Fying/driving after altitude diving

[triton94949]

A reference to a rec.scuba post where MHK described going into a plane right after a dive with "His hair still wet". Here it is

Kane is famous in SoCalif as a recovery diver. I dont know of any evidence that makes him an authority on diving safety rules, other than recovery protocols.

 

[DepartureDiver]

You don't have to convince me either. MHK has convinced me. I see no reason not to use the "wet hair rule" if you decompress properly.

The reason is the reduction in atmospheric pressure. Are you decompressing properly to surface after a dive (to 14.7 psi) or decompressing for a reduced pressure? If the ocean decompression is conservative, then flying right after may be a possibility. If you are decompressing just to reach an ambient pressure of 14.7 psi, then you need to off gas more. If the pressure you are surfacing to just doesn't matter, then why even do a 10' stop or even a 20' stop. Once you are okay to ascend from 30' to 20', why not just use this "wet hair" logic and ascend directly to the surface? So just like the 20' and 10' stops are necessary, so is a "stop" at 14.7 psi (the hidden stop) before flying. Either the decompression in the ocean is overly conservative which will allow a greater reduction in pressure or it is not and a surface interval is required. Flying to a cabin pressure of 8000' is a 25% reduction in atmospheric pressure compared to sea level pressure. While I have great respect for Mike, I disagree with this approach - which would be compounded if there was a loss of cabin pressure while flying.

 

[TheAvatar]

Oh it's now OK to execute the wethair procedure? I guess all those doctors and researchers who say you should wait before flying are a bunch of morons. :icorolley

Departure diver is correct.

Consider there is more going on here than what you recognize from your PADI dive table and anecdotal evidence from some guy you heard of on the internet.

There is some research out there that suggests people shouldn't fly for 48 hours! It's another significant risk factor one adds to the list.

How long should you wait? 0? 12? 24? 48? Whatever the NOAA chart says for 8000? Do you understand what lies behind each number? What do you really base your choice on? Are you qualified to determine "no wait is safe" without doing the research (not anecdotal stories of "it worked that time for that guy").

If you don't get away with salty wet hairing to 8000ft and you get yourself bent on the plane, not only do you get the joy of a chamber ride, you just screwed over the other 200 people on the flight if it diverts for your preventable condition. You probably also cost the airline tens of thousands of dollars because then they still have to get all of those people where they were going.

Though there may still be some pressurization in the plane in a depressurization event, maybe the cabin altitude changes to oh say 17,000ft equivelent. That won't bend anyone else on the plane... but did you plan on that with your wet hair? There are many reasons why an aircraft might depressurize. I'm sure mempilot can tell us how a airliner may be intentionally depressurized to help stifle an inflight fire.

Please stop advocating practices that are known to be unsafe. The general wait time is NOT zero.

 

[Dr Deco]

Hello readers:

Individual Limits

It is very important to note that the NDLs are for a group of divers and is experimentally determined. The NDL is the experimental limit (plus some extra margin) for the most sensitive member of the test group. All others in the group could have sustained more decompression.

The question is, where do you stand with respect to other divers? Are you “sensitive” or “resistant?” Do you want to test it on an airplane?

Activity Levels :umnik:

All NDLs are determined in a laboratory on individuals basically seated in a hyperbaric chamber and then seated after they reach the surface. If you are not seated after you reach the surface, then the NDLs are different.

Certainly there is a safety margin built into all tables, and for this reason, activity levels after surfacing do not play a role within certain limits. Hiking up a hill is way beyond those limits.

Dr Deco :doctor:

 

[mempilot]

There are many reasons why an aircraft might depressurize. I'm sure mempilot can tell us how a airliner may be intentionally depressurized to help stifle an inflight fire.

Modern pressurized passengerliners have two redundant systems and are composed of controllers, packs (air conditioning units), and outflow valves. Aircraft are not airtight vessels by design, but do contain rubber gasket seals around doors, cockpit escape windows/hatches, and any other through hull doors or hatches to limit the transfer of pressure from the inside to the outside. Pressure can leak from these areas at an acceptable rate well less than the ability of the packs to provide positive pressure from the inside.

The controller takes readings of outside ambiant pressure and inside ambiant pressure to calculate differential, and monitors the rate of ascent and descent via dynamic pressure differential to determine when to open and close the outflow valves. The OFV's open and close to maintain a predermined differential schedule in the climb, level, and descent phases of flight. Any one of each of the paired components can fail, and the system maintains integrity. There is also manual reversion, if the auto control system fails, and the pilots can control the outflow valves from the cockpit. Outflow valves are spring loaded and fail closed, so if the pilots do not respond immediately, the aircraft will try to maintain the inner pressure.

