My reasoning is quite simple; I enjoy flying using VORS and NDBS, and implementation of this switch allows the stand-by RMI gauge to be used more effectively when using these “older style” navigational aids.
LEFT: RMI Switch Assembly dated stamped 1967. RMI switch has been custom fitted to blanking plate (click image for larger view).
VOR and NDB Flying (NG)
The NG 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 B737 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).
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. We will will look further at the NG RMI knobs in the next post.
The use of the older style 737 Switch Assembly duplicates the functionality ( if it existed) of the stand-by RMI buttons on the MIP.
The card was interfaced to Flight Simulator using a PoKeys55 interface card. In my set-up 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.
LEFT: Early Boeing N737 RMI Switch Assembly showing detail of two switches, Canon plug, wiring harness and front panel (click image for larger view).
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 bus bar located in the center pedestal and are 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 to determine the correct pin outs for the plug a multimeter was used to determine conductivity from each pin in relation to the mode toggled. A longer wire harness was made to allow the harness to reach the System nterface 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.
In the Boeing 727 and earlier 737 airframes, the RMI Switch Assembly is mounted within the Main Instrument Panel (from memory). In this era (circa 1967) modern-style EFIS units had yet to be developed.
As such, the switch does not require a blanking plate as it’s 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 dremil cutter. The switch assembly could then by placed in the cut hole and attached directly into the blaking plate via the four screws.
LEFT: The 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. In the "real world" this panel would never be seen in three-bay center pedestal.
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 pre-mapped and you do not need to "delve" into the world of FSUPIC offsets in an attempt to get something working.
Not Standard Airline Order
Although it's not standard NG equipment - "you would NEVER see this switch panel in a B737 NG", the Switch Assembly is very enjoyable to use and makes using the alternate RMI gauge more user friendly - at least until I add functionality to the NG RMI knobs or aquire a OEM RMI guage.
In the next post we'll discuss the Next Generation (NG) RMI knobs.
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