March 8, 2012

The Strange and Difficult to Find MD80 Standby Compass

MD80 Standby Compass
Have you ever been in the cockpit of an MD80 series aircraft?  If you have, then you probably weren’t able get over the scrambled and chaotic design of the cockpit long enough to notice that something important was missing…the standby compass. 

I learned to fly in a 1980’s era Cessna 152 with a compass located in clear view in the center of the instrument panel.  The compass had to be checked against a standby “wet” compass and manually corrected by the pilot every few minutes.  If the pilot forgot to make the necessary corrections, then the compass would develop an error over time that would eventually lead the aircraft off course.  Most modern aircraft are equipped with a slaved compass system that continually and automatically updates and corrects the heading information displayed on the cockpit instrument panel, removing the need to continually check the accuracy of the compass system.  A standby compass is still required equipment and must, at a minimum, be checked before each flight and would be used in the event of a failure of the slaved compass system.

The cockpit picture below is that of a Boeing 757-200.  The standby compass is prominently and conveniently located on top of the center window post.  In the event of a failure of the compass system on this aircraft the pilots would be able to easily and clearly view the compass.  Except for the light within its casing, the standby compass needs no electrical power to operate and is functional during a complete loss of electrical power.

Boeing 757-200 Cockpit


The cockpit picture below is that of a McDonnell Douglas MD83.  Now that I’ve mentioned it, you might notice the absence of a standby compass.  Clearly this aircraft does not have a center window post, but it seems logical to me that the compass could have been placed above the post nearest the Captain.  If that wouldn't work, I could come up with at least two or three other options...in the ceiling behind the First Officer would not be one of them, but that's exactly where it is.

McDonnell Douglas MD83 Cockpit


In order to view the standby compass on this aircraft, the pilot must first flip up one of two mirrors on top of the instrument panel…one for the Captain and one for the First Officer.  No, the mirrors are not for checking the condition of your hair, but they are probably used more often for grooming tasks than for checking the compass.

After flipping up the mirror, the pilot must position the mirror in just the right position so that he can see a second mirror in the ceiling in order to view the compass.  The second mirror makes it so that the compass is not viewed backward as it would if viewed through only one mirror.  There’s even a light switch on the overhead panel that turns on a small light to illuminate the face of the compass. 



The video below shows the procedure in full.  After viewing the pictures and video maybe you will have the same thought that I’ve had for years…I sincerely hope I never have to use it on a dark and stormy night.


10 comments:

  1. How ridiculous is that? I'm sure it's fine when you are sat at a desk with your slide rule but the time you may actually need it, with the confusion of failed main instruments and perhaps a little turbulence, no chance!

    Reminds me of the Fisher Space Pen, American engineers spent millions designing a pen that would work in zero-G, the russians? Oh, they just used a pencil........

    All the best, and thanks for a great blog!

    Dave from the UK

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  2. I suspect there's more to it than meets the eye. I don't doubt they would have preferred to put the compass in an easy to use location if they could have. The issue with magnetic compasses is that their accuracy is compromised by large masses of metal that can alter the magnetic field. Even more, the direction and magnitude of this deviation is depends on your particular heading! When on a naval ship, the engineers will develop something called a "compass deviation card" which shows you how much the compass deviates from magnetic north, depending on your particular heading. This deviation is carefully measured when in port through a process known as "swinging a compass" and can be up to 10 degrees depending on many factors.

    In a ship that moves relatively slowly, you have the time to add or subtract the deviation depending on your particular heading. But I suspect the MD engineers decided this wasn't practical for jet aviation, and instead decided to place the compass in a location where the deviation was minimized. Unfortunately for the pilots, this location appears to be set well back in the cockpit and not as user friendly.

    I recognize that other jet cockpits do not have this problem, but that is perhaps due to better design that places large metal objects far away from the compass, or perhaps a higher tolerance for magnetic deviation.

    All of this indicates yet another reason why GNSS and other radio navigation systems are so revolutionary and valuable...

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  3. I flew the MD-80 for a little over a year in my first airline job. Coming from the C-141, I just accepted its quirks as normal to civilian jetliners. Having flown newer aircraft since then, I just look back and scratch my head at some of the design decisions on that aircraft.

    There is a very good and humorous comparison of the MD-80 and 737 cockpit designs on the "Jethead" blog. He talks a lot about the seemingly random locations of things in the MD-80 cockpit.

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  4. About this and other quirks of the MD-80 - maybe it was supposed to be the next Bond plane, and essential items for flight were hidden so that the bad guys couldn't steal it :-)

    @jeremy: don't forget, that in the unlikely event that you are left flying a completely unpowered airplane, you need a bunch of indicators that show you how high you are, how fast you go, the plane's attitude and the direction without any power supply - the compass is one of those.

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  5. @Ted - I don't get your point? Of course I fully understand the need for a standby compass, in addition to a standby altimeter, ASI, horizon, turn coordinator, etc. And you want one that's accurate too! My point is that the engineers may have been forced to put the standby compass in that obscure location in order that it remain accurate. I highly doubt they were deliberately making it difficult.

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  6. This info is too much informative for me, kindly in the future keep updating us like this...
    Airlines Contacts

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  7. The location is to reduce the interference from all the electrical items around the instrument panel. (it is a magnetic compass) simple as that =)

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  8. Yeah I know I bumped an old thread, but the answer is not on here lol

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