By Robert N. Rossier, EAA 472091
This piece originally ran in Robert’s Stick and Rudder column in the August 2024 issue of EAA Sport Aviation magazine.
We don’t have to fly long to learn that not all flights are equal. Every now and then we encounter a flight that requires us to consider factors beyond our normal scope of planning. In some cases, it’s a whole new set of rules to consider.
For example, imagine as a private pilot, we’re planning a flight from Rocky Mountain Metro Airport (KBJC) in Broomfield, Colorado, to Santa Rosa, New Mexico (KSXU), for a weekend scuba diving trip to the Blue Hole. On the surface, it might seem like a typical flight, but the considerations run a bit deeper.
A Simple Plan
Let’s say that we’re planning to make this flight under VFR using VOR and pilotage as our primary means of navigation. Sure, we might use GPS and could theoretically fly direct, but there’s high terrain along the way that we might want to circumvent. We could make the flight under IFR, but the MEAs (the lowest published altitude between radio fixes) in those parts could easily necessitate the use of oxygen, which we don’t happen to have available.
We can break our flight down into three legs. The first 100-nm leg from BJC south to Pueblo (KPUB) overflies a stretch of terrain with elevations near 8,000 feet MSL, so we’ll likely climb to 9,000 to 10,000 MSL for terrain clearance.
The second leg continues from Pueblo south to the Cimarron (CIM) VOR — a roughly 100-nm span that crosses the border of Colorado and New Mexico, again overflying terrain that stretches to well more than 8,000 feet. Here we’ll likely climb to 9,500 MSL or higher for terrain clearance.
The final leg proceeds southeast about 50 nm from the VOR to Santa Rosa, along which we’ll likely fly at altitudes as high as 9,500 MSL. Elevations along the final 50-nm stretch to Santa Rosa are a bit lower, allowing us to descend to perhaps 8,000 MSL or so.
Depending on what we’re flying, our 300-nm flight would likely take about 2.5 hours. FAR 91.211, Supplemental Oxygen, states: “No person may operate a civil aircraft of U.S. registry — 1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration.”
It goes on to include “(2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes.”
Making this flight VFR and flying at the altitudes described would not necessarily require any oxygen, at least as far as the FARs are concerned. But there is more to this picture, particularly for our return flight.
Diving the Blue Hole
Located just 4 miles from the airport (field elevation 4,791 feet MSL), the Santa Rosa Blue Hole is a body of fresh water roughly 80 feet in diameter by 80 feet in depth, and it sits at an elevation of just under 5,000 feet.
Its crystal clear waters attract scuba divers for both training and recreational scuba diving. At this elevation, divers must use special dive tables to limit their time at depth to minimize the risk of experiencing decompression sickness after surfacing.
Decompression sickness (DCS) is a medical condition caused by dissolved gases (primarily nitrogen) emerging from solution as bubbles inside the body tissues after a person transitions from a high-pressure environment to a lower pressure environment. Examples include a scuba diver emerging from depth or a pilot ascending to high altitude in an unpressurized aircraft. The bubbles can form in or migrate to any part of the body, resulting in various symptoms from joint pain and rashes to paralysis and death.
Divers use specialized high-altitude dive tables to determine appropriate depth and time limits for their dives in order to minimize the risk of decompression sickness when diving at higher altitudes.
However, these tables assume that after surfacing, the diver will remain at that same altitude — and not higher — until excess nitrogen has dissipated from their body through normal respiration.
The Second Ascent
The problem in this case arises when, after diving, a diver decides to travel from Santa Rosa to another location that requires ascending to a higher altitude where lower ambient air pressure is experienced. It doesn’t matter what mode of transport is involved (car, train, airplane, etc.), the exposure to a lower pressure can result in DCS unless sufficient time has passed to allow the excess nitrogen to off-gas from the body.
Most often, such a situation arises when a diver has visited a sea level vacation destination for some recreational scuba diving, and then takes a commercial flight home. Divers in this case must wait typically for 12 to 18 hours or more before taking that commercial flight, depending on the number of dives they have made, and the depth and time of those dives. The industry guidelines for flying after diving have been developed by an organization called Divers Alert Network (DAN), which recommends the following:
- A minimum 12-hour surface interval before flying after making a single no-decompression dive.
- A minimum 18-hour surface interval before flying after making multiday or repetitive no-decompression dives.
- Substantially longer than 18 hours before flying after diving involving mandatory decompression.
Back to BJC
Making the 300-nm and roughly 2.5-hour flight back to BJC after diving in the Blue Hole requires careful consideration. First of all, our dives at the Blue Hole have been made at an elevation significantly higher than sea level, which requires use of the specialized dive tables described above, with the expectation of not ascending to a higher altitude following the dive. The FAA has no specific regulations regarding flying after diving, but we might consider the recommendations from DAN.
However, the DAN guidelines are predicated on commercial flights, which operate at cabin pressures no higher than 8,000 MSL. Our flight back will require climbing to altitudes considerably higher than the normal 8,000 MSL cabin pressure of an airliner, so our guidance from DAN for flying after diving is invalid. No matter how we slice it, a pilot considering such a flight might be taking on considerable risk for decompression sickness.
We still have one additional regulatory item to consider. FAR 61.53 prohibits a pilot from operating an aircraft if the pilot knows of a medical condition that would “make [them] unable to meet the requirements for the medical certificate necessary for the pilot operation” or — for those not requiring medical certification — make [them] unable to operate the aircraft in a safe manner.”
Having been scuba diving could be considered a known medical condition that could affect our ability to safely operate an aircraft. Beyond our own safety concerns are those of any passengers we might carry. Even if we were not involved in diving ourselves, we would need to carefully consider carrying a passenger who has been diving — either at sea level, or at some higher