Cheers,The Secret of 617 Squadron’s High Standard of Practise Bombing – the S.A.B.S. Pilot/Navigator/Air Bomber Team (By Flt Lt Hay)
The excellent results gained by crews of 617 Squadron using the S.A.B.S. have only been achieved by the fullest, most practical use of the ‘bombing team’. Before any bombs are dropped, some 4 hours training on the specially adapted A.M.B.T. are carried out by the pilot and air bomber to give manipulation practise to the latter and to familiarise the pilot with the B.D.I. (Bombing Direction Indicator). The navigator is trained to carry out computation of true height and airspeed, and settings for a given course of attack with the instruments and computers at his disposal. Some 2-4 hours are then spent in the air doing ‘dummy runs’, firstly on objects ‘on track’, then choosing targets and ‘turning on’, and finally on to targets and setting up sight in accordance with settings computed from known navigational data. The sight is only accurate when correct height above target is set. Thus the pilot must fly at the indicated height he states he will be at, the navigator must correctly compute this to the true height above target, and the air bomber set this accurately.
True height is dependent upon:
i. Sea Level Pressure at Target. This is gained by setting the aerodrome height for practise bombing or, operationally, from Form 2330. It is the Q.F.F. which is set.
ii. Indicated Height Above Sea Level. All 617 Squadron altimeters have been accurately calibrated for every 1,000’ for 140 and 180 m.p.h. I.A.S. and from the appropriate card the navigator allows for this error, which may be up to 300’.
iii. Temperature. ‘Thermometer, Air, Direct Reading, Mk I’, now fitted, doesn’t give an accurate reading for temperature of the outside air as a) the stem is heated by cabin temperature and b) the outside air bearing against the bulb is under pressure varying with airspeed. Both factors tend to give a ‘warmer’ reading than true. Again, the navigator computes from a special computer to get an accurate air temperature.
With these factors and the use of an ICAN computer, true height (c) can be computed. True height above target will need a deduction of target height above sea level and there will be a further allowance (addition) to be made where stick bombing is being used.
When the pilot advises I.A.S. with aircraft trimmed and bomb doors open, the navigator computes a T.A.S. which is used against a Trail scale for the appropriate bomb number. Errors of + or – 5 m.p.h. make negligible ground errors. Most errors in range can be traced to (1) flying at other than the indicated height stated by the pilot, (2) incorrect computation of height and/or T.A.S. by the navigator or (3) incorrect settings or bad manipulation by the air bomber.
The sight will automatically correct for drift and ground speed if switched on with the target in the graticule and held there by proper manipulation of the sight by the air bomber, and by the pilot following the direction of his B.D.I., until such time as the point of release is attained. Those who remember the A.B.S. and who used it to its utmost efficiency realised that the length of run could be considerably reduced if settings were applied before the run. As the heading of attack is generally known, the navigator can pass a drift and G/S to the air bomber just prior to the run. So, after practise, the air bomber need only wind his sighting head back for a 25 second run, whereas 40-50 seconds may be required without these settings.
Let us listen to a typical 617 Squadron bombing run:
Pilot: “Turning onto a heading of 250°.”
Nav: “250° - drift 4° port, G/S setting 17.”
Air bomber: Acknowledges, directs pilot on and calls “Run started” at the appropriate moment.
After 30 seconds of concentration, but silence, by pilot and air bomber, we hear:
Air bomber: “Bombs gone. Good run here, drift 3° port, G/S 16.5.”
Once the bomb hits the ground:
Air bomber: “Bomb plotted, ten yards overshoot.”
Pilot: “Sorry, my fault, I was 120’ too high!!”
When results are received from the Range, the team assembles about the plotting table to further sort out factors causing any errors.
Richard
