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    Re: Did a precessing gyro lead to the loss of Earhart?
    From: Gary LaPook
    Date: 2010 Sep 13, 14:20 -0700

    I was looking at the documents on the Purdue archive website last night and came across a photo of Earhart in the cockpit which is attached. After zooming in on it I found even more convincing evidence showing that a precessing gyro did not not cause the disappearance. Earhart is tuning the radio with her left hand. Below her hand you can see a white card. Zooming in on that card reveals that it is a compass correction card which lists the compass deviation on various headings and lists the headings to fly to correct for the measured deviation. For example, according to this card, on a heading of south the deviation is 1 degree west so you would steer 181 on the compass to accomplish a 180 degree magnetic heading. All compasses in airplanes are required to have their deviation determined (by "swinging the compass") and must have such compass cards prepared. What makes this particular compass card significant on the issue of gyro precession is that at the top of the card it is marked "APERIODIC." This proves that an aperiodic compass was installed in the airplane. Such a compass is not the normal type of compass installed in airplanes and is not the compass seen above the Cambridge analyzer above the instrument panel. An aperiodic compass is much larger and more accurate and are read while looking down on them, not through a window on the side as with a normal aircraft compass. They are "aperiodic" because they do not have a "period" meaning that they do not oscillate when disturbed like the usual aircraft compass but remain dead steady. They are also called "dead beat" compasses. They are also known as the "navigator's compass" as different from the "pilot's compass" and can be seen at the navigation stations in WW 2 aircraft. So Noonan would have been referring to this much more accurate compass when checking the headings that Earhart was flying and would not be looking at a directional gyro.

    An aperiodic compass is read differently than the pilot's compass. In order to eliminate the oscillating tendency, the mass of the compass card was eliminated and instead there is just a compass needle. There is rotating bezel with two lines across the face over the top of the compass card that you set to the desired heading and you then turn the airplane until the compass needle lines up with the lines on the bezel. This is just like using a "marching" compass or a scuba diving compass and it makes it easier to check that you are maintaining the correct heading, no numbers to think about, just be sure the needle is lined up.

    In addition, there is no reason to believe that a duplicate DG was installed at the navigation station for Noonan's use and this would have been unlikely since the purpose of a DG is for the pilot to use it while making turns as the normal compass exhibits many errors when the airplane is not flying straight and level at a constant speed. DGs were powered by vacuum from a vacuum pump mounted on the engine so it would have required running a vacuum line back to the nav station. I have been through many WW 2 aircraft and have seen many aperiodic compasses at the nav stations but I have never seen a DG there.

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