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The mysterious DC9/MD80/717 standby compass

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  • The mysterious DC9/MD80/717 standby compass

    Today’s modern commercial aircraft rely heavily on advanced computer systems for navigation, but what happens if these systems fail. Every aircraft has backups to these systems so that in the event of a failure, the pilots can still navigate to a suitable airport and land safely. One of these backups is the stand by compass. This device is a very basic, old school way of direction finding. Also known as a whiskey compass, its basic design is a rotating disk with magnets and heading markings that floats in a liquid. The magnets in the compass will align with the earth’s magnetic poles and stay that way while the aircraft turns, giving the pilot the direction they are flying based on the fixed points of the earth’s poles.
    In most commercial aircraft, the standby compass is located on the aircrafts centerline below the over head panel. In some cases, such as the A320 family, the compass can be hidden and hard to find as it can be stowed to be out of the way and out of sight since it is rarely used. But in no aircraft is it harder to find than on the DC-9, MD80 and 717 aircraft series. Although the same case on many other Douglas jet aircraft, in today’s video we will be using the Boeing 717 as an example.
    So if the standby compass isn’t located in front of the pilots like a normal aircraft, where does it live? Well strangely it can be found in the ceiling behind and above the first officer. Despite being behind the crew, in order for it to not spin opposite of what it should, it is still oriented to face the same direction a normal standby compass would in any other aircraft. Now you might be asking, if it is located above and behind the FO, how does the crew see it? In another strange design aspect, a series of 3 mirrors are used to form a Z of site lines from the pilots. 2 different mirrors are located on top of the glare shield, one for the captain and one for the first officer. These mirrors are individually adjusted to line up with a 3rd mirror which is located on the aft flight deck bulkhead inside the cubby where the standby compass is installed.
    Lots of rumors as to this compasses strange placement like it being an afterthought and that’s the only place they had room left. Another rumor is that because of the center window, there was simply no way to mount it. This is just another false rumor though as many other airplanes such as the falcon were able to mount a standby compass in a center window configuration. Only one rumor actually has some validity to it and it takes us back to Douglas’s first jet powered aircraft, the DC-8. Early series DC-8s were equipped with a standby compass on the aircrafts centerline above the glareshield in plain view of the pilots. It was determined that because of all the avionics and radio equipment in the glareshield, there was too much electrical interference with the compass’s magnetic field, causing false or unreliable readings. The compass was moved during the production of the later DC-8 models and would stay that way in the design of the DC-9, which would go on to the updated MD-80 series, MD-90 and MD-95 which we know today as the Boeing 717.

  • #2
    I blame aeroengineers.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.