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7 Answers

Heading Indicator query

Asked by: 20376 views
General Aviation

Bearing friction causes the heading indicator to precess from the heading to which it was set.  However, unless you properly align the HI to the magnetic compass at regular intervals, you may fail to notice this error.  Consequently, you may begin to drift off course by attempting to maintain the desired course by following an inaccurate heading.

Now, my question is this.  It is my understanding that precession is the reaction to a force applied to a gyro acting in the direction of rotation and approximately 90 degrees ahead of the point where the force is applied. 

Then why isn’t the heading indicator always 90 degrees off?   

Also, if my autopilot is slaved to the GPS, and I forgot to match the heading indicator to the compass, will this have an effect on my course?  The autopilot would continue to follow the correct course since it's slaved to the GPS, right?

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7 Answers



  1. John D. Collins on Feb 11, 2011

    The basic heading indicator has no connection to the magnetic heading and must be adjusted manually to align with the magnetic heading. The heading gyro remains rigid in space and the aircraft effectively moves around the gyro providing different heading indications.  The precession causes the heading to drift over time, and the pilot has to reset the indicator to match the actual heading.  Some heading indicators are interfaced to a magnetic flux sensor that automatically and continuously adjusts for the gyro precession.  This is normally termed a slaved heading indicator, as the magnetic heading is constantly corrected.
     
    Autopilots have two main theories of operation, one is attitude based  and the other type is rate based.  The attitude autopilots get their control information from an attitude gyro, examples of these type of autopilots are the Century III and the KFC200.  Rate base systems are controlled by a rate gyro in the turn coordinator, examples of this type of autopilot are the Century I, the KAP140, and the Stec autopilots.
     
    Most autopilots have several modes of lateral control, from a wing leveler, to heading hold, to Nav radio CDI tracking, to Course Intercept, to roll steering control.  Usually only heading mode is affected by the DG and even then you set a reference heading (called the heading bug) and the autopilot turns until the heading bug is the current heading.  A simple Nav coupled autopilot will react to the CDI deviation, if the CDI moves to the left, the autopilot will turn to the left, and if the CDI deflects to the right it will turn to the right.  In some instances, the heading bug is required to be set to provide the autopilot a course reference and this along with drift rates of the CDI will enable the autopilot to correct for wind.  In those cases, if the heading bug is too far off the desired course, the autopilot will be unable to keep the track centered.  The Nav mode is limited to following a straight course.  With an HSI, the same reference heading function is handled by the course select pointer. There are many advantages of a HSI over a DG, in that it combines both a Nav CDI and a DG presentation onto the same instrument, and HSI’s are more often to be found with a slaved heading capability.  It is nice to never have to set the DG.
     
    Roll Steering is a more precise method of controlling the autopilot because unlike a Nav based CDI deviation, the roll steering will guide the autopilot thru curved courses such as holding patterns, DME arcs, and will anticipate turns so they may start early to cut the corner and roll out precisely on the new course. Roll steering flies by telling the autopilot what angle of bank to have at all points in time to follow the desired course. It will nearly instantaneously direct the autopilot to correct for wind drift, in other words it doesn’t have to figure out wind drift, it already is aware of the correct track for the course.  The roll steering commands are generated directly by the GPS, so it knows the course, the track, how far off the track the aircraft is, what the ground speed is, when the next turn is coming up, what the course for the next turn is, and so on.  Since most autopilots don’t have a roll steering input, adapters have been designed that go by the acronym of GPSS (GPS Steering).  These adapters go between the heading bug on the DG and the autopilot heading error input signal. A switch is provided that switches the real DG to the autopilot or the GPSS which simulates its own DG bug to the autopilot. In this setup, when DG is selected, the autopilot follows the heading bug as set by the pilot. When GPSS is selected, the autopilot is faked by the adapter into following a heading bug generated by the adapter from the steering information provided by the GPS.  So when you use GPSS, you have to select the autopilot into heading mode and not a Nav mode, as the CDI position is not relevant.  For that matter, neither does the DG heading matter.

