ful aerial reconnaissance. As a result of following this policy there 

 occurred one interval of 11 days between flights, 2 intervals each 

 of 10 and 9 days, 5 intervals of 7 days and 2 intervals of 6 days. 

 The remaining intervals between flights were 4 days or less. 



On the basis of assumed average ground speeds of 100 and 150 

 knots respectively for the PBY-5A and PBIG aircraft the total 

 distance flown during the 1947 season amounted to 83,100 miles. 

 As no change has been made in the policy of usually laying out 

 parallel search courses 25 miles apart, the area covered is estimated 

 at a total of 2,077,500 square miles. As it is seldom that visibility 

 in the entire area covered by a flight is suflficient to assess the area 

 as 100 per cent swept, it is difl^cult to make a numerical estimate 

 of the total area swept by aerial reconnaissance during the season, 

 but it was probably in the neighborhood of one and a half million 

 square miles. 



The effectiveness of aircraft for ice observation is limited by 

 the low visibility conditions resulting from the fog and low stratus 

 which characterize the Grand Banks area, especially in the latter 

 half of the ice season, for even with airborne radar working at 

 peak performance, it is often impossible to investigate a radar 

 target and identify it visually without unsafe flying practices and 

 hazard of collision with a berg. Wooden fishing vessels and their 

 dories give a radar return which is indistinguishable from the 

 return from a berg with growlers around it. Airborne radar, 

 therefore, is a big help but, at least in its present stage of devel- 

 opment, it cannot be looked upon as permitting complete air cover- 

 age of an area during periods of low visibility. Aerial reconnais- 

 sance must be considered, therefore, as supplementing during pe- 

 riods of good visibility, rather than as replacing, surface-vessel 

 ice observation. 



During the first part of the season good visibility usually occurs 

 with sufficient frequency so that the progress of the developing 

 ice season can be followed with unbroken continuity with aerial 

 reconnaissance alone. This was true during the 1947 season until 

 the end of April. In fact, the great mobility of aircraft and the 

 comparatively vast areas which could be searched by the ice-patrol 

 planes during a succession of days of good visibility made it possible 

 in March to search the upstream areas to a sufficient distance so 

 that for the first time in the history of International Ice Patrol it 

 was considered safe to recommend on 14 March a shift of Canadian 

 traffic northward from the normal seasonal North Atlantic Track 

 Agreement track D to track E 28 days ahead of the date on which 

 track E usually becomes effective. Aerial reconnaissance, with con- 

 tinued good observing conditions, showed that the United States- 

 European track C was not menaced by ice as it usually is and made 

 possible a further recommendation on 3 April that United States- 