A plane depressurizes for the following reasons:

1. normal depressurization schedule during a normal descent
2. unscheduled slow depressurization due to loss of positive pressure differential
3. rapid depressurization due to loss of control of the outflow valves
4. explosive depressurization due to hull breach
5. intentional depressurization to clear smoke (below 10,000 feet)

With regards to number

(1) if the a/c has an operable controller (manual or auto), it is barely noticeable (situation normal)

(2) can happen when there is a partial loss of positive airflow into the system which is insufficient to maintain a positive differential. The aircraft is descended normally to a safe altitude where the system can keep up. The passengers are typically unaffected or unaware that this has happened. (very rare)

(3) if a single controller fails, the second will control the OFVs. If both fail, the OFVs fail closed. If for some strange reason, an OFV fails to close (shouldn't happen due to design) then air can leak out (I've never heard of this actually happening) Maybe due to foreign object debris blockage?

(4) is bad. Really bad and you won't care whether you are bent or not.

In regards to (5), the PPO2 is kept within life-sustaining limits by maintaining this positive pressure through the climb and level phases of flight. Fires, in most cases, need O2 to burn; however, people need O2 to live. Depressurizing an aircraft in the upper flight levels to starve a fire could have devastating impact on the people aboard. Therefore, at the very first indication of fire or smoke anywhere on the aircraft, an emergency descent is iniated until the aircraft is at or below 10k feet and the plane can be safely depressurized.

Note on 5: The crew of the Swiss Air flight that went down off Nova Scotia due to fire turned off the packs which actually sucked the fire into the cockpit through the vents. By degrading the pressurization system, they allowed the fire to enter the cockpit. They were at an altitude where even without pressurization, there was enough O2 for fire to breath. This information was released recently as a result of the pursing investigation.

So, do we depressurize on purpose? A. Very very rarely and not at cruise altitude. We typically try to get below the dehibilitating hypoxic altitudes above 10000 feet.

We actually just had an aircraft with smoke in the cabin and cockpit do an emergency descent and landing in Talledaga several weeks ago. Both the captain and first officer are friends of mine. The aircraft held normal pressure until they got below 10k at which time they flushed the smoke on the way to the runway by forcing the OFV's open and allowing ram air to sweep through the aircraft. The passengers felt only a small spike in cabin pressure when the plane went through 10000 feet, at which point they were descending at 6000 ft per minute. There was a 'burp' in the cabin pressure as it went from 8k to about 8500 as the plane caught the cabin pressure on the way down. - Almost seamless.

 

[TheAvatar]

For those who are interested
From Table 9-5 "Required Surface Interval Before Ascent to Altitude Ater Diving"
Increase in altitude of 8000ft:
Repet group/time h:mm
A/0:00
B/0:00
C/0:00
D/3:28
E/6:54
F/9:34
G/12:05
H/14:09
I/15:58
J/17:35
K/19:03
L/20:23
M/21:37
N/22:46
O/23:49
Z/24:00
Wait 48 hours if diver went into decompression.

If you are going to assume the aircraft will absolutely not decompress, that's your table.

It's really simple to come out with a repet group that requires 12 hours. Doing multiple dives it's pretty easy to come out where you should be waiting 18 hours. If you were on a multiday dive trip with multiples dives every day, you may want to consider waiting even longer!

5. intentional depressurization to clear smoke (below 10,000 feet)

In regards to (5), the PPO2 is kept within life-sustaining limits by maintaining this positive pressure through the climb and level phases of flight. Fires, in most cases, need O2 to burn; however, people need O2 to live. Depressurizing an aircraft in the upper flight levels to starve a fire could have devastating impact on the people aboard.

I was told: the depresurization wasn't to be done at 33K or anything that extreme, but did help in slowing the flames since there would be less ambient O2 available... and of course the onboard chemical O2 systems were to be deployed so that the passengers didn't suffer "devestating impact" from depressurization (not to mention toxic smoke and fire O2 consumption) during fire supression and descent.

I don't understand. Are you saying that the O2 masks don't drop???

So, do we depressurize on purpose? A. Very very rarely and not at cruise altitude. We typically try to get below the dehibilitating hypoxic altitudes above 10000 feet.

I thought that passengers could be exposed to 14K equivelent per FAA without supplimental O2 but pilots had to have supplimental O2 at 10K or 12K equivelent.

