Flying the LOC/DME BC-A into KSMX

Provides assistance in flying the LOC/DME BC-A into KSMX

Open or print the KSMX LOC/DME BC-A (Santa Maria Localizer DME Backcourse Alpha) approach plate, referenced at the bottom of this document.

This non-precision approach is based on a navigation aid, I-SMX, which is the localizer used for the KSMX ILS RWY 12 approach.

Whilst the front course of I-SMX (running from the station to the northwest) services the ILS approach, the back course of the localizer (running from the station to the southeast) services the LOC/DME BC-A approach.

Reverse Sensing.  What the.... ?

Unlike VORs, localizers only transmit a single radial. This is why technically possible to fly an ILS or LOC approach without setting the OBS on your NAV receiver (although it is considered good practice to do so to aid in visualization and situational awareness).  For this reason, though, when flying the backcourse approach, you will experiencing reverse sensing when interpreting the Course Deviation Indicator (CDI), or the 'needle' on your VOR receiver.

This is the same experience that can be had by tracking towards a VOR with the needle centered and the TO/FROM flag indicating FROM.  When dealing with a VOR, you can adjust the OBS to alleviate the reverse sensing issue. With a back course localizer approach, however, you must deal with it.  If you are using an autopilot to maintain lateral control on the back course of the localizer, you must TELL the autopilot that you are using the back course by using the cunningly named "BC" button. If one is not available, do not attempt to have the autopilot track the localizer back course, unless you are a fan of seeing your autopilot become depressed, or worse, angry.

When flying the front course, if the needle is to the left, then the localizer is on your left, and you need to make a left turn to recapture. You fly 'to' the needle.  On the back course, the opposite is true. If the needle is offset to the left, you need to turn RIGHT to recapture. You fly 'away' from the needle.

A reasonable way to practice reverse sensing is to takeoff under VFR, find a nearby VOR, become established either to or from the VOR, using the appropriate OBS setting, and then either turn the airplane 180 degrees, or spin the OBS 180 degrees.  You will then experience reverse sensing and can practice holding the course and making reversed corrections.

If you are lucky enough to be flying an airplane equipped with an HSI. Try setting the HSI to the front course heading, this will cancel the reverse sensing.

Flying the approach from RZS

With the plate in hand, let's fly the approach from San Marcus (RZS). Set your NAV1 radio to the I-SMX localizer with the OBS set to 300.  Set your NAV2 to RZS, with GLJ in the standby freq, we'll need it to set up for the missed. Departing RZS, the NAV2 OBS should be set to 286, to track the RZS R-286 outbound. Maintain at or above 6000, as published to 14 DME (MADOO), then at or above 4600 until KOAKS.

KOAKS is located at the intersection of the I-SMX back course, and the RZS R-286, at RZS 29 DME.  Approching KOAKS, the needle on the NAV1 should swing from a right deflection to being centered (even though we were on the RIGHT side of the back course, we were on the LEFT side, relative to the heading of the front course, which is all I-SMX is good for telling us about, as discussed above.)

At KOAKS, we join the back-course and proceed on the 300 course, using the NAV1 needle to remain on track. Descend to cross CAMCO at or above 3000 and PATER at or above 1700. After PATER, we can descend to the Minimum Descent Altitude (MDA), which is likely to be 1100 MSL, unless you're flying an SR-71 (Category D), in which case....good for you.

Once established on the localizer, and when time permits, drop RZS from the NAV2 and bring GLJ to life, it'll be needed for the missed. Set the NAV2 OBS to 300.

You may receive a glideslope indication during this approach. Treat this as you would your spouse, mother-in-law, or low oil light in a rental vehicle....ignore it. It's a really great thing to follow on the ILS RWY 12, but not so good for this approach.  It says it right there on the vertical profile on the plate, too.

Reaching I-SMX 0.5 DME, we execute the missed approach.

LOC/DME BC, I get it, but why -A and not RWY 30?

The final approach course is suspiciously well-aligned with runway 30, isn't it?  This is not surprisingl since the front side signal is used for a PRECISION approach to the same piece of asphalt, runway 12.  A fair question, then, is why this is a circling approach without any straight-in minimums.  The descent gradient of this approach exceeds the maximum stipulated for straight in approaches, so this is a circling approach only, hence the Alpha designation.  The VOR-A into KSMO is another example of such an approach.

Does it mean that you must physically circle the aircraft even if you plan to land on runway 30? No. If you can safely land the airplane with a straight-in approach, then do so.

Flying the missed

Reaching the missed approach point, execute the missed approach, by climbing to 2000ft and proceeding direct Guadalupe (GLJ).  Reaching GLJ, enter the hold as published.  Given the direction from which we are arriving, a parallel entry would be the appropriate method.  Depart GLJ parallel to the holding course (or in this case, directly on it see we arrived at GLJ already on the parallel heading).  Continue outbound for 1 minute then start a right turn. During this time, spin the NAV2 OBS to the inbound heading for the hold, 120.

Continue the right turn to set up a 30 degree intercept to the holding course, 150 will work here.  Continue on that heading until the NAV2 needle centers, and turn left to join the holding course, 120.  Once this happens, you are established in the published hold at GLJ.  Passing over the station, make a left 180 degree turn, proceed north for one minute, make another 180 degree turn and join the holding course with the centered NAV2 radial. Your inbound leg should take 1 minute.  You've just flown a full lap of the hold.