Japanese sightings of U.S. Marine Corps PB4Y-1 Liberator reconnaissance planes sent to reconnoiter the area. Attack The three carrier task groups committed to Hailstone moved into position and began launching their first fighter sweep 90 minutes before daybreak on 17 February 1944. No Japanese air patrol was active at the time, as the IJN's 22nd and 26th Air Flotillas were enjoying shore leave after weeks on high alert following the Liberator sightings. Similarly problematic for the Japanese, radar on Truk was not capable of detecting low-flying planes — a weakness probably known and exploited by Allied intelligence organizations. Because of these factors, U.S. carrier aircraft achieved total surprise. Japanese pilots scrambled into their cockpits just minutes before TF 58 planes arrived over Eten, Param, Moen, and Dublon Islands. Though more than 300 Imperial Japanese Navy Air Service (IJNAS) and Imperial Japanese Army Air Service (IJAAS) planes were present at Truk on the first day of attacks, only about half of them were operational compared with over 500 operational aircraft among the carriers of TF 58. U.S. Navy fighter pilots in their Grumman F6F Hellcats, with the advantages of speed, altitude, armor, and surprise, achieved a one–sided victory against IJNAS pilots flying the outdated Mitsubishi A6M Zero. As many as 30 of the 80 Zeros sent up in response to the fighter sweep were shot down, compared with four Hellcats reported lost. Only token aerial resistance was encountered for the rest of the morning; almost no Japanese aircraft were present by the afternoon. Due to the lack of air cover or warning, many merchant ships were caught at anchor with only the islands' antiaircraft guns for defense against the U.S. carrier planes. Some vessels outside the lagoon already steaming towards Japan were attacked by U.S. submarines and sunk before they could make their escape. Still others, attempting to flee via the atoll's North Pass, were bottled up by aerial attack and by Admiral Spruance's surface force, Task Group 50.9, which circumnavigated Truk, bombarding shore positions and engaging enemy ships. Torpedo bomber and dive bomber squadrons from the carrier air groups (CAGs) were responsible for the bulk of the damage inflicted on Japanese ground facilities. Early on the first day of Hailstone, Grumman TBF Avenger torpedo bomber squadrons from Enterprises Carrier Air Group 10 (CAG-10) and Intrepids CAG-6 dropped fragmentation and incendiary bombs on runways at Eten Island and the seaplane base