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Avionic basics

panel dimensions
It occurred to me that there's a standard for avionics equipment sizes. There's a lot of equipment that are fitted with a bajonet type of locking screws at the sides. I started looking for more information and after some research I found a lot of usefull information. I found the first pieces of information here: After studiung the information I found more related infomration like patents and specifications. Below is a summary of the most important information to know...

Equipment examples
On the image show below are three devices that I've bought recently.

The left device is the control panel of a Rohde & Schwarz UHF communication radio model XD 613 P1. (This is the panel of a 200...399,975 MHz aircraft radio.)
The top right device is a control panel for 108...117,95 MHz VOR (omnidirectional radio range), ILS (instrument landing system) and DME (distance measurement equipment) radio navigation.
The bottom right device is a Racal CDU (control display unit) of a Seaking helicopter.

The common thing of these devices is that the width and mounting type are identical. The size of modular avionic equipment is based on a standard. The result is that the composition of a instrument panel can be easily changed if desired.

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Three devices with the same width and the same locking mechanism.

Locking mechanism
The typical locking mechanism are quarter turn fasteners. On the image below are examples of the font and rear of these fastener studs.

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The studs are 'flanged' to the panel. By pressing the slot with a screwdriver and rotating the stud a quarter turn counterclockwise, the fastener is unlocked for removal of the panel.

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It turned out that the used mounting system is based on a rail system. The rails (of sometimes called line receptacle strip) is known as a DZUS mounting system. The mounting rails are usually riveted to the environment. The rail has repeating holes to create a repetetive spacing for the locking bolts. The system is in the Imperial system, thus premerely documented in Inches (and therefore rather odd Metric numbers). Here are the 'ground rules':

The holes in the rail are spaced 3/8" = 0,375" = 9,525 mm.
The width of the panel is 5,75" = 146,05 mm.
The center distance of the locking holes is 5 3/8" = 5,375" = 136,53 mm.
The (aluminium) panel thickness is usually 1/16" = 0,0625" = 1,59 mm.
All the panels need to be trimmed for easy fit: 1/64" = 0,016" = 0.40 mm.

For a graphical view, see the image below.

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The height of a device can be expressed in the number of DZUS units. Here are the DZUs units for the three shown devices above:
Rohde & Schwarz radio panel height: 123,83 mm = 4,88" = (4,88"/0,375" =) DZUS 13 units
Radio navigation contol panel height: 66,68 mm = 2,63" = (2,63"/0,375" =) DZUS 7 units.
Racal CDU height: 114,30 mm = 4,50" = (4,50"/0,375" =) 12 DZUS units.

There's a (MIL-F-25173A) standard from 1956 where a lot of details are mentioned like the specific material alloys for panels, heat treatment of materials, locking torque, mechanical load limits, corrosion resistance and so on... These details are very relevant for the original use. For me these details are not that relevant since I only want to build a sample panel for some devices. Therefore the dimensions are more than enough for me. If you're interested in more details, see for example

Source: Panel dimension information

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