Finding the Height

After this warning the pilot would have to fly on his altimeter. Therein lay a new problem. The standard aneroid (pressure operated) altimeter was not sensitive enough to provide the degree of accuracy required at low level. In any case, ambient barometric pressure over the target would be impossible to predict.

The dropping trials of Upkeep, taking place in daylight, used a radio altimeter.

A pulse from the aircraft, bounced off the water below, was timed and translated into height. Ideal over open water for heights up to 150 feet, it was less reliable over the enclosed waters of a reservoir surrounded by hills. It also created a false sense of security, giving only an indication of the ground directly beneath the aircraft. The instrument would not detect any sudden rise in terrain height ahead before the aircraft flew into the ground. Flying a heavily laden aircraft low at night, the pilot needed to be looking outside the cockpit, not concentrating on his instruments.

There seemed no easy solution until the Royal Aircraft Establishment (RAE) at Farnborough produced a report dating from September 1941. This described optical methods of calibrating a conventional altimeter fitted to an aircraft flying over the sea at night. The first method used two lights fitted to the aircraft: one in the nose, the other in the tail, shining downward and angled so that their beams intersected beneath the aircraft at a given altitude.

A trial installation had been carried out using a Vickers Wellington. The front beam had been inclined slightly forward and to the left of vertical, the rear beam set to intersect this at a height of 200 feet. The point of intersection would be forward and to the left, visible to the pilot. With the latter being close to the light source, the front spot appeared to remain stationary. The rear beam spot appeared ahead of the fixed spot when the aircraft was too high, and behind it when too low, making corrections easy.

A different version of this method was developed by removing the front light and giving the pilot what was, in effect, a «ring and bead» sight fitted to the outside edge of his port window. At the correct height the spot from the rear lamp would be central in the sight. A further variation used a single spotlight mounted in the tail and a tubular, rearward angled sight looking downwards towards the point where the rear beam would strike the water when the aircraft was at the correct height.

Test flights of the twin spotlight method proved that it was possible, but impractical, for the pilot to observe the spots on the water. However, the lights could be seen from the bomb aimer’s position and instructions issued to the pilot via the intercom. There was no difficulty in determining whether the aircraft was above or below the correct height by both the position and brightness of the spots.

After a little practice it was found that it was possible to judge the height to about plus or minus five feet. The system worked well over water and even better over fairly level land. The single lamp and rearward sight method also worked, but suffered from the tubular sight having a restricted field of view.

The 1941 report also acknowledged the idea’s pedigree, dating back to the First World War. During this conflict a Captain Jenkins had developed an «Optical Night Height Altitude Indicator». A trial installation was fitted to a BE 2c and tested at Orfordness in August 1917. Jenkins’ system comprised two fixed four-volt lamps, spaced five feet apart fore and aft on a tubular frame mounted to the side of the aircraft. These projected a pair of marks onto the ground. A third adjustable light then was positioned to shine between the other two, its angle being read off. This angle could be converted to height using a set of tables. It was workable up to 500 feet; a twelve-volt version was planned for use up to 1,000 feet. In practice, the lamp spacing was too short to give great accuracy; the set up too cumbersome for service use.

The idea re-emerged briefly during the inter-war period as a possible night landing aid for flying boats. At about the time of the 1941 Wellington-based experiments it was also being considered for use by 44 Squadronc Handley Page Hampdens as an aid to minelaying (although it is unclear as to whether it was ever used for this purpose).

The idea was soon adapted for the Lancaster. On 4 April 1943, Squadron Leader Henry Maudslay flew one of 617 Squadron’s original standard aircraft from Scampton to Farnborough for a trial installation.

Two Signalling Lamps, Type B, were used. The front one was mounted in the bombing camera position, to port of the centreline and just forward of the bomb doors, directed 30 degrees to starboard of vertical.

The rear lamp was some forty feet to the rear, aft of the bomb bay, in a hole cut into the Lancaster’s redundant under turret blanking panel. The rear beam shone 40 degrees to starboard and 15.1 degrees forwards giving an intersection of the beams at 150 feet — the original intended height for the release of Upkeep.

With a beam spread of 6 degrees, at a height of 150 feet each spot was fifteen feet wide.

The spots could be observed by the navigator standing with his head in the observation blister on the starboard side of the cockpit canopy. As the aircraft descended the front one appeared to be stationary but the rear was either ahead if the aircraft was too high, or behind if too low. At the correct height the touching spots formed a figure «8».

The equipment installed, on 8 April Maudslay flew the aircraft back to Scampton. En route trial runs were flown over The Wash for three quarters of an hour shortly after sunset with conditions equating to those of a full moon.