Side note:

I find it funny that altitudes above 10K are marked as "debilitating hypoxic" environments for people who are seated (low O2 demand).

Why just yesterday I was at 13K making split second decisions and performing precision movements during extreme physical exertion (high O2 demand).

Then again... I am somewhat acclimated.

Activity Levels :umnik:

All NDLs are determined in a laboratory on individuals basically seated in a hyperbaric chamber and then seated after they reach the surface. If you are not seated after you reach the surface, then the NDLs are different.

Certainly there is a safety margin built into all tables, and for this reason, activity levels after surfacing do not play a role within certain limits. Hiking up a hill is way beyond those limits.

[/font] Dr Deco :doctor:

Out of curiosity, have there been quantitative studies on how much effect this has? Something that says "exertion at this level post dive could shift the NDL left by this many repet groups on a table" (not sure how to expess that properly)?

 

[mempilot]

Are you saying that the O2 masks don't drop???

Oh yes. They drop. But at that altitude, and in that chaos, only a few will get them on before passing out. Think: a couple of breaths. Also, if the aircraft doesn't descend rapidly, the on board O2 will not last very long. The other factor is that the masks don't seal to the face like ours in the cockpit.

Anyway you slice it, people are going to get hurt and potentially in a life threatening way if we rapidly depressurize the plane at altitude, so we don't do it. Those masks, in severe decompression cases, are a placebo. It don't happen like the movies.

I thought that passengers could be exposed to 14K equivelent per FAA without supplimental O2 but pilots had to have supplimental O2 at 10K or 12K equivelent.

You are thinking general aviation here. But you are right in the fact that one can function above 10k ft. The dehibilitating effects start at 10k for most people. The effects are worse for some than others. They are exponential as altitude increases. Above 12k for only 30 minutes or so and some people pass out. (think smokers) I've been to 18k and 33k in a chamber and removed my mask. I have video. It ain't cool at 33k and it's mildly funny at 18k after a bit.

I find it funny that altitudes above 10K are marked as "debilitating hypoxic" environments for people who are seated (low O2 demand).

Dehibilitating is just that. You are reading too much into it. It progresses over exposure and impacts less fit people quicker. Lowest common denominator and all. Go from 8000 to 36000 rapidly and you won't even see the mask fall in front of you.

Why just yesterday I was at 13K making split second decisions and performing precision movements during extreme physical exertion (high O2 demand).

For how long?

 

[mempilot]

For those who are interested
From Table 9-5 "Required Surface Interval Before Ascent to Altitude Ater Diving"
Increase in altitude of 8000ft:
Repet group/time h:mm
A/0:00
B/0:00
C/0:00
D/3:28
E/6:54
F/9:34
G/12:05
H/14:09
I/15:58
J/17:35
K/19:03
L/20:23
M/21:37
N/22:46
O/23:49
Z/24:00
Wait 48 hours if diver went into decompression.

The 48 hours if deco is what I have problem with. This is an arbitrary number not based on anything other than being a disclaimer.

The majority of my dives are planned decompression dives. I know what the theory says my tissue loading is upon exit. I can control these ending M values via conservatism in my plan.

The 48 hour disclaimer is more pointed at a non-planned deco where the diver has no idea what the RN is.

After a planned deco, I can exit the water with less residual N2 than someone using the NDL tables to get out as soon as available.

I don't advocate doing non-planned deco without a full deco computer, because even a standard cx that will give you mandatory stops will throw wild numbers at you leaving you with no real idea where you're at since it tells you to cool it for 24 hours anyway.

 

[TheAvatar]

Why just yesterday I was at 13K making split second decisions and performing precision movements during extreme physical exertion (high O2 demand).

For how long?

Bout six or seven hours. It was fun! Hard work though.

Being in shape and having an elevated hematocrit and BPG concentration helps (acclimation to altitude). If you sliced me up I'd probably have a higher density of cappilaries in my muscles too (more acclimation).

 

[triton94949]

...If you are going to assume the aircraft will absolutely not decompress, that's your table.

It's really simple to come out with a repet group that requires 12 hours. Doing multiple dives it's pretty easy to come out where you should be waiting 18 hours. If you were on a multiday dive trip with multiples dives every day, you may want to consider waiting even longer! ...

If you are going to surmount a mountain pass in your car, that is your table.

It makes much more sense simply to wait 24 hours before flying, after NDL diving, and 48 hours, after deco diving. That is what the training agencies teach (I know for sure that NAUI and TDI teach this).

There are so many accounts of Joe Diver on his way home in an aircraft ending up in a recompression chamber because he got on the plane too soon after diving.