ADF Qustion

[Buzzerd]

I have a question that I can't seem to get a definitive, satisfactory, answer to.
My Cessna 150 is equipped with a Bendix ADF, which is deadly accurate within, let's say, 3 NM.
(I know this because occasionally I'll deliberately concentrate on keeping the ADF needle pointed absolutely on the nose, observe a "station passage", (as it were) and then afterwards I'll review my GPS track and scale it off. I've never been more that 200' off, even at 4500 msl, according to Garmin.)
Outside of that range, let's say, 10 NM, the heading "to" difference between my ADF and my Garmin GPS will vary by as much as 20°.
So, my question is: Is that common? Is it an acceptable variation? Does my ADF need calibration? Or is this the wrong forum to post this question in?
Thanks,
Buzzerd

[flybymike]

Rely on your Garmin. ADF is notoriously unreliable especially at a distance.

[Pilot DAR]

20 degrees does sound a little high, though as said, ADF's do wander somewhat, particularly at certain times of the day. Comparing the ADF to a GPS is really comparin technologies of two different eras. ADF's have their place, but don't expect the performance of a GPS form an ADF. Yes, you could have it serived, but ask yourself, Is it worth it? You could spend the value of a GPS, brining the ADF back up to the perfect specs. Most people I know with VFR aircraft are not investing in their ADF's very much these days.

Pilot DAR

[Johnm]

ADF/NDBs are affected by all sorts of things and the further away you are the less reliable. The distance that they would be expected to work reasonably reliably is published in the AIP.

Thunderstorm activity, hills coasts and night versus day will all influence accuracy.

However the most important issue is that you get a relative bearing so if you are aiming off drift that'll skew the reading relative to the GPS.

Example if you are trying to track 080 to an NDB and you need to deal with 10degrees drift you would be heading say 070 and the ADF would be pointing 10degrees right if you are staying on track. The GPS will report 080 as track.

It's interesting from an academic point of view but ADF's are not a very useful Navaid unless you've got nothing else! I only use mine for IR training!

[englishal]

It is a real shame that the CAA don't approve GPS Overlay approaches for NDB approaches....so if you had an IFR certified GPS you could use that instead of NDB as the primary nav instrument. This is very common in the USA and would mean the carriage of ADF would no longer be nescessary.

[IO540]

An ADF should be accurate at both short and long ranges, so long as there is nothing around that distorts the electric field from the NDB.

NDBs were used for accurate long range (hundreds of nm) navigation for decades, before there was anything else. In principle the system is as accurate as a VOR.

The problem is that a lot of the time there are distorting factors e.g. coasts, hills, etc.

[Mark1234]

20 degrees seems a rather large variation indeed; but also consider that, close in, 20degrees will not be much distance as the error is angular. To answer the OP's question, I reckon that's unacceptable - I'd be inclined to suspect there's something not right with the ADF.

It's interesting how perspectives vary; In my experience they're very accurate, though you need to be aware of things like coastal refraction, and terrain effects. Perhaps less relevant here, but having recently done a big trip around Aus, found it the single most useful nav instrument in the cockpit. From 10k-ish you could often get them at far greater distances than published, and much further away than VOR's - yes, you can't rely on them at those ranges, but the extra needle saying 'roughly thataway' is comforting, particularly in big open spaces. Also useful for 'inflight entertainment' (music radio), and NDB's are more common (certainly out there) / easier to maintain / aforementioned music stations can double as navaids (indeed their ersa lists co-ords of said music stations and frequencies). However, the thing does require a bit more wit to use.

[Whopity]

My Cessna 150 is equipped with a Bendix ADF, which is deadly accurate within, let's say, 3 NM.You cannot express the accuracy of an NDB in NM it has to be in Degrees. A good ADF may have a typical accuracy of 2-3 degrees within the protected range of the beacon, beyond that anything may happen. If the needle is wandering, that implies a poor signal level or interference from other signals.

A three position line fix taken from NDBs is much more accurate than using 3 VORs, that's why the military use TACAN as opposed to VOR.

Many non calibrated NDBs at GA airfields use poor quality antennas which do not produce a vertical cone of silence, the radiated signal may be weak and bearings may also be affected.
I recall flying a Piper Dakota away from Filton and the NDB was still rock steady at Enstone.
Another thing you should ask is has the ADF receiver been calibrated; a good ADF compensates for quadrantal error. Has it got a corrector and when was it last adjusted?

When you are over the top of an ADF you know exactly where you are; a GPS may give you the position in more digits but you can never be quite as shure!

[IO540]

IME an error of 30 degrees is not unusual if there is a coast nearby, your track is well away from being perpendicular to the coast (so the distortion is nicely assymetrical) and you are just the right distance away.

For example fly towards the Lydd or Shoreham NDBs, at say 45 degrees to the coastline, on a precise GPS track (autopilot coupled), and see where the ADF is pointing as you are in the 2-5D range. Closer in it is OK and further out it is also OK.

This is why most people fly NDB approaches using a GPS, checking the ADF at the FAF or whatever but ignoring it afterwards.

Over flat open country, say middle of N France, I find NDBs are very accurate - within 1-2 degrees.

