Section I: Equipment and Environment

Part V, Chapter 15: The LORAN/GPS RMI Combo (High Tech)

When Bush Flying was first written, Loran was all we had and GPS was still on the horizon. As a result, Chapter 15 is now in the process of being revised for the next edition; however, I decided to include this old version because the concepts are readily transportable to GPS.

When I started flying the bush, navaids, mostly NDBs, were few and far between, so it is not surprising that I developed a fondness for the radio magnetic indicator (RMI). But of what real use in an area rapidly converting to VORs is an expensive instrument that, for all practical purposes, takes a nice reasonable VOR and turns it into a NDB? One that, in fact, forces us to exchange our nice HSI or CDI -- in a strange M. C. Escher transformation -- for a bird-dog pointer just like an ADF's?

Well, a few years back, the answer would have been simple -- not much. But that was before the current revolution in avionics, and the widespread development of sophisticated Loran and emerging GPS technology. These new devices, with their interfaces and internal algorithms designed to take advantage of the RMI's unique capabilities, are resurrecting the RMI from the dark ages of instrument flying and bringing it to the forefront of modern navigation.

In the second part of this chapter I will describe some of the new uses for the RMI that the bush pilot will appreciate, but for now let's take a look at how the instrument operates and how the pilot flies it.

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In most incarnations, the RMI has a slaved compass card, along with two needles, one for the ADF and the other for VORs or Loran/GPS waypoints. The two needles always point TO the stations tuned, and your bearing TO or FROM the stations is found by reading the numbers under the head or tail of the needles. This makes your position relative to the stations and aircraft heading quite clear.

Most airplanes with RMIs and HSIs also have DME; thus, when the RMI in tuned to a VORTAC, you'll not only have bearing TO or FROM the station, but distance as well.

In its most basic application, the RMI is used to track TO or FROM a station, or to intercept a radial or bearing. To illustrate the principles involved, let's assume we're on a mid-winter vacation down south, and use a short flight out of Tucson, AZ along V-202. This airway departs TUS on the 107-degree radial, goes 25 miles to MESCA intersection, and intercepts the CIE 245-degree radial. The course, therefore, will be 107 degrees to MESCA, then left to 065 degrees.

For this demonstration flight, tune both navigation receivers to the TUS VOR, and place CIE's frequency in the standby windows. Also, switch the HSI and RMI to nav 1, and the number 2 VOR/LOC indicator to nav 2. Set the number 2 VOR/LOC indicator's OBS to 107 degrees.

Departing Tucson, the 107-degree radial is intercepted off the end of the runway, and climbing through 5,000 feet the compass cards on both the HSI, with the D-Bar centered, and RMI are showing 117 degrees. This is our first interesting comparison of the HSI and RMI, for both instruments show the same offset for a southerly wind correction, and both needles show 107 degrees. The head of the HSI needle is on 107 with the TO-FROM indicator showing FROM; and the tail of the RMI needle is also on 107, which indicates FROM.

As long as the D-Bar is kept centered, or the RMI needle is on 107, you will be tracking the correct radial. Heading will naturally shift as the fickle wind does, but everything else will remain the same.

Now put your autopilot (if you have one) in heading mode and use nav 2 to continue tracking the TUS 107-degree radial while you switch nav 1 to CIE and dial in the 245-degree radial on your HSI. Passing the 12 DME (TUS) point, the D-Bar on the HSI will begin to come alive and move off the stop. The RMI will show approximately 072 degrees under the head of the needle, which is the present bearing TO the station.

The bearing FROM the station, which is shown under the tail of the needle, would be the reciprocal: 252 degrees. At half-scale deflection on the HSI, the RMI will show approximately 068 degrees. Both instruments will still indicate a magnetic heading of 117 degrees.

As the target radial is intercepted, three things will happen: the TUS DME will show 25 miles, the HSI D-Bar will center, and the RMI needle will show 065 degrees under its head (and the 245-degree reciprocal -- which is the radial we're on -- under its tail).

To fly to the station on the 245-degree radial, all you have to do is keep the D-Bar centered on the HSI, or 065 under the head of the RMI needle. In a no-wind condition, a heading of 065 degrees will line up with both the HSI and the RMI needles.

However, no-wind conditions are rare, so compensation is required. With the HSI, if wind starts to drift you off course while tracking TO or FROM a station, you simply turn toward the D-Bar offset. Then it's just a matter of cut-and-try until you find the magnetic heading that will keep the D-Bar happily centered.

