IMC Confuser

[thing]

While I know that confusers can be wrong, can anyone work this one out?

"You are passing to the right of an NDB on a magnetic track of 170 with 4 degrees of starboard drift. What will the RBI read when exactly at right angles to the to the NDB?"

a. 274

b 284

c 094

d 084

The answer doesn't seem right to me. The answer apparently is 284. I would have said it was 274.

[Johnm]

Yet another reason to switch off NDBs and forget about all this drivel.

The ADF shows relative bearing. Relative to what I hear you ask, answer to the aircraft heading.

You are tracking 170 and facing 4 degrees of starboard drift so your heading needs to be 166

Your ADF will probably have 360 at the top. So lets suppose that you were heading due south in nil wind your heading would be 180 and as you passed the NDB the ADF would read 270. Then if you turn left onto 166, the ADF will move clockwise by the corresponding 14 degrees and read 284. Therefore as you pass at right angles on a heading of 166 you'll see 284 on the ADF.

Easy isn't it:ugh:

[RTN11]

This can be handy to try to figure it all out

VOR navigation system (http://www.pilotfriend.com/training/flight_training/nav/vor.htm)

And - agree with the above, switch the NDBs off now and save us all a load of time

[24Carrot]

For a fixed card RBI in zero wind, with 360 at the top, right angles to track would show as 270.

If you are laying off 4 degrees of right drift, you are heading 4 degrees left of track, so I think 274 should be on the card.

After a couple of glasses of wine, mind...

[thing]

Your ADF will probably have 360 at the top. So lets suppose that you were heading due south in nil wind your heading would be 180 and as you passed the NDB the ADF would read 270. Then if you turn left onto 166, the ADF will move clockwise by the corresponding 14 degrees and read 284. Therefore as you pass at right angles on a heading of 166 you'll see 284 on the ADF.


But it wouldn't be at right angles to the NDB in that case would it? Because your track is actually 170M. I give up, I hope there aren't too many questions about NDB. VOR I'm absolutely fine with.

[Big Pistons Forever]

While I know that confusers can be wrong, can anyone work this one out?

"You are passing to the right of an NDB on a magnetic track of 170 with 4 degrees of starboard drift. What will the RBI read when exactly at right angles to the to the NDB?"

a. 274

b 284

c 094

d 084

The answer doesn't seem right to me. The answer apparently is 284. I would have said it was 274.

At right angle means 90 deg. So 170 track + 90 deg = 260 plus 4 degrees for the left crab = 264 deg for the RBI to the right. For the other side it would be 170 - 90 = 80 - 4 deg for the crab = 076.

However In the real world the needle will be wiggling between 261 and 267 so with small crab angles like 4 deg when the needle point to the 90 deg chevron on the ADF indicator you are there.

Number of times I have actually had to use a calculation like this in the air since I got my MEIFR in 1987.........ZERO.

[Johnm]

But it wouldn't be at right angles to the NDB in that case would it? Because your track is actually 170M. I give up, I hope there aren't too many questions about NDB. VOR I'm absolutely fine with.

The plonkers who write the questions don't think the way you (or I) do when they say "at right angles" they really mean that your aeroplane is due West of the NDB. They expect you to do the calculation I did which actually does show the picture you would see in the cockpit when due west of the NDB on a heading of 166.

[GeeWhizz]

Maybe it's the way the question is worded. When at right angle to the NDB, is the important bit rather than when the NDB is at a right angle to the aeroplane..subtle. I understood it as when the aircraft was 270 from the NDB (a 'radial' if you like) but with a heading of 170 + 4 drift. That does equal 284 on the RBI.

Didn't have any questions on the NDB in my IMC exam. A few on ILS, but most on a route that had to be plotted and plogged exactly as per the Nav exam.

[jecuk]

Stupidly worded question. 'Right angle' is imprecise.

[thing]

Thanks for the replies chaps, glad it's not just me that's confused by the aptly named confuser.

[Johnm]

It is sad but true that in my experience of PPL, IMCR and IR the theory around navigation and flight planning is of absolutely no value to practical flying of light aircraft in either VFR or IFR conditions.

So it's vital to for newly qualified pilots to find a mentor or two, either informally through clubs and the like or formally through the AOPA scheme.

I'm more grateful than I can say for having had such help from members of the old European flyers group at Blackbushe.

[24Carrot]

Looking at it again this morning, I think:

a) 274 is correct for a fixed card RBI, at right angles to track, passing to the right of the NDB.

b) 284 is correct for a fixed card RBI if you are exactly West of the NDB, not at right angles to track.

c) 094 is correct for a fixed card RBI if you are passing to the left of the NDB, not to the right.

d) 084 is correct for a movable card RBI with the 170 track on top instead of the more sensible 166 heading that an RMI would have.

So I would still go with a), because it fits the question best.

[GeeWhizz]

Thre are two of us here that came up with the correct answer to the OPs question as per his book. Ok so the answer was given anyway and it's only my integrity that you may judge. Either way your working/answer was incorrect.

The original question and your Q31 are different in more than just numbers. And your method/answer for your example is correct. Have a read and fully understand what they each say before doing the working. Then have a look at the subtleties as presented in my previous post. If you still don't understand speak to a ground school instructor.

[FlyingKiwi_73]

I'm confused,.... i agree with BPF it should be 264?Mag heading 170 + 90 for the right angle + 4 for the drift.if it means you are Due West then its 274?? (which is not a right angle) in NZ we have a mg deviation.... 'east is least' is this why i'm 10 deg out from the answer.... could somebody do this S.L.O.W.L.Y.we didn't have to do alot of radio nav for our PPL's :-)

[GeeWhizz]

OK...

