ADF dip

[jessfix]

ADF dip (in the turn in a hold) is mentioned in some places, but not in the following

PPSC ATPL notes
Trevor Thom vol 5
IFR Principles & Practice

Can anyone explain either about dip

or

where to find mention of it in a good publication
???

[Noggin]

Dip is caused because the ADF loop antenna is tilted from the horizontal. The modern ADF loop antenna is optimised to pick up horizontal magnetic fields, when tilted it picks up vertical components which produce bearing errors. Night effect can cause similar problems when sky-wave signals are picked up by vertical parts of the antenna.

[ 08 January 2002: Message edited by: Noggin ]</p>

[MJR]

Are you sure that's correct Noggin?

[Noggin]

Basically yes. The transmission is vertically polarised giving rise to a Vertical E field and a Horizontal M field in phase with the former.

Perhaps I could have simplified things by saying the ADF works on the basis of looking for a signal Null. At this point the emf induced in one side of the antenna is equal and opposite to the emf generated in the opposite side and thus creates a nul. If we rotate either the signal field or the antenna, the emf induced is not equal and thus does not cancel, the ADF servo then drives the goniometer and ADF needle to a false null ( Dip or Night effect)

Admiralty Handbook of Wireless Telegraphy VolII 1938.

[MJR]

Is night effect not the increased multi-path effects experienced by the ADF receiver due to the change in the tropospheric propagation of the original NDB signal. Additionally is it not also the case that the ADF receiver will also receive signals from normally out of range NDB's on the same frequency during the changed atmospheric conditions experienced at night.

[Noggin]

Night effect can be a combination of both, but the primary cause is due to polarity change of the signal when reflected from the ionosphere. Ground and sky wave arrive at different times, this is of no consequence to the ADF receiver but the angle of incidence of the skywave is. Other transmissions within the pass band, not necessarily NDBs, as few are on the same frequency(synchronous transmissions), can add to the problems. Multipath may also occur to the groundwave, called terrain effect.

Dip and Night effect are both in their simplest form, due to a rotation of the incoming signal relative to the antenna.

[fireflybob]

Yes - dip error certainly exists but seems to feature more prominently on light aircraft installations.

If you are flying a hold, for example, the amount of "lead" when on the inbound turn when turning through a heading which is at 60 degrees to the axis is 7 degrees (assuming negligible wind effect). On may light aircraft the 7 degree lead is cancelled out by "dip error" so if when you turn through said heading the RBI shows a relative bearing of +/- 60 degrees then the turn should connect up. If you have the luxury of an RMI then the desired inbound track would be displayed.

[MJR]

Surely if both the ground waves and the skywaves arrive at the ADF receiver then there will no doubt be a phase difference leading to an erroneous interpretation by the ADF. (Not too dissimilar to ghosting on a TV set.) The ADF is a phase interpreting instrument hence the presence of a number of phases of the same original signal can only lead to inaccurate readings.

[Chuck Ellsworth]

The ADF is the least accurate of the nav aids due to many errors it is subject to, when discussing ADF errors one must also factor in such errors as precipitation static, electrical static such as st. elmos fire, mountain/valley errors, sea/land errors and on and on go the reasons for the ADF being inaccurate.

Several years ago I was required to write the U.S. Commercial rotorcraft exams for a U.S. licence and to my surprise there were seven ADF time and distance / bearing change questions on the exam. Can anyone tell me what advantage knowing the answers to such inaccurate methods would give me?

Of course it is nice to be able to chat about the errors that are inherent in the ADF but at the end of the conversation we still are left with the same result...the ADF is an aid for cloud breaking procedures when flying IFR and is without doubt a true non precision aid. Knowing this very simple fact we also must be aware that it will soon go the way of the radio range method of navigation.

As a closing comment I defy anyone that is flying the ADF to convince me that any given swing of an ADF needle is due to any one cause.


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:) The hardest thing about flying is knowing when to say no. :)

[jessfix]

Glad to see we've started a bit of discussion on ADF dip.

With my combination of NDB & aircraft equipment, the ADF is a waste of time, because I'm not prepared to accept an error of +/- 180 deg!

