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3 Answers

Confused about relative magnetic bearing

Asked by: 943 views Flight Instructor, General Aviation, Private Pilot, Student Pilot

I have the following question which I cannot answer and I do not understand, even though I know the right answer:

An aircraft is tracking away from an NDB maintaining a track of 300 with 5 of starboard drift. What bearing should the relative bearing indicator (RBI) indicate?

The correct answer is "185 relative", but I absolutely don't get it. Can anyone please explain it to me? Thanks!

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3 Answers

  1. Russ Roslewski on Aug 26, 2016

    Wow, I didn’t think anybody learned about NDBs anymore!

    I’ll give it a try.

    Assume first, NO WIND. In this case, the aircraft track is equal to the aircraft heading (the nose is pointed where you’re going).

    So if the aircraft is tracking directly away from the station in a NO WIND situation, then the station is directly behind the aircraft, right? Makes sense, you’re flying away from it, it’s behind you. If the station is right behind you, then the “relative” bearing (“relative to the front of the airplane”) is 180. Relative bearings are always measured with the nose of the airplane being at 0 and increasing clockwise as viewed from above.

    If, instead, there IS WIND, and it is blowing you to the right as in the example, you must correct for the drift by pointing the nose slightly to the left. In the question you gave, this amount is 5 degrees. This will keep your TRACK headed straight away from the station, though the nose is a little bit off to the side, so the HEADING is different. If the nose is a little left of the track, then where is the station? It’s no longer perfectly behind you. Rather, it is now a little bit left of directly behind you – in other words, a little bit closer to the left wing.

    If directly behind you is 180, then a little bit toward the left wing is a little bit more than that, or 185 in this situation.

    Notice that the actual track (given as 300) is extraneous information in this question.

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  2. sixzero on Aug 27, 2016

    This isn’t an answer, more of a follow-up question – I assumed the difference in the reciprocal (185 rather than being 125) was due to the question being based on the fixed compass card, but it seems there’s some missing info, i.e. a pictoral representation of the aircraft, the RBI, and where the needle is plus the relative location of the NDB to the acft. I need this picture for it to make any sense! This question and answer confuses me as well.
    I would say that if I am flying a 300 after station passage I should see 120 on my needle, unless it was on the fixed compass… which I will never ever fly with. I remember reading once that flying the fixed compass card and interpolating its values is cruel and unusual punishment. They do not even teach that thing in the IE course at Rucker which is taught by Vietnam UH1 pilots. So my question is – using the RBI fixed card, your airplane icon is always pointed at 360, and you have to go off of the mag to see actual track/hdg and then do the math from what the NDB needle is pointing at to get yourself situated?

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  3. Russ Roslewski on Aug 29, 2016

    The answer to the “relative bearing” asked in the original question does not depend on whether there is a fixed compass card on the ADF indicator or a rotating one. It’s irrelevant. “Relative Bearing” is measured with respect to the nose of the aircraft at all times. Heading is also irrelevant.

    By way of explanation, your right wing (assuming a standard, straight-winged training airplane) is always at a 090 relative bearing to the nose. Always. Doesn’t matter which way you’re pointed or what equipment you have on board. If your right wing is pointing at the NDB station, then it’s at a 90 degree relative bearing to you. To keep it at that 90 degree relative bearing you’d have to fly a circle around it. Your heading would be constantly changing, and the _magnetic_ bearing to the station would be constantly changing, but the _relative_ bearing stays exactly the same, 90 degrees.

    Incidentally, I actually flew in an aircraft with a fixed ADF card yesterday!

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