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2 Answers

Heading Indicator alignment knob

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Aircraft Systems

I understand that a Heading Indicator has an "alighnment knob" in order to keep it alighned with the Magnetic Compass.

I came across this statement in a flight training book: "Before releasing the alighnment knob, always twist it to ensure compass card disengagement." (Private Pilot Flight Training Manual, Ralph Buther, page 2-26)

What is that supossed to mean? what does it mean that a compass card is "engaged"? does this knob also need to be pushed in/released out?


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2 Answers

  1. skyboyCFI on Jan 23, 2011

    Twist the knob after you let it pop out of engagement, to make sure it’s not still engaged. Just a check. If the knob is still engaged, the indicator won’t work. It’ll be in a neutral, floating or error state.

    The knob releases the disk from the mechinasim that controls it and the knob then controls the disk when you push in on it..

    Not a bad question at all. I’ve never seen one not disengage though. But I would assume it’s possible.

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  2. Steve Pomroy on Jan 23, 2011

    Hi Levi.
    As you probably know, the heading indicator is not a compass, but a gyroscope.  So it doesn’t actually track North the way a compass does.  Instead, it remains oriented in space in a constant direction acording to how it’s set.  At least that’s the theory.  In practice, there is friction in the bearings, which leads to a small amount of unpredictable precession.  This problem is compounded by “apparent precession” caused by the rotation of the Earth (gyros remain oriented relative to space, not relative to Earth).  So we need to reset the gyro every once and a while (every 15 minute is a good rule).
    Resetting the gyro itself can be quite difficult once it’s spun up.  So instead, we leave the gyro alone and reset the face of the instrument by rotating it.  But since the face of the instrument is mechanically linked to the gyro, it has to be disengaged first, and then re-engaged after the reset.  We disengage the face from the gyro by engaging the adjustment mechanism (by pulling or pushing the knob), and vice versa.
    Once the adjustment is made, we need to re-engage the face to the gyro (i.e. – disengage the adjustment mechanism).  Usually this is taken care of for us by the spring loading of the mechanism.  But ocassionally, especially with older instruments, cogs may not align perfectly, or the spring may not be stiff enough.  When this is the case, you may need to help the mechanism disengage by pushing, pulling, or turning the knob.  Failure to do so could result in the heading indicator reading constant regardless of your direction since the adjustment mechanism is engaged, and the gyro is disengaged.
    As noted by skyboyCFI, this isn’t very common.  But I’ve seen it happen a few times, and it’s worth the half-second of attention required to prevent it.

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