Topic Author
Posts: 135
Joined: Mon Apr 23, 2001 7:33 pm

RMI Orientation

Sat Jul 07, 2001 3:42 am

Hi everybody, this question is adressed to all the fellows who are familiar with navigation mainly with VOR-NDB orientation.
I just took a check with a major european airline to land a job as a pilot and with my disappointment I noticed that I had troubles in determining my position with respect to a beacon and a VOR.
Premitting I was not familiar with the RMI and that the check took place in a Cheyenne 3 simulator equipped with EADI and EHSI, they gave us radar vectors to intercept an ILS FAC of 205; even though the vectors seemed to be just fine to intercept the localizer, the FAC was 205, the heading was 180 and the deviation bar was on the left hand side on the course pointer, I was,nt able to intercep the localizer cos my heading was not going to carry me to intercept the 205 bearing inbound based on the outer marker, how was I supposed to know it?
Secondary they gave us instructions to takeoff from runway 21 with a heading of 205, intercept the radial 180 outboud, follw that radial for 15nm and then turn right to intercept the radial 240 inbound. I took off, set my course pointer on the 180 radial, follwed it as far as 15 nm from the VOR then I turned right to intercept the 240 radial inbound, I set my course pointer on 060 and with the deviation bar offset from the center on the left hand side I turned to a heading of 030 for an interception angle of 30 degrees, once again I was told that I was not going to intercept that radial before hitting the VOR and they suggested me a heading of 340.
Now how was I supposed to know that I had to stop the turn to a heading of 340?
Finally how can I know the heading to intercept a radial/bearing inbound outbound right away just looking at the RMI? I was tought to parallel my course and then looking at the ADF, turn towards the head of the needle of a number of degrees corresponding to the double of the needle displacement and then wait until the needle deflection equals the interception angle, but I have no clue on how to do that right away.
Please if there is anybody out there willing to help me out with the RMI he will be very welcome.
Do not hesitate to contact me on my email adress diegoair@yahoo.it.
Posts: 1216
Joined: Thu Jun 21, 2001 6:19 pm

RE: RMI Orientation

Mon Jul 09, 2001 3:31 am

RMI orientation is not really easy if you have not had practice with one. But I can assure you that when you have some practice with one you will never go back to some other pointer style navigation. It is a VERY useful tool for orientation if used properly.

I did all my IFR training in Switzerland. I do not know where you did yours but mine taught a very systematic approach to all things. I am not able to comment on the ILS approach that you had trouble with because I don't know all the factors. You do not mention wind. I am sure that the selectors wanted to see a proper wind correction or your reaction if their vectors would not lead you to the right place. Also, an RMI will not point to an ILS transmitter. Was there an NDB or VOR at the "airfield" to use as a reference on your RMI? When possible, always crosscheck your position with the RMI while under vectors to the ILS final inbound course. This could oneday help you prevent yourself from being vectored into terrain. Always know your current position. This is especially important here in Switzerland where we have some dangerous terrain.

Regarding your second problem I have a little bit better answer. Here we learned what they call the "Six Point Rule". 1. What is my current radial or bearing/QDM-QDR (to or from)? 2. What is the requested radial or bearing?3. Difference in degrees? 4. What type of intercept required? 5. First turn? 6. Intercept heading?

We learned that 0 - 15° requires a correction. For example - you are on radial 180° and you need to be on radial 190°. If you are travelling away from the station you just need to turn right and wait for the OBS/CDI to center. 15°-30° requires a 45° intercept. For example, on the radial 180° outbound you need to intercept radial 210. You will turn right to a track (not heading - don't forget wind correction) of 255°. 30°-70° requires a 90°/45° intercept. For example, you are again on the 180 and need to intercept the 245. You will turn right to a track of 335° and when your RMI shows a difference of 15° (230°) you turn LEFT to a track of 290°.

Of course, wind and distance from the station must be considered. If you are 30 miles from the station a correction on a 10° difference might take many minutes to complete. In this case you would make an interception of up to 30°-45°. Your RMI needle will always tell you which radial or bearing you are on. An RMI is a slaved instrument so you will not need to worry about correcting for precession.

We also learned a little rule when making these manoevers. "Frequency-OBS-Ident". If you do this you will not go wrong. Assuming you have the frequency and have identified it in your example you may go ahead and set the OBS on your HSI. From the 180°radial you would set (as you correctly said) course 060° and then follow the 6 point rule I mentioned. This would give you an initial intercept course of 330° (non wind corrected). When the RMI indicates that you are on the radial 225° or the needle points to 045° you can begin to turn right again to a final intercept of 45°. This will give you a track of 015°. Draw it all out on a sheet of paper. This helped me a lot.

You say that you were 15 miles out. This is plenty far out for a proper intercept despite the high speed of a Cheyenne.

You sound like a good candidate for instruction on a computer based IFR "simulator". I spent many, many, many hours in an ELITE sim here in Switzerland getting my orientation down.

You sound like me in that you are a pretty new pilot. I just passed the selection with a Swiss airline. I wish you luck. Did you pass your selection? If not, or even if so, I would recommend that you find someone who can give you some systematic instruction on basic orientation. This is not to sound demeaning in any way, but proper orientation is is crucial information. My colleagues who fly for the big airlines here say that they are constantly refering to the RMI for orientation while enroute. This is their personal crosscheck for the internal nav systems.

Hope I could help. You may contact me also if you wish at kile@bluewin.ch.

Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: RMI Orientation

Mon Jul 09, 2001 12:55 pm

Too difficult to explain in words alone, but very easy to understand with a little hands-on demonstration and practical exercises. Find a good Instrument Instructor who has a plastic RMI learning tool and pay for the 2 hours or less it will take to fully grasp how easy it is to understand how VOR/NDB/ADF needles work.

1. Head Falls, Tail Rises

2. VOR needle is always relative to compass card (RMI).
NDB/ADF needle is always relative to aircraft's nose
(top index of RMI, not the compass card itself).
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