Three years later, in 2012, now repaired La Triomphant, paid a friendly visit to Faslane Naval Base (HMNB Clyde), Scotland. Photo courtesy Dougie Coull Photography http://dougie-coull-photography.blogspot.com.au/2012/08/le-triomphant-s616-french-submarine.html
CNN Europe, February 16, 2009 report: " A British Royal Navy nuclear submarine and its French equivalent collided while on operations in the Atlantic Ocean earlier this month, defense ministries in Paris and London confirmed Monday. The French submarine Le Triomphant suffered damage to its sonar dome. Both vessels, HMS Vanguard and Le Triomphant, were armed with nuclear warheads and suffered damage but were able to return to port under their own power following the collision..." From TimesOnline, Feb 17, 2009 a submarine commander comments "The sea is not homogenous — you do get some funny effects. When you go above 23 knots, if the other guy is directly ahead you get a bow blanking effect: all his noise source is shrouded and you cannot hear him...The most dangerous thing you ever do is bring that submarine up from depth to periscope depth. The millionth percentage of chance is that someone else is going to be exactly where you are going to be."
CNN Europe, February 16, 2009 report: " A British Royal Navy nuclear submarine and its French equivalent collided while on operations in the Atlantic Ocean earlier this month, defense ministries in Paris and London confirmed Monday. The French submarine Le Triomphant suffered damage to its sonar dome. Both vessels, HMS Vanguard and Le Triomphant, were armed with nuclear warheads and suffered damage but were able to return to port under their own power following the collision..." From TimesOnline, Feb 17, 2009 a submarine commander comments "The sea is not homogenous — you do get some funny effects. When you go above 23 knots, if the other guy is directly ahead you get a bow blanking effect: all his noise source is shrouded and you cannot hear him...The most dangerous thing you ever do is bring that submarine up from depth to periscope depth. The millionth percentage of chance is that someone else is going to be exactly where you are going to be."
Comment
The near risk of disaster and political embarrassment alone would put the career of Britain's First Sea Lord and his French equivalent in doubt. This would also normally be career enders for the respective captains, watch keepers and sonarmen.
Autopilot on but sonars being left unmonitored would be a worrying scenario for multi-billion dollar vessels each with a nuclear reactor and 16 ballistic missiles. With each missile carrying 6 to 8 nuclear warheads.
Intentional ramming is out of the question. These vessels are very delicate, hugely expensive and submarines don't intentionally ram.
Intentional monitoring is also unlikely because these (SLBM) or SSBN subs are built to act as hidden, mobile, missile bases rather chase other subs (the job of attack/SSN subs).
Given the huge cost of Vanguard and Le Triomphant it is likely they both carry "black boxes" which will reveal what happened.
Possible Reasons for Collision
A more likely explanation for the collision is a combination of human error, mainly environmental causes and possibly technical malfunction.
Vanguard may have heard Triomphant and decided to make itself noisy (making its presence known) by blowing out water ballast. This would also move it upward to hopefully avoid collision. The surfacing sounds would have been suddenly loud and confusing for the French sonarmen. The motion of Vanguard may have pushed it upward and onto forward moving Triomphant which could not avoid collision in time.
Its likely that each then sounded active sonars or other noise makers to identify themselves and each other. Both then probably surfaced for any possible damage control, limited damage assessment and rather surprised communication.
It may be months until limited explanations are released.