Flying after Scuba Diving with Nitrox

[steve zodiac]

It seems to me that the rules concerning scuba diving and flying don't take account of the use of Nitrox which is in wide use in the scuba diving environment. My company use a blanket rule of 10m depth or less - 24 hours before flying, more than 10m depth - 48 hours before flying.

For instance, using Nitrox 32, 10m depth is equivalent to only 7.2m using air and only 6.2m if using Nitrox 36. The maximum depth to achieve 10m equivalent air depth using Nitrox 32 is 13m and 14m using Nitrox 36 (all conversions rounded in the safe direction).

If you are like me and try to get a dive in whenever possible downroute, I would like to be able to dive to the Nitrox equivalent air depths as this is likely to be a more rewarding dive than sticking to 10m regardless.

My company (as with most I guess) have no interest in getting this rule changed, however, I am sure there are enough scuba divers in the pilot fraternity who might benefit from a change of ruling. If there is enough interest, perhaps an approach to the CAA might be the way ahead (through BALPA?).

Personally I would like to see the 48 hours rule reduced to 36 hours; I have spoken to many PADI instructors who view 48 hours as gross overkill. Maybe 36 hours 'dive to fly' for greater than 10m (Equivalent air depth) and 24 hours for less than 10m (Equivalent air depth). Certainly 36 hours would allow much more flexibility for 2-3 day layovers. Any views?

[N1 Vibes]

Are you the pilot on this service?

Answer "Yes" - then whatever PADI says, when you have the responsibility for 100-400 pax behind you, wouldn't it be better to be 200% sure?

If it is "No" - then it's your life, you will become incapacitated in a pax seat and nobody else is at risk, oh except if they have to divert the a/c to offload you.

Sorry if this puts a damper on your hobby. The alternate is to fly with an airline with a route structure and layover period that allows you to enjoy your hobby without any risk. IMHO.

[osbo]

Rumour? or news?

[anotherthing]

Steve,

there is no differentiation as to what the PADI or BSAC or SSI books say regarding what you are diving on. It's ususally a blanket 24hrs regardless of mix. I dive TRIMIX and after a series of long deep dives, use the 24hr rule to be safe.

If you want to shorten it (the 24hrs), get a dive computer that you can input nitrox mixes into (there are lots around), and this will automatically count down your 'no fly' time based on your tissue compartment half times.

[steve zodiac]

N1 Vibes - You're missing my point. The present rule is 10m and makes no allowance for gas used. Nitrox is not standard air and is much safer than using air as far as decompression sickness is concerned. Of course I could be 200% safe and not dive at all, but then maybe I shouldn't have a beer within 48 hours of flying either.
I spend alot of time down route and being able to dive does help relieve the boredom of the umpteenth visit to The Maldives when drinking or sunbathing is the only alternative.

[steve zodiac]

Thanks for the reply. I don't want to shorten below 24hours, who would? I just would like the rules to come into the 21st Century and state '10m or Equivalent Air Depth if using Nitrox'. Not too much too ask and a very simple and reasonable amendment? I do have a dive computer, however, like most law abiding pilots, I don't want to break the rules; much simpler to try to get them changed. Rules are often written but never subsequently updated to reflect the modern environment unless we subject them to scrutiny.

[stressmerchant]

Steve, the rules you quoted are in common use, but actually have very little proper science behind them.
The problem with "flying after diving" (FAD) is the increased levels of inert gasses resulting from the dive. The inert gasses could be nitrogen or helium, depending on the mix. Could be a few other exotics, depending on the type of diving.
The amount of inert gas absorbed during the dive is dependent on a number of factors, including the depth of the dive, the time at depth, and the decompression methodology adopted. The common FAD rules quoted only really account for depth. Sometimes an additional penalty is added for decompression, but again no account taken of the true tissue loadings after the dive. If we were to consider a single tissue only, and divers who dive to the same maximum tissue loading, then a diver who had decompressed properly from a shallow dive should be at exactly the same risk as a diver who has decompressed properly from a deeper dive.
The problem is getting a simple rule that can be easily implemented. In addition, the risks of in-flight emergencies must be taken into account. Most models that do allow a "time to flight" estimate base their cabin altitude at about 8 000 ft. Obviously the diver is at increased risk during a cabin decompression incident. The extent to which this risk is acceptable depends on whether the person is flight crew or passenger. The usual 24 or 48 hour rules seem easy to use, despite being overly cautious.

