 As the infantry moved to the attack contact @-@ patrol aircraft flew low overhead , two being maintained over each corps during the day . The observers were easily able to plot the positions of experienced troops , who lit flares and waved anything to attract attention . Some troops , poorly trained and inexperienced , failed to co @-@ operate , fearing exposure to the Germans so aircraft flew dangerously low to identify them , four being shot down in consequence . Although air observation was not as vital to German operations because of their control of commanding ground , the speed by which reports from air observation could be delivered , made it a most valuable form of liaison between the front line and higher commanders . German infantry proved as reluctant to reveal themselves as the British so German flyers also had to make visual identifications . Reports and maps were dropped at divisional headquarters and corps report centres , allowing the progress of the infantry to be followed . During the pause on the ridge crest , an observer reported that the Oosttaverne line was barely occupied , at 2 : 00 p.m. a balloon observer reported a heavy German barrage on the II Anzac Corps front and a counter @-@ attack patrol aircraft reported German infantry advancing either side of Messines . The German counter @-@ attack was " crushed " by artillery fire by 2 : 30 p.m. Each corps squadron kept an aircraft on counter @-@ attack patrol all day , to call for barrage fire if German troops were seen in the open but the speed of the British advance resulted in few German counter @-@ attacks . Artillery observers watched for German gunfire and made 398 zone calls but only 165 managed to have German guns engaged . The observers regulated the bombardment of the Oosttaverne line and the artillery of VIII Corps to the north of the attack , which was able to enfilade German artillery opposite X Corps . 
