RMI Switch Assembly Panel — Articles / Posts — Flaps 2 Approach

RMI Switch Assembly dated stamped 1967 (727 or 737-100/200). RMI switch has been custom fitted to blanking plate

It probably seems an oddity to install into the center pedestal a switch manufactured in 1967 that in all probability was used in a an early model Boeing 737 or more likely a 727.

My reasoning is quite simple. I enjoy flying using VORS and NDBs and the use of the older style 737 switch assembly replicates some the functionality of the stand-by RMI buttons on the MIP. In time, the panel will be replaced when I find OEM 737-800 RMI knobs.

VOR and NDB Flying (NG)

The Next Generation allows tracking of the primary and secondary VOR/NDB with a visual display on the Navigation Display (ND). The display can be turned on and off from the either the Captain or First Officer side EFIS. Tuning to the VOR and NDB is accomplished by dialing in the correct frequency on the NAV and ADF radio panels.

The navigation output is duplicated and shown as dual needle movement on the RMI gauge which is the third gauge within the stand-by instrument cluster. In the real 737 aircraft, the mode of the RMI gauge can be toggled between VOR 1/2 and ADF 1/2, or a combination, by the small knobs on the front of the RMI that protrude through the Main Instrument Panel (MIP).

RMI Knobs

It’s unfortunate that many manufacturers of reproduction Main Instrument Panels (MIPS) do not include functionality to these two small knobs and provide only a rough facsimile of an original knob.

Early Boeing N737 RMI Switch Assembly showing detail of two switches, Canon plug, wiring harness and front panel. The original Canon plug and pib-outs was used in the conversion

Interfacing, Wiring and Blanking Plate

The switch assembly was interfaced to function with ProSim737 using a PoKeys55 interface card. In my simulator ,the PoKeys card resides in the System Interface Module (SIM) and the five wires from the 737 switch were run through a piece of conduit (plastic piping) beneath the platform to the System Interface Module (SIM) located forward of the MIP.

The five wires correspond to VOR 1/2 and ADF 1/2. The fifth wire is the common (earth). Two additional wires (positive and negative) connect to the 5 Volt busbar located in the center pedestal and is used to power the backlighting of the panel.

Canon Plugs - Why Change a Perfect System

The switch assembly included a male Canon plug in very good condition; therefore, it was decided to use the Canon plug system rather than wire separately. A female Canon plug was purchased from E-Bay and a multimeter, set to continuity mode, was used to determine the correct pin-outs for the plug.

A longer wire harness was made to allow the harness to reach the System Interface Module forward of the MIP. Using Canon plugs keeps the wiring very neat and allows for an easy disconnect should you need to remove the panel from the pedestal.

oem 727 early 737 cl RMI Switch Assembly installed to the center pedestal. Selection can be either ADF1/2, VOR1/2 or a combination. Switches and panel are backlit by 5 Volts which is the standard voltage used in many panels. This panel would never be seen in a 737 Next Generation center pedesta

Blanking Plate

In the Boeing 727 and earlier 737 classic airframes, the RMI Switch Assembly is mounted to the lower part of the MIP (from memory). In this era (circa 1967) modern-style EFIS units had yet to be developed.

As such, the switch does not require a lightplate as it is attached to the MIP by four screws. To facilitate the switch being installed to the center pedestal, a blanking plate had the center portion cut out using a dremel cutter. The switch assembly could then by placed in the cut hole and attached directly into the blanking plate via the four screws and the panel secured to the pedestal by DZUS fasteners.

Mapping Functions

To configure the functionality of the Switch Assembly to ProSim737 was straightforward, as the functions have already been mapped within ProSim's configuration menu. This is one of the major advantages to using ProSim737 as the avionics suite; many functions have been mapped and you do not need to delve into the world of FSUIPC offsets in an attempt to get something working (This what must be done if you use Sim Avionics).

Never on a Next Generation

Although you would never see the panel on a 737 Next Generation aircraft, the switch assembly is very enjoyable to use and makes using the alternate RMI gauge more user friendly - at least until OEM RMI knobs are obtained and configured for use, or an OEM RMI gauge acquired.

Acronyms & Glossary

ADF – Automatic Direction Finder
Blanking Plate - An aluminium plate used to cover a gap in the pedestal or overhead. The plate is equipped with DZUS fasteners for attachment to the DZUS rail VOR - Omni Directional Radio Range
EFIS – Electronic Flight Instrument System
IMM – Interface Master Module
MIP – Main Instrument Panel
NDB – Non Directional Beacon
PANEL – Refers to actual avionics module. Panel and module are interchangeable
RMI – Radio Magnetic Indicator. The gauge that displays VOR and ADF mode. Part of B737 NG stand-by instrument cluster