The spots were switched on and could be seen from 500 feet from the starboard cockpit blister, the bomb aimer’s position and the wireless operator’s starboard window. Maudslay was guided by verbal instructions given by either his navigator or an RAE observer, who was also aboard. Neither had used the equipment before.

After a few minutes practice there was no difficulty in achieving runs to within plus or minus ten feet of the specified height, measured against a specially calibrated sensitive altimeter. As a further experiment, a hand held signalling lamp was shone downwards from the bomb aimer’s window and two other lamp fronts that spread the beam into a line were tried. Neither was deemed to be as good as a plain glass front.

With the installation proven, six sets of lights were to be made available at Scampton by 16 April to permit the squadron to train. A further twenty sets would be prepared by 25 April, ready for fitment to the modified, operational aircraft.

Installation and set-up took approximately two man-days per aircraft, involving a modified wiring circuit and switch for the lights, cutting the aperture for the rear light and mounting the lamps. With the latter installed, each aircraft was jacked up into flying attitude and the distance between the lamp centres marked off on the hangar floor.

From these points, the position on the hangar floor through which the beams should shine to intersect at the required height was calculated. The lamps were then switched on and adjusted until the centre of the each spot shone on the correct point on the floor. It was precise work, requiring accuracy of adjustment to 1/4 inch. The lamp brackets were then drilled and locking bolts fitted to fix the lamps firmly in position.

All seemed to be going well until Sir Arthur Harris, Commander-in-Chief of Bomber Command, heard of the proposed solution. Once again his mistrust of inventors came to a head. Annotating a Minute on 15 April 1943, he wrote tersely: «I will not have aircraft flying about with spotlights on in defended areas. Get some of these lunatics controlled — if possible, locked up.» As an afterthought he added: «Note — beams of spots will not work on water if glassy calm. Any fool knows that.»

After further discussion, Air Vice Marshal Sir Ralph Cochrane, AOC No.5 Group, contacted Ben Lockspeiser at the Ministry of Aircraft Production: Harris had a good deal of experience with this type of height finder (Harris had been involved with flying boats during 1932-33) and a surface of calm water did not reflect the light so that intersection of the beams took place some distance beneath the surface. Cochrane suggested that a check be made to see whether this was in fact the case.

Further trials by 617 Squadron duly proved Harris’s fears unfounded since most water had a slight «chop». Nevertheless, the crews too had concerns about flying in to attack a defended target at low level «lit up like a Christmas tree».

By 23 April, four of 617 Squadron’s aircraft had been fitted with the «spotlight altimeter calibrator» and ten crews had experienced its use. The following day Barnes Wallis summoned Wing Commander Gibson to Weybridge for a progress meeting. There Gibson was informed that trials had demonstrated that for Upkeep to work it must be released from a height of only sixty feet.

The Dambuster crews carried out training over The Wash and over the airfield, where observers with theodolites checked the aircraft height. By 1 May eighteen crews had practised with the system.

The equipment was simple to use and extremely effective. If they descended below the required height pilots were instructed to pull up immediately to a safe altitude before making another attempt. Training with the original standard aircraft continued at 150 feet, but the operational aircraft, adapted and equipped to carry Upkeep, were to have their lights fitted further forward in the bomb bay so that the travel of the spots at sixty feet was roughly the same as those in the training aircraft. This position also shaded the lamp and obviated a problem previously experienced of oil smearing the lamp glass. To reduce visibility from the ground, the forward lamp would be fitted with a projecting tubular shade.

The first of the Upkeep aircraft was fitted at Farnborough and returned to Scampton on 2 May. To help fit the remainder, two RAE technicians stood by, ready to travel to Scampton if required.

Progress was good. By 7 May eighteen operational aircraft had been fitted and their crews were proficient. Two more aircraft would be fitted, the final one when a new Lancaster was delivered to replace that damaged by Squadron Leader Maudslay who released an inert practice Upkeep from too low a height at Reculver on 12 May.

One final machine would be delivered to the Squadron on the afternoon of 16 May, the day of the operation, to be used as a reserve, but it arrived too late for the lights to be fitted. In the event this aircraft was taken on the raid by Flight Lieutenant McCarthy’s crew, who were detailed to attack the Sorpe. Fortunately the lights were not essential for their attack profile, releasing Upkeep from as low as possible without spinning.

Like the bomb sight, the spotlight altimeter employed basic trigonometric principles. Simple but effective pieces of equipment, both contributed in no small measure to the success of Operation Chastise.

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