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I am part of a team developing a Cessna 172 P model for FlightGear, an open source free flight simulator. We are now programming how the panel and radio lightning rheostat should work, but we found a lot of unclear and/or contradictory information about it online. Our main problem is to understand how the radio light knob (labelled as RADIO LT) work in these older models of the 172.

The POH from 1982 states the following:

The engine instrument cluster (if post lights are installed), radio equipment, and magnetic compass have integral lightning and operate independently of post or flood lightning. The intensity of this lightning is controlled by the inner knob on the light dimming rheostat labelled RADIO LT; rotate the knob clockwise to obtain the desired light intensity. However, for daylight operation, the compass and engine instrument lights may be turned off while still maintaining maximum light intensity for the digital readouts in the radio equipment. This is accomplished by rotating the RADIO LT knob full counterclockwise. Check that the flood lights/post lights are turned off for daylight operation by rotating the PANEL LT knob full counterclockwise.

This makes it look like the RADIO LT knob controls both the brightness of the radio digital readouts (i.e. the frequency digits) as well as other types of lights (radio cluster buttons and compass), but I found out that other renowned simulations of the 172 for other simulators do not dim the digital readouts with that knob. It also seems strange to me that one would be able to dim the radio digits under any condition, let alone turn them completely off.

So does anyone here have experience with these models of the 172? Would anyone be able to describe how to best model the behaviour of the radio lights?

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  • $\begingroup$ Well, we are modelling the P variant, but as far as I understand the variants from the late 70's and 80's are all very similar to one another. Also, in case you do have the information concerning other variants, we would be more than happy to hear them because maybe some conclusions could be inferred from it. $\endgroup$ – gilbertohasnofb May 12 '16 at 20:18
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    $\begingroup$ Have a look page 355 (of the pdf viewer page number) of this maintenance manual. Here is a snapshot of the related circuit. There are other manuals on the site that may be useful. $\endgroup$ – mins May 12 '16 at 20:26
  • $\begingroup$ The dual knob is clear to us. The outer knob controls the panel lightning, which is some sort of backlight for all gauges. The radio lightning on the other hand is what is confusing us, particularly whether the radio displays with digits should also be dimmed or not $\endgroup$ – gilbertohasnofb May 12 '16 at 20:54
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    $\begingroup$ From the drawing, the INST LT rheostat (referenced 6 in an hexagon) should be the PANEL LT mentioned (it could have its name changed over time). INST LT controls both the instrument/panel lighting via the red-white wire, and the compass lighting via the red-black wire and pin 3 of the top connector (which parts are referenced 4 and 5). The RADIO LT which dims the display is completely separate. Here an annotated drawing. $\endgroup$ – mins May 12 '16 at 21:14
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – mins May 12 '16 at 21:41
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This depends more on the radios installed than the plane its self, older era planes may have had King radios with analog numeric readouts. These radios are generally back lit and controlled by what ever rheostat controls the panel those radios generally look like this

enter image description here

(source)

later planes may have had digital readouts which in some cases had another control to dim the intensity. This is often because in a sense the lights are always on you may need to control them with out turning on or dimming the panel. As a matter of fact you generally want them to work opposite, during the day when the panel is dim or off you want the radio lights to be brightest they would then be dimmed in the evening as you bring the panel lights on. Those radios often looked like this,

enter image description here

(source)

The dim function may appear on units like the DME and Auto Pilot if they are similarly lit.

Its been a while since I have night flown with one but if memory servers the newer GPS units like the GNS430/540 have backlit buttons that come on when the panel lights are activated. These units also tend to have separate ways to control the screen brightness/contrast

enter image description here (source)

You may have more luck referencing the manuals for the radios them selves than the POH for the plane. You should design for the types of radios you are putting in (looks wise)

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The Bendix-King KX-155, KR-87, and KT-76C dim their digits based on built-in photosensitive resistors. The faceplate backlighting and the illumination intensity of the NAV instruments is controlled by the radio dimmer button, which adjusts the voltage they receive via a voltage stabilizer circuit (LM350K). The engine and flight instrument' lighting is controlled by another voltage stablizer adjusted by the instrument dimmer.

Note; the lighting of the magnetic compass is controlled by the pedestal light dimmer!

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The light rheostat is generally used to brighten or dim the overhead light used to shed some light on the control panel on older 172's (their all old). It really is useless. Newer ones have a few lights under the edge of the glare shield and the knob controls their intensity. The post light is a light in the post between the glare shield and the cabin roof. It's a white light and not something you want on at night. If you are designing a simulator then make it real. Make the panel almost impossible to see at night. This way the player can get the feel of a real 172. Check out Sporty's pilot shop. That's why they sell lights you can strap to your head. Or lights that clip somewhere so you can see the instruments.

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