Here's how to do it:
Fly a constant course and note whether the distance indication remains CONSTANT, is INCREASING or is DECREASING.
If the distance indication is CONSTANT then the relative bearing of the station is 90 degrees to the left or right of the aircraft.
If the distance indication is INCREASING then turn 180 degrees to a new heading which will then give a DECREASE in distance indication.
When a heading which gives a DECREASE in distance, note the RATE OF CLOSURE - (e.g., 5 miles in 2 mins. 10 secs.) - see Notes.
Turn 30 degrees LEFT and again note the RATE OF CLOSURE.
Then either:
If the RATE OF CLOSURE has INCREASED then turn another 30 degrees LEFT and note the new RATE OF CLOSURE. Now bracket the course which gives the highest RATE OF CLOSURE in order to find the course on which MAXIMUM RATE OF CLOSURE CAN BE OBTAINED.
or
If the RATE OF CLOSURE has decreased (after Step 6) then turn 60 degrees RIGHT and note the new RATE OF CLOSURE. Now bracket the new course in order to find the course on which MAXIMUM RATE OF CLOSURE can be obtained.
Having determined the course on which to fly, make frequent checks of the RATE OF CLOSURE. If the RATE OF CLOSURE remains CONSTANT maintain the heading until over the beacon - if the RATE OF CLOSURE decreases then make small alterations, e.g., 10 to 15 degrees to the heading in an endeavour to bracket the heading on which a constant RATE OF CLOSURE can be obtained.
If the track made good is not in fact the direct track to the beacon then, when abeam the beacon, i.e., when the distance indication ceases to decrease and then increases, turn 90 degrees to proceed to the beacon - if the turn is not in the correct sense then continue the turn another 180 degrees in the same direction as the original turn.
NOTES
(i) If on any course the rate of closure is such to indicate a closing speed approximately the same as the T.A.S. and provided that as the aircraft proceeds further the rate of closure remains relatively constant, it is advisable to hold the course being flown until either over the top or abeam the D.M.E. beacon.
(ii) If on any course the rate of closure is obviously very slow then do not wait to measure a fixed change in distance but turn 30 degrees left and proceed in accordance with Step 6.
(iii) When the rate of closure is being determined it is desirable to measure a precise distance between two calibration marks on the D.M.E. indicator and measure the time in minutes and seconds, and then compare the times taken for similar distances. If, however, the distance indication appears constant then the course should be held for, say, one minute and note made of the distance covered. When using the X10 scale it should be remembered that if the indicator is watched only for a matter of seconds a rate of closure of one mile per minute will not result in an obvious pointer movement. Pilots should guard against assuming a constant distance indication without observing the indicator for at least one minute.
(iv) During homing procedures, when within 10 miles of the D.M.E. beacon, it is suggested that the course being flown should be continued until either over the top or abeam the D.M.E. beacon. This will assist in orientation prior to commencing descent.
(v) When on a course that originally gave a reasonable rate of closure it is observed that the rate of closure is decreasing, then "bracketing" may be employed to find a more satisfactory course. When bracketing, if a turn of 15 degrees is made and it is in the wrong direction then a turn of at least 30 degrees in the opposite direction should be made. Turns of less than 10 degrees will not result in an obvious change of rate of closure.
(vi) It has been proved theoretically that in drift conditions it is possible to obtain the maximum rate of closure when flying on a course other then the course which gives a direct track towards the D.M.E. beacon. However, this very slight increase in rate of closure is not pronounced and as the aircraft proceeds further towards the beacon the rate of closure will decrease, whereas if the aeroplane is tracking towards the beacon the rate of closure will remain constant.
(vii) It will be found in practice that after the initial 30 degree turn, the problem of orientation becomes simply a matter of turning the aeroplane to a heading which gives a reasonable rate of closure and then bracketing, initially with turns of 15 degrees, to obtain the course which gives maximum rate of closure.
(viii) It is desirable, to assist in orientation, that all turns should be made to the LEFT unless it is apparent that such a turn is in the wrong sense. This procedure will prevent confusion when trying to remember the last direction turned.
The stuff of nightmares at 400kts..................
