Why the wait to fly?

[tursiops]

Why is a 12, 18, or even 24 hour wait time recommended given these factors?

Because people get bent with shorter pre-flight times. See https://scubaboard.com/community/resources/flying-after-diving-dan-workshop-report-2002.19/
and

Info - The DSAT Recreational Dive Planner (RDP), 1994

Document Name: Development and validation of no-stop decompression procedures for recreational diving: The DSAT Recreational Dive Planner Authors: R.W. Hamilton, Raymond E. Rogers, Michael R. Powell, Richard D. Vann, with Richard Dunford, Merrill P. Spencer, Drew Richardson Attached is the...

scubaboard.com

 

[Nick_Radov]

I assume I'm missing something, but:

Doesn't this make it far more likely that the divers in the 3, 6, and 9 hour groups who got DCS were already bent before the "flight?" If their supersaturation levels correlated to a GF99 of 97% upon surfacing, and only 38% (in the 3 hour case) upon "flying" they underwent far greater decompression stress upon surfacing than upon flying.

What this suggests to me is that relatively short intervals between flying and diving are perhaps sufficient to reduce DCS risk to well below what is generally assumed to be a reasonable level for diving.

Exactly. The first symptom of DCS is denial. While I can't really prove this, I strongly suspect that most divers who supposedly got bent by flying after diving were actually already bent before they ever boarded the airplane: the flight just made the symptoms worse. Conversely, if you have normal physiology (no circulatory system shunts or lung conditions or anything like that) and you do the right deco (for some definition of "right") then you can safely go straight from the dive boat to the airport. Please don't shoot me, I'm not actually recommending that anyone do this.

Most divers like a certain level of conservatism, say a GF hi of 80, so assuming sufficient surface interval has passed that an 8000 foot pressurized cabin will result in a GF99 of under 80, then wouldn't flying be reasonable?

(All of this puts aside the idea that putting time between diving and flying allows time for DCS symptoms to manifest, treatment to be acquired, etc.)

In many ways, this makes sense considering that an 8000 foot cabin pressure corresponds to under 10 fsw in terms of pressure differential.


Where am I wrong? Why is a 12, 18, or even 24 hour wait time recommended given these factors?

Many minor Type 1 DCS cases resolve on their own with no clinical treatment so waiting 12+ hours allows some time for that to happen. Or failing that it allows time for a bent diver to move past the denial stage to the acceptance stage and realize they need to go to the chamber — or at least postpone their flight.

There is also a tiny risk of cabin depressurization (like if an airplane door blows out in flight), which in theory could cause a diver with remaining excess tissue saturation to start bubbling. But even in such an unlikely scenario the pilots will typically make an emergency descent below 10,000ft within a few minutes for passenger safety.

 

[Jai Bar]

Because you are trying to apply linear extrapolation to a non linear solution.

 

[John C. Ratliff]

In practice it's not an issue worth worrying about for anyone on a commercial flight regardless of whether they have been diving or not. The pilots will descend to 10000 ft or lower within a few minutes. Bubbles don't form instantly.

Ask the Captain: How fast can a plane descend if cabin pressure is lost?

This week, retired US Airways Captain John Cox discusses what is done in the event a cabin loses pressure and recalls his scariest flying experience.

www.usatoday.com

Risk of Decompression Sickness (DCS)
Johnny Conkin, PhD1, Jason R. Norcross, MS2, James H. Wessel III, MS2, Andrew F. J. Abercromby, PhD2, Jill S. Klein, MS2, Joseph P. Dervay, MD3 Michael L. Gernhardt, PhD3
1 Universities Space Research Association, Houston, TX
2 Wyle Science, Technology & Engineering Group, Houston, TX
NASA Johnson Space Center Houston, TX

https://ntrs.nasa.gov/api/citations/20140003729/downloads/20140003729.pdf

SeaRat