Sunday, May 13, 2007

Procedure Turns

What really is the purpose of a procedure turn? It’s pretty simple – you’re going the wrong way and need to get turned around. We all learned about them – those little arrows on an approach plate, showing the direction to fly away from the course, generally for one minute followed by a 180° turn back to intercept the course going in the right direction. The one shown below is the Jeppesen method of charting a procedure turn



Instrument students were taught to fly it exactly that way. To deviate was to invite the dreaded pink slip from a disapproving examiner. However things are now starting to swing to a less strict interpretation.

But there are a couple of different types of procedure turns that must be flown exactly as depicted. One of these is the teardrop reversal, as shown below.




This procedure turn is from the ILS-18 at Lincoln, NE. To do this procedure turn, the pilot must fly out on the 324° radial of the Lincoln VOR and then turn right and intercept the inbound localizer course of 174°. Funny thing, it looks exactly like a teardrop entry into a holding pattern. The 324° radial is exactly 30° offset from the outbound localizer course.

The teardrop procedure turn must be completed within the limit specified on the profile view. The default value for this is 10 nm from the specified point or fix, but it can vary. The one for ILS-18/LNK is 12 nm, as shown below.



Another procedure turn that must be flown exactly as depicted is the holding pattern, as shown below for the ILS for Red Wing, MN.



If you have any doubt about whether the holding pattern is actually the procedure turn, check the profile view. That will clearly indicate that it is a holding pattern. In this case it is based on time, the tradition one minute. More and more however, the holding pattern is based on a DME leg length.



The purpose of both the teardrop procedure turn and the holding pattern in lieu of a procedure turn is to allow the pilot to lose a lot of altitude within a constrained amount of airspace.

As mentioned above, unless the procedure turn is one that must be flown exactly as depicted, then the pilot is given a fair amount of latitude as how to accomplish the procedure turn. The AIM actually states that the manner in which the standard procedure turn is accomplished is left to the pilot. This includes the point at which to start the turn, as well as type and rate of turn. Some of the methods include the 45° procedure turn, the racetrack pattern, the teardrop procedure turn, and the 80°-260° procedure turn. The only restriction is that it must be done within the limits specified on the profile view and it must be done on the protected side of the course.

One situation in which the charted 45° procedure turn makes a lot of sense is when doing an NDB approach (without a GPS). Then the charted procedure turn provides a 45° intercept to the final approach course. And when you are doing this type approach with only an ancient ADF providing guidance, that makes intercepting the final approach course a lot easier.

Years ago a retired Navy pilot showed me the 90°-270° procedure turn, and it has become my method of choice. As he explained it to me, this method is utilized by the Navy for man overboard, because it pretty much brings you back to the point at which you start the turn. The method is simple, and it involves no timing. Simply turn 90° in the direction of the protected side of the course and immediately start a 270° turn back in the opposite direction. This is a graphic of the 90°-270° procedure turn done in the sim. I did a screen capture of the track and made it into this graphic.



It’s easy, it’s efficient, and you don’t have to time anything. In actual practice, I teach instrument students how to do it both ways. This is a precaution against having an examiner who insists that it must be done in the traditional manner.

This brings us to the interesting question of when do you have to do a procedure turn. The AIM is pretty clear on this. You don’t have to do a procedure turn if you are being vectored, or if you are on a published portion of the approach that states NoPT. But with more and more GPS approaches being published, this brings up the question of why do a procedure turn when common sense clearly says one is not needed. Take the following example, the GPS-28 into Maple Lake (KMGG). It is clearly not the newer TAA style of GPS approach, yet it has something of the same shape and format – three IAF fixes, with one of the them being in the center.



Maple Lake lies west of the Minneapolis metro area, and is often used for practice instrument approaches. Clearly if you are coming from the east and are cleared directly to NAZMY, doing a procedure turn makes no sense whatsoever, but it doesn’t indicate it to be a NoPT entry. This has puzzled a lot of instrument instructors in the area, and the debate raged on for quite some time. Finally a senior controller with Minneapolis Approach was contacted and asked as to what was expected. He said he would not expect a pilot cleared to NAZMY from the east to do a procedure turn, and in fact it could conceivably cause problems if he had other traffic in the area close to the fix. The situation has been resolved, at least locally, by an amendment to the wording of the clearance. Now ATC will state that the pilot is five miles from NAZMY and is cleared straight in for the approach or words to that effect. The 2007 AIM says a procedure turn is “a required maneuver when it is necessary to perform a course reversal.” The bold-face emphasis comes from the 2007 AIM itself. It further states that a procedure turn is not required when an approach can be made directly from a specified intermediate fix to the final approach fix, which somewhat applies to the Maple Lake GPS approach, since NAZMY is both an IAF and an IF. But then the AIM promptly muddies the water by saying in such cases the term “NoPT” is used. Fortunately the TAA GPS approaches have resolved these ambiguities.

One final item should be noted on the topic of procedures turns, and that is the definition of the phrase “procedure turn inbound”. Many instrument students as well as instrument pilots interpret this phrase to mean the point at which you start the turn back towards the final approach course. It is not. Procedure turn inbound is the point at which you turn onto the final approach course.



The bottom line is that unless the procedure turn is a teardrop track or holding pattern in lieu of a procedure turn, today you can pretty much do what you want when it comes to course reversal. Just do it on the protected side and stay within the specified limit.

4 comments:

Ron Talcott said...

Linda,

I am looking for some ICAO information on procedure turns and cannot find it easily. I believe you have to be alligned withing 30 degrees of the outbound procedure turn course when you cross the IAF to begin the procedure turn. Do you have any ICAO information? ron@talcottaviation.com

Alan said...

Another thought I had on the procedure turn was in VOR / Localizer approaches. As a student, I always had some form of DME, but recently found myself practicing a VOR approach in an airplane not equipped with DME or GPS. I realized that if you had to make the approach without the aid of ATC (comm failure, radar out, et cetera), the VOR would give you an accurate fix to let you know when it was safe to descend and you could simply follow a timed approach.

Anonymous said...

A problem I have noticed with your article regards your recommendation of 90-270 degree procedure turns. You screenshot your procedure turn and stressed how great the turn is because it brings you back to exactly where you start. But, this will not always be the case, and is likely not to be the case in real life scenarios. With any sort of wind at all, regardless of the 90-270 turn, you will drift either left or right of course.

Anonymous said...

Ya, I agree. I'm not sure how the 90-270 pans out with a signifigant wind. Especially if it is perpendicular to the maneuver. I guess I'll have to try it.