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  2. Kent Shook on Feb 12, 2011

    The heading indicator is not always 90 degrees off because what you are seeing is simply a movable card connected to a gyro. You can set your DG to any heading you want, and the gyro will still work – If you set it to 180 degrees off what your actual heading is, for example, it will remain roughly 180 degrees off. So, by adjusting that card to the correct heading, it will read the proper heading, provided you adjust it every so often.
     
    The autopilot will only “pay attention” to the DG if it’s in Heading mode, and even then it’s only paying attention to where you set the bug, rather than the actual heading. If you’re heading directly East, for example, and you adjust the card to read South, but you put the bug at South too, the airplane will continue heading East – It’ll simply try to keep the bug at the top of the DG, regardless of what the DG actually reads. If you’ve got the autopilot following the GPS (regardless of whether it’s in Nav mode, or if equipped with a GPSS system as John describes, in HDG mode with the GPSS switched on) the autopilot is no longer paying attention to the heading bug and will follow the track from the GPS regardless of what the DG reads or where you have the heading bug set.
     

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  3. Wesley Beard on Feb 13, 2011

    I’m simply going to answer the gyroscopic part of the question.  Whenever a force is applied in the vertical axis of the gyroscope; that force will react 90 degrees in the direction of the turn.  A force applied in any other axis will not be “felt” by the gyroscope.  There is an excellent picture in the Pilots Handbook of Aeronautical Knowledge on page 7-16 (Figure 7-19) that explains what I am trying to explain.  Notice that the force is only “felt” on a single axis.
     
    The heading indicator is a single gimble gyro meaning it can only sense a force on the lateral axis.  (The most common attitude indicators is a double gimble gyro meaning it can feel a force from two axis; lateral and longitudinal axis).  When an aircraft begins to turn, the force is applied to the lateral axis.  The gyroscope will resist that force and remain in the same “plane of rotation” it was before.  This is call rigidity in space.  If at any point, the “plane of rotation” is changed that force will be applied 90 degrees in the direction of rotation.  Hopefully, this “plane of rotation” doesn’t change in flight so there is no 90 degrees in direction of rotation phenomena.
     
    It is important to note that even though the gyroscope stays in the same “plane of rotation” with respect to the Earth.  From the aircraft’s point of view, the gyroscope is moving inside the instrument housing.  This movement is transferred via gears and linkages to the heading indicator face and it works as we expect it to. The only exception is the fact that we don’t want to spin the gyro as fast as we need to eliminate precession because the gyro destroy itself.
     
    I hope this helps.

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  4. Student Pilot on Feb 24, 2011

    If the autopilot is set to heading and the gyro gets “off” due to precession, does the heading bug precess with it?
     
    I seem to recall someone telling me that that one can push in the heading bug button to keep the bug where it is and rotate the directional gyro seperately in order to align it back to the compass.
     
    Therefore, if the bug does precess with the DG then why would you want to perform the above procedure?  You would be keeping an incorrect heading by keeping the bug where it is and aligning the DG back in place.

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  5. Wes Beard on Feb 24, 2011

    I have not seen a heading bug that stayed where it was when the heading indicator was reset due to precession.  With that said, there might be some that do that but I can’t imagine why.  The easiest way to check is to turn on the autopilot in heading mode and see what happens.

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  6. Flight Student on Feb 25, 2011

    While in heading mode, if the heading indicator becomes unaligned with the magnetic compass due to precession, will the heading bug precess WITH the DG?  Or, will the heading bug remain in place even if the heading indicator becomes unaligned?

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  7. Kent Shook on Feb 25, 2011

    The heading bug will precess with the DG – It doesn’t have any other reference to stay “still” other than the DG itself.
     
    I think the procedure that Student Pilot is talking about is simply there so that if you are looking out the window or using some other reference for your desired heading, and the airplane’s ground track is proceeding as desired, you can keep the airplane on the same heading while adjusting the compass card. However, if you’re on vectors or otherwise are interested in the heading itself (“Bugsmasher 123A, fly heading 280, vectors for sequence”) you’d want to leave the bug alone and adjust the DG so that the airplane will go back to that 280 heading.
     

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