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  I am watching a History Channel show about the loss of the "Lady Be
Good," a B-24 lost in the Libian desert due to a navigational error.
They were  showing artifacts recovered from the plane and they just
showed the aperiodic compass!

gl



On 9/13/2010 2:20 PM, Gary LaPook wrote:
>
> 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.
>
> gl
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