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I'm going to be taking my checkride soon on a Skycatcher 162. It has an all glass cockpit utilizing the Garmin G300, and its my understanding that it does not utilize gyroscopes or vacuum pumps for any of its instruments.

Seeing that if I were in something like a Cessna 172L, which uses both gyros and vacuum pumps for many of its instruments, questions such as "how does the attitude indicator work?" are fair game, I assume that the same questions are fair game for avionics like the Garmin G300.

The problem, I don't know how they work and its looking like the top google result for "garmin g300 gyroscope" returns a 404 error.

Also, do digital pitot/static systems work in essentially the same way as the old school ones in planes like the C172L? I'd hate to tell my examiner anything along the lines of "I have no idea".

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  • $\begingroup$ These sound like questions for your flight instructor $\endgroup$ – acpilot Sep 19 '16 at 0:23
  • $\begingroup$ My flight instructor is now an airline pilot but fair point. $\endgroup$ – boulder_ruby Sep 19 '16 at 1:06
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    $\begingroup$ A helpful term to search for is "laser gyro" $\endgroup$ – DJClayworth Sep 19 '16 at 2:07
  • $\begingroup$ @DJClayworth I doubt that the Garmin 300 system uses a laser or fiber optic gyro. I've used FO gyros and the sensors alone cost $50,000. $\endgroup$ – Ron Beyer Sep 19 '16 at 2:08
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    $\begingroup$ If Google returns a "404 error" then you have a problem with your connection. $\endgroup$ – mins Sep 19 '16 at 7:33
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Looks like the following link takes you to the G300 user manual. You'll find your answers here, and this document would be a good one to be familiar with for exactly the sorts of questions you're anticipating.

G300

The short answer is, the G300 has, instead of mechanical gyros, an AHRS -- "attitude and heading reference system", which is essentially an INS but without the navigation function. And, the same unit also includes an air data computer to process the pitot/static inputs. So, "it's all done in the GSU-73" -- GSU standing for Garmin Sensor Unit.

System Schematic (from linked operating manual)

Source: the above manual, page 4 (page 22 of the PDF file)

There is more in the linked document on this, and the more comfortable you are explaining it, the better. Hopefully, though, nobody will ask you to explain "how to build" the GSU! But having a good idea of what components are in the system, what each does, and what the expected (possible) failure modes are, is probably fair game.

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  • $\begingroup$ "Its all done in the GSU-73". Roger $\endgroup$ – boulder_ruby Sep 19 '16 at 1:10
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    $\begingroup$ Its fair to note that the AHRS still has a gyro, just not an older spinning mechanical one. It uses MEMS sensors to achieve the same (or close enough to the same) effect. $\endgroup$ – Ron Beyer Sep 19 '16 at 2:07
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    $\begingroup$ flyingmag.com/… $\endgroup$ – abelenky Sep 19 '16 at 19:29
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    $\begingroup$ @abelenky Good find, thanks -- added the link into the answer. $\endgroup$ – Ralph J Sep 19 '16 at 20:14
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If you haven't already done so, you should have a look at section 7 of the C162 POH. That section gives an overview of all the aircraft's systems and the information is at a good level of detail for a checkride (at least at the private level). The examiner isn't expecting you to explain how a ring laser gyro works, but he probably will expect you to be able to give an explanation like this (from the POH):

AIR DATA, ATTITUDE AND HEADING REFERENCE SYSTEM (ADAHRS) AND MAGNETOMETER (GRS)

The ADAHRS provides airplane attitude and flight characteristics information to the G300 displays integrated avionics units. The ADAHRS unit, located behind the instrument panel, contains accelerometers, tilt sensors and rate sensors that replace spinning mass gyros used in other airplanes. The magnetometer, located in the tailcone, interfaces with the ADAHRS to provide heading information.

The air data portion of the ADAHARS compiles information from the airplane's pitot-static system to calculate pressure altitude, airspeed, true airspeed, vertical speed and outside air temperature. An outside air temperature probe, mounted on top of the cabin, is connected to the ADAHRS.

The G300 Pilot's Guide goes into more detail about the system's components (see Section 1), although it's still at a fairly high level.

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I can't say for certain what is actually inside a G300, but it seems likely that it uses a set of MEMS solid-state gyros, accelerometers, and magnetometers to sense

  • three axes of acceleration (including gravity)
  • three axes of rotation
  • three axis of earth's magnetic field lines

Using these inputs, sometimes helped with GPS inputs, you can mathematically estimate the full orientation of the aircraft similar to the way an artificial horizon maintains its reference over the course of a flight -- but without moving parts.

Additionally, inputs from static pressure, pitot pressure, and external air temperature are used to determine altimeter altitude, indicated, and true airspeed. These sensors are electronic transducers that output a voltage for a given input, so there are no moving parts for these systems either.

All of these inputs are fed into essentially a computer, which is estimating the state of the system, taking new inputs into account, adjusting its state, etc etc and continuously displaying this state estimate on the instrument panel.

Note that you need to draw a distinction between the estimated state and actual state, because even with an electronic system, it's possible to 'tumble' the artificial horizon under certain conditions. Some units are better than others!

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To expand on Daniel's answer -- a modern ADAHRS consists of three sections:

  • An inertial-magnetic sensor frontend, referred to as a 9DoF or MARG (magnetic, angular rate, gravitation) sensor package. Older or higher precision units (such as those which support inertial navigation) use stuff like the ring laser gyros Pondlife mentioned, while simpler AHRS units can use MEMS 9DoF frontends -- they are available in integrated circuit form from several manufacturers.
  • A processor that takes the raw 9DoF data and converts it into attitude and heading information. This is a mathematically complex operation; in the past, Kalman filtering has been used for this, but Kalman filters are quite computationally intensive and can be vulnerable to singularities if implemented naively. More advanced algorithms, such as Madgwick's work, that use quarternion techniques to simplify the calculations are available -- this also eliminates problems with singularities, which manifest themselves as "tumbling" or "gimbal lock" type phenomena (or simply a red X, for that matter). Error processing is also found here -- some AHRS implementations will simply reject attitudes beyond certain limits, which is something you should be familiar with for the sake of unusual attitude recovery/training if nothing else.
  • Finally, there is the air data sensor frontend. Simpler units simply have two pressure sensors -- one senses static pressure for altitude and vertical rate measurement, while the other can either sense the difference between pitot and static pressures or simply sense pitot pressure directly in order to measure airspeed. This air data section would also include an angle of attack vane frontend and air temperature probe interfaces, if the unit supports such functionality.

All of this is plugged into the avionics communications bus in your airplane, where it provides attitude, heading, and air data info to whatever subsystems need it, as well as error flags to allow the displays to show you the red X instead of bogus garbage from a busted sensor.

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  • $\begingroup$ Kalman filtering can be done with Euler angles or quaternions. Madgwick's algorithm is less computational intensive, but also less capable once the Kalman filter has been tuned to the application. I would bet that the G300 uses Kalman filters, but have no inside information. $\endgroup$ – Peter Kämpf Sep 28 '16 at 14:56
  • $\begingroup$ @PeterKämpf -- yeah, Euler angles are the "naive" implementation of Kalman filtering, if you will. $\endgroup$ – UnrecognizedFallingObject Sep 28 '16 at 21:13

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