As a private pilot-in-training, you discover that the FAA wants you to learn a modicum of information about the physiology of flight. This need-to-know training continues in instrument and commercial training, and gets more specific if you pursue training in high-altitude flight. Most of modern physiology training centers on hypoxia, night vision, and fatigue, for good reason. Early on, the FAA also talks about some basic knowledge for combining flying and scuba diving. In the interest of simplicity, the feds simply want you to say no flying after diving, and if you say no flying for 24 hours after diving during the oral portion of your checkride, the designated pilot examiner will nod in approval and move on to the next topic.
But what is the real answer? And why?
These are valid questions. Scuba diving is a popular recreational activity, and depending on which source you want to cite, anywhere from 60 to 70 percent of pilots also are certified divers. That should not be a surprise. The two activities have much in common: Each requires specific training; safety is paramount; there are some rules in each that are flexible, and some that are inviolate; pilots have flight plans and divers have dive plans; an attention to detail is key. Most important, each operates in a three-dimensional setting, so that divers often describe the sensation of being “like flying.”
In flying, we concern ourselves with effects of too little oxygen at high altitudes, where atmospheric pressure is low, thus making it difficult to consume as much oxygen as we need in each breath. Divers concern themselves with consuming air while under an increased amount of pressure. The air in the tank is under pressure, and the diver underwater is experiencing greater pressure from the surrounding water. The concern under water is not oxygen, but nitrogen.
The common answer to “Why can’t you fly after scuba diving?” is that you will get “the bends.” But what really happens, and what are the real rules?
When a diver goes underwater, the water exerts pressure on the body, as well as on the scuba tank and the air it contains. At 33 feet of sea water, the body is under twice as much pressure as it is on the surface. At 66 feet, the pressure is three times as much. At 99 feet, it’s four times as much. Divers, like pilots, feel the change in pressure in their ears. They body itself doesn’t feel any different.
As a diver inhales air from his tank, the pressure from the water causes the tissues to absorb more of the gas than is normally absorbed on land. As long as the diver stays within the acceptable limits of recreational diving, this is generally not a big deal with the oxygen. However, the body also absorbs the nitrogen that is in the tank (air is primarily a mix of oxygen and nitrogen, with about 1 percent being other inert gases). Our body needs oxygen, and so metabolizing it is no big deal. However, the body doesn’t need the nitrogen in a gaseous form. In fact, the body wants to get rid of it. But, under the increased pressure experienced underwater, the nitrogen is absorbed.
If this sounds familiar, it should: This is the exact same process that goes into making a carbonated drink. In a soda, the gas is added to the liquid under pressure, so that the saturation level of the liquid is increased. Under water, a diver’s body has an increased saturation point because of the pressure, and so the tissues absorb more of the gas that is inhaled.
As a diver ascends toward the surface, the pressure on the body decreases, and some of the gas that was absorbed into the tissues begins to come out. This is similar to slowly cracking a soda bottle to release some of the pressure before opening it all the way. Divers slow this process by ascending to the surface in a slow, controlled manner, and then perform a safety stop. That is, they ascend to 15 feet, then stay there for three minutes to “off-gas” some of the nitrogen before surfacing.
If the diver ascends too quickly, the nitrogen does not leave the tissues in a controlled fashion, and a very painful injury or even death can occur as the nitrogen bubbles—comes out of solution—in the blood stream. This is commonly called “the bends,” and is more accurately referred to as “decompression illness.”
Even after surfacing, a diver still has nitrogen in the tissues. If a diver was to go flying, the ambient pressure around the diver would continue to decrease as the airplane climbs. That would allow the nitrogen to be released from the body even faster, thus increasing the risk of injury.
So, what is the proper protocol when it comes to diving and flying? If you are flying first, you can safely dive immediately. If you are diving first, PADI (the Professional Association of Scuba Diver Instructors) and DSAT (Diving Science and Technology, Inc) recommend that a diver who has not exceeded decompression limits wait a minimum of 12 hours after a single dive before flying, and 18 hours if the diver has engaged in repetitive or multi-day dives. If the diver has participated in decompression diving (whether intentionally or unintentionally), then a preflight interval of more than 18 hours is suggested.
The blanket rule that some airlines or flight departments have about waiting 24 hours after diving before flying is more of a catch-all designed to eliminate as much risk as possible. But the guidance listed above can safely be followed, as it will allow the diver to safely expel the excess nitrogen before flying. It is very possible that a diver who does a single-tank dive in the morning can take a sightseeing flight at sunset.—Chip Wright