One of the lesser known modifications to the Hudson was the fitment of a magnetometer boom. Adastra's Works Manager, Jack McDonald, wrote a letter to the Senior Aircraft Surveyor of the Department of Civil Aviation on 20 March 1962 seeking advice on the installation of a magnetometer boom on the Hudson. This letter describes the construction of the boom, which was 16 feet long, but its method of attachment to the aircraft was still under development. The aircraft file in the National Archives also includes a page of hand-written notes on weight and balance calculations for the boom.

Subsequently, the boom installation was photographed fitted to VH-AGS (see below). The exact date of this photograph is unknown, but at the time of the fitment, VH-AGS was still carrying Westralian Aerial Surveys titles, which it did until late 1962. It will be noted that VH-AGS is also carrying a towed bird installation which is known to have been fitted around May 1962. This would date the photograph in the latter half of 1962. A report in the Journal of the Aviation Historical Society of Australia (Vol 3 No 7, July 1962) states that the boom was painted "bright red". This may have been so but the photograph below indicates that at least two colours were applied.

Hudson VH-AGS with the boom fitted. (Photo: Peter Ricketts)

Although it has been reported that a Hudson never flew with the boom installation, Ted McKenzie, then Chief Pilot with Adastra, recalls that he test flew Hudson VH-AGS with the boom fitted. (Unfortunately, Ted's log books from this period in 1962 are unavailable):

"There were several tests of the installation in the latter half of 1962 and I'm pretty sure I was the only one to fly the aircraft with the boom. The test flying was preceeded by a run along the east-west runway at Sydney. I was the only one on board and the idea was to get the tail up and check for any abnormalities up to take off speed. I remember I was just starting to enjoy the run to the west when I realised how close the end of the runway was. I hit the brakes and managed to stop right on the piano keys. The second test was a full blown air test. As I recall, I had John Thorpe, Ted Barden and another DCA officer on board (see below). The aircraft flew normally and we did quite a bit of turning, pitching and yawing without remarking any unusual effects. Why it was never pursued I really don't know, but it could have been difficult to compensate, especially with a tailwheel aircraft. There was also the question of airspeed v. response time with the magnetometer of the day but that's speculation on my part."

In April 2005, the previously unidentified DCA officer from the 1962 test flight turned up in our Guestbook and identified himself as John Fincher. John recalls the flight as follows:

"In mid-1962, I was transferred to NSW Region of DCA as Airworthiness Aircraft Performance Engineer. One of the first projects I was involved with was a flight test on Hudson VH-AGS fitted with a magnetometer boom. This was carried out in August 1962. The flight crew consisted of John Thorpe, Ted Barden and myself representing DCA, and with Ted McKenzie as the pilot. The flight handling characteristics were virtually unchanged and the aircraft was 'bounced around' a little to satisfy us that the boom did not significantly move in flight. To this end, the floor hatch was removed and a rearward-facing mirror attached so that someone lying on the floor behind the hatch could look rearward by the mirror and view the afterpart of the boom. This task was 'allotted' to my colleague Ted Barden who, in order to more or less attach himself to the aircraft, had his feet wrapped around some webbing attached to the rear bulkhead. I remember after one particularly energetic switchback manoeuvre there came a large bang from the rear of the aircraft. I was sitting in the right hand pilot seat next to Ted McKenzie and remember us both looking at each other in horror as we thought something had broken. It was however no more than Ted Barden rising from the floor, hitting the ceiling and then bouncing back onto the floor again. That was the only flight test that we carried out at that time on this particular aircraft and I don't know what became of the installation after that time, however, the date of August 1962 is correct as my period of tenure in NSW Region was for twelve months only from July 1962 to July 1963."

THE HUDSON MAGNETOMETER BOOM - A REPRISE

The following table summarises another series of magnetometer test flights by Ted McKenzie in Hudson VH-AGE during September and October 1964. All flights involved magnetometer tests, although the flights on 26 & 27 September also involved Doppler tests. Ted is unsure if these flights were with a boom magnetometer or a towed bird. However, DCA files in the National Archives of Australia record that Hudson VH-AGE was test-flown on 2nd October 1964 with a "Boom Support Modification". The 1962 photo of VH-AGS fitted with the boom confirms that VH-AGS was fitted with both the boom and a towed bird, so conceivably VH-AGE could have been tested with both also. On the other hand, DCA records state that VH-AGE was air-tested on 3 June 1965 with a towed bird, from which it could be inferred that VH-AGE was not previously fitted with the towed bird system. This may also date the removal of the boom installation. Ted McKenzie subsequently flew VH-AGE on an aeromagnetic survey out of Maralinga in January 1965 at which time the only magnetometer fitted to the aeroplane was a towed bird.

Why the magnetometer boom should have been tested on two different Hudsons two years apart and apparently not used operationally on either occasion remains a total mystery.

This possible explanation has been provided by Doug Morrison, a specialist and historian on aeromagnetic survey operations.

"Although those Adastra personnel who could possibly provide the answer why the magnetometer booms were mounted on Hudsons in both 1962 and 1964 aren't with us now, it was more than likely to see whether suitable magnetic data could be obtained. I suspect it turned out to be very unsuitable and the location probably made it impossible to compensate for the changes in the magnetic field from moving aircraft control surfaces (just too much movement going on near the sensor). It is possible that there may have been an attempt to get a 'gradiometer' system operational - such magnetic detection systems were becoming popular in the late 1960s. The idea was to measure the differences between the readings of two magnetometers mounted either horizontally (wingtips) or vertically with one mounted in the stinger and one towed below in a bird. The difficulty with such systems was that the sensor closer to the aircraft was generally noisier, certainly in the 1960s but not so much nowadays with modern computer controlled active magnetic compensation. Both horizontal and vertical gradient techniques are still used but it is not common. Aero Service Corp solved the problem by towing two birds thus removing the interference from the aircraft although problems were still encounted if the birds didn't fly properly e.g., bird swing (wandering left to right)."