Root Causes: Some Accidents Are No Accident

Interesting flights and interesting dives provide an opportunity for post-event introspection; debriefing if you will.

Professionally, I am called upon to analyze fatalities and near-misses for the Navy and, occasionally, the Air Force. Personally, I spend even more time analyzing “what ifs” for my own activities.

For example, recently I was preparing a video of one of my more beautiful nighttime flights with a passenger, departing the coal-mining regions of Pennsylvania, heading south over the valleys and mountains of Appalachia as the early morning sun began to brighten our part of the world. Sunrise crop Editing that video gave me a chance to reflect on the pre-flight and in-flight decisions I made that day. There were many decisions to be made, and those decisions resulted in not only a safe flight, but a spectacular flight.

But like most things, there was also a risk, calculated, and weighted, and recalculated as conditions in flight and on the ground changed in the face of aggressive weather.

In very real ways, single pilot IFR (instrument flight rules) flight is akin to cave diving. They are both technically challenging, rewarding solo activities. However, you better be on your game, or else not play.

I was cave diving before cave diving was cool; before it was considered a technical diving specialty, before safety rules and high quality equipment was available. Trimix, scooters, and staged decompression were all decades in the future, and frankly the safety record at that time was atrocious. I am alive because I had the good sense to limit my penetration; “just a little” was enough of a sobering experience, about which I have previously written.

But this posting is not about moderation; it is a warning to those who would, for whatever reason, deliberately make bad decisions, one after the other. If after a chain of such deliberate misadventures, a fatality results, then I would say that fatality is no accident. It is a procedure; a flawed process of decision making with a more or less guaranteed fatal outcome.

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Dr. Tom Iliffe, Texas A&M University at Galveston

Lest you lose interest in reading this post because you believe all cave divers are loonies, rest assured that could not be further from the truth. Where I work we have four very active cave divers, highly intelligent, experienced, diving deep breathing trimix (helium/nitrogen/oxygen) when necessary using scuba and rebreathers. They are safe divers who are on the cutting edge of diving research when they’re not diving for pleasure. In fact, two of them are the U.S Navy’s diving accident investigators, so they know all too well about underwater misadventures.

Friends met early in my career have been the cave explorers of the 70’s and 80’s; names you may know like Bill Gavin and John Zumrick. Another long-time friend from the Navy’s Scientist in the Sea Program, and of whom I am quite envious, is Dr. Tom Iliffe, a biologist constantly on the front edge of underwater cave biology. (My draft novel, Children of the Middle Waters, includes a story about his beloved Remipedes.)

All these cave divers have survived due to their sane and balanced approach to risk management; moderation in all things. But sadly, not all divers I’ve come to know, one way or the other, have been so sensible and measured.

One was a wonderfully gracious man, a Navy diver who had a hobby: free diving. He’d tell me how he enjoyed surprising divers in the main cave at Morrison Springs, Florida when he would swim up to them and wave, while wearing no breathing equipment at all except that with which he was born.

I’m sure they were shocked; I know I would be.

After a while, as he gained experience with this solo recreation, he began to confide in me, and ask me questions about events he’d experienced. He told me how pleasant it was sometimes when he would surface. I warned him about shallow water blackout, loss of consciousness on ascent, and explained the physical laws that made breath-hold diving so dangerous; at least in the manner in which he practiced it.

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Morrison Springs, Florida. Photo licensed under Wikimedia Commons.

The last day I saw him alive, he once again came in for consultation, and told me about the euphoria he had experienced a few days before. I was of course extremely concerned and told him that what he described sounded like a near death experience. The next time he might not be lucky enough to survive, I told him. Later I heard more of that story; the previous weekend he had been found floating unconscious on the surface, but was revived.

Soon after that, this diver was again found, but this time his dive had proven fatal. His personal agenda for thrills exceeded all bounds of either training or common sense. And those thrills killed him.

The only solace I could find was that he wanted to share his experience and bravado, but he clearly was not interested in really hearing the truth, no matter how hard I worked to educate and dissuade him. While some might call this young man’s mental status as a perpetual death wish, I would argue that he never consciously thought he would die; at least not that way. Life was good, in his perspective, and I suspect he thought he was smart enough to make sure it continued that way.

Unfortunately when we were talking, we did not know just how close the end was.

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Jackson Blue Spring, Marianna, Florida. Photo by Paul Clark, released under Creative Commons license.

The same was true I suspect for another well-liked diver who was the subject of a fatality report I helped write several years later. It was a rebreather fatality at Jackson Blue Spring in Marianna, Florida. The decedent was reportedly an experienced diver. I won’t belabor the story because the NEDU report is available on the internet (released by his family and available on the Rebreather Forum).

Nevertheless, the sequence of events leading to his demise involved a surprisingly long list of decision points which should have prevented the fatal dive from occurring. As each opportunity to change the course of events was reached, poor choices were made. In combination those choices led inexorably to his demise.

By now we know that even the U.S. Navy is not immune to poor decision trees. In fact, I would argue that wishful thinking is a common factor among people with intelligence and technical ability, and those with a “get it done” attitude. People who fix problems for a living are seemingly resistant to admitting that sometimes the bridge really is too far, and some problems are better fixed in the shop than in the field.

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Gareth Lock of Cranfield University, Bedfordshire, U.K. is currently collecting data on diving incidents through a questionnaire on “The Role of Human Factors in SCUBA Diving Incidents and Accidents”. Like me, he has both an aviation and diving background. Gareth is serious about trying to understand and reduce diving accidents. Links to a description of his work, and his questionnaire can be found here and here. If you are a diver, please consider contributing much needed information.

