LOW ALTITUDE Flying after diving

[Trace Wilson]

I'm not sure this is the correct forum for this, so if not, my apologies...

I'm a new OW diver and a private pilot in NE FL. Unfortunately, it seems all the 'good' offshore diving is clustered around the keys, which is a 5-8 hr drive, depending on how far down I go. However, I also have access to my own plane. I've got several springs within 2-3 hrs, but have really enjoyed the small amount of ocean diving I've done on vacations.

For the short term, I'm expecting most of my trips to be with groups from my LDS until I get a little more experience under my belt. They typically dive the West Palm Beach area, which is a 4-5 hr drive each way. They generally leave 0-dark:30 on Saturday, meet the boat for a 2-tank dive Sat around noon, then another 2-tank dive Sunday monring, then drive home.

Our OW class went over the restrictions on diving after flying, which I understand. But I also believe those are recommendations aimed at commercial airliner flight, which have a cabin pressure of 8000-10000 ft. In my own plane, I have the option of flying at pretty much any altitude above 1000 ft. (Technically I should be at 1500 or 2500, but below 3000 it's technically my choice).

I discussed it with my instructor, and he maintained that the 24 hr restriction was for any flight, to any altitude, no if's, and's or but's, based on DAN's advice. So, I scanned around on DAN's site and found the Proceedings from a 2002 "Flying After Recreational Diving" document (https://www.diversalertnetwork.org/files/FADWkshpBook_web.pdf). On page 12 of the document, I found the following:

Finally, a minimum threshold altitude below which guidelines were unnecessary was
discussed. Although no data were available that specifically addressed the question, a
previous publication had suggested that restrictions were unnecessary below a threshold
altitude of 2,300 feet (701 meters) (15). The U.S. Navy Diving Manual had followed this
recommendation without apparent problem until introduction of the 1999 procedures that
reduced the threshold to 1,000 feet (305 meters) for internal consistency (5). The
workshop participants agreed that 2,000 feet (610 meters) was a reasonable, if uncertain,
threshold, as no problems with 2,300 feet were cited. ​

Additionally, page 26 of the document shows 'Required Surface Intervals' based on Repetitive Dive Group and target altitude, and shows that for Group G and below, to 3000 ft or lower, the Surface Interval is 00:00. Lower groups have 00:00 surface intervals to higher altitudes.

From aviation training, we learn that standard atmospheric pressure at sea level is 29.92 inHg, and we loose close to 1.00 inHg for each 1000 ft of altitude we ascend. So, with 1ata = 29.92 inHg, if we ascend to 2000 ft, we find ambient pressure of 0.93 ata. This would be the equivalent of losing an additional 2.5 fsw of pressure from depth.

[Begin thinking of a new diver with just enough knowledge to be dangerous]

If I'm a diver at 90 fsw (yep, beyond the 60 I'm certified to now), I'm allowed to ascend at 30 ft/min. So, I can lose 3.00 ata of pressure on my body in 8 mins, with a 5 min safety stop. If instead I'm at 102 fsw, I can still lose that additional 3 fsw in basically the same 8 mins. So, is there any significant additional risk to ascending from 102 fsw compared to ascending from 99 fsw and jumping into a plane and ascending to 2000 ft. over the coarse of a couple of hours? Since our body doesn't know the difference between pressure from water vs pressure from air, it only really knows the ambient pressure, is there REALLY any SIGNIFICANT additional risk of DCS?

Assuming the answer above is 'no', I looked at my potential schedule:

Sat
Dive 1 - 60 min @ 60 ft @ EAN32 (RDG: H) + 60 min SI (RDG: G)
Dive 2 - 50 min @ 50 ft @ EAN32 (RDG: K) + 16:00 hr SI (RESET RDG)
Sun
Dive 1 - 60 min @ 60 ft @ EAN32 (RDG: H) + 60 min SI (RDG: G)
Dive 2 - 50 min @ 50 ft @ EAN32 (RDG: K) + 4:00 hr SI (RDG: D)

Now, these were worst case numbers since I'm sure not everything will be at 60 ft, and I don't think my SAC rate will give me 60 mins on an AL80 @ 60 ft.

Using the SSI Dive Tables, the second I come up from the last dive Sunday I calculate myself in Repetitive Dive Group "K", and group "D" after the 4 hr surface interval. The last 4 hr SI is assuming: time to return to dock, return rental gear, return to hotel, shower & checkout, grab lunch with the rest of the group from the LDS, drive to the airport, turn in rental car, load and preflight the plane, taxi for take off.

From the DAN Workshop Proceedings, they show for RDG K an assent to 2000 ft can be accomplished after a 4:37 SI. Interestingly, given a 4 hr SI, the SSI tables put me in group "D", which the Proceedings table show no additional SI needed for an assent to 6000 or below. But, using the table the most conservatively, if I extend the SI to 4:37, the table shows I should be good up to 2000 ft., which allows my quick flight home (2hrs).

For this particular trip, I'm not saving a huge amount of time (4 hr driving vs. 2 hr flying), but extend the dive location to Key Largo/Marathon, and suddenly what is an exhausting weekend trip could be become quite doable. This gives me incentive to dive more if I'm not dreading the trudging back and forth to SE FL.

In addition to the non-pilots who will have great insights, I'm really hoping to hear from other diver/pilot's to see what they actually do.

Also, does anyone know of any more recent studies that include effects of LOW ALTITUDE flying after diving?

Thanks for hanging on through the ramble...

