As the training flights grew in frequency and duration, so did the complaints from locals, who thought the bomber crews were just out enjoying themselves. Section Officer Fay Gillon , a WAAF Intelligence Officer attached to 617 squadron, was given the task of alerting local authorities in advance of such flights. This quickly proved an impossible task, as crews were allowed to decide their own routes, and invariably ‘forgot’ to inform her. Also, well planned and scheduled missions were delayed or cancelled, through weather and technical issues, at the last minute. However local anger was nothing! compared to the incandescent rage of Flt Lt Joe McCarthy when, as he thundered across Lincolnshire, only forty feet above the ground, another Lancaster passed BENEITH him. “Low flying is one thing but this was absolute,bloody lunacy”, he told Gibson. In fact such incidents became so frequent that Gibson initiated a fund into which any crew flouting the rules, to the point of endangering their own or another aircraft, would have to contribute……..the fund grew steadily. As the training progressed, the strengths and weaknesses of crews began to emerge. Throughout April a number of crews were returned to their old units and new crews brought in and Gibson replaced his own bomb aimer….No 617 Squadron was taking shape. At the start of April the unit had received more specific instructions. Barnes Wallis had been working on the best height from which to effectively drop ‘Upkeep’, this he calculated to be 150 feet. However the instrumentation in the Lancaster was not sensitive enough to gauge such a low level accurately. A device using 2 Aldis signalling lamps was developed from an idea tried earlier in the war. One lamp was fitted in the front camera aperture, pointing downwards. The second was placed just behind the bomb bay, angled forward. Both lights were angled slightly to starboard so as to be clearly visible from the cockpit. The 2 beams would bisect 150 feet below the aircraft, enabling the Lancaster crew to stay at the correct height for delivery. On April 4th, Maudsley flew W4926 AJ – Z , down to the Royal Aeronautical Establishment at Farnborough to have the device fitted. He carried out the first successful trial over the sea a few days later. He then demonstrated it to the other crews with a series of runs up and down the aerodrome. Within days the squadrons electricians, led by Sgt Bill Cammack, had fitted it to W4921 AJ – C, W4940 AJ – B, and ED756 AJ – H. While 617 sq continued to train, tests continued with the weapon. On almost every drop the casing broke or was so badly damaged it severely affected it’s run, in other cases it simply hit the water and sank. Wallis quickly realised the problem lay in the dropping height, after some re-calculation he concluded this had to be reduced to just 60 feet…..alarmingly, less than the wingspan of a Lancaster, any miscalculation by a pilot and disaster was unavoidable. Gibson promised Wallis that his boys would give it ‘a damn good try’ however, to AVM Cochrane he made a chilling prediction, “ This Squadron will either make history or it will be completely wiped out ”………. The spotlight altimeters were adjusted to the new height and on April 25th Bill Astell took W4940 AJ - B to Wainfleet.He made several low level passes at a height of 60 feet, dropping 12 bombs, the first time such a low bomb drop was recorded, other crews soon followed.