Monday, May 28, 2007

Using an RMI

Those of us who did instrument training before GPS took over, had to learn how to do NDB navigation. An ADF is a very simple instrument. It simply points at the NDB. Follow the needle, and sooner or later you will get to the NDB – or possibly a thunderstorm. But such a seemingly simple instrument has been the source of a great deal of frustration to generations of pilots who had to learn how to use it for navigation. If our instrument instructors were successful in beating it into our heads, we eventually learned how to intercept specific bearings and track towards or away from the NDB in a more or less precise manner. The degree of precision often depended on the degree of wind.

VORs have radials, and NDBs technically have bearings. But to simplify this discussion, I am going to use radials in talking about NDBs. If we wanted to track inbound on the 180° radial towards the station, we learned to turn to an intercept heading, usually 30, 45, or 60° for ease of use. When the head of the ADF needle fell to the intercept angle, we were on the desired radial. If our airplane was really fancy, it had an ADF card that could be rotated. This simplified things for the pilot – more or less. If you remembered and had the time to do so, you could rotate the ADF card to match the aircraft’s heading. By doing this, you didn’t have to calculate intercept angles. When the head of the ADF needle fell to the desired radial, you had made the interception. Suppose you were southwest of the NDB and wanted to intercept the 180° radial and track inbound on it. You could turn to an intercept heading of 045° and also rotate the ADF card to match your heading. The head of the ADF needle would fall, and when it reached the desired reading of 360° (reciprocal of 180°), voila – you were on the desired course.

The ADF with a rotating card was often called the poor man’s RMI. It simplified NDB navigation, but having to constantly rotate the card really added to the pilot’s workload. But what if the card rotated automatically, just like your DG (heading indicator) or HSI. That essentially is what an RMI is. It is a DG with either one or two needles that can be set to respond to either NDBs or VORs. So using an RMI is basically just remembering how you did NDB navigation.

The RMI combines the DG and ADF into one instrument. As you turn, the card moves just as the DG or heading indicator would move.

Recently I was asked about using an RMI for hold entries. So let’s investigate how you use an RMI to get into a hold. To do this, set the RMI to respond to a VOR rather than an ADF. Assume you are tracking north on the 180° radial towards the VOR.

I have marked off the face of the RMI with the lines to determine holding entry for a right-hand hold. If you don’t recall this, please refer to Creating Holding Clearances. Our holding clearance at Podunk VOR was given as:

Hold northeast of PODUNK VOR on the 050° radial, right-hand turns, EFC……

This calls for a teardrop entry. To make this entry, we would turn to a heading of about 020° after passing over the VOR and fly for one minute (if it was timed legs).

Turning right to intercept the inbound course on the 050° radial, the RMI tells us right away that we need to stop the turn momentarily to intercept the radial.

Remember the RMI acts just like an ADF. The head of the needle is going to fall. Right now the plane is on the 40° radial, but we need to intercept the 050° radial. Since the head of the needle will fall, we can stop the turn temporarily on a heading of 200° and wait for the needle to fall. When it gets to 230°, we are on the 050° radial.

Modern RMIs include those that can track two different stations simultaneously and can be independently set to either VOR or ADF. This one is from a King Air panel.

It takes a little bit of readjustment to your thinking, but once you get the hang of it, it’s really a great instrument.


Mohan said...

Awesome explanation!! Helped me a lot.......I went through all of your blogs.......particularly impressed with the explanation on RMI and Holding mentioned I am an instrument rated pilot who finds this hard to understand at times, but this article really helped!! I have bookmarked this site for quick references. Thank you......

osman karaca said...

Thanks for simple explanation.

Capt. Radic said...

To make entry in to holding pattern even more simpler, I would like to add the following: The no-wind outbound heading of the holding pattern will be in one of three entry sectors.