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Radio Magnetic Indicator (RMI)

The RMI is a navigational aid providing aircraft magnetic or directional gyro heading and Very High Frequency Omnidirectional Range (VOR), GPS and Automatic Direction Finder (ADF) bearing information. The RMI was made to and to be installed on aircrafts is so that it could compensate for errors in and the limitations of older types of heading indicators

How It Works

The RMI consists of a compass card, a heading index, two bearing pointers and pointer function switches. The two pointers are usually driven by any two combinations of GPS, ADF and/or VOR. As for the case for the Learjet 35, the RMI two pointers combinations are the ADF and the VOR. A pilot has the ability to select which navigation aid that he/she wants to be indicated.

 

The green pointer is indicating the station tuned on the ADF and the yellow pointer indicates the course to a VOR of GPS waypoint.

Actual Location of RMI in LearJet 35 @ The ARCH

VHF Omni-Directional Range (VOR)

The VOR system is well known as to provide magnetic bearing information to and from the station which is also being used in the Course Deviation Indicator (CDI) for navigation. The prefix “Omni” means all and an omnidirectional range is a Very High Frequency (VHF) radio transmitting ground station the projects straight line courses or radials from the station in all directions.

How It Works

The standard VHF frequency is 30 MHz – 300 MHz but for the VOR, the frequency that is used is 108.0 MHz – 117.95 MHz. The VOR course or radials are always projected from the station and it is referenced to magnetic north. This would make a radial defined as a line of magnetic bearing extending outward from the VOR station.

 

Radials have its own identification numbers beginning with 001 which would be 1° east of magnetic north. This would the progress in sequence through degree of a circle reaching 360°. To aid this orientation, the compass rose reference to magnetic north is superimposed on aeronautical charts at all station locations.

VOR on Board Aircraft

Advantages

VOR Ground Station

Disadvantages

  • More accurate, precise flying

  • Reliable

  • Not susceptible to interference (Weather Conditions)

  • Voice Capable

  • Costly to maintain

  • Line-of-Sight Restrictions

Types of Navigation Instruments used with VOR

  • Course Deviation Indicator (CDI)

  • Horizontal Situation Indicator (HSI)

  • Radio Magnetic Indicator (RMI)

Automatic Direction Finder (ADF)

The Automatic Direction Finder (ADF) is used to navigate the plane when the pilots tune the receiving equipment to a ground station which is known by a Nondirectional Radio Beacon (NDB). An NDB station has a low or medium transmission which transmits a frequency band of 200 to 415 kHz.

How It Works

The pilot will tune to the desired frequency of the NDB on his Automatic Distance Finder.

This will then make the needle point to the NDB immediately 

Advantages

Disadvantages

  • Uses Low frequency or Medium frequency

  • Not affected by line-of-sight

  • Signals follow curvature of earth regardless of its altitude

  • Very susceptible to electrical disturbances such as lightning’s

  • With this disturbances, the needle may point to the weather not to the NDB

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