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Original military aircraft radio magnetic indicator with its identification plate and a Lexington Bluegrass Army Depot label documenting overhaul and testing in November 1981. A distinctive piece of analog cockpit instrumentation with documented applications in the Bell UH-1 Huey, Bell AH-1 Cobra, and Beechcraft U-21A.
Purpose and historical context
The ID-998/ASN Radio Magnetic Indicator, commonly called an RMI, combines aircraft heading, selected heading, and radio-navigation bearing information in one panel-mounted instrument. Its two bearing pointers display information supplied by ADF (Automatic Direction Finder) and VOR (VHF Omnidirectional Range) equipment.
This instrument type was used with the AN/ASN-43 gyromagnetic compass system, which combined remote magnetic sensing with a directional gyro to provide stabilized heading information. It is not a standalone magnetic compass and requires external aircraft equipment for normal operation.
U.S. Army aviation documentation lists the ID-998/ASN for the UH-1, AH-1, and U-21A. These are documented applications of the instrument type—not proof that this particular example was removed from any specific aircraft. Its original aircraft assignment, manufacture date, and operational service history are unknown. No claim of Vietnam or combat service is made.
Identification and historical markings
- Identification plate: “INDICATOR, RADIO MAGNETIC COMPASS”
- Military designation: ID-998/ASN
- Reference National Stock Number: 5826-00-897-4889
- Associated compass system: AN/ASN-43
- Maintenance reference: TM 11-5826-299-34
- Depot label: “OVERHAULED & TESTED BY LEXINGTON BLUEGRASS ARMY DEPOT”
- Depot service date: November 1981
- Additional handwritten tape annotation appears to read “GOOD / 30 MAR 93”
The depot label documents historical maintenance, not current functionality. The handwritten annotation is included as historical context only and is not treated as a formal test record. The manufacturer is not identified on the visible plate.
Approximate measurements
- Main case: 3 × 3 inches
- Front bezel: 3¼ × 3¼ inches
- Overall length: 10 inches, including projecting controls and threaded rear connector
- Unpacked weight: 4 pounds, 1.8 ounces
Observed condition and operation
The housing and bezel show scratches, finish wear, paint loss, and surface discoloration. Dial markings remain readable. The rear multipin connector and historical labels are present.
All external controls move freely. Turning the heading-selection knob smoothly repositions the selected-heading marker around the dial. This confirms only the observed mechanical adjustment; it does not establish electrical operation or navigation accuracy.
The instrument has not been powered or bench-tested. Electrical functionality, illumination, calibration, and airworthiness are unknown.
Scope of sale
Offered for parts, repair, or collectible display. An interesting addition to a military aviation collection, historical cockpit display, or restoration project. It is not a plug-and-play flight-simulator accessory.
Sale includes the pictured indicator only. No external navigation equipment, wiring harness, power supply, or current serviceability documentation is included. Not represented as ready for aircraft installation or flight use.
