confusing explanation for RMI

[SpeedBird43]

hi, I encountered this explanation for RMI, which I already used a lot in flight, and this doesn't make any sense to me:






as you can see, the last sentence says:

"the distances between the center of the needle and the end of the needle indicates the distance of the airplane to the station"

and then there's the picture which shows the station at the center and the airplane at the back of the needle. why does it that way? and what is that with the distances? the distance between the middle and the back of the needle is constant, so how does that make any sense

 

[Salty]

I think it’s a typo. I think distance was supposed to be direction in both places. It makes sense if you do that substitution.

 

[Boone]

It’s talking about establishing your position relative to the airport, I believe. The scale is set by your distance to the station. You can then reference the location of the airport from the beacon and place it along an imaginary line using your distance from the beacon as the scale. It’s one way to do point to point (but not how I do them), so you could fly directly to the airport instead of tracking to the beacon, then to the airport.
For instance, if you are 30 miles from the station and the airport is 10 miles south you can see you’d need a heading of about 155° or so to get there. You could also make a rough guess at the distance by placing a direct line back over the 30 mile scale.

 

[SpeedBird43]

I think it’s a typo. I think distance was supposed to be direction in both places. It makes sense if you do that substitution.

if that's the case so you will also have to change a few more words:
"the distances between the center of the needle and the end of the needle indicates the distance of the airplane to the station"

and then the chance it's a typo is decreased.. still I tend to think it is actually a typo

It’s talking about establishing your position relative to the airport, I believe. The scale is set by your distance to the station. You can then reference the location of the airport from the beacon and place it along an imaginary line using your distance from the beacon as the scale. It’s one way to do point to point (but not how I do them), so you could fly directly to the airport instead of tracking to the beacon, then to the airport.
For instance, if you are 30 miles from the station and the airport is 10 miles south you can see you’d need a heading of about 155° or so to get there. You could also make a rough guess at the distance by placing a direct line back over the 30 mile scale.

The RMI suppose to have a very simple purpose, and it works in a very simple way, I doubt this interpertation is the author's meaning here..

 

[Boone]

if that's the case so you will also have to change a few more words:
"the distances between the center of the needle and the end of the needle indicates the distance of the airplane to the station"

and then the chance it's a typo is decreased.. still I tend to think it is actually a typo





The RMI suppose to have a very simple purpose, and it works in a very simple way, I doubt this interpertation is the author's meaning here..

The simple purpose is to determine your position relative to something else. He refers to that very thing being the airport. It’s the way I’ve used it for decades and the way it’s taught in the military. I suppose he could have a different method, but nobody seems know what that is.

 

[midlifeflyer]

The RMI suppose to have a very simple purpose, and it works in a very simple way, I doubt this interpertation is the author's meaning here..

It kind of sounds like he's trying to do the old "rule of 60s" time and distance to the station calculation using the RMI. It's been (thankfully) gone from the IFR knowledge tests for some years but, if you remember, it goes something like, "if you violate your IFR clearance by turning off course for a few minutes, you will be able to figure out that, if you hadn't turned off course, you'd be there by now."

 

[EdFred]

Still not sure how you're getting a distance off an RMI. If it's pointing 30 degrees right of your course, you could be 1, 10, or 100 miles away and it could still show that same offset.

 

[Maxnr]

I spent more than 3,000 hrs looking at an RMI and I'm baffled by this description. Find another reference.

 

[Boone]

Still not sure how you're getting a distance off an RMI. If it's pointing 30 degrees right of your course, you could be 1, 10, or 100 miles away and it could still show that same offset.

By the rate at which the bearing changes.

 

[midlifeflyer]

Still not sure how you're getting a distance off an RMI. If it's pointing 30 degrees right of your course, you could be 1, 10, or 100 miles away and it could still show that same offset.

I've forgotten most of it (truthfully, never really learned it - way to much cockpit math for me with no practical application I could think of), but it works like this:

You keep going in the same direction. The needle will, of course, turn further away. You record the time it takes the needle to move 10 degrees.

Time in seconds/10 = the number of minutes it will take to get to the station.​

You apply your ground speed to that to get the distance.
​

 

[EdFred]

By the rate at which the bearing changes.

I've forgotten most of it (truthfully, never really learned it - way to much cockpit math for me with no practical application I could think of), but it works like this:

You keep going in the same direction. The needle will, of course, turn further away. You record the time it takes the needle to move 10 degrees.