On one hand, we should allow for dip as it would seem to be the only quantifiable (at least in apporximate terms) of all potential ADF errors.

On the other, from reading fireflybob's remarks, whilst we should be aware of it, it has little practical effect in light aircraft.

Does anybody agree/disagree - and is there anubody who flies "grown up" aircraft that can explain how they cope with?

[Chuck Ellsworth]

Jessfix:

What do you mean by "Grown up airplane"?

No matter what you put an ADF in it will still be an inaccurate navigation aid.

For instance tracking outbound from a radio transmitter using ADF the further you go from the transmitter the less accurate the track information.

Why are we even discussing antiquated nav aids when we should be concentrating on modern aids such as GPS?

Unfortunately for flying training we seem to cling to old tecnology rather than learn new and far better methods. One aid that comes to mind here in Canada is the use of the E6B flight computer in flight schools when we have far better computers to do our calculations with. Never could figure that one out.

.................

:) The two most dangerous words in aviation.. "WATCH THIS" :)

[CaptAirProx]

Cat, we still have to study the ADF here in pretty little England cos its scattered with them. Unfortunately even in a western country, I still have to do NDB approaches on a regular basis. Always ends up being a lottery as to the actual final approach track.

[bluskis]

Is not the simple fact that NDB's are cheap to install, and in many locations they are already there. Better than nothing, and nobody is about to replace them with something more expensive.
Some people like crosswords, I suppose some people like NDB's.

[whats_it_doing_now?]

NDB's are featured all over the place. I did an approach into a West African airfield in a 767 a few months back using an NDB procedure. Similar problems encountered to light aircraft, plus the ever present tropical thunderstorms thrown in just to liven it up.
The other problem with the 757 and 767 with regard to NDB's is the design philosophy. At the time the aircraft was designed, it was thought that the ADF was an unnecessary peice of ballast, as a result you can't display the needle on the HSI, and have to use the RMI needle, which is small and usefully placed out of view behind the control yolk (in my seat position anyway!). On the plus side we do have a track line, so once established on the inbound course, you just steer the trackline to match it and (in theory) you should stay well on track!

[Noggin]

Compared to most other aids its prety poor, but if you have nothing else it can be pretty useful, especially when you are overhead.

MJR
The only phase sensitivity in an ADF is the sense circuit. Otherewise, it just responds to the modulation envelope comprising of the alebraic sum of all the different signals in the pass band.

[A and C]

A lot of very good theory above none of this i doubt but from practical observation of the two most common ADF,s in use in GA the king KR87 has considerably less tendancy to "dip" than the Collins ADF650 this i find surprizing because they both use the same type of system to find the sorce of the radio signal to which the unit is tunned and both units have a good reputation with the avionics guys that i know.

The King KR87 recever has a tighter band width than the ADF650 but in theory this should not effect the dip when near the station ,i am left to speculate that it is something to do with the "damping" of the signal within the unit but this all goes far beond my electrical know how.

Comments from an avionics engineer please.

[Noggin]

Collins make real radios, perhaps its down to "Quality" in the end.

[Dan Winterland]

A better way of describing dip is to imagine an aircraft with wings level with a beacon on a relative bearing of 045 degrees on an intercept heading, the needle will of course 045 degrees off the nose. Now bank the aircraft, i.e turn onto the inbound track. You now have a horizontal and vertical component of the bearing. The vertical component increases, the horizontal reduces. If you have problems imagining this, consider if the aircraft was to use 90 degrees angle of bank. The horizontal component will be zero as the needle attempts to point directly at the beacon indicating a relative bearing of zero. This is the dip, and it only occurs with bank causing the relative bearing to reduce.

To counter it, anticipate by lagging your roll out a bit . I'm sure someone more clever than I could work out how much lag would be necessary depending on the cosine of the angle of bank, but I was taught to use th TLAR method. (That Looks About Right). It is a function of the angle of bank, regardless of aircraft type.

[Chuck Ellsworth]

Another way to describe the actions of an ADF needle would be to compare it to the actions of the magnetic compass.

Both are subject to multible errors and in practice you strive to adverage their errors.

