How did German submarines know a Sunderland was approaching?

Short answer

Through a radar warning receiver: the Metox, informally nicknamed the "Biscay Cross" for the look of its receiving antenna, was tuned to the frequency of the British ASV radar and gave submarines early warning that an aircraft was in the area. Sunderlands received the ASV Mark II radar from October 1941.

Published: November 4, 2026 · Updated: November 4, 2026 · Edited by Above Ten Thousand · Editorial policy

How did German submarines know a Sunderland was approaching?

Curtiss Model E Flying Boat (hull only)

The Metox and ASV radar

The Sunderland was heavily involved in the Allied effort to counter the threat posed by German U-boats during the Battle of the Atlantic. From October 1941 the aircraft received the ASV Mark II radar, for "Air to Surface Vessel." In response, German submarines began carrying a radar warning system formally known as "Metox" and informally as the "Biscay Cross," after the look of its receiving antenna, tuned to the ASV's frequency, which gave the submarines early warning that an aircraft was in the area.

The deception of August 1943

In August 1943, a captured RAF airman misled the Germans by telling them that aircraft homed in on the signals emitted by the Metox itself; as a result, U-boat commanders were instructed to switch it off. Centimetric radar was invisible to the Metox and initially baffled the Germans.

Sunderland attacks and combats

On 17 July 1940, an RAAF Sunderland from No. 10 Squadron achieved the type's first unassisted submarine sinking. The Germans are reputedly said to have nicknamed the Sunderland Fliegendes Stachelschwein, "flying porcupine," for its defensive firepower. On 3 April 1940, a Sunderland operating off Norway was attacked by six German Junkers Ju 88C fighters: it shot one down, damaged another badly enough that it withdrew and ended up making a forced landing, and drove off the rest.