Question about no fly time

Why do we have a no fly time since the airplane cabin is pressurized?

Someone asked me & I was wondering myself.

If you can ascend from 4 ATA to 1 ATA safely in 3-5 mins why can't we take a plane where cabin pressure is (from what I heard) 0.7 ATA?

thanks

Cabin pressure depends on flight altitude

One concern could be related to what happens if the cabin loses pressure....

I believe there are also issues with ascending to the equivalent of 8000 ft which is generally advertised as the standard plane pressurization.

Sorry that I can't give you a scientific answer. But, I KNOW the rule is there for a good reason because our flight from Bonaire to Houston had a medical evacuation on our arrival due to DCS.

In talking with the Doc at the local chamber, even a 15' dive before flying can present problems.

The main reason is you're starting saturated at 1 atm, then diving, then ascending back to your saturation level, then flying. In other words it's like a saturation dive from .7 atm to 1 atm, then further short dive, then ascent from saturation to 0.7 atm.

I'm not sure how the table is to be interpreted. For cabin pressure differential what is it measuring?

Scott:

Cabin pressure depends on flight altitude
View attachment 164224

Click to expand...

---------- Post added August 26th, 2013 at 06:59 AM ----------

I too am convinced there must be a reason why the rule is there. I just want to understand why.

driftwood:

Sorry that I can't give you a scientific answer. But, I KNOW the rule is there for a good reason because our flight from Bonaire to Houston had a medical evacuation on our arrival due to DCS.

In talking with the Doc at the local chamber, even a 15' dive before flying can present problems.

Click to expand...

---------- Post added August 26th, 2013 at 07:08 AM ----------

Hatul:

The main reason is you're starting saturated at 1 atm, then diving, then ascending back to your saturation level, then flying. In other words it's like a saturation dive from .7 atm to 1 atm, then further short dive, then ascent from saturation to 0.7 atm.

Click to expand...

When you start your ascent at 4 ATA at say 30m on your last dive of the day, you are already fully saturated with nitrogen. But you are allowed to ascend from 4ATA to 1 ATA with stops in a rather short time without DCS.

Compare this with flying where you are "ascending" after a pretty long surface interval (boat ride back to shore, wash gear, shower, pack, ride to airport, check in, etc.) we are looking at a 3-4 hour SI at least. And we are changing in pressure from 1 ATA to approx 0.7 ATA. Still doesn't add up.

Unless as giffenk says its in case the cabin loses pressure...

WhiteSands:

I'm not sure how the table is to be interpreted. For cabin pressure differential what is it measuring?...

Click to expand...

I wouldn't worry too much about that particular table. I'm pretty sure it's from: Aircraft Pressurization Systems and is for the S-3 aircraft.
The takeaway from the table is that cabin pressure will likely vary with flight altitude (and time, and aircraft).

For US commercial aircraft, FAR Part 25 Section 841 says not more than 8000 feet.
WHO says 6000-8000 ft. here: WHO | World Health Organization
[FONT=&amp]http://en.wikipedia.org/wiki/Cabin_pressurization gives other cabin altitudes, including 4500 ft.[/FONT]

Hatul's explanation is good - you're saturated at sea level (all "compartments"), and if you've been diving some compartments are super-saturated. Then you ascend to a lower pressure.

And recall that it is the ratios between pressures that matter, not the absolute pressure change.

see also boulderjohn's post #19 in this thread: http://www.scubaboard.com/forums/basic-scuba-discussions/460793-altitude-diver.html

Did you get out your course textbook and re-read the chapter on decompression before posting this question ? On a tangent, supposedly Navy pilots of high altitude aircraft breathe pure oxygen prior to take-off to de-gas nitrogen to protect against the bends in case of cockpit depressurization.

WhiteSands:

I'm not sure how the table is to be interpreted. For cabin pressure differential what is it measuring?



---------- Post added August 26th, 2013 at 06:59 AM ----------

I too am convinced there must be a reason why the rule is there. I just want to understand why.



---------- Post added August 26th, 2013 at 07:08 AM ----------



When you start your ascent at 4 ATA at say 30m on your last dive of the day, you are already fully saturated with nitrogen. But you are allowed to ascend from 4ATA to 1 ATA with stops in a rather short time without DCS.

Click to expand...

Saturation means the nitrogen in your tissues reaches steady state, so no more is taken on. To model the tissues in the body most dive computers use tissue compartments with nitrogen half times up to 6 hrs or so. To saturate this takes some 5 half times or about 30 hrs. None of our dives approach saturation even after repetitive diving.

If you have a software program that links to your dive computer, many of these show nitrogen bars, and you can follow these through the dive. You will see that through the dive the bars are going up and down indicating that your tissues are not saturated.

The problem is not so much to do with the amount of nitrogen in the body (although it obviously has an effect); the problem is more to do with the pressure differential (or pressure gradient) and the rate at which the pressure gradient increases.

Two analogies: Firstly, the classic bottle of soda. Shake it up and imagine the difference between twisting the cap off really slowly (safe ascent rate, slow off-gassing) to twisting it off really quickly (soda everywhere)
Secondly - a 40 metre high hill. If the descent from 40 metres to 0 metres is spread over 10 kilometres, you will roll down that hill on your skateboard relatively sedately. If it drops from 40 metres to zero metres over a 10 metre horizontal distance, you will wipe out.

The important thing is that the pressure in the bottle, and the height of the hill, are identical in both circumstances. What's different is the rate at which they change over time. Although an aircraft cabin is pressurised, it is pressurised to less than the equivalent at sea level, which means that the ambient pressure (that which surrounds you) actually decreases pretty rapidly. It's what makes your ears go pop a few minutes over take off.

Also an addition to the note about saturation; which is not the specific issue - saturation with regards to absorption of Nitrogen whilst diving means the body tissues are "full" of gas and have reached a state of equilibrium with the ambient pressure - no more gas can go in, and no more gas can go out - but according to the decompression model, a tissue does not have to be "saturated" to become dangerous; there are separate limits for the different theoretical tissues in the model - and it's important to remember that despite extensive testing, it is still purely theoretical. Supersaturation, on the other hand, simply means that the pressure outside the "vessel" is less than that on the inside - in both of the earlier analogies, one could consider that both the hill and the soda bottle are "supersaturated" - but not necessarily dangerously so.

In very basic terms, for no-decompression recreational diving, the danger is in the rate at which things happen, rather than the amount we have absorbed, although the two are inevitably linked.

Hope that helps

C.