Most torpedoes of the WWII era had limited on-board guidance. They used a gyroscope to keep them aimed down a particular path. If this failed, then the torpedo would constantly adjust it's course, causing a circular run. There were several possible causes for such a failure. In March 1942, HMS Trinidad was hit by a circular running torpedo while escorting a convoy to the Soviet Union. This circular run was likely caused by the extreme cold preventing the gyro operating correctly. With the Mark 14 used by USN subs, poor manufacturing quality meant that the gyros frequently failed to work correctly. Flooding of the gyros and faulty steering mechanisms could also be at fault. The Germans were the first to introduce a sub-launched acoustic torpedo. This would home in on the noise of a ship's propellers. However, if the launching sub was making more noise than the target, a circular run would ensue. Modern torpedoes have several protections against circular runs. Most are wire-guided from the launching craft, giving them sufficient control to avoid a circular run. They also have built-in anti circular run devices, which detect if the torpedo is on a circular path. These do rely on having a separate gyro, which can fail, but this is a much lesser risk.