The Norden bombsight consisted of two primary parts, the gyroscopic stabilization platform on the left side, and the mechanical calculator and sighting head on the right side. They were essentially separate instruments, connecting through the sighting prism. The sighting eyepiece was located in the middle, between the two, in a less than convenient location that required some dexterity to use.
Before use, the Norden’s stabilization platform had to be righted, as it slowly drifted over time and no longer kept the sight pointed vertically. Righting was accomplished through a time consuming process of comparing the platform’s attitude to small spirit levels seen through a glass window on the front of the stabilizer. In practice, this could take as long as eight and a half minutes. This problem was made worse by the fact that the platform’s range of motion was limited, and could be tumbled even by strong turbulence, requiring it to be reset again. This problem seriously upset the usefulness of the Norden, and led the RAF to reject it once they received examples in 1942. Some versions included a system that quickly righted the platform, but this “Automatic Gyro Leveling Device” proved to be a maintenance problem, and was removed from later examples.
Once the stabilizer was righted, the bombardier would then dial in the initial setup for altitude, speed, and direction. The prism would then be “clutched out” of the computer, allowing it to be moved rapidly to search for the target on the ground. Later Nordens were equipped with a reflector sight to aid in this step. Once the target was located the computer was clutched in and started moving the prism to follow the target. The bombardier would begin making adjustments to the aim. As all of the controls were located on the right, and had to be operated while sighting through the telescope, another problem with the Norden is that the bombardier could only adjust either the vertical or horizontal aim at a given time, his other arm was normally busy holding himself up above the telescope.
On top of the device, to the right of the sight, were two final controls. The first was the setting for “trail”, which was pre-set at the start of the mission for the type of bombs being used. The second was the “index window” which displayed the aim point in numerical form. The bombsight calculated the current aim point internally and displayed this as a sliding pointer on the index. The current sighting point, where the prism was aimed, was also displayed against the same scale. In operation, the sight would be set far in advance of the aim point, and as the bomber approached the target the sighting point indicator would slowly slide toward the aim point. When the two met, the bombs were automatically released. The aircraft was moving over 110 metres per second (350 ft/s), so even minor interruptions in timing could dramatically affect aim.
Early examples, and most used by the Navy, had an output that directly drove a Pilot Direction Indicator meter in the cockpit. This eliminated the need to manually signal the pilot, as well as eliminating the possibility of error.
In U.S. Army Air Forces use, the Norden bombsight was attached to its autopilot base, which was in turn connected with the aircraft’s autopilot. The Honeywell C-1 autopilot could be used as an autopilot by the flight crew during the journey to the target area through a control panel in the cockpit, but was more commonly used under direct command of the bombardier. The Norden’s box-like autopilot unit sat behind and below the sight and attached to it at a single rotating pivot. After control of the aircraft was passed to the bombardier during the bomb run, he would first rotate the entire Norden so the vertical line in the sight passed through the target. From that point on, the autopilot would attempt to guide the bomber so it followed the course of the bombsight, and pointed the heading to zero out the drift rate, fed to it through a coupling. As the aircraft turned onto the correct angle, a belt and pulley system rotated the sight back to match the changing heading. The autopilot was another reason for the Norden’s accuracy, as it ensured the aircraft quickly followed the correct course and kept it on that course much more accurately than the pilots could.
Later in the war, the Norden was combined with other systems to widen the conditions for successful bombing. Notable among these was the radar system called the H2X (Mickey), which were used directly with the Norden bombsight. The radar proved most accurate in coastal regions, as the water surface and the coastline produced a distinctive radar echo.
Since the Norden was considered a critical wartime instrument, bombardiers were required to take an oath during their training stating that they would defend its secret with their own life if necessary. In case the bomber plane should make an emergency landing on enemy territory, the bombardier would have to shoot the important parts of the Norden with a gun to disable it. As this method still would leave a nearly intact apparatus to the enemy, a thermite grenade was installed; the heat of the chemical reaction would melt the Norden into a lump of metal. The Douglas TBDDevastator torpedo bomber was originally equipped with flotation bags in the wings to aid the aircrew’s escape after ditching, but they were removed once the Pacific War began; this ensured that the aircraft would sink, taking the Norden with it.
After each completed mission, bomber crews left the aircraft with a bag which they deposited in a safe (“the Bomb Vault”). This secure facility (“the AFCE and Bombsight Shop”) was typically in one of the base’s Nissen hut (Quonset hut) support buildings. The Bombsight Shop was manned by enlisted men who were members of a Supply Depot Service Group (“Sub Depot”) attached to each USAAF bombardment group. These shops not only guarded the bombsights but performed critical maintenance on the Norden and related control equipment. This was probably the most technically skilled ground-echelon job, and certainly the most secret, of all the work performed by Sub Depot personnel. The non-commissioned officer in charge and his staff had to have a high aptitude for understanding and working with mechanical devices.
As the end of World War II neared, the bombsight was gradually downgraded in its secrecy; however, it was not until 1944 that the first public display of the instrument occurred.