As Whopity says they are also super accurate when crossing bang over the top. In fact I would say that is their only accurate aspect in terms of practical flying, because one doesn't need them for enroute nav :)

[Captain Smithy]

I quite enjoy using ADF, dead simple to use. Useful for locating CBs too:) Many ADFs are being decommissioned when they go U/S and most people don't use them anymore, but I still like using them purely for simplicity (sadly there are less and less around).

A lot seems to depend on the site's transmitter power. Many ADFs at airfields are low-powered locators and are only effective when within 10nm or thereabouts radius and seem more susceptable to interference from Wx and terrain. Higher-powered Tx sites (and hence longer-ranged) aren't as susceptable but can still suffer from the same problems. As mentioned earlier the aircraft's ADF Rx and the aerial/its position on the airframe also can be a problem.

Not too long ago I managed to pick up EDN once whilst way out past Dunbar on the East coast, usually I'm lucky to pick it up anywhere past the Eastern boundary of the Edinburgh CTA, but it was a very nice day Wx-wise, as I got close to the coastline however the needle moved towards Fife... perhaps a good demonstration of coastal effect.

Smithy

[ab33t]

Loads of factors can cause this , transmitter power , antena in aircraft, terrain, weather , reception equipment , altitude so to give an informative answer would be difficult at best

[niknak]

Many non calibrated NDBs at GA airfields use poor quality antennas which do not produce a vertical cone of silence, the radiated signal may be weak and bearings may also be affected.


Airfield NDBs aren't regulated calibrated like other Nav Aids, they're installed, licensed and thereafter, their output is monitered by the CAA.
If they start transmitting beyond their licensed power output, the operator will receive a call from the Department of Wigglyamps instructing them to tone it down.

Which leads on to:

Many ADFs at airfields are low-powered locators and are only effective when within 10nm or thereabouts radius and seem more susceptable to interference from Wx and terrain.

That hits the nail on the head, the licensed range of most airfield NDBs is only 15 - 20 nm largely because of the lack of available frequencies and interference factor from other NDBs (or even radio stations) in Europe.

The next time we get high pressure and suitable flying conditions, tune into an airfield NDB 30nm or further away, fly at 3000ft or above and try and track it accurately to 15nm.

[flighttest-engineer]

I am familiar with the „good old” Bendix T-12C/-D ADF system and I must confess that it was reliable equipment. I have repaired, installed and used this ADF system myself years ago.
Presuming that installed equipment shall work according to its specification, the allowable accuracy of ADF-bearing indication (not depending on the specific system) according to the ICAO Annex 10 is +/- 5° with a radio signal from any direction.

The rated coverage for NDB locator beacons is approx. 10-25 NM, where the rated coverage of NDB’s is approx. from 25-150 NM.

To determine the cause of your observed abnormal bearing deviation I would suggest the following procedure:

A good start would always be to have the ADF-receiver bench-checked by an avionics-shop to verify the proper function of the receiver parameters (sensitivity, selectivity, AGC, proper alignment etc.) and spend some money for it, even if you have GPS already available.

Thereafter perform some easy checks in the airplane (on gnd & in air).

Check if the bearing-deviation is constant for all headings which means you have the same offset (+ / -) for every heading.
Check if the bearing deviation depends also on the fieldstrenght of the received NDB-signal, .e.does a stronger signal give you more accuracyAs a first try you can check your ADF also on the ground, while taxiying around, but not in the vicinity of strong electrical fields like lightings etc.
For checking you shall perform it on a day with good weather conditions (no thunderstorm with lightning nearby etc, no twilight).

If you have the same offset deviation on all headings, it can also be a problem caused by the inaccurate installation of the Loop-Antenn (not aligned to the aircrafts centerline, if this problem was from the beginning).
If you observe different bearing-deviations on different headings (presuming you are receiving the same NDB, maybe there is a problem with the “quadrantal error correction” of the installed Loop Antenna.

The “HI-CAP” of your ADF sense antenna can also have an influence to the indication accuracy. It is importand, that the signal level of the sense-Antenna is matching the signal level also of the Loop Antenna.

Let me know if I can give you some more hints to get your system functioning correctly.

Have fun,

Armin

[RatherBeFlying]

There's lots of poorly calibrated compasses out there. The first thing I would do if dependent on an ADF is get the compass properly swung.

On my IFR ride, I did an NDB approach and when it came time to lift the hood, the field was a mile to my right -- and the examiner passed me. Well, I had followed the needles correctly, even if they pointed in the wrong direction:}

I once used a back bearing for a considerable distance over unsettled country and found myself considerably offset from where I thought I was going.

Newly installed equipment can do funny things to the compass. My instructor and I once entertained ourselves by seeing which radio moved the compass needle the most when switched on and off. The ILS receiver moved the compass 20 degrees which made tracking to the LOM interesting.

[silverknapper]

As alluded to above before you do anything I would check the radius of coverage that the beacon you use has. Then check for weather, coasts, large metal buildings, etc etc.
Do you use it IMC? If not I wouldn't worry about it. If so and all the above don't give any reason for doubt then yeah have it checked out. But i wouldn't give the shop a blank cheque, especially as you have a GPS.
I regularly fly outbound on an NDB to turn for a Loc/dme. With the localiser set up, and the GPS I can see I am flying a direct straight line outbound. If I were to attempt to keep the ADF on the exact course I would be weaving about the sky all the way!