Draw a picture with an aeroplane exactly West of an NDB (aeroplane right angle to the NDB), on the 270 radial heading South, 180. The ADF needle (RBI) is pointing left at 270 on the compass card because ADF needles always point at the NDB. A fixed card ADF will always have 360 (North) at the top, and to be honest I usually leave my movable card ADF set to North too unless its needed for something specific.

Now turn the aeroplane to head 170 i.e. 10 degrees to the left. The RBI needle appears to move up toward the top of the compass card onto to 280. That is to say the needle rises. In fact the needle has remained constant relative to the compass card and NDB, and it's the aeroplane that has moved.

Now for the drift. 4 degrees of starboard drift means you turn 4 degrees into wind i.e. 4 degrees left in this case. So the aeroplane is now heading 166.

If the aeroplane is heading 166 on the 270 radial from an NDB, the RBI needle will point a further 4 degrees toward the top of the compass card onto 284.

All of the numbers are magnetic as the heading was given in magnetic and nav aids (VOR/NDB/RBI/RMI/HSI) are all magnetic too. There is no mag deviation, variation, or any other 'ation' in this question therefore assume that it doesnt require any working out other than the above.

These are all subtly different and will therefore change the answer:

- aeroplane at a right angle to the NDB (NDB radial 270 + track 170 + drift 4 = heading 166 = RBI 284)

- NDB at a right angle to the aeroplane (on the wingtip (i.e. heading 166 = NDB Radial 256 = RBI 270)

- NDB at a right angle to track (170 = RBI 270 = NDB radial 260 (+ 4 drift = hdg 166 = RBI 274))

It is a poorly worded question by all accounts and I would agree with BPF if the NDB was at right angles to the aeroplane. It's a very subtle difference and there are no questions this finnicky in the IMCr exam thankfully!:D

Hope that clears it up.

GW

[24Carrot]

And peace broke out!

Is this a first for PPrune?:)

This post got mis-placed too. Is there something wrong with the PPrune clock?

[24Carrot]

I still think, with the NDB at right angles to the aircraft track, the RBI needle points at 274.

Incidentally, I find it helps to imagine the RBI mounted horizontally so that the RBI card "rotates underneath the fixed needle". After all, the needle always points at the NDB, only you and the aircraft are rotating.

Take the no-wind case first.

360 on the RBI represents straight ahead, which will be in the track direction (no wind).
The NDB is on the left, and at right angles to the track, so it is on the wingtip, the needle points to it and is on 270.

We don't need to know the track to work this out. The 170 info is a red herring.

Now rotate the aircraft 4 degrees to the left because of the wind correction angle.
The NDB is in the same place, the aircraft is in the same place, the track has not changed, all we did was rotate the card 4 degrees under the needle. The needle now points at 274.

We still didn't need to know the track!

[GeeWhizz]

We don't need to know the track to work this out. The 170 info is a red herring.

A fundamental flaw. A track of 170 in nil wind = heading. Heading is integral to the operation, interpretation and use of an NDB/ADF combination.

Giving up now :ugh:

[24Carrot]

A fundamental flaw.

I don't think so. This particular question is constructed so you don't need to know the track.

From the question details, specifically the starboard drift, we know the heading is 4 degrees left of track.

Separately, we are told that the NDB is 90 degrees left of track.

RBI measures the Relative Bearing, ie the difference between NDB direction and Heading. Both are stated as offsets to a track, so the actual track is irrelevant.

For the mathematicians:
RBI = (Track+270) - (Track-4) = 274.

I also dug out my 2006 5th edition of the IMC Confuser which has a similar question, but a different conclusion.

Q31: When maintaining a mag track 330, you experience 8 degs of starboard drift. What will the RBI show when passing to the left of an NDB when it is exactly at right angles to the track? A=074, B=090, C=082, D=098
The correct answer is D, according to the book.

Applying my "track free" method:

With no wind, and the NDB at right angles to the right this time, the RBI shows 090. With 8 degs of starboard drift we have rotated the aircraft 8 degs to the left, so the RBI shows 098.

Which matches the answer in the 2006 Confuser, and we didn't need to know the track, because of the way this particular question type is constructed.

[24Carrot]

Gee Whiz, your latest post appeared out of sequence for some reason.

And I am certainly not questioning your integrity. Surely there is a right answer and we can arrive at it through rational thought?

I still get 274 even if I do use the track.

I think we agree the track is 170, it is given in the question.

In the specific case where the NDB is at right angles to the track, and on the left, the NDB bearing from the aircraft is 170-90 = 80. In your working you say the NDB "radial" is 260, which is the reciprocal of the bearing from the aircraft, ie 80+180 = 260 so I think we agree on this also.

The aircraft heading is 166 (170 track - 4 wca), I believe we agree on this also.

Arithmetically, the RBI is NDB bearing minus the heading.

RBI = 80 - 166 = -86.

This is a negative number, so we add 360 to get 274.


edited to add:
PS If it helps, I agree that the wording of the original question was not very clear. I also agree with your results for the first two interpretations you looked at. I only disagree with the final "right angles to track" case where you got 264, and I get 274.

[GeeWhizz]

I only disagree with the final "right angles to track" case where you got 264, and I get 274.

Ahhhh I see; you're absolutely correct. I'll go edit that. 274 would make much more sense! :ouch:

Memo to self: don't involve oneself with silly numbers at silly-o'clock in the morning! :(