With the RMI, things are slightly different (though no more difficult), and this is the point where most pilots new to an RMI have their initial problems with it. When tracking TO a station, if wind drifts you off course, the head of the RMI needle will point the way to turn to get back on course. In other words, if your RMI needle moves off the bearing you want because of wind, all you need do is turn toward the direction the needle points in order to get back on course.

Once the proper number is back under the head of the needle (in this case, 065 degrees), all you need do is find the magnetic heading that will keep the needle where you want it, just as you'd do with the HSI. Keep in mind that since you are tracking TO the station (and this is a very important point), the head of the RMI needle will be pointing toward the nose of the airplane.

However, when tracking FROM a station -- and this too is very important -- the head of the RMI needle will be pointing toward the tail of the airplane. Yet here, too, all you need do is turn in the direction the head of the needle indicates to get back on course: head of needle on the right, course to the right; head of needle on the left, course to the left.

Now that the basics of RMI use have been covered, let's go back up to Alaska and take a look at how the instrument is used for a DME arc approach with an NDB segment.

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In the more interesting corners of the world, instrument approaches tend to be a bit interesting too, and the Sparrevohn NDB/VOR DME 34 is no exception. Admittedly a bit of a job using just an HSI and ADF, it turns into a piece of cake with an RMI. All you need do is get yourself to the initial approach fix (IAF), make a 90-degree turn, then fly the arc.

The turn will put the RMI VOR needle directly off your right wingtip, where it will immediately start moving toward the tail. Once the needle has moved 10 degrees toward the tail, turn 20 degrees towards the station. If the wind isn't too horrible, this will put the needle 10 degrees ahead of the wingtip, which will once again begin to move toward the tail. Continue this process, keeping a sharp eye on your DME to make sure you don't get too far off the arc. Two tenths of a mile on the outside is about as far as you should allow yourself to stray, though slightly more on the inside is permissible.

All this time, of course, you would be making the first stepdown, while monitoring your RMI ADF needle. When the ADF needle reaches 347 degrees, turn right and track directly to the CRN beacon, at the same time making your next stepdown. The final stepdown is at the 5.5 DME fix. It's good practice to use available cross checks; as a cross check for this fix, you'd have the SQA 148-degree radial. At this point, descend to the MDA. It's important to keep the ADF needle right on target (the RMI VOR needle will be defining your missed approach point for you). If the ADF needle drifts off more than a couple of degrees, execute a missed approach, which has to be flown very promptly and carefully, as the approach plate warns.

So here we have the basics of RMI use in its traditional guise: using it enroute TO and FROM various stations, intercepting radials and bearings, and flying approaches -- NDB, VOR, VOR/DME arc and the occasional cuties like Sparrevohn's, which really demand its use. Yet the RMI's real utility, and, for me, its primary interest, lies in its new high tech applications.

Interface: the LORAN/GPS-driven RMI

We are moving into an age of VOR/NDB independent navigation, one in which the RMI is being put to increasing use in Loran, and shortly, GPS, navigation. At the time this chapter is being written, the only IFR-certified Loran that has provision for an RMI interface is the King KLN-88, which I have in my Turbo Cessna 206. That's the model I'll use as an example here, but more Loran and GPS units will be available soon, and the most useful models will have the RMI interface.

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With the new Loran/RMI interfaces, it's obvious that the only limit on how a pilot utilizes this equipment will be his imagination. Strings of waypoints around MOAs and other restricted areas, complex STARs to an IAF in difficult terrain, keeping an eye on ATC: it's all easy to accomplish.

And once entered into the computer, following the charted waypoints is as simple and straightforward as can be. Using this system in my daily flying for the past few years, I have come to depend on and trust it, and the Loran/RMI combo has become my favorite instrument combination.

It will remain that way until the IFR-certified GPS/RMI combo becomes a practical reality.

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As this book was going to press, I installed a King KLN-90 GPS with RMI interface in the B36TC. My initial impressions are that GPS is far more accurate, and noticeably more solid, then Loran. I would suggest that those contemplating the purchase of a Loran get GPS instead. It is the wave of the future, for the VOR/DME system (and Loran with it) will soon join the old aural LF/MF ranges as a quaint and nostalgic part of aviation history.

$Date: 2005/05/06 19:16:05 $ Copyright © 1993 by F. E. Potts CSS XHTML 1.0 Strict