Monday, February 10, 2014

Instrument - descent rates and back courses!

I recently conducted a PCATD ("simulator") lesson with one of my instrument students.  When I "fly" on any type of simulator, I like to choose approaches which are interesting, appropriately challenging depending on their stage of training, and most definitely not local!  Why would I want to practice local approaches in the sim?  It's my goal to train the student to be a good instrument pilot, not a good "Southwest Ohio" instrument pilot.  (Insert your local area as appropriate.)  So I search out approaches which meet the training needs of the student and the syllabus.

Okay, enough philosophy.  Two of the approaches we flew recently were in Oregon, and they had some interesting challenges and educational value.

One was the KMFR VOR-A:

I like this approach (remember, I have a weird definition of fun).  It has three different radials involved all off the same VOR, so setting up the radios is important.  It has a little bend at the FAF, which could be easily overlooked if you're rushed as you begin your descent.  But what really caught my (and my student's) attention was the descent within the Procedure Turn.  Notice it requires 1700 feet of descent.  The maximum for a 10nm PT is 2000, so this is up there.  Now picture a normal 1-minute PT.  By the time you get turned around and lined upon the R-342 inbound, even if you do it perfectly, you now have barely more than a minute to lose 1700 feet.  That is a pretty excessive descent rate, especially in something like a Cessna 172. 

So how do you fly it safely?  Really the only way is to not fly a 1-minute PT, but to extend that outbound leg to 2 minutes.  Or even 3.  But you have to be careful, remember you need to stay within 10 nm of the VOR.  Naturally, the place to figure this out is not once you're inbound on the PT, by then it's too late - proper review of the procedure beforehand and a thorough approach briefing are important.

This procedure also brings up the question of "why is it a circling-only procedure? It's lined right up with the runway."  You do see this sometimes.  When the altitude to lose from the FAF to the runway is too steep for a normal approach (defined in the TERPS as 3.77 deg for Cats A-C, and 3.5 degrees for Cats D/E), the approach is labeled as circling only.  I calculate about a 5.3 deg glidepath for this one.  This means some maneuvering may be required once in visual conditions to land.  Of course, if you get the runway in sight in time to land safely straight ahead, there is no requirement to perform a circling maneuver.


The other approach we "flew" and I wanted to discuss is the KSLE LOC BC RWY 13:

One of the topics of the lesson was to introduce LOC Back-course approaches.  This is where the localizer needle reads "backwards" from normal - to get it to center, you have to fly away from it!  This is referred to as "reverse sensing", and to fly it, you need to "pull the needle" back to center.  It's really not that difficult once you flip that little switch in your brain, but if you're maneuvering to get lined up on final it can be a bit of a mental exercise.  Once on final everything usually goes pretty smoothly, unless of course you forget it's a back course!

For this approach, I let the student enter the procedure from the UBG VOR.  Even I thought it would be cruel and unusual punishment to require him to fly the route from CVO, with a Procedure Turn, on his first exposure to back courses (there's always next time though...)

This procedure has an unusual note, notice the third line in the notes box "ARTTY INT not authorized for final approach fix."  What on earth does that mean?  Simply, it means that when you're inbound on final and identifying ARTTY as the FAF, you HAVE to use either DME, or the outer marker to do so.  Notice the route from CVO also proceeds to ARTTY as the IAF - for this route, ARTTY can be identified as the intersection of the CVO R-359 and the localizer course.  But once inbound on final, you must use DME or the marker beacon.  Why?  Look at the distance from CVO to ARTTY - 31.5 nm!  Since VOR radials spread out as you get farther from the transmitter, this means that the signal accuracy is okay to use when you're just entering the procedure (and at a higher altitude).  But once you get lower and are inbound approaching the FAF, the signal accuracy is no longer acceptable.  Logically, FAFs require somewhat tighter signal tolerances than IAFs and other fixes further out.

The missed approach also brings up a useful discussion - tracking inbound on the back course is the same as tracking outbound on the front course, so as you pass over the runway and fly the missed approach, nothing changes - you're still pulling the needle.  This is also true for the first half of the entry for the missed approach hold.  But once you get turned around, you're flying inbound on the front course, in which case the needle now reads normally!  Once in the hold, the needle remains reading normally, so once you flip the switch in your head back to "normal", leave it there.

It was an interesting and educational day.  (And yes, the student did great.)

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