Likely CAUSES of Indonesian Submarine Nanggala's Sinking

On April 21, 2021, the Indonesian Navy reported that its submarine KRI Nanggala (402)  disappeared in waters about 95km north of Bali.^[16] Nanggala had been conducting a torpedo drill, but failed to report its results as expected.^[17] The navy stated that Nanggala had requested permission to dive to perform firing of a SUT torpedo^[18] at 3am (Indonesian time) Wednesday April 21.  Around an hour after being given clearance, Nanggala  lost contact with surface personnel.^[20][21] The Indonesian Navy subsequently sent a distress call to the International Submarine Escape and Rescue Liaison Office to report Nanggala missing and presumably sunk.^[22] The Indonesian Navy stated it was possible that Nanggala experienced a power outage before falling to a depth of about 700m.^[20]

No usual radioing-in from Nanggala to Indonesian ships, naval bases or satellites or other distress signals (eg. flares) have been heard or seen. Based on a similar case, that of Argentine submarine ARA San Juan, in 2017, total loss of contact of a sub, usually indicates it has been lost with all hands (ie. no hope for Nanggala's 53 officers and crew). 

On April 23 it was reported that magnetic anomaly sensors had detected an object of "high magnetic force" floating at a depth between 50m and 100m near to where Nanggala was last seen. Hopes are that this is Nanggala.

KRI Nanggala's approximate last position. (Map courtesy Sydney Morning Herald)
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A much larger scale, locating KRI Nanggala's last known position, map (Courtesy BBC) 

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Around 7am Indonesian time, an aerial search revealed traces of oil spill on the surface of the water near the location where the submarine was believed to have dived.^[25][19] Oil slicks, if from Kri Nanggala, can indicate its diesel fuel tanks have ruptured under extreme crush depth water pressure - which may indicate KRI Nanggala was "lost with all hands" ie. sunk with the whole crew. The seabed is reported to be a submarine crushing 700m deep where Nanggala was lost.

Indonesia has requested assistance from the Singaporean and Australian Navy submarine rescue ships which can deploy deep diving rescue submersibles. Indonesia has an agreement with Singapore for submarine rescue. The Singaporean Navy's MV Swift Rescue has already departed Singapore's Changi naval base and can be seen on this marine satellite tracking site.

Significantly KRI Nanggala is 41 years old -  a German built TKMS (was HDW) Type 209 conventional diesel-electic submarine. It was commissioned into the Indonesian Navy in 1981 and its last reported major maintenance/overhaul was in South Korea, in 2011. 

A 41 year old submarine, perhaps with decades old torpedoes, can suffer many technical defects.

Kri Nanggala exterior and specs (Courtesy Indonesian Navy via the Straits  Times)
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5 POSSIBLE CAUSES FOR KRI NANGGALA's SINKING:

1. Human error and/or failure of torpedo tube equipment in the torpedo firing drill Nangalla intended to conduct. The main danger is to leave the tube's outer and inner doors OPEN simultaneously - thus allowing seawater to rush in. This quickly makes a sub "nose-heavy" with too much weight for Nanggala's usual buoyancy air-tanks to counteract. This would cause Nanggala to sink to around 300m "crush depth", and then imploding, totally destroying, Nanggala.

2. seawater leaks through the snorkel when sucking in surface air at shallow depth. In the  2017 (Argentine submarine ARA San Juan) lost submarine case seawater gushed through the snorkel down to the submarine's 100s tonnes of lead-acid Batteries causing a release of poisonous, highly flammable and explosive hydrogen gas. This sank San Juan. It is possible similar has happened to Indonesia's KRI Nanggala. 

See Pete's very early (proven correct) judgement that ARA San Juan was lost with all hands, just a few hours after its disappearance in 2017.

3. Deploying and/or dropping very old "live torpedoes" during test firing always carries risks that the torpedoes' highly flammable/explosive propellent manages to leak. Any sparks/electrical equipment can set the propellent on fire - leading to smoke/poison gas and ultimately catastrophic explosion. This is what happened to the sunk Russian submarine Kursk in the year 2000.

4. Submarines are normally decommissioned after 20 to 25 years. One major reason is too many "cycles" of immersion/contraction of their Pressure Hulls and cutting into the hull for engine maintainence accelerates Metal Fatigue. This makes a submarine unsafe to dive anywhere below about 200m deep. If Nanggala dived to (say) 250-300m its pressure hull may have "failed" ie. collapsed inward/imploded instantly destroying Nanggala.

5. The Indonesian Navy said an electrical failure may have occurred during the dive, causing the submarine to lose control and become unable to undertake emergency buoyancy procedures that would have allowed it to resurface. Still, the equipment to make a sub buoyant, allowing it to then resurface, can often be manually operated without electrical inputs needed. 

KRI Nanggala (see video here and above) if still intact, is running low on oxygen, due to run out 3am Satuday, April 24, morning.
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FINALLY DETERMINING WHAT HAPPENED

Sidescan sonar and deep sea manned or unmanned submersibles might locate Nanggala, or fragments of it, on the seafloor in the next 24 hours, or longer,

Failing that undersea sensors, belonging to the US Navy, that are nearby eg. about 100km  away from where KRI Nanggala was lost, will be able to detect any loud sounds, like an imploding submarine hull. Due to secrecy the US may need to claim that it was the loud sound/nuclear test sensors of the international civilian "Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO)"  that picked up Nanggala's last sounds (especially any metal pressure hull cracking implosion). 

There is a remote chance  Nanggala is intact and on a shallow seafloor, with most crew alive. If that were the case Nanggala's use of its loud-booming active sonar could be heard 100s kms away, by fixed undersea sensors or on surface ships, aircraft or on other submarines. But unfortunately it is unlikely any crew are alive to trigger active sonar pulses from KRI Nanggala. This is because the seabed is reported to be a submarine crushing 700+m deep where Nanggala was lost.

This is the international naval hymn for submariners to mark Nanggala's sinking.  

Peter Coates
Director, Submarine Matters (Australia-US)