As US forces prepared for Operation Olympic, the invasion of the Japanese home islands, the Japanese prepared for Ketsu-Go, their own defensive operation, which involved an obsolete yet deadly force:
By the time the Battle of Okinawa was winding down, “Ketsu-Go” was in full swing on Kyushu and a key part of the defense was the use of massed kamikaze attacks. But the state of Japanese aircraft industry was in disarray with the B-29 attacks and the ongoing ore shortages. In July 1945 to meet the required numbers of kamikaze aircraft, all of the training units were converted to kamikaze units which added thousands of experienced pilots but over 5,000 antiquated biplane trainers made of wood and fabric.
But again, at this point in the war, no one in Japan had realized that an elderly biplane trainer was a lot harder to spot on radar — plans at that point were to offset the slow performance of the biplane aircraft by shifting the kamikaze attacks to the night time, the traditional sanctuary period for the American fleet. However, somewhere in the Japanese command structure connected all the dots — on the night of 28 July 1945, the Fletcher-class destroyer USS Callaghan was on radar picket duty off the coast of Okinawa.
In a time before the advent of airborne early warning aircraft, radar picket destroyers patrolled the edges of the fleet to search for inbound kamikazes. On this night, an elderly biplane floatplane, most likely a Yokosuka K4Y1 trainer, was warded off on its first pass, but it came around undetected for a second pass and struck the destroyer, sinking it with the loss of 47 sailors. The following night, another elderly biplane struck another radar picket, the USS Cassin Young — though not sunk, 22 sailors were killed and the ship had to withdraw from action for repairs. A third destroyer, the USS Prichett, was aiding the stricken Callaghan, was very nearly sunk by another elderly biplane on a kamikaze mission.
The destroyers had difficulty on downing the attackers for three reasons — it was night, not the usual time kamikazes attacked, secondly, the wood and fabric biplanes were difficult to spot and track on radar, and lastly the wood and fabric construction threw off the proximity fuses of the anti-aircraft guns — the proximity fuse’s sensor that triggered the detonation of the round was optimized for metal aircraft; against the old wood and fabric biplanes, the proximity fuzes detonated the round too late, or in some cases, not at all.
The action that night against those three radar picket destroyers changed thinking on the role of the 5,000+ elderly biplane aircraft that were going to be used as kamikazes for “Ketsu-Go”. Here was an unexpected weapon that could counter the American technological advantages in radar and proximity-fuzed shells fired by anti-aircraft guns. American intelligence analysts had seen the massive change in the air forces of Japan in the summer of 1945 and were well aware of Japanese interests in wood, but it hadn’t occurred to the Navy that this was a possibly game-changing combination that would have threatened initial phases of Operation Olympic.
It was assumed that fuel shortages would keep most Japanese aircraft grounded and this misconception was reinforced by the increasing lack of air action against the B-29 raids and that US warships even managed to get close enough to the Home Islands to shell coastal targets without getting attacked. In fact, the Japanese had stockpiled fuel just for the use of the kamikazes in “Ketsu-Go”.