In Diving, What is Best is Not Always Good

A Closed Circuit Rebreather diver in a Florida spring.

In technical or recreational rebreather diving, safety is a matter of personal choice. Wrong choices can turn deadly.

Some poor choices are made for expediency, while others are made with the best of intentions but based on faulty or incomplete information. As a diving professional, it is those latter choices that concern me the most.

David Shaw

A poignant and well documented diving fatality involved a record setting Australian diver, David Shaw. David was an Air Bus pilot for Cathay Pacific.

Professional pilots are immersed in a culture of safety, a culture that makes airline travel the surest means of long distance transport. David applied that same sort of attention to his diving, recording on his personal web site his detailed plans for a record setting dive to recover the body of a diver who died in the 890 feet (271 meter) deep Boemansgat Cave of South Africa 10-years prior to David’s ill-fated dive.

Despite his extensive preparations, David Shaw made a fatal mistake. Like those who fail to appreciate the threat of an approaching hurricane, David failed to recognize the risk of really deep diving with a rebreather.

Unlike other types of underwater breathing equipment, a rebreather is entirely breath powered. That means you must force gas entirely through the “breathing loop” with the power of your respiratory muscles. On a dive to 890 feet, you are exposed to 28 times normal pressure, and breathing gas more than five times denser than normal. The effort involved is enough to dismay some U.S. Navy divers at depths far less than David Shaw intended to dive. Yet in David’s own words, he had previously never had a problem with the effort of breathing.

“The Mk15.5 (rebreather) breathes beautifully at any depth. WOB (work of breathing) has never been an issue for me. Remember that when at extreme depth I am breathing a very high helium mixture though, which will reduce the gas density problem to a certain extent.”

He goes on to say, “I always use the best quality, fine-grained absorbent on major dives. The extra expense is worth it.”

“I have had 9:40 (9 hrs, 40 min duration) out of the canister and felt it still had more time available, but one needs to qualify that statement with a few other facts. Most of the time on a big dive I am laying quietly on deco (decompression), producing minimal CO2 (carbon dioxide).

In those words lie a prescription for disaster.

A rebreather scrubber canister containing granular absorbent through which a diver has to breathe.

David wanted to use a single rebreather that would accomplish two tasks — provide a long duration gas supply and CO2 absorbing capability for a dive lasting over nine hours, and provide a low work of breathing so he could ventilate adequately at the deepest depth. To ensure the “scrubber canister” would last as long as possible, he chose the finest grain size, most expensive sodalime available. His thought was, that was the best available.

Arguably, the two aims are incompatible. He could not have both a long duration sodalime fill and low breathing resistance.

Cartoon of breathing through a scrubber canister.

As illustrated in a previous blog posting, the smaller the size of granules you’re breathing through, the harder it is to breathe. Think of breathing through a child’s ball pit versus breathing through sand.

Perhaps if David had maintained a resting work rate throughout the deepest portion of his fatal dive, he might have had a chance of survival. After all, he had done it before.

But the unexpected happens. He became fouled and was working far harder to maintain control of the situation than he had anticipated. That meant his need to ventilate, to blow off carbon dioxide from his body, increased precipitously.

A sure sign of high breathing effort is that you cannot ventilate as much as is necessary to keep a safe level of carbon dioxide in your blood stream. CO2, which is highly toxic, can build rapidly in your blood, soon leading to unconsciousness. From the videotaped record, that is exactly what happened.

Purer A, Deason GA, Hammonds BH, Nuckols ML. The effects of pressure and particle size on CO2 absorption characteristics of High-Performance Sodasorb. Naval Coastal Systems Center Tech. Manual 349-82, 1982. (Click for larger image.)

Had David been fully aware of the insidious nature of carbon dioxide intoxication from under breathing (hypoventilating), he probably would have chosen an alternative method to conduct the dive.

One alternative would be to use a larger granule size absorbent in a rebreather at considerable depth (say, 100 meters and deeper), and reserve the fine-grain absorbent for use in a separate rebreather shallower than 100 meters.

David chose the fine-grain absorbent because of the longer dive duration it made possible. Although fine grains are more difficult to breathe through than large grain absorbent, fine grain absorbent lasts longer than large grain absorbent.

But that long duration is only needed during decompression which is accomplished far shallower than the deep portions of the dive. The time spent deep where work of breathing is a threat is quite short. He did not need the capabilities of a long duration, fine grain absorbent.

From the U.S. Navy experience, there are other problems with this dive which might have hastened the end result. A rapid and deep descent causes the oxygen pressure within the rebreather to climb to potentially dangerous levels; a phenomenon called oxygen overshoot. Thus he might have been affected somewhat by oxygen toxicity. A rapid descent might also have induced the High Pressure Nervous Syndrome which would affect manual dexterity.

While those contributing factors are speculative and not evident on the tape, the certainty of the physics of dense gas flow through granular chemical absorbent beds is an unavoidable fact.

No doubt, many have offered opinions on what caused David’s accident. I certainly do not claim to be intimately involved in all the details, nor familiar with all the theories offered to date. Nevertheless, David’s mistaken belief that using the “best absorbent” was the best thing for his dive, is a mistake that needs to be explained and communicated before this accident is repeated with a different diver in some other deep and dark place.

Click to go to the source document.