Time in seconds/10 = the number of minutes it will take to get to the station.​

You apply your ground speed to that to get the distance.​

Yeah, and when you use actual trig, the numbers don't work, which is always a fun discussion when you give the DPE a math lesson during a check ride. And who the F is using a stopwatch to time the swing on an RMI?

 

[midlifeflyer]

Yeah, and when you use actual trig, the numbers don't work, which is always a fun discussion when you give the DPE a math lesson during a check ride. And who the F is using a stopwatch to time the swing on an RMI?

No one. At least not in the last 20 years or so

I suspect that, like a lot of things, it was a standard thing to do once upon a time in a galaxy far, far away. Pilots probably used a crossing navaid to be able to give a position report , including the next ETAs, in a nonradar environment.

 

[dtuuri]

as you can see, the last sentence says:

"the distances between the center of the needle and the end of the needle indicates the distance of the airplane to the station"

"Distances" is a typo, should be singular. @Boone has it right, of all the above. For another look at the same thing, see my slide #67 HERE: An IFR Approach on eBay?

For the OP's original article, see:
https://sites.google.com/site/omniflightschool/home/navigation/nwwtwt-mksyryt-1/nav10/rmi

 

[Arnold]

An RMI is/was in the top two or three most useful pre-gps tools in the cockpit. Example: Shooting ILS with multiple step down fixes identified by multiple crossing radial from nearby VOR/NDB. Set #1 radio for primary approach aid. Set #2 radio for the crossing VOR and use RMI to identify fixes. Nothing could be simpler.

 

[Maxnr]

All the HSI's and EHSI's that I use have at least two RMI pointers. Even use them on occasion.

 

[Tarheelpilot]

I think whomever wrote that did not actually understand RMI’s. Ignore it.

 

[flyingron]

Yeah, and when you use actual trig, the numbers don't work, which is always a fun discussion when you give the DPE a math lesson during a check ride. And who the F is using a stopwatch to time the swing on an RMI?

Much of these sort of short cuts avoid doing trig by noticing that a 60 NM radius circle has a circumference of 377NM. If you assume that a degree is one NM, you're off by only 17/377 or less than 5%. Pilots aren't all that accurate in their calculations anyhow.

Of course, the FAA in their incredibly inane stupidity, assumes that the 1-in-60 shortcut is an EXACT VALUE, yeilding some really horrendous questions on the instrument rating writtens. Couple that with the fact that nobody at the FAA even proofreads the final versions distributed to their monopoly contractor, means the test is full of bogosity.

 

[Salty]

Much of these sort of short cuts avoid doing trig by noticing that a 60 NM radius circle has a circumference of 377NM. If you assume that a degree is one NM, you're off by only 17/377 or less than 5%. Pilots aren't all that accurate in their calculations anyhow.

Of course, the FAA in their incredibly inane stupidity, assumes that the 1-in-60 shortcut is an EXACT VALUE, yeilding some really horrendous questions on the instrument rating writtens. Couple that with the fact that nobody at the FAA even proofreads the final versions distributed to their monopoly contractor, means the test is full of bogosity.

And beyond that virtually nobody is using an RMI to calculate time to station for navigation, other than for novelty sake.

 

[flyingron]

I can count on one hands the number of times in over 40 years of flying that I've been in a plane with an RMI to begin with.

 

[midlifeflyer]

An RMI is/was in the top two or three most useful pre-gps tools in the cockpit. Example: Shooting ILS with multiple step down fixes identified by multiple crossing radial from nearby VOR/NDB. Set #1 radio for primary approach aid. Set #2 radio for the crossing VOR and use RMI to identify fixes. Nothing could be simpler.

They are actually still pretty useful in their modern incarnation - the extra bearing pointers on a glass PFD or HSI. In something like a G1000 everything is glass, so it's SOP for flying a VOR approach using the latest AIM guidance. Magenta CDI uses GPS; green bearing pointer uses VLOC.

 

[EdFred]

They are actually still pretty useful in their modern incarnation - the extra bearing pointers on a glass PFD or HSI. In something like a G1000 everything is glass, so it's SOP for flying a VOR approach using the latest AIM guidance. Magenta CDI uses GPS; green bearing pointer uses VLOC.

And on the G5 the RMI to the VOR is light blue.

 

[MauleSkinner]

Allow me to translate:

“There is only one way to understand an RMI, it’s MY way, and this is what that way is.”

The method is not actually wrong, he’s just using terms and descriptions that don’t line up with the way most people’s brains work.