ADF testing

[SCaro]

I'm in the process of testing a new ADF installation. Certain countries still require this to be installed as a backup for IFR flight. I'm not old enough to have actually used ADF during normal operations, so I have some questions. What is the best accuracy one can expect from ADF? How much needle movement back and forth on a steady course (exactly towards the station for example) would be normal / acceptable (2-3 deg?)? How much error can / should one expect in a 15 bank turn? The error should be minimum at 0, 90, 180, 270 relative bearing, but what about the 45 deg positions (it is worse there). Some of the airplane's systems when turned on cause a larger error (up to 15 deg). Does anyone have experience on how to get rid of that? Any help is appreciated. The airplane is a single engine, 4-seat, low wing, fixed gear, with the antenna mounted under the fuselage, near the wing leading edges.

[Boltzman]

There are many environmental factors such as terrain, coastlines, night error (don't flight-test ADFs within 2 hours of sunset/sunup), storms, etc. However, if I remember correctly, at the maximum quadrantal error points (that's the error you mentioned which is worse on non-cardinal relative bearings), and within the promulgated range of the station, you should get a worse-case error of +/-5 degrees.

You should expect approximately a 10 degree error ("dip") in turns. That's just the way an ADF works.

As for 15 degree errors caused by other systems, which systems, specifically, are causing this sort of error ? (would it be RT on transmit or systems involve digital electronics, by any chance ?)

Would definitely recommend reading BCAR Section R (it's downloadable from the CAA site) - there's a section on testing ADFs. Even better - find a friendly neighbourhood licensed avionics engineer and ask them about it. They should know the required sign-off standards and have suitable ramp test gear for tracking down any tricky EMC problems far more quickly and cheaply than flight testing (though it's not as fun for the TP, obviously).

B.

[SCaro]

The two systems which increased the error were the position lights and the panel lights.

[angelorange]

send an email to flight precision - www.flightprecision.co.uk
they test NDBs on a regular basis all round Europe.

[Hand Shandy]

Check out CAP 208 part 2 If memory serves. The error at 45 degree stations is known as quadrantal error , ironed out with a quadrantal error corrector [strangely enough] , which plugged in series with the loop and sense ant , and will be particular to type.

[safetypee]

See AC 25-7A Flight Test Guide for Transport Aircraft (www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/MainFrame?OpenFrameset) Chap6 (page 568-570).

[Boltzman]

Hi SCaro,

Does the installation use an RMI (moving fluxgate driven compass card) ? If so, you may need to isolate compass errors from ADF errors. Position lights are more likely to affect a fluxgate unit and cause the card to move. However, it shouldn't cause the ADF needle itself to move (relative to the aircraft), unless there are "pulsing" things on that circuit, like strobes (which might emit RF that can be picked-up by the ADF receiver). Not sure why the panel lights should affect it, but certain dimmer units can cause problems.

If you turn-up the ident volume on the ADF receiver, do you get any audible noise in the headset/speaker when you turn on those systems you mentioned ?

You've probably already checked, but if you're outside the useable range of the beacon, the receiver will be much more prone to on-board interference, so it's worth double-checking the published range of the beacon you're flight testing against.

If it really is a serious on-board interference issue, then there is no simple adjustment or fix - it's engineer time (maybe that's you). Worth checking the installation's earthing for a start, to ensure nothing has come loose.

B.

[AngloPepper]

One further point. If I recall correctly, accuracy is to be tested at 100% rated range of the ground station, and you need to check for ident readability at 150% rated range. That may be from Def Stan 00970 rather than BCARS. Definately get a book out time :)

[Boltzman]

AngloPepper,

You are right - BCAR section R says something similar about "loop swinging" (i.e establishing quadrantal error corrections) being done as close as possible to the edge of promulgated range.

Anyone know why that is ? I would have thought that you wouldn't necessarily want worst-case SNR for quantifying quadrantal errors. Maybe it is to minimise multipath effects.

The AC 25-7A Flight Test Guide (credit safetypee) is similar in content to BCAR-R as regards ADF testing, but talks about testing at half-range.

B.

[SCaro]

It does have a fluxgate compass. When the position lights are turned on the compass does not change, but the ADF needle moves. I tried it at different distances from the NDB, the further away, the more the needle moves. There is no audible noise if I listen to the station identifier and turn the lights on/off. We did find improvements with a different ground plane when we attached a large aluminum plate between airplane antenna and airplane, but only for a ground test. Maybe the antenna location is not ideal. Fortunately I'm not the responsible engineer for this, I'm just testing it.

[ICT_SLB]

Scaro,
It may not be a problem with your installation but on one ADF we installed the vendor-supplied attachment screws for the antenna were anodized & thus insulated. The install required conductive screws to carry the ground plane through to the underlying metal - changed the screws & fixed the ADF.

Some ADFs (Collins' ADF-462 for instance) have QE Corrections that can be strapped in by program pin but I doubt that's on you GA receiver.

[Boltzman]

Hi SCaro

Did you get to the bottom of that ADF problem in the end ?

B.

[SCaro]

We have not solved the issue yet. It may take a while since we are also working on a bunch of other projects. But thanks for everyone's input. I'm confident we can figure it out once we get back to working on it.

[forget]

Maybe the antenna location is not ideal.

Take a look at CAA Approved Aerial Positions at -

http://www.caa.co.uk/docs/33/CAP457.pdf

[Boltzman]

Hi Scaro

Thanks for the update. Don't know if it'll help but I've recently been working on an installation with a mis-behaving ADF which started working again after we changed the suppression capacitor on the alternator and re-adjusted the voltage regulator. Although in an ideal world the ADF shouldn't be bothered by such things, it might have been sensitive either to alternator noise or incorrect bus volts. Might be irrelevant in your case, but both things are fairly easy to check.

Regards,
B.

[Rich Lee]

Spurious nodes.:ugh: