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The directional gyro needs to be realigned every so often, but the attitude indicator does not, and I have a hard time finding why.

Pondlife
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vector
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  • knowing which brand/model instruments you are talking about might help us answering you, as not all are guaranteed to be the same. – Federico Sep 20 '20 at 13:19
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    ... I don't have a specific make and model in mind, let's say 'traditional' vacuum-driven ones. PHOAK specifically calls out the directional gyro, but does not mention the attitude indicator. – vector Sep 20 '20 at 15:46
  • Are these truly just gyros? Because the typical reason is that your attitude gyro has an absolute reference with which another instrument can be used (such an accelerometer, for example) to correct long-terms errors against: gravity.

    Your directional gyro doesn't have such a thing unless it also involves a compass.

     – DKNguyen Sep 20 '20 at 22:08
  • @DKNguyen I think the OP is referring to old school AIs & DGs from back before they were known as steam gauges. There was a time before we had accelerometers, magnetically slaved instruments, or flux gates. Six-packs were powered by pitot-static systems and vacuum pumps solely. Some even dispensed with vacuum pumps and were powered by Venturi tubes. The backup instruments of some glass panel trainers still rely on old-school technology. They are not all Aspen, glass or evening electric driven like your 1970s Turn Coordinators. These aircraft are still flying. Especially in the training fleets – Dean F. Sep 21 '20 at 01:01
  • @DeanF. Even so, I imagine it would be much easier to gang an attitude gyroscope to a weight than to mechanically link any kind of absolute heading reference to a heading gyroscope. – DKNguyen Sep 21 '20 at 01:50
  • @DKNguyen - I am really not sure what you mean by your comments. Of the aircraft I rent, those with accelerometers, HSIs that are slaved to magnetometers or glass cockpits, rarely to never require the need to periodically adjust or align the DG or AI after initial alignment. However, the Cherokees, Arrows, C150s, C152s, and C172s that are old enough to have nothing more advanced than a G530 or less in them, most of them have AIs and DGs that are solely powered by vacuum pumps. One C140 has a Venturi tube instead. There are no accelerometers nor magnetometers integrated into the six-pack. – Dean F. Sep 21 '20 at 03:23
  • @DKNguyen - That is why I made the point that there are a multitude of aircraft flying in the training fleet that would have their DGs and AIs as “truly just gyros”. Now, as previously mentioned, gravity and load due to things like centripetal force on gravity switches and pendulous vanes may effect AIs more than DGs to keep them from precessing in straight and level, unaccelerated flight. Yet, they are still just gyroscopic instruments. – Dean F. Sep 21 '20 at 03:35
  • Heading indicators can be slaved to a magnetic sensor giving them the ability to continuously self adjust similar to the attitude indicator. (More commonly found with the HSI style) – Max Power Feb 25 '23 at 22:11

4 Answers4

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The self erection device in the attitude gyro that levels it when you start up on the ground also keeps it erect (its axis pointed at the center of the earth) when precession tends to send it off kilter during flight.

Air driven gyros use a pendulous vane arrangement that controls the exhaust ports for the air being sucked through, and which always work to keep the gyro aligned with vertical when stabilized in non-accelerated flight. This system is imperfect, because the pendulous control system is influenced by accelerations and causes its own errors, but works well enough for light aircraft.

John K
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    Precession and pendulous vanes operate on a scale magnitude of around a degree per minute, and maneuvers with significant acceleration last less than 1-2 minutes so the max error is limited by the slow response of the vertical correction. And the significance of the error is limited by the useful display resolution. – Max Power Feb 25 '23 at 22:07
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I am no expert. My understanding of the Gyroscopic Attitude Indicator is that it is free to move relative to the fuselage in all three axis of motion until you go past 60° in any of the directions of motion. No outside force is applied to the gyroscope until the mechanical limitations of the instrument are reached. Once the mechanical limitations of the instrument are exceeded, the attitude indicator may tumble.

