Instructions for firings against aircrafts

The complex question of the organization and the employ of the a/a units was fully defined with the Инструкция за артилерийските ПА взводове, картечни постове и команди пехота по отбраната срещу въздушен противник (Instructions for the artillery a/a sections, machine guns posts and infantry commands on the defence against air enemy) published on 14 December 1915. The aim of the text was to fix a uniform firing method, to arrange the observation and the reconnaissance, to provide the knowledge of the appearance of the Bulgarian aircrafts, and to favour the cooperation between the a/a artillery and the air force. The Instructions were constantly updated, with additions containing corrections and improvements, according with the innovations introduced by the German Army.

At first the fire against air targets was regarded like the fire against land targets moving quickly. The usual procedure in this case was to obtain a 100 m bracket, and to shoot the target with a progressive fire, beginning with the sight at the low bracket, if the target was moving back, or at 100 m less than this limit, if it was moving forward. Since at that time aircrafts flew at a speed of 120 km – 150 km/h, and the flying time of a common field artillery gun with a muzzle velocity of 500 m/s was 10’ for a distance of 5000 m, the Instructions recommended to fire about 330 m before the target. Nevertheless this could hardly be achieved, so as a rule the fire against air targets should be carried out at a given distance, with firing data fixed before (barrage fire). The projectiles employed were H.E. shells with time fuze. The a/a units were supplied with weapons and ammunition by the Inspection of the Artillery, according with a shift set for all the artillery units of the Field Army.

After the return of the officers, who had attended a course in Germany, the Bulgarian a/a artillery begun to employ the firing methods established by the German Army :

– direct fire (директна стрелба), that could be carried out in three different ways :

a) free fire (свободна стрелба) : the gun commander used only the firing data, without any special device for firing against air targets. He estimated the actual distance of the aircraft by means of a rangefinder, the altitude by means of an abacus with graduated ruler fixed at the right side of the rangefinder, the angle of sight and the actual azimuth, then he obtained the “future” datas, determinig directly the movement of the aircraft by tables and the survey of measures made during an exact lapse of time;

b) fire with tables (стрелба с таблици) : the gun commander estimated the distance, the altitude, the actual speed and the flight angle of the aircraft, then he obtained the firing data deductively by means of tables which divided the sky in sectors with a fixed altitude, that is 2000m, 2500m, 3000m and 3500m. These tables had been obtained firing a great number of testing shots with a 77mm field gun 96 n/a arranged to fire against aircrafts, and were truly effective only with this kind of gun.

c) fire with flight predictor (стрелба с летимер) : the gun commander obtained all the firing data by means of the “Auswanderungsmesser (Am.) Peres”, developed by the firm Karl Zeiss (Jena) in 1916. This device was able to measure a horizontal, straight line and the continuous moves of the target, combining an altimeter, a rangefinder and a stopwatch. The rangefinder measured the distance of the target, while the altimeter gave its altitude. A telescope with a grid inside was connected with it. During the flight the aircraft, at first in the middle of the grid, was gradually moving its position away from the centre. With the help of a special stopwatch the “Auswanderungsmesser” was able to provide a real time prediction of calculated inclination to the target and the required data for time of flight for the fuze setter.

– indirect fire (индиректна стрелба) : the gun was aimed directly at the point where the aircraft was coming thanks to the “Kommandotafel Jacob”, that gave the azimuth, the declination of the barrel and the setting of the fuze; the elevation angle was obtained with the range dial and the lateral deflection with the firing table;

– barrage fire (преграден огън) : it was used to fire at night against targets that could not be seen. It required the development of a rigorous firing plan, assigning to every gun a sector to be fired on and the height of bursting of the shells.

As for the observation and the reconnaissance, the Instructions ordered that every battery and section established observation posts, with a specially trained observer equipped with binoculars. He should carefully watch the skyline, trying to locate the arrival of enemy aircrafts also by means of the noise of their engines. A net of observation posts was established around Sofia, all the post being connected with the post office of the capital. It received reports about air raids also from the frontier towns, and forwarded them to the a/a sections assigned to the air defence.

In order to favour the knowledge and the cooperation with the Bulgaria air force, sketches, showing the identification marks of the Bulgarian and allied aircrafts were attached to the Instructions. The a/a artillery had to shot at every aircraft that lacked such identification marks.

For his part, the Bulgarian air force should inform the a/a units about all the flights of friend aircrafts. If the flight of an aircraft had not been reported, to avoid hitting it by mistake, the heads of the airports should call it back, before it entered into the firing area. The Bulgarian aircrafts were signalled also by green and red rockets.