 

[RainPilot]

I suspect many pilots have done what you are discussing. I also suspect none of them will talk about it since it would be a great way to not have insurance pay out for any incident or accident. Also FAA may have some additional issues.

Physiologically I think your reasoning is fine. Legally? Yeah that’s the big question.

 

[Duke Dive Medicine]

Hi Trace,

You're off to a good start on your research.

Briefly, most diving decompression tables (including the no-D tables) are designed for use at sea level, so they're developed with the assumption that a diver will be surfacing at sea level and staying there. They are not designed to eliminate all the built-up inert gas from the dive or dive series, but rather to eliminate enough that the diver won't get DCS symptoms, with a considerable safety factor built in. If a diver ascends to altitude too soon after a dive or dive series, especially one that results in a significant inert gas load, the change in altitude can be enough to produce decompression sickness symptoms. You're correct in assuming that the recreational "flying after diving" recommendations are generally meant for commercial flight with cabin pressures at around 8000' msl, but your training agency may have developed stricter recommendations. In any event, what you're looking for are recommendations for ascent to altitude less than 8000' msl after diving. Revision 7 of the Navy Diving Manual contains a good discussion on this beginning on page 9-57. You can download it from the Supervisor of Salvage website here: //www.navsea.navy.mil/Home/SUPSALV/00C3Diving/DivingPublications.aspx

Page 9-62 of Rev 7 has a required surface interval table similar to the one you found in the 2002 recommendations, with some changes. The catch with using this table is that the repetitive group designators come from the Navy decompression and no-decompression tables. The air no-D table is found on the next page, 9-63 and the air RNT table is on the page after, 9-64. They use equivalent air depth calculations when diving nitrox so we'll stay away from that for demonstration purposes, but lets' say for example you do a series of repetitive air dives that leave you with a Navy RGD of J. Using the Navy's required surface interval table, you could ascend to 1000' msl right away if you were feeling froggy and wanted to dodge cell phone towers all the way home. You'd have to wait 41 minutes before ascending to 2000' msl, 2 hours and 25 minutes before ascending to 3000' msl, 4 hours and 15 minutes before ascending to 4000' msl, and so on.

Of course you assume all the risk when using these tables, so it's up to you to become proficient in them and take your own physiology into account if this is the route you want to go. I hope this helps.

Best regards,
DDM

 

[flyboy08]

I suspect many pilots have done what you are discussing. I also suspect none of them will talk about it since it would be a great way to not have insurance pay out for any incident or accident. Also FAA may have some additional issues.

Physiologically I think your reasoning is fine. Legally? Yeah that’s the big question.

There are no exclusions in your insurance policy that would void coverage if you had an incident after diving....just sayn....

 

[RainPilot]

There are no exclusions in your insurance policy that would void coverage if you had an incident after diving....just sayn....

Not even on the aircraft insurance? The phrase “pilot error” gets real expensive in my experience. Especially if you’re not around to explain which tables you used.

“Aircraft full of scuba gear? When did he dive? Two hours before the flight? Sounds open and shut to me. Who’s buying the first round?”

 

[RainPilot]

The GCAA regulations here (our FAA) stipulate 24 hours before flying. Not even the DAN changes made a difference.

 

[Altamira]

Hi Trace,
You will find many threads covering this topic on SB, and I have made numerous comments sopporting the current PADI and USAF guidelines pertaining to flying after diving. As a retired USAF and American Airlines pilot, my primary concern is that you cannot guarantee you will be able to stay at or below planned altitude,even if you are the pilot in command. As you know, there are many contingencis that may require a climb to a much higher altitude. You do not want to have to chose between flying safely and possibly getting bent. The risk is not worth it.

 

[Trace Wilson]

@RainPilot, leaving the legal stuff aside, I'm more trying to focus on the physiological issues. A similar, legal, situation could occur say in the Bahama's. Book a puddle jumper between islands for the afternoon, and then get in a couple of dives in the morning. As short as the flights would be, it's cheaper for the pilot to stay low, so the question becomes, how short of a surface interval do you really need for a low altitude flight.

As an aside, regarding the legality of the flight, I'm not aware of anything in the FAA regs that would prevent it. There are regs regarding how long between drinking and flying, but I've seen no other specific restrictions. The closest are some general, vague reg's that basically say 'The pilot will not fly if they are in no condition to fly', but there is nothing that specifies a litmus test for when you are 'in no condition to fly', beyond alcohol and a list of prescription drugs.

For insurance purposes, yeah, I could see them denying a claim if they could prove your were impaired in some way at the time of the flight.

 

[flyboy08]

Not even on the aircraft insurance? The phrase “pilot error” gets real expensive in my experience. Especially if you’re not around to explain which tables you used.

“Aircraft full of scuba gear? When did he dive? Two hours before the flight? Sounds open and shut to me. Who’s buying the first round?”

Not sure what your saying here? No aviation insurance company will deny coverage in the event of an accident or incident after one has been diving....there are NO exclusion that would prevent such.....pilot error is not excluded either!

Best,

 

[RainPilot]

I’d hate to be the test case. I have friends who have had the insurance companies come after them to recover money after a PE verdict. In one case it was posthumously. If it’s different there in the US then no worries.

Physiologically speaking, I have flown several individuals who were still wet and salty with no adverse issues. Most of them were diving O2 rebreathers but a couple of times on OC.

I have also done fixed wing casevac of DCS cases in unpressurised aircraft. Slow ROC to 500’ over the ocean, 1000 AGL over land and they didn’t seem to get worse.