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Sandel SG102 AHRS Gyro

Has anybody installed one of these?

A local avionics upgrade project was unfinished and I have managed to buy, at a very good price, an unused SN3500 EHSI which will go on the RH side, so I will have two of them. The first one went in 2 years ago. Glass cockpit pilots - eat your hearts out

The first EHSI was done as a gold plated FAA Major Alteration, with enough paperwork to exceed MTOW. The 2nd EHSI will not control anything in the aircraft - it is doing monitoring only - so the paperwork is very simple. On the plus side, because there is no need for supporting any existing NAV/GPS switch and autopilot functionality, one can connect to it absolutely every input which it supports i.e. GPS1 GPS2 NAV1 NAV2 ADF1 ADF2 DME1 DME2 or whatever one has available, and it will display any of the nav sources as CDI or RMI presentations.

But I also got an unused SG102 AHRS gyro, with the MT102 solid state fluxgate sensor. This is a really interesting bit of kit. Unlike the common KCS-55 slaved compass system, the compass calibration is done wholly in the gyro, which then outputs the heading as either the usual XYZ form, or digital ARINC429 high speed serial. Both come out at the same time. So there is no compass calibration in the indicating instruments.

It's interesting reading the installation manual for the SG102. The compass calibration is done by pointing the plane onto each of the four cardinal headings and setting it up with a laptop. There is also a special 50ft extension cable (which one can buy; I made it up) which one uses to measure the residual field at the calibration location.

The SG102 needs to be mounted within 0.25 of a degree of the aircraft axis, which is going to take some doing. Getting the roll axis within 0.25 deg is trivial, but there is no objective pitch axis, since the pitch depends on how fast you are going (AoA) etc. Yet it does apparently matter because this is a strap-down gyro so - unlike the KG102A it replaces - it has no gravity-based self-erection.

Nevertheless the SG102 is plug-compatible with the KG102A and apart from having to rewire the wingtip fluxgate magnetometer, it can directly replace a KG102A, with nothing else in the aircraft having to be touched.

I wonder how this differs from say a G1000 system... there should be a lot of similarity.

Shoreham EGKA, United Kingdom

The SG102 needs to be mounted within 0.25 of a degree of the aircraft axis, which is going to take some doing. Getting the roll axis within 0.25 deg is trivial, but there is no objective pitch axis, since the pitch depends on how fast you are going (AoA) etc. Yet it does apparently matter because this is a strap-down gyro so - unlike the KG102A it replaces - it has no gravity-based self-erection.<

Yepp I read the install manual also.the mechanical part is the most challenging I think...To my understanding maybe I am wrong the neutral pitch is referenced form the pitch neutral level point. E.g on the Mooney you have a point on the fuselage that can be used to rig the horizontal stabilizer in neutral. (when the pitch trim is centered the aircraft should be in level flight. If you can find this point and you note it down how many degrees pitch this is with reference to the ground level. In the avionics bay you need then to install the SG102 into the same number of degrees. Anyway this is just my 0.5 cent of input but to be X-checked with Sandel Avionics. I am very much interested in this my self. On the other hand I would not be surprised if you need to do some post rigging after a level flight in calm conditions and selecting the ATT reversionary mode so the ATT indicator shows 0 degrees pitch..the SG102 is MEMS based no..? Angel of attack depends on your loading so technically this is a dynamic thing (this is why the old INS's had Mercury sensors to align (erect) the gyro platform...

Thinking some furtehr on it it's all accelerometer/ and rate gyro based based so I would think the 0.25 degrees is written for installation purposes and the system will use this as a baseline to compute acceleration rate etc...from this baseline data XYZ...maybe ther is still a mercury switch inside to align it...

I found this on the internet: During the straight and level parts of the flight, the information from the accelerometers is dominant in determining your attitude while during any maneuver (or turbulence induced rotations) the gyros become dominant. Should you be flying a continuous maneuver without ever giving the AHRS a change to re vector Earth's gravity, your horizon will eventually drift out. Just how long this takes is dependent on two factors: The quality of the system as a whole (including the rate gyroscopes) AND the quality of your installation


The alignment issue arises because (in the extreme scenario) in a 90 degree roll, the two axes swap over.

However I don't think perfection can be achieved because obviously the varying AoA will be more than 0.25 degrees.

What that probably means is that there will be a heading error of say 1-2 degrees if you are in a lot of roll, but this is only during the roll, and does anybody actually care about that?

Yes I think it is MEMS. What else is there? It certainly isn't FOG

Shoreham EGKA, United Kingdom

..nor RLG (Ring Laser Gyro)......would be a bit expensive..


Isn't it the same thing? The laser ring is made up with fibre wound around a spool, I think.

Shoreham EGKA, United Kingdom

Nope , ring laser gyros are mostly use in airline IRS systems look here it works with mirrors..

You are right. I did some reading. The two work on a similar principle but FOGs are not as low drift as RLGs.

Shoreham EGKA, United Kingdom

I would think that you put the airplane on jacks and level it IAW the maintenance manual.

KUZA, United States

I have the stuff working on the bench, where I will run it for a few weeks before it gets installed.

The video quality could be better but it shows the basic functionality

Shoreham EGKA, United Kingdom

I found some interesting things with this gear...

Sandel say the SG102 will not restart if power is interrupted in the air.

The old KG102A gyro will restart though obviously if you interrupt the power when flying a coordinated turn, for long enough for it to spin down, it cannot possibly know how to erect... and when you return to level flight it will indicate a roll.

What I found is different:

Firstly the SG102 has an internal power storage for about 29 seconds, so a power loss up to 29 secs doesn't matter at all.

Secondly, I cannot make it go wrong anyway.

I put the unit on a 15 deg pitch and a 15 deg roll and turned off the power for a few minutes. When it came back on, the unit recovered just the same. I then tried some other combinations e.g. power down with it level and power up with it at some weird pitch/roll. I could not make it fail. It seems to always work perfectly after power up, with the correct indications in all respects.

It's also possible that there is a further internal power reserve on the gyro, which continues after the LEDs have been extinguished.

Or perhaps there is an "electronic level" inside, on both pitch and roll, which is used to initialise the unit? Otherwise, I cannot see how it can possibly power up showing the correct pitch and roll. Obviously one could fool such a level in flight because in a coordinated turn it would indicate straight-up, regardless of the roll angle, but that will be true for any gyro.

There is no objective "up" in space

The gyro is also incredibly stable. If I put it at say a 10 deg roll and leave it for an hour, it still shows the same. This however may simply be confirming that they are using some kind of level sensing, and feeding that in as an auto erection mechanism.

Normally, to provide auto erection, one needs either airdata (constant altitude plus constant heading is assumed to be straight and level flight) or GPS data (constant GPS altitude plus constant GPS track is assumed to be straight and level flight). Aspen do the former, AFAIK, while Garmin use the latter. The SG102 does has neither input which is probably why the system is not certified as a primary horizon.

But it is clearly completely capable of an air restart, to the same extent as any gyro.

Sandel won't comment on what they have done...

Shoreham EGKA, United Kingdom
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