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Tinstaafl
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The display in the aircraft has an influence on doing things.
An RMI allows you to look at your position using the RMI & needle as a plan view of your position w.r.t. the aid & desired track/bearing. Think of the instrument as an 'astronaut's eye view' of the aircraft, the navaid & 360 deg bearings around the aid.
In this view the following applies:
* The pivot point of the needle represents the NDB (or VOR if you have a VOR/ADF selectable RMI),
* The compass rose represents the earth underneath the aircraft, oriented 'heading up'. Similar to 'track up' on a moving map GPS.
* The aircraft is positioned on the tail of the needle, heading 'up' the instrument panel, and
* Scale is proportional to the most distant item to be visualised. Usually it's the aircraft range but it could be a bearing/distance to be intercepted.
With the above in mind the following information is shown or can be derived:
* Bearing from the navaid (radial if using a VOR data source) by reading the compass rose adjacent to the tail of the needle.
* Bearing to the navaid from the head of the needle
* Relative position & orientation of any desired bearing to or from the navaid.
* With a little bit of practice, the approximate heading needed to intercept another bearing/radial at any distance along the new bearing/radial. This is often referred to as the 'point to point' method. If you plotted this on a CR type flight computer then you can also read the track distance from your current position to the desired intercept point using the CR's underlying graticule.
Unfortunately this is difficult to describe without using diagrams or a model but I'll have a go. Bear in mind pictures.....thousand words...etc etc. so forgive me if prose fails.
Orientation first:
Scenario 1: Imagine flying North on a bearing/radial of 180 deg. That would have the RMI showing a 360 deg Hdg for the a/c, the tail of the needle would show 180 deg (the bearing from the navaid/radial the aircraft is on), the head would show 360 deg (bearing to the station). Try to imagine a miniature a/c mounted on the tail of the needle **always** pointing up the instrument panel & the navaid in the centre. The RMI would also show the 150 bearing from the station to be to your right, crossing at an angle from rear R --> front L & meeting your current track at the navaid. Do the same exercise for other bearings & try to picture where they are in relation to the a/c's position. You could use the wind side of a flight computer to do this exercise.
Scenario 2: Flying N. but positioned on the 270 deg bearing from the navaid. The RMI would still show 360 for the Hdg but the tail of the needle will be on 270 (ie bearing from the navaid) & the head on 090 (the bearing to the station). Still try to imagine the miniature a/c mounted on the tail of the needle. That would show the a/c on the 270 bearing and a right turn of 90 deg is needed to track 090 to the station. The 300 bearing crosses from front L --> rear R, the 240 bearing behind from rear L to abeam at the station.
Similarly to Scenario 1, picture other bearings on the RMI.
Interception:
Once you can picture the a/c position on the RMI and where a desired bearing is then it's also possible to see how much to turn to have a track that will cross the desired bearing at some point before or after the navaid.
A development of this is to use the circumference of the RMI to represent the largest distance in the intercept problem eg if you're at 20 nm & want to intercept a bearing at 10 nm then the circumference equates to 20 nm and the point to intercept would be midway between the circumference & the centre along the desired bearing. Using this then a straight line between the two points is also the track to get there. To find the required heading, match a parallel line that starts from the centre of the RMI & goes to the circumference - I use my pen or pencil. Whatever compass direction is under your pen is the heading needed to get to the intercept point**.
This sort of technique - particularly the orientation part - can be used on a standard fixed card ADF in combination with the the a/c's Direction Gyro. You'll need to use your pen (or count convenient divisions from the head/tail/left or right abeam to the head/tail of the ADF needle. Transpose the pen's orientation (or the same number of divisions in the same rotational direction as on the ADF) onto the DG. This is a pseudo-RMI since those same divisions you count mark where the ADF needle would be *if* the DI was also an RMI.
You could also rotate the ADF card if it's a rotating card type. I generally don't because I can't be bothered having to do it for every single heading change. I'd rather count the convenient divisions on the ADF & then count the same amount on the DI to find where the ADF needle would be
Once on a heading to intercept it's important to confirm that the heading is appropriate to achieve what you intend. On an outbound intercept the TAIL of the needle must be in a position to RISE towards the desired bearing (on an RMI) or rise towards the same number of divisions on a fixed card as there are between your HDG & the bearing on the DG. For an inbound intercept the HEAD of the needle must be on the opposite side of the nose as the heading is from the desired bearing ie HDG to the left of the desired bearing then head of needle to the right of the nose) AND in a position to fall to the bearing.
**In nil wind. Any wind will put an error in this no different to flight planning on the ground using track as heading techniques. Bear in mind it's all based on an NDB so utmost precision is hardly to be expected.
An RMI allows you to look at your position using the RMI & needle as a plan view of your position w.r.t. the aid & desired track/bearing. Think of the instrument as an 'astronaut's eye view' of the aircraft, the navaid & 360 deg bearings around the aid.
