16,000 and 17,000 foot MEAs on nearby airways, so descent planning becomes a very real consideration!

Notice the PT Completion Altitude of 13,000 is also the first stepdown fix altitude at HADLA, 10 DME. Further descent is allowed to 11,800 at 4 DME, then crossing the VOR is the FAF at 10600. When is the best time to figure this all out? Obviously on the ground during flight planning!

I think that's enough about PTs for now. Thanks for reading (and listening to the show), and let me know if you have any comments or questions!