From a practical viewpoint I am making these obversations based on starting my airline flying in the DC 3, as I recall we became quite proficient in out thinking the damn thing and actually did some rather impressive approaches.

Also the old Lear ADFs with a bfo select were great in the Arctic for long range relative bearings.

Then again I am half senile now so don't pay to much attention to my ramblings.( I was told when I was young I would go blind doing what I did , but all it did was ruin my memory. )

.......................

:) The hardest thing about flying is knowing when to say no. :)

[18greens]

You must be doing an IR to be interested in this topic.

I've never seen anything in publications about it. All I ever heard was from the instructors.

Unfortunately you need to learn it a lot for the IRT. RANT does a very good job of simulating it.

[MJR]

For some bizarre reason you have to track the inbound QDM within 5 degrees on a non-precesion approach for the IRT, often this is done asymetrically as well just for a laugh! For best enjoyment do your IRT on a really buffety day with lots of windshear and enjoy the challenge of keeping that little needle within the tolerances.

On a simulation note I bought Jeppersen FlitePro which dissappointingly has no ADF dip. I recently contacted them and asked them if they were likely to include it on a later release, the answer NO! :)

[fireflybob]

MJR - what's bizarre about having to track within 5 degrees? If you were doing it for real wouldn't you want to be on track?

5 degrees is the tolerance required for the IRT. Examiners have discretion and whereas you might stray outside the tolerance and pass, you can also stay within the tolerance and fail!

E.g. on a good day in perfect conditions if a candidate made no attempt whatsoever to correct a 5 degree error one might consider a fail. In other circumstances, weather/crew workload etc. a candidate might stray outside the tolerance but you would be happy to pass him. Essentially the examiner would have to ask himself whether he might have done the same thing in the circumstances.

Pilots sometimes refer to "limits" which is not the correct terminology - they are "tolerances" but essentially it all boils down to the same thing in the sense that the "professional" pilot is the one who is striving to fly the procedure accurately at all times.

[Wee Weasley Welshman]

I have had to perform 27 ADF only approaches in the UK in the last 3 months in a Boeing 737 on scheduled ops. 3 of those were to minimums and one resulted in a go-around.

Dip is present in my aircraft type to about 7 degrees. The years spent in various holds teaching Dip to students allows me to somewhat enjot these approaches without any aprehension.

If you can hold and approach to minima on and NDB then you can do most things. A non-precision night circle to land on one engine not withstanding...

WWW

[Chuck Ellsworth]

Triple W:

It is interesting that you are getting the privilege to do ADF approaches in Boeings neat jet.

The ADF approach was for years the only approach we used in the Arctic, one gets so used to them you never really think of the physics of why the needle does what it does you just fly the thing and experience allows you to recognize when something is really out of whack.

The secret of course is to always be aware of where you are and where the fu##ing thing is headed. If any doubt exists that is what the missed approach is for.

By the way I don't know if you have used the ADF in mountinous regions, it can really get goofy way down in deep valleys, but the D.I dosen't know you are in a valley.

Hmmm a 737 doing ADF's must be fun?

.............
:) The hardest thing about flying is knowing when to say no. :)

[SEbby87]

hey guys, looking for some in depth info on ADFs wondering if any of you could give me a hand with it.

firstly im trying to sort out which parts connect to which

[BigEndBob]

Dip seems to have something to do with the poor location of antenne.
We had a fleet of PA28's.
The one which produced no dip had its loop aerial mounted half way up the tail instead of between the wings.
Logic would suggest the wings shield the aerial during turns thus producing errors. Errors being seen to be greater on multi's.

[Croqueteer]

:) No-one has mentioned the fact that an NDB on an airfield (Half-a-crown a week maintainance) is an invaluable aid to keep positional awarness when under radar vectors, and I have had occasions when radar has "forgotten" me!

[GOLF_BRAVO_ZULU]

Lifted direct from AP3456:
14. Bank Error. When an aircraft is banked, the aerial is tilted and some of the signal is detected by the horizontal elements of the aerial. This will produce an error in the ADF bearing until the wings are levelled again.

