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Thread: QDM (Magnetic heading) v Compass heading

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    Default QDM (Magnetic heading) v Compass heading

    I'd appreciate some help in respect of an accident in Iceland during March 1945.

    A Hudson returning to Reykjavik from a routine met reconnaissance sortie was considerably off course when it encountered extensive low cloud, base 500 ft, over the coast of southwestern Iceland. The aircraft's altitude at the time was 1200 ft. Flying Control passed a number of QDMs including one of 012 degrees seconds before the aircraft flew into high ground about 13 nm from the airfield.

    Working from Google Earth Pro the compass bearing from the crash location to Reykjavik is 345 degrees.

    I understand there is a considerable magnetic variation at Reykjavik, but could anyone advise if these two bearings are compatible. To my untutored eye they seem too much at variance.

    Perhaps I should add the QDM comes from the Squadron's ORB as the F1180 gives no numerical values to its references to QDMs. No ranges were passed, just headings.

    Brian

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    Default Re: QDM (Magnetic heading) v Compass heading

    Brian: Reykjavik at 64.12 degrees N and 21.86 degrees West. This website for March 15, 1945 provides an estimate of -27.3 degrees for magnetic declination.

    https://www.geomag.nrcan.gc.ca/calc/...e_direction=-1

    So a heading of 345 degrees True (NNW) would equate to 345+27-360 or a compass heading of 12 degrees, i.e. NNE. Does my logic make sense?

    "D: magnetic declination, defined as the angle between true north (geographic north) and the magnetic north (the horizontal component of the field). D is positive eastward of true North." This makes sense as Magnetic North is over Canada, pulling the compass bearing considerably "West" in the vicinity of Reykjavik that far north. In order to correct, you would have to steer considerably East, so a compass heading of 12 degrees, QDM makes sense.

    Here are a couple of other considerations. 1) the nature of the metals on the aircraft will affect the compass. A compass card was used to correct for these but I have not been able to determine how this was corrected in calculations. 2) there would undoubtedly be local magnetic anomalies that would affect compasses. These may not have been properly charted in WWII. Edit: update. Charted magnetic anomalies in the region of the crash site:

    https://notendur.hi.is/leo/pdf/J67p43-49LKGJ.pdf


    NB: I have found the estimates for magnetic declination from this Gov't of Canada model provide results within a degree or so of those indicated on Bomber Command navigation charts.

    Jim
    Last edited by JDCAVE; 1st September 2021 at 03:34. Reason: Clarification and checking my logic!

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    Default Re: QDM (Magnetic heading) v Compass heading

    Brian: could you please provide the coordinates for the airfield as well as the crash site? I can provide more information.

    Jim

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    Default Re: QDM (Magnetic heading) v Compass heading

    Firstly, Jim, many thanks for your comprehensive response. If I've done it correctly I think the coordinates for Reykjavik are 64.12818, -21.93718, and for the crash site 63.91177, -21.81773; I've estimated the latter from the only information I have of 63d 55m N, 21d 48m W but, as ever, I'm open to correction.

    Brian

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    Default Re: QDM (Magnetic heading) v Compass heading

    And when you hear the angels tapping out their faultless morse , you’ll know then that your QDM was a reciprocal course

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    Default Re: QDM (Magnetic heading) v Compass heading

    Brian: I have run it through my spreadsheet calculator and have the aircraft with a bearing to the airport of 346 degrees and 13 nm away so you are close. This website also provides a calculator. As the a/c was so close to base, relative bearings would change very quickly with time and distance.

    https://www.movable-type.co.uk/scripts/latlong.html

    Did the base provide a QDM, by way of HFDF ("HUFFDUFF")? This is my introduction to a circumstance like this.

    Jim

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    Default Re: QDM (Magnetic heading) v Compass heading

    I honestly don't know, Jim. As I explained in my first post the little information I have largely comes from the F1180, although the QDM I quoted comes from the ORB. The Reykjavik Flying Control - or should I say the Flying Control Officer - did not cover itself with glory as only bearings were broadcast, but not ranges. Unfortunately that was just one of many errors.

