The minimum range of early airborne search radars was too long to allow their use to guide a night attack; the ASV (air-to-surface vessel) Mk. II set was only effective between 1 and 36 miles. Dropping flares gave a submarine sufficient warning to escape before the aircraft could attack.
Squadron Leader Humphrey de Verde Leigh, together with the firm of Savage & Parsons, developed a 24-inch, 22 million candlepower searchlight that could be switched on just before losing the target on radar, giving the submarine only 15 to 20 seconds to evade attack. Initial testing began in March 1941, but operational aircraft equipped with Leigh Lights did not enter service until mid-1942 because the Air Ministry initially preferred the Turbinlite, an existing but less powerful (and less effective) airborne searchlight.
Leigh Light–equipped aircraft over the Bay of Biscay greatly increased their contact rate but sank only two submarines during 1942, largely because the U-boats began carrying radar-warning receptors. Nevertheless, the ease with which the Leigh Light–equipped aircraft intercepted submarines led Admiral Karl Dönitz to order U-boats in transit to remain submerged even at night, greatly reducing their operational effectiveness. Moreover, the combination of Leigh Lights and centimetric radars, which entered service in 1943, subsequently proved deadly to Germany's U-boats, most notably during the Biscay Offensive that summer.
Paul E. Fontenoy
Franks, Norman L. R. Conflict over the Bay. London: William Kimber, 1986.; Franks, Norman L. R., and Eric Zimmerman. U-Boat versus Aircraft. London: Grub Street, 1999.; King, H. F. Armament of British Aircraft. London: G. P. Putnam, 1971.