Welcome Guest. Sign in or Signup

2 Answers

Heading Indicator alignment knob

Asked by: 10326 views , ,
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?

 

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes



  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.
     
    Cheers,
    Steve
    http://www.flightwriter.com

    +5 Votes Thumb up 5 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.