Normally, battery power is only good for a up to twelve hours of operation and such subs spend most of their time propelled by their diesel engines. The diesels have to be run while the sub is surfaced, or semi-submerged (using a schnorkling device first widely used by the Germans during World War II.) The schnorkel device (a venting system attached to a reinforced periscope type mast) allowed a sub to run at periscope depth using the diesels. But the schnorkel vents automatically closed anytime the waves got too high, thus causing the diesel to shut down, and the air pressure in the sub to suddenly change (causing ears to pop). Sailors hated it, but the alternative was running on the surface and risking an attack from allied aircraft. Today, such patrol aircraft have radars that can pick up a schnorkel mast, although other ships have a harder time spotting the mast than they would the surfaced sub.
The Ming class subs are Chinese built, but based on the German Type XXI subs produced at the end of World War II. The Russians were so impressed by the few Type XXIs they captured, that they copied the design and produced the Romeo class of subs. The Mings are copies of the Romeos. The Mings are 249 feet long and displace 1600 tons when on the surface. The Chinese had problems building the Mings and did not get them into service until the late 1970s. It is believed that China is still building Mings, with 21 delivered so far. Number 361 entered service in 1995. China has had a terrible track record building submarines, and those it has built it uses very carefully, and infrequently. But lately, diesel-electric subs have been sent on more frequent patrols, especially in the waters off Taiwan. The crews appear to be poorly trained and American ships and subs that have encountered Mings have not been impressed with the Chinese boats. However, in the hands of a good captain and crew, at least a few Mings have proved capable of sneaking around quietly. In theory, a diesel-electric sub, when moving slowly underwater (when it uses its batteries) can be very difficult to detect.
Initially, all sorts of theories emerged to explain the sudden death of the crew. One possibility tossed about was that 361 may have been equipped with an Air Independent Propulsion system that was being tested on the surface, had an accident, and released a large amount of noxious fumes that killed 70 people. If such a system was being tested, it could be done more safely with the sub cruising on the surface, but with all hatches closed. Normally, when doing any kind of experimental testing, the hatches between the various compartments in a sub would be closed. But discipline in the Chinese navy tends to be sloppy, and the compartment hatches could have been left open simply because it was more convenient. It is known that at least one of the latest Mings built had an Air Independent Propulsion system. Such a system allows a sub to operate for extended periods under water.
The cause of such massive deaths was also thought to have been something more prosaic, like a battery malfunction (which can produce large quantities of poisonous chlorine gas). Another possibility was massive damage to a torpedo, sending large quantities of poisonous gas through the sub.
But in this case, the most likely culprit appears to be the diesel engine and the control systems that, normally, insure that the engine is shut down when the exhaust and air intake vents are closed before diving. Since the sub was still on the surface when the dead crew was found, an examination of the diesel engine and related systems should indicate if this was the cause of the calamity.
Last week, China announced that there had been a "mechanical malfunction" aboard one of its Type 035 Ming class diesel electric submarines (identified as sub number 361). They reported that 70 sailors had died and that the sub was towed back to it's base. This information confused more than clarified. The Ming class boats have a crew of 55, not 70, and it would be unusual for everyone on board to be killed while the sub was on the surface. Chinese navy officials, also perplexed about the accident, speculated that the mechanical problem was the failure of diesel engines to shut down as the sub prepared to dive. The navy reported that all the sailors appeared to have died at their duty stations, and within a few minutes of each other. This would be the result of such a diesel engine accident, which has been known to happen in the past. But such an incident is rare, as it requires a poorly built or maintained diesel system on board, and a poorly trained and supervised engineering crew to allow something like this to get out of hand. A subs diesel engines require a large amount of air to operate, which is why they are only used on the surface. When preparing to dive, the vents that allow air in, and diesel exhaust out, are closed. If you keep the diesels running after these vents are closed, you quickly suck all the oxygen out of the sub, killing all on board.