I was watching this video about the MD-83 magnetic compass and I found it interesting that it's designed the way it is. I'm not sure which of the airplanes in this series featured this strange arrangement, but it was the case on at least the MD-80 and MD-83.

The magnetic compass on these machines was located behind the first officer's seat and a series of mirrors were used to make it visible to the pilots.

Why was it built this way? Were there any advantages to it? Did they go back to a more conventional design in later models?

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    $\begingroup$ Pure guesswork, since I don't know. The standby compass is positioned away from sources of interference. Electrical generators and components, rotating components, the airframe itself etc. It might be that on this particular design, a "normal" position could not be found with an acceptable level of interference. $\endgroup$ – Simon Feb 12 '16 at 7:41
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    $\begingroup$ See also: The Strange and Difficult to Find MD80 Standby Compass $\endgroup$ – mins Feb 12 '16 at 11:26
  • $\begingroup$ I would agree with @Simon that the positioning is most likely related to magnetic interference (though I don't think generators would be a part of that interference). The mag compass can be hard enough to read when it is right in front of your nose; I can't imagine it is any easier when it is optically 6-8 ft away from you. $\endgroup$ – J Walters Feb 12 '16 at 13:36

In most aircraft, the standby magnetic compass is mounted well above the main instrument panel, usually above the central window post, in order to reduce interference. In the three piece window of the MD-80 (without a central window post), this is not possible, resulting in the different location for compass.

All the Douglas jet aircraft has this type of mounting. According to this thread, the compass was first mounted on the instrument panel, which resulted in severe interference, resulting in its relocation. From the thread:

I understood that, early in the 60's, the compass was relocated in all DC-8 production due the three section type of the windshield (the compass has a lot of error/deviation at the center of the glareshield panel).

Note: I'm not able to find original documents supporting this. All the documents I can get simply instruct to check the compass using mirrors in the glare shield.

This design decision was used in all the later Douglas models, DC-9, MD-80/90 right upto Boeing 717 (which was developed as Dc-95); incidentally, all these models used a three piece front windshield devoid of center post.


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