In the case of the Directional Gyroscope, on the other hand, it is constrained to move freely only upon one axis relative to the fuselage. Movement upon the other two axis is possible, but more constrained due to mechanical limitations. When the fuselage reaches attitudes that will exceed those limitations, force will be applied to the gyroscope. That force will be translated to movement in the plane of motion displayed along the axis of freedom of movement. In other words, a pitching motion of the longitudinal axis around the lateral axis, will be translated into precession or yawing motion of the longitudinal axis around the vertical axis.

Dean F.
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3

In the artificial horizon the spin axis of the gyro is tied by the earth's gravity to remain in the earth vertical. That's the reason why this gyro is also known as an Earth gyro. As John K said, this gyro uses a system of erection. Air driven uses pendulous vanes and electrical gyros use a system with electrical motors with gravity switches (with mercury) to orient and continuously maintain the axis to local vertical.

This type of gyro has two degrees of freedom. Those system are not free of errors. In air driven gyros for example, the pendulous system can give you false indications when doing manoeuvres. Basically there are acceleration and turniing errors.

Examples:

During an acceleration phase at constant altitude, the resetting principle of the artificial horizon results in the horizon bar indicating a nose-up attitud.

Following 180° stabilised turn with a constant attitude and bank, the artificial horizon indicates too high pitch-up and correct banking.

These errors are related to mechanical gyros. In the new electronic system these errors doesn't exist anymore

See this link to fully understand the erection system: https://www.theairlinepilots.com/forum/viewtopic.php?f=35&t=905&p=1204&hilit=artificial+horizon#p1204

O'Terror
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  • "These errors are related to mechanical gyros. In the new electronic system these errors doesn't exist anymore". That depends on the accuracy. Differentiating drift from a slow banked turns is still tricky for an AHRS, particularly on the low-end without GPS aiding. – Cody P Sep 21 '20 at 21:49
  • @Cody P, yes you are right, in purity all systems have errors, but from the pilot point of view I would say they are minimal compared to the old mechanical gyros. – O'Terror Sep 21 '20 at 22:41
  • Yes, I agree that especially from a pilot perspective theses errors are much smaller than for mechanical gyro, especially with GPS-integration on AHRS becoming more and more common. – Cody P Sep 21 '20 at 23:14
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Both attitude and directional indicator are using a gyroscope to remain stable inside the aircraft. This means that their orientation is not perturbed by the aircraft evolutions, the horizon of your AI will remain on the real earth horizon whatever the plane attitude and your DI will keep it's internal orientation constant thus helping you in a turn to evaluate your real heading.

On one hand your attitude indicator is aligned with the horizon and display your pitch and inclination. Those informations are only related to your airplane and gravity (normally the horizon is perpendicular to gravity). Most AI have mechanism that keeps them aligned with the gravity otherwise you might develop small error in your pitch angle due to the curvature of the earth on long trips covering large distances.

On the other hand the directional indicator should be aligned with the earth magnetic field to display your heading. Those informations are related to the airplane orientation and the magnetic field. And this field is rotating 15°/hour due to the earth rotation. This information has no other way to be transfered to your system than manually realign the display with a magnetic compass (or electronically in new ones).

MaximEck
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    ... regarding the DG, it makes sense in the context of a cross country flight, what about - say an 1.5 hr flight of shooting local instrument approaches, where the aircraft operates within some 20 nm corridor between longitudes? – vector Sep 20 '20 at 16:17
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    "Some AI have mechanism that keeps them aligned with the gravity" – I thought all AIs had such mechanisms. What's one that doesn't? – Tanner Swett Sep 20 '20 at 16:22
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    The gyroscope keeps an absolute orientation. Your aircraft is following the earth rotation but not the gyroscope. The fact that your flying on the same longitude doesn't change. Let's say you can be in space looking at the earth and gyroscope, 1h later the earth will have rotated 15° but the gyroscope on its surface will have kept the same orientation from your point of view – MaximEck Sep 20 '20 at 16:24
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    @terran Swett I've seen some old ones that you might now encounter in flight schools display for learning purpose that didn't had this, but they were from early ages of aviation ^^ – MaximEck Sep 20 '20 at 16:26