It's a bit like bank error in a magnetic compass. The needle, in this case the aerial, moves relative to the thing it's pointing at and moves to re-acquire it. Relative bearings of 360, 090, 180 and 270 provide the least errors (not to be confused with Quadrantal Error).

[High Wing Drifter]

Jessfix,

Thom 5 has a tiny paragraph on 'dip' but calls it "Turning Error" and defines as the loop taking time to resolve the signal.

BGS notes define dip as being something related to some Bendix King installations and is at its maximum with relative bearings +/- 45 and 135 deg.

RANT defines ADF dip as when the sense antenna shields the loop antenna. Says dip error reduces eventually to zero as the beacon moves abeam.

Take your pick. Guesstimating 10 degress with a rate 1 turn seems to work well enough every time.

I recall reading somewhere, that some Bendix King installations have been modified to remove the problem.

[pilotbear]

The best way to fly ADF is to;
not read the instrument unless wings are level. You should be targetting a heading anyway.
not to read read any moving instrument.

Big End Bob, I have noticed the same, In particular,high wing aircraft seem to suffer less from dip.

Tracking within 5degrees is very relevant in some of the valleys in Canada where I started my flying. If you try to be within 5deg and get 10deg then not good but OK, but if your limits are 10deg you will get 20deg etc.

ADF is a good instrument because it always shows you where the station is pictorially.
Imagine you are a little aeroplane on a pivot on the tail of the needle always pointing vertically up the instrument panel. The beacon is always in the centre of the instrument.
Just turn in the direction that will push or pull the needle where you want it to go.
Remember also that the needle head always wants to move towards the wind.
I believe that single needle tracking is a good and satisfying skill to posess for situational awareness.

[BigEndBob]

This is a bit sureal, but imagine the ADF needle is connected via a piece of elastic to the NDB beacon.
When you bank, as in the hold, the elastic gets caught on the wing tip producing error.
When you roll level the elastic becomes taught again giving a true bearing.

[EGBKFLYER]

I asked a similar question on dip in Tech Log a little while ago - will see if I can find it... Had some good explanations.

Found it - http://www.pprune.org/forums/showthread.php?t=234057 See especially post #4 from Dan Winterland - neat explanation that worked for me.

The search button is your friend, as Jerricho would say...

[JulieFlyGal]

To counter it, anticipate by lagging your roll out a bit . I'm sure someone more clever than I could work out how much lag would be necessary depending on the cosine of the angle of bank, but I was taught to use th TLAR method. (That Looks About Right). It is a function of the angle of bank, regardless of aircraft type.

Dan, when you say "to counter it, anticipate by lagging your roll out a bit", do you mean stop the turn and rollout before the needle reaches the inbound track, OR delay the rollout unti l the needle has gone past the inbound track?

[Dan Winterland]

Blimey, this is a blast from the past! I seem to remember first posting on this topic in 2001. Since then, I've moved on for instructing and flying classic jets to flying Airbus wonderjets where all you have to do to fly a NPA is press the Approach button and sit there arms folded while the kit does it all until MDA when you press the red button and land it.

But as far as I remember, at GA aircraft speeds of about 90 to 120 knots, I used to reach anticpate by about 2 degrees and for airliners about 5 degrees. This means you start to roll out when the needle has either 2 or 5 degrees before it reads the approach course.

But this theory relies heavily on the TLAR premise. (That Looks About Right). Hope this helps.

[ReverseFlight]

Has anyone mentioned ADF lag yet ? I fly GA aircraft at the speeds DanW mentioned and the ADF needle typically lags some 15 degrees behind the DG for it to be spot on coming out of a base turn into the final approach. If they were running neck to neck then you're undershooting.

[belowradar]

not read the instrument unless wings are level. You should be targetting a heading anyway.
not to read read any moving instrument.

Good advice Pilotbear - look at the needle and decide what heading you need to turn to then ignore the dip during the turn (if you add or subtract your drift to that heading even better).:ok:

[BigEndBob]

Every aircraft i have flown the adf reacts differently.
We had one warrior where the adf loop was mounted between wing and tail.
Never suffered any errors.

So why aren't all adf loops fitted on the rear fuselage?