    Brian

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    Default Re: QDM (Magnetic heading) v Compass heading

    Good morning gentlemen

    Please allow me to clarify what is meant by 'Compass Heading' if I may. I am an ex Navigator of forty years' experience and an ATPL Instructor in General Navigation.
    True heading: The direction the aircraft is 'pointing in relation to True North (i.e. the North Geographic Pole - the northernmost point of the axis around which the Earth rotates.
    Magnetic heading (not QDM!): The direction the aircraft is pointing with reference to Magnetic North (i.e. the Magnetic North Pole). The problem with the Mag North Pole is it goes on walkabout, very roughly circulating in a random pattern the Geographic North Pole once every 960 years or so. The difference between True N and Mag N is called declination, or more commonly, Variation, being measured East or West. If Mag N is to the west of True N at a given position, it is referred to as Variation West, and vice versa. A useful poem is "Variation West, Magnetic Best (Hdg M is greater than Hdg T), Variation East, Magnetic Least (Hdg M is less than Hdg T). For example, True Hdg 053, Variation 15W, Hdg M will be 053 plus 15 = 068 degrees Magnetic.
    Now we come to Compass Heading; Every individual aircraft has its own magnetic field, caused largely by all the electrical wiring and the ferrous materials of the aircraft itself.. This is known as "Deviation" and is the direction that the aircraft's compass actually points. The amount of deviation is calculated during a 'Compass Swing' where some Deviation can be eliminated by adjusting the aircraft's magnetic compass. Any remaining Deviation is noted and written on a 'Compass Card' and placed in the cockpit. The amount of Deviation is dependant on the aircraft's Magnetic Heading, but i have never seen a value of more than 4 degrees. This card will tell the pilot what Hdg C to fly to maintain a Hdg M. Another poem: " Deviation West, Compass Best" (i.e. Compass North is to the West of Magnetic North, therefore Hdg C will be greater than Hdg M) and "Deviation East, Compass Least". For example if a pilot wishes to fly a Hdg M of 242, he will refer to the Compass Card to check the Deviation on that Heading, let's say 2 West. So he must fly a Compass Heading of (242 + 2) = 244 degrees C.
    So to convert from Hdg T to Hdg Compass we use the saying Cadburys Dairy Milk is Very Tasty, i.e. Compass - Deviation - Magnetic - Variation - True.
    So sadly, without the Compass Card for your aircraft, the Compass Heading would be impossible to determine.
    Now QDM is the Magnetic Great Circle Track (or Course) along the surface of the earth between the aircraft and a radio station. Unless the wind is calm or the wind is directly ahead or behind, there will be an element known as "Drift".
    An aircraft will fly along a track by applying the correct amount of drift and maintaining a Heading. QDM is NOT a Heading!
    I hope that helps a bit chaps, please contact me if you need more help!
    Best wishes
    Matt

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    Default Re: QDM (Magnetic heading) v Compass heading

    Thanks Matt: Hopefully my response, post #2 wasn't too far off from your response. I believe Brian and I were referring only to what we know from the data. We can calculate the "True Bearing" from the crash site to the airport, based on the information at hand and we do know, from the ORB that "This A/C did not return from this Sortie and was last in touch with F/C (flight control) at 08.55 when it broke off in the middle of making for a QDM. The A/C was then flying at 1200 ft. and a QDM obtained was 012 [degrees], which placed the A/C in the hills south of the airdrome or beyond them. The weather on the south coast was very poor and the cloud base was 500 ft. The Control Lone Scheme was in force." We determined that the base was 345 degrees True from the Crash Site. So we were trying to determine the magnetic course to steer if the magnetic variation for the location was 27 degrees. The term "QDM" is new to me, but from this reference:
    https://en.wikipedia.org/wiki/Aeronautical_Code_signals
    seems to be Magnetic bearing from the aircraft to the station.

    I agree. This does not take into account any calculation pertaining to Compass Deviation. Sadly, I have found it poorly documented in my search for how it was documented in practice in Bomber Command. For example, I have navigators logs for some operations and while there is a column for Compass Deviation in the calculations, it is not filled out.

    Jim
    Last edited by JDCAVE; 2nd September 2021 at 15:54.

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    Default Re: QDM (Magnetic heading) v Compass heading

    Thank you Matt for your comprehensive reply. I'm afraid it was my ignorance and naivety which led me to introduce 'heading' into my first post - I won't do it again.

    Likewise, Jim, thank you again for your valuable replies..

    As background my interest in this particular accident was prompted by a Met Air Observer, now 96, who flew with 251 Squadron at Reykjavik and had flown with the crew not long before the accident, asking if the cause was ever established as he had been unable to find the F1180. In the event there was a definite error in the handwritten F1180 in that the location of the crash was given as 63d 35m N, 21d 49m W, which would have placed it off the southwest coast of Iceland. The '35m' was possibly a typo since the actual crash site was 63d 55m N, 21d 48m W (https://www.stridsminjar.is/en/a-lis...catid=0&id=128). It was finding that the compass bearing from this spot to Reykjavik differed considerably from the QDM quoted in the ORB that made me wonder if this was another error.

    Brian

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