Realistically, practically, we're talking about literal milliseconds - mercifully likely quicker than the mind could process the beginning of a thought, much less to formulate the conclusion that there was a problem and the realization of the implications of that.
Catastrophic failure of the carbon fiber pressure chamber would have been "instantaneous". And it was an
implosion, not an
explosion, and the implications for human bodies at those depths and those pressures are such that should in the unlikely event any remains could be recovered, you would be talking about nothing more than tissue samples.
No good aspect for the victims except to say they never knew it when it happened.
My wife and I had the same simultaneous response when a member of the press asked a Coast Guard admiral about efforts to recover the remains of the passenger and crew:
“What remains?”
The admiral was much more tactful and evasive, starting into a discussion of the extreme environment and nature of the event and then veering off into more or less “I don’t know about that yet.”.
In aviation, by regulation after a certain number of flight hours, air frames are required to be x-rayed for microscopic stress fractures then fluxed if needed or scrapped/decertified if not possible.
Regulations, in the form of Airworthiness Directives (ADs) only require inspections of spars or critical structures in circumstances where there is reason to believe that the structure is at risk of failure to to stress or fatigue cracks. They are usually the result of cracks or failures being found once the aircraft has been in service for awhile.
They often are initiated as a Service Bulletin issued by the manufacturer or current type certificate holder. If the FAA sees it as a safety issue, it may follow with with its own AD.
For example, there were recently two rudder failures on PA-20 aircraft where the top portion of the rudder folded over due to rust in the rudder post. The PA-20 is a 1950s era aircraft. The two in question had long histories of being operated on floats, where they were frequently exposed to spray, etc. Both aircraft also had rotating beacons mounted ion the rudders, which both increases stress on the rudder, and provides a potential entry point for moisture.
A few more rudders were found that had been removed due to corrosion issues. The manufacturer issued a service bullet requiring replacement of the rudder with a new one with a thicker rudder post.
SBs are only required for aircraft operated under part 135 (unscheduled charter operations) or part 121 (scheduled air carrier operations). They are not required for part 91 operations.
The FAA may or may not at some point publish an AD. It probably won’t as neither of these rudder failures resulted in a loss of control or any injuries or deaths, and they occurred only on float planes, and only on float planes with rotating beacons on the rudder.
That same rudder assembly is also used on a wide range of Piper aircraft including the Tri-Pacer, Super Cub, Pawnee, etc. Consequently, the FAA could also expand the scope of the AD if it published one, but it has to balance the cost involved (around $$3000 per aircraft) plus the limited availability of new rudders for combined fleets of thousands of aircraft, with the extremely low probability of the possible fault causing death or injury (very rare occurrence and when it occurs it has not rendered the aircraft uncontrollable and caused a crash).
It’s also more practical and arguably just as safe to have the AP/IA performing the required annual or 100 hour inspections just push and pull on the top of the rudder to ensure it doesn’t bend. For tailwheel aircraft like the PA-20, Pawnee and Super Cub the push pull test is also something that the pilot can check during pre flight (provided it’s not on floats).
The FAA could also issue an AD but only make it applicable to aircraft that have or have had float fittings attached and thus that may have been operated as float planes, and or to aircraft with rotating beacons on the rudder.
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Those SBs and ADs specify a means of compliance. An AD, as a form or regulation does out for public comment in a Notice of Proposed Rule Making Process (NPRM) required by the Administrative Procedures Act. During that public comment alternate means of compliance can be be proposed and approved by the FAA in the final rule.
Mandatory inspections after so many hours in service, or on some sort of weighted scale based on different types of operations might be required by the manufacturer based on how the structure was originally engineered and an estimated life span for the structure. Similarly, they can be developed or revised in conjunction with the manufacturer.
This AD is an example of one that imposes various spar cap limits ranging from 13,300 hours for early radial engine Air Tractor 400 aircraft with progressively lower limits for later higher performing 401 and 401B aircraft as well as the even later turbine powered 402 series aircraft where the spar cap life is as short as 2000 hours before mandatory inspections must be made at intervals of 600-1000 flight hours based on model and serial number, with a multiplier for aircraft with winglets.
https://www.federalregister.gov/documents/2008/12/10/E8-29165/airworthiness-directives-air-tractor-inc-models-at-400-at-401-at-401b-at-402-at-402a-and-at-402b
Using carbon fiber materials, which are strong for the weight but can be called 'brittle', one has to wonder if previous trips stressed the materials that resulted in the company pushing their luck one time too many. Given there was no accepted safety standards ever certified for the vessel there are hard questions to be answered and liabilities explored.
This is one of the safety issues raised by a previously fired employee, his director of operations and safety. Apparently Rush rejected the recommendation by that employee (who was one pr those non inspiring 50 year old white guys he preferred not to hire) to have the 5” thick carbon fiber pressure hull non destructively inspected to ensure there were no voids in the layup of the structure.
The concern was that even a very small void will increase in size with each pressure cycle, until it becomes large enough to cause a catastrophic failure.
In this case the pressure cycles in the prior dives to various depths including two all the way to the Titanic in 2021 and 2022 most likely caused a propagation of a small void to the point of causing a failure on this last dive.
In a just world, Stockton Rush would have been struck by a small stream of water just long enough before the hull imploded for him (but no one else) to recognize what was about to happen and realize he’d just killed himself and the other 4 people on board.
There’s also a second reason why this is the first manned submersible to use a composite hull. Way back in the day when I was involved in public safety diving and also worked part time for a local dive shop and hydro test facility, we saw a lot of SCBA tanks (Scott Air-Paks, etc) that used composite carbon fiber wrapped tanks. They both had a limited 15 year life and were also immediately condemned of the outer layer of resin was compromised exposing the wrap. Any water getting into the wrap would eventually cause a failure.
As a rebreather diver I occasionally see carbon fiber wrapped tanks in use on rebreathers for oxygen and diluent tanks, but as a cave and wreck diver they are not something I would ever consider due to the inevitable damage they would receive.
When an engineer looks at a submersible with a carbon fiber pressure hull, they have to start considering all the attachments to that pressure hull, and the potential for chafing or other damage to the outer layer of that hull that absolutely must keep water out of the inner wraps. And in the case of a 4000 meter rusted hull, being about to do that with over 6000 psi of pressure.
It becomes a race to see whether an undetected void or in detected salt water damage will cause the implosion first.