In this view the following applies:
* The pivot point of the needle represents the NDB (or VOR if you have a VOR/ADF selectable RMI),
* The compass rose represents the earth underneath the aircraft, oriented 'heading up'. Similar to 'track up' on a moving map GPS.
* The aircraft is positioned on the tail of the needle, heading 'up' the instrument panel, and
* Scale is proportional to the most distant item to be visualised. Usually it's the aircraft range but it could be a bearing/distance to be intercepted.
With the above in mind the following information is shown or can be derived:
* Bearing from the navaid (radial if using a VOR data source) by reading the compass rose adjacent to the tail of the needle.
* Bearing to the navaid from the head of the needle
* Relative position & orientation of any desired bearing to or from the navaid.
* With a little bit of practice, the approximate heading needed to intercept another bearing/radial at any distance along the new bearing/radial. This is often referred to as the 'point to point' method. If you plotted this on a CR type flight computer then you can also read the track distance from your current position to the desired intercept point using the CR's underlying graticule.
Unfortunately this is difficult to describe without using diagrams or a model but I'll have a go. Bear in mind pictures.....thousand words...etc etc. so forgive me if prose fails.
Orientation first:
Scenario 1: Imagine flying North on a bearing/radial of 180 deg. That would have the RMI showing a 360 deg Hdg for the a/c, the tail of the needle would show 180 deg (the bearing from the navaid/radial the aircraft is on), the head would show 360 deg (bearing to the station). Try to imagine a miniature a/c mounted on the tail of the needle **always** pointing up the instrument panel & the navaid in the centre. The RMI would also show the 150 bearing from the station to be to your right, crossing at an angle from rear R --> front L & meeting your current track at the navaid. Do the same exercise for other bearings & try to picture where they are in relation to the a/c's position. You could use the wind side of a flight computer to do this exercise.
Scenario 2: Flying N. but positioned on the 270 deg bearing from the navaid. The RMI would still show 360 for the Hdg but the tail of the needle will be on 270 (ie bearing from the navaid) & the head on 090 (the bearing to the station). Still try to imagine the miniature a/c mounted on the tail of the needle. That would show the a/c on the 270 bearing and a right turn of 90 deg is needed to track 090 to the station. The 300 bearing crosses from front L --> rear R, the 240 bearing behind from rear L to abeam at the station.
Similarly to Scenario 1, picture other bearings on the RMI.
Interception:
Once you can picture the a/c position on the RMI and where a desired bearing is then it's also possible to see how much to turn to have a track that will cross the desired bearing at some point before or after the navaid.
A development of this is to use the circumference of the RMI to represent the largest distance in the intercept problem eg if you're at 20 nm & want to intercept a bearing at 10 nm then the circumference equates to 20 nm and the point to intercept would be midway between the circumference & the centre along the desired bearing. Using this then a straight line between the two points is also the track to get there. To find the required heading, match a parallel line that starts from the centre of the RMI & goes to the circumference - I use my pen or pencil. Whatever compass direction is under your pen is the heading needed to get to the intercept point**.
This sort of technique - particularly the orientation part - can be used on a standard fixed card ADF in combination with the the a/c's Direction Gyro. You'll need to use your pen (or count convenient divisions from the head/tail/left or right abeam to the head/tail of the ADF needle. Transpose the pen's orientation (or the same number of divisions in the same rotational direction as on the ADF) onto the DG. This is a pseudo-RMI since those same divisions you count mark where the ADF needle would be *if* the DI was also an RMI.
You could also rotate the ADF card if it's a rotating card type. I generally don't because I can't be bothered having to do it for every single heading change. I'd rather count the convenient divisions on the ADF & then count the same amount on the DI to find where the ADF needle would be
Once on a heading to intercept it's important to confirm that the heading is appropriate to achieve what you intend. On an outbound intercept the TAIL of the needle must be in a position to RISE towards the desired bearing (on an RMI) or rise towards the same number of divisions on a fixed card as there are between your HDG & the bearing on the DG. For an inbound intercept the HEAD of the needle must be on the opposite side of the nose as the heading is from the desired bearing ie HDG to the left of the desired bearing then head of needle to the right of the nose) AND in a position to fall to the bearing.
**In nil wind. Any wind will put an error in this no different to flight planning on the ground using track as heading techniques. Bear in mind it's all based on an NDB so utmost precision is hardly to be expected.
dmspilot00
Independent
- Joined
- Feb 22, 2002
- Posts
- 712
RideTheWind
Well-known member
- Joined
- Sep 16, 2003
- Posts
- 400
User546
The Ultimate Show Stopper
- Joined
- Jan 24, 2004
- Posts
- 1,958
Interesting you mention that, cause I was out at the local airport last week, adn sure enough there was a notice there from the local FSDO explaing how they were going to be decommishing our local NDB.paulsalem said:
The notice said they were eliminating a large number of NDB's. Main criteria for decommishing is when the NDB is located on a field where other navaids are located (ie. VOR, ILS, etc).
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