A British TV documentary about warships at sea revealed, and the Royal Navy confirmed that a British anti-submarine frigate was operating in the North Atlantic during late 2020 when its towed sonar array collided with a Russian submarine the frigate was tracking. The submarine was catching up with the frigate when the collision occurred and, when it moved closer to the surface, it was running parallel with the frigate whose crew, and TV cameras could clearly see the sub’s periscope and communications mast above the surface. The periscope was to identify what the ship they were having problems with while the communications mast enabled the sub to radio its headquarters to report the situation. While the two masts were visible, they continued moving parallel with the frigate separated by less than a hundred meters. The sub soon submerged to a deeper depth and the frigate deployed a helicopter and used its onboard sonar to try and track the sub as it sought to escape from the frigate, which had to slow down to retrieve its damaged TSA (Towed Sonar Array). The TSA is the most effective submarine detection system on the sub and ship must slow down when using it, which is how a submerged sub was able to catch up and pass it.
The sonar array was damaged and had to be replaced. TSAs consist of a thick cable that is often several kilometers long. Along most of the cable, there are hydrophones to detect underwater noises. TSAs improve two aspects of onboard sonar. The TSA detects anything behind the sonar “blind spot” created by the ship propeller. The hydrophones are facing different directions and at the end of TSA there is a ROV (remotely operated vehicle) that can take the TSA down to a deeper depth, to measure thermal layers, which sonars often have problems with. TSAs are passive, they do not transmit a sound like an “active sonar” does to detect submarines farther away. Western warships often used their TSAs to seek out and track Russian submarines. The TSA is undetectable by a submarine but the ship using it has to slow down while the TSA is deployed. The sub apparently knew it was near a ship but did not know if it was military or commercial. The slow speed indicated a commercial vessel. The pictures and video of the two masts confirmed that the sub was Russian. The Royal Navy would not release any more information, such as the type of Russian sub involved. Western warships often track Russian subs via their TSA. Russian subs are still noisier than Western models and their sonars are not as capable. The Russian sub was unaware it had been detected and apparently closed in to get a better idea of what was in front of them. There was no comment from the Russians, which is normal.
Subs are easier to detect and more prone to collisions the closer they are to the surface. That is why the oldest adage for successful submariners is to “Run Silent, Run Deep”.
Some submarines also have TSAs, including a few of the more recent Russian boats. Sub TSAs enable the sub to improve the quality of information it has about nearby ships or subs. It’s not unusual for submerged subs to run afoul of fishing nets or surface ships as they surface, but accidentally making contact with a TSA is rare. In one case during the Cold War a British nuclear sub was equipped with a device that enabled it to cut and capture a TSA from a Russian ship. During the Cold War Western subs were often quite effective during espionage missions, like getting close enough to a Russian aircraft carrier to take photos of its bottom hull. This use of subs for “special missions” was kept secret until after the Cold War ended in 1991. Over the next decade, more details of these operations became public. While most collisions, or near misses, but surfacing subs can be avoided, detecting fishing nets or TSAs is currently much more difficult. There are procedures submarine officers are supposed to follow while surfacing but shortcuts are often taken and when that happens in an area known to have lots of surface activity, there are accidents.