ADF outbound - needle always swings towards the tail?
 

I was in the SIM today with my instructor, finishing some IF time for my CPL, when he mentioned that no matter what, the ADF needle always swings towards the tail the further outbound you are.

Now I know my Pythagoras, so the only way the ADF needle would move further towards the tail the further out you got is if you were the same distance off track the entire way (same as the 1-in-60 rule).

For example, if you are tracking 090 and laying off 10 degrees of drift on a heading of 100, then unless the wind changes or you steer off track, the angle between your heading and the NDB will remain the same the entire way - 10 degrees (right) off the tail. It doesn't slowly starting moving left as you get further out . . .

If you move off track, however, the deflection will obviously be larger the closer you are to the aid, and lessen as you move away from it.

My instructor said this was true in theory, but not in practice: even if you are 100% on track and maintain the same heading to offset the same wind (as in my example above), the needle will still slowly centre itself towards the tail.

I can't see how this is possible, and haven't been able to find any reference. The only thing I can think of is that, practically speaking, you are never going to be exactly on track, and so yes: the further out from the NDB you are, the less needle deflection will be for any given track error (one degree for every mile off track for every 60 miles outbound).

Am I missing something? He says pilots learn this when they do their IFR.

Why bother with all that ?


You are the needle, when using an RMI simply look at the tail of the needle to see what bearing you are on.


That's all you need.

I probably won't explain this well but "Live on the tail". When tracking to or from (ignoring layoff for wind):

If the tail is at the bottom of the card and its to the left of bottom, turn to the right.

If the tail is at the top of the card and it's to the left of top turn, to the right.

The head always falls, the tail always rises - Only when travelling in a straight line.

In theory, unless you pass directly over the station and continue outbound in a straight line, you'll never have the needle point to the tail. It's like the old analogy of halving the distance to your destination then travelling it. You travel half way, then halve the remaining distance, then travel that distance, then halve it again. You'll get closer, but never get there.

In practice, the accuracy and errors of the NDB, ADF and turbulence will make the theory imeasurable.

Bringing wind into the situation simply means that the whole head falling and tail rising rule is offset by the drift angle. Still, you'll never achieve a true 'needle on the tail' condition (+-drift angle) unless you have either flown over the station or changed from a straight path to conduct an intercept.

What's an ADF? :}

The first time honeymoon trick..:E

ADF= Australian Drug Foundation

As stated before:

You are on the tail of the needle (pointy bit points at the NDB, most of the time).
The tail always rises (when you are in a straight line)

Look at where you want to be. If it is right of where you are (the tail) turn right. If it is left, turn left. Works whether you are outbound or inbound.

To intercept a track, both "where you want to be" and the tail of the needle must be on the same side of the fore-aft line, with the tail below where you want to be. (If the needles are split either side, you will never get there.) Maintain a straight line, and the tail will rise to intercept.

Remember to allow for drift when tracking.

If you pass a station (not overhead) and take up a heading of, say, 090, you will never get onto the 090 track from the station in nil wind. The tail will get closer and closer to 090, but never make it, UNLESS you get blown onto the track or you change heading towards the track.

A lack of simplicity in what were taught, and how we see it is exactly why everyone screws up the NDB in flight and in tests.

Many instructors don't know, so I can't imagine how the students are supposed to.

Correct, most of the time.

Even more correct!

In general, the head of the needle will tend to move from top to bottom. In general, you will not know precisely whether you're on track or not - but you should generally have a fair idea of a heading which will reduce the track error, such as it is.

I second the remark above, that drift angle is a fixed correction to apply. With wind, the tail will not end up at twelve o'clock.

Actually, I think it is a result of focussing too much on tolerances and not enough on technique. Spend most of the time looking at the AH, some reasonable time stopping trends on the Alt or ASI or DI, and adjust heading in the correct direction to reduce the track error. That will take care of about 90% of flight test fails, right there.

God fancy trying to do an NDB let down with a fixed card on one donk (twin) hand flying at night in rain..........Christ am lucky to still be above ground!:E
The old basic trick to regain Trk fly toward the head of the needle inbound & away from the tail outbound. Twin Loc's, to think we actually did them in anger for real!...scarey stuff:-)
Thank God for that trestle bridge Nth of En....:E
At least the 5 Deg's wasn't gunna kill ya if ya went 6+ deg's off track momentarily as NDB App's are surveyed out way beyond that.

Wmk2

And that's where the debate began . . . Because in the SIM it's possible to measure the theory pretty accurately, as wind is constant, there is no turbulence and you have almost perfect "flying" conditions.

On an outbound track of 145, on a heading of 165, offsetting 20 degrees of drift, my instructor says: "You'll notice the needle (which at this stage is 20 degrees right) will tend to fall towards the tail (swing towards 180) the further outbound you are".

My question was: "Why? The needle should remain in the same position."

On a constant track, on a constant heading, with no change in wind, the angle should remain the same no matter how far out you are.

But I accept it's not a perfect world, and so obviously what he was trying to explain was that the further out you are, the less the needle deflects for each mile off track.

Thanks for all the explanations, but I still think the theory is correct - if not in practice.

Unfortunately, nearly all the old planes we fly have fixed-card ADFs and not RMIs.

Well then, that's easy. In the artificial world of a simulator set to "easy" (!!), your instructor was wrong. As you were in fact measurably and precisely on track, the needle would not have moved.

The rest of us were talking real world... But I appreciate you have to get the basics right first, before tackling real world vagaries.

Yes, that's what the argument boiled down to: theory versus reality. Problem was, he was applying his real-world experience to the theory and we had the SIM set up for simple ADF exercises which involved no change of wind or any other nasties. In which case, the theory should have been correct.

(I will add, I already have my NVFR with an ADF endorsement, so I do know the basics.)

My instructor and I get on pretty well - and he does know his stuff (maybe doesn't communicate it as clearly as I'd like at times) - but we do tend to have a few of these "idealogical debates" . . . which aren't always conducive to learning how to fly when it happens in the middle of exercises! We have a good laugh about it when we get back on the ground . . or out of the SIM, as it were in this case. :}

"(On any intercept)the needle always moves to the back of the aiecraft". Nick Tanner, ACST 1970 RIP Nick, best piece of advice I ever had.

Regarding the above scenario of 165 HDG and 145 TRK:

If you nail the heading and no other variables change (wind etc) you will remain perfectly on track with the needle 20 degrees off the tail until you lose NDB coverage.

The needle will NOT drift closer to the tail as distance increases. What does happen, is that the ADF needle becomes less sensitive with increased distance, causing less deflection when the variables DO change.

That's exactly what I thought. Thanks.

My simple ADF rule which I was taught is;

The head of the needle always points to where the track lies. To get back on track turn towards the head of the needle.

Easy!!!!!!!!!

'V3' yr instructor was correct with regards to yr first couple of lines in yr post.
"5-in-50" said ti well but here's anther way of looking at it.
Look at it this way. You pass abeam a station by say a mile or so, it's irrelevant anyway the distance. Seeing as yr still close in notice that the needle head drops fairly quickly at first towards the bottom of the compass card. But as you get further & further away from the station that rate of change (towards the bottom of the card) slows, that's providing nothing else changes in a windless perfect tracking environment. Look at it like the spokes on a wagon wheel they are close together at the hub (the radio beacon) & way out near the outer ring of that wagon wheel (a theoretical never ending ring limited only to radio range)the spokes are a long way apart so it takes longer for the needle to drop or transit between spokes so to speak assuming each spoke is one degree for Eg.


That's the way I see it & most likely yr instructor

Wmk2