[100m down/10,000m up]

The various dive tables rules do not make sense. It is better to do a DEEP (eg, 30m) short dive than shallow and long: fast tissues get loaded and offloaded quicker.

Good diving computers will sometimes give as low as 8 hours between diving and flying. And, do not forget, all of this (tables and computer) cover the worst case scenario: a sudden decompression at high altitude.

As far as approaching local CAA, forget it, it will never happen. The various diving agencies cannot agree on the rules, let alone the feds.

[bbrown1664]

Having got my Open Water PADI certificate a couple of months ago, I can confirm that the PADI limits are much lower than I would consider.

The theoretical limts now are 12 hours after a dive less than 18m or 24 hours after a dive of more than 18m. That said, consideration of the RDP must be taken.

In my book, 24 hours is more than enough unless you have been doing multiple deep dives.

[Wino]

If you are FLYING the airplane, just fly low enough so that at max differential the cabin is at sea level or lower, then it doesn't matter.

You will have to explain your fuel burn however, but many companies I have worked for have never tracked it sooooooooooo good luck.

Dispatch: "Captain PADI, Why are you putting 20k extra pounds of fuel on your jet?"
Capt. Padi: "It aint extra, I'm gonna BURN it!"



Its been done.

Cheers
Wino

[stressmerchant]

And, do not forget, all of this (tables and computer) cover the worst case scenario: a sudden decompression at high altitude.
Are you sure of that? The majority of the FAD calcs I have seen are simply based on a pressure altitude of about 8 000 ft.
In my book, 24 hours is more than enough unless you have been doing multiple deep dives.
The problem is that the generic rules must cater for the worst case. There is a reference in one of my textbooks (JSSG-10 Crew Oxygen Systems Handbook) to a case occuring in flight 4 days after the dive. Trouble is that the textbook gives no other details, so one cannot be entirely sure of the situation.

Somehow I don't see anyone changing the generic rules in a hurry

[steve zodiac]

Thanks for your informed response. I agree, a simple rule is required, however, just change '10m' to '10m Equivalent Air Depth'. This seems a simple and reasonable step.

[Keygrip]

So - having done a search and found this rather than posting a new question (Yay, me!).....what is the problem with diving at, say, 30 feet max (Bahamas shallows) and flying the Cessna 172 back as pilot in command at 2,000 feet?

That's less of a pressure change than diving to 60 feet and then sitting on the beach for the next 24 hours with beer in your hands for only 16 of them.

[Avtrician]

I think it would be wise to wait the 24 Hrs, Bubbles of N2 in your joints would not be pleasant, nor would the bubbling of your skin be a good look.

The place of the emregency landing, may not have recompression equipment available either. Better safe than sorry.

[Keygrip]

Avtrician - I understand the suggested/legal side - but I don't understand the actual physical effects.

If I went diving to 60 feet and then sat on the beach for 24 hours - the pressure differential is MORE than diving to 30 feet and then flying home at 2,000 the same day, or morning after.

What I'm trying to understand is why doesn't a 60 ft dive and sit on the beach give me decompression sickness whilst I'm sat on the beach - if a 30 foot dive and 2,000ft cross country would affect me if I was in an aircraft.

The latter (30 feet down, 2,000 up is a lesser pressure diferenetial).

Keygrip scratches head again.:bored:

[mad_jock]

keygrip personally I wouldn't have a problem flying after what you suggested but......

The difference in pressure is 0.9Bar diving compared to 0.1Bar flying.

But the overall pressure difference is not the full story you have to look at the partial pressure of the different gases to compare. Its why nitrox works.

The whole decompression table lark was discovered by sticking goats in a chamber and seeing when they got bent. Once they sussed out what was lethal they tried out combinations on navy divers.

http://www.dive-tech.co.uk/bert%20and%20haldane.htm

Even now the whole thing is a statistical risk managment exercise.

Some tables are statistically more liable to give you a bend US navy tables from memory 1% chance and PADI 0.01%. Even the ultra cautious PADI tables have had cases of people getting bent while complying with them (usually fat and unfit and outside the research group that was used to test them).

As a note even not having dived for 48 hours plus. A rapid decompression at above 30,000ft will give at least 0.7bar drop in pressure and your into the realms of a bend being statistically possible.

There has been research into ischemic brain lesions. At Aberdeen Uni they found that Pilots have the same amount of damage as commercial divers. The whole area of decompression is still in a relatively young state. The amount of changes in methods in only the last 15 years is astounding. And now with the advent of mass recreational diving instead of a few perverts flatie bashing is creating serious amounts of data that the researchers can work with.

[Keygrip]

MJ, got that, thanks.

Actually just read the PADI history of the RDP (dive plan table) and it's conversion from Navy statistics yesterday (am studying for the open water certificate).

I fully appreciate the decompression side of a pressurised aircraft at high level, and maybe even cabin pressure altitude of 8,000 feet - but cannot get my head around the difference of a C172 at 2,000 feet above sea level, especially following a particularly shallow dive.

The "Nitrox" in the title is misleading slightly as I actually did a search for the potential frequently asked question and then resurrected a dying thread rather than starting it again.

I'm considering putting together a Bahamas/Caribbean fly/dive/party weekend out of Florida for pilots.

[mad_jock]

It will make a slightly higher risk of a very small chance of having a bend.

Actually the long exposure to shallow depths loads you up more than deep short exposure dives. I have actually managed to get up to 56 hours desaturation diving 3 times a day for 3 hours per dive in 11 meters of salt water over 6 days. Everyone thinks deep dives are the most dangerous ones but all they do is load up the fast tissues. Actually these fast tissues are the best at off gassing as well. Its the slow tissues which cause you the problems with flying and these are the ones which tend for you not to feel you have a problem with. You will proberly only find out after 15 years when your bones start breaking up that diving for 9 hours a day then flying home for 2 hours at 2k is a bad idea. Once off no problem.

Don't think of it as flying and diving just as a pressure differental and an exposure period. And thats exposure to diving and flying. They are all interlinked ie if you are flying for 16 hours at 8k your body is starting at a different level of saturation than someone who has who is sat at sea level for the last 2 weeks. If I am flying anywhere to dive I always turn the bends box on for the trip so it gets the complete profile.

[sibcald]

The point of safe to fly after diving is a somewhat arbitory one. Nitrogen loading depends on length of dive, depth of dive, gas mix and undefinables such a weight, age, hydration etc etc.
Nitrox is no safer than air if used to its max. no decompression time (it just gives you longer). it is safer if used on air tables. (ie a nitrox 32 on a nitrox 21table).
Fitness to fly after diving is therefore laregly quesswork, esp. after mutliple dives. tables and computors lose their accuracey after 2 -3 dives and certainly are of little use after the typical multi dive holiday.
DCI is often difficult to explain (unless one finds a PFO) and therefore I suspect most airlines are operating in extremes of safety. This I could certainly go along with if I I was behind a captain that had been diving.

I regularly deal with divers who come back from Sharm after a diving holiday who get bent or re bent on a plane journey. I have even met someone who got bent driving over the Scottish Highlands after a dive.

This is not an exact science. You are asking the employers to take a risk on your behalf - why should they?
Ian Sibley-Calder
UKSDMC referree and PPL

[Keygrip]

You are asking the employers to take a risk on your behalf - why should they?
I'm not asking anybody to take anything.

I'm trying to understand why *I* cannot fly myself home at 2,000 following a max 30 feet dive, if I can sit on a beach at sea level for 12 hours after a 60 foot dive.

I say again - the "Nitrox" in the question is irrelevant, as is the law (as I have no intention of *actually* doing a flight within 12 hours).

I just want to understand. Bit like a pilot learning the systems of the aircraft - he's not likely to want to make an undercarriage system, or repair a generator, he just needs to know how they work (in case they decide not to).

sibcald - you subscribed PPRuNe specifically to respond to this question?

[sibcald]

No - have been a member for a while.

And - you can fly. Its just that the risks are unquantifiable.Its all a question of how far you push it.
Ian

[mad_jock]

Ian as a matter of interest if your allowed what was the dive profiles for the bend driving in the highlands.

We always had a bit of a concern for the Oban, Glasgow over the rest and be thankful.

[sibcald]

Sorry about delay. Was many years ago so cannot remember details. Was a weekend of diving but nothing exceptional for the time. No tech diving.
Sorry cannot remember more
Ian