Weaponry

This is a short clip of a practice weapons attack. We are simulating attacking a target with a 500 / 1000 lb retarded bomb from a level flight attack. Retarded means the weapon after release from the aircraft is slowed (retarded) by a parachute so that the aircraft can move forward relative to it. Otherwise when the bomb hits the ground with the aircraft above it - it destroys the target and the releasing aircraft! We are actually dropping a small 3 kg bomb that contains a smoke and flash generator so that the range safety officer can spot and score it. To hit the target we have to fly at a precise height (150 ft above ground) at a precise speed (400 knots, 667 feet per second) and release the bomb at a precise range from the target whilst allowing for wind, temperature, aircraft mass / fuel weight and pressure.

What you can see in the clip is an old fashion sight that has 2 parts. If you watch the horizon during the clip you will just see 2 indistinct symbols. This is the fixed part of the sight. One symbol is a dot with a horizontal line through it (the longitudinal datum of the aircraft but often called the flight path bug - FPB) and below it an inverted T (the direction the gun is pointing - called the T). Because of the angle of attack of the wing the FPB is above the horizon and the T below. This relationship varies dependent on the weight of the aircraft and the speed but we can use it to hold the aircraft level once we are at 150 feet. You may notice it move up and down relative to the horizon as we use it to try and adjust our height (and also because of turbulence).

The second part is below the 'fixed part of the sight' - the reticule. We can adjust the reticule’s depression below the FPB. On this attack the Reticule is set at 101 milliradians below the the FPB. (One milliradian of arc subtends 1 unit at 1000 of those units i.e at 1000 yards the angle of 1 mil subtends a distance of 1 yard). Because we know the forward throw of the bomb (from trials) we simply use trigonometry to work out the angle below the FPB (allowing for the items previously mentioned) that marks the spot on the ground where the bomb will hit. Easy!

What you can see on the film is a practice attack at a coastal range called Donna Nook on the east coast of the UK. We fly the range pattern at 500 feet and what you will first see is the turn onto the target run and the descent to 150 feet. I then move the camera to film the sight. You will see the sight 'depress' soon after I move the camera to the sight. We keep the reticule high in the sight early to aid us line up the target. You may notice that the FPB and T are not directly above the reticule; this is because we have moved the reticule 12 mils right relative to the FPB. This actually points the nose of the aircraft into wind to the left (there is 5 knots of wind from the left) so that the aircraft crabs towards the target. Once we are happy with the line up we depress the sight to 101 mils. We then fly to the target ensuring our height and speed is correct. The bomb would be released at 38 seconds into the clip. If you frame by frame through the clip after that point you can make out the target as a black x with a circle of orange barrels around it. The circle is 150 foot diameter (300 radius) so the pipper (the centre of the reticule) is only inside the circle for less than .5 seconds. The target X is 20 foot across. So the pipper crosses it .03 of a second. We press a button (pickle) to release the bomb - this take a bit of practice and anticipation but the brain is an amazingly accurate computer. Once I'd sorted my students technique and accuracy out he was dropping the practice bombs within 30 foot of the target. You can't actually do much better with a computed sighting system.

You will notice that when the sight is depressed to 101 mils half the reticule disappears - this is because I am not holding the camera at my eyeline. I needed to see what my student was doing and therefore the camera is at the bottom of the design field of view of the sight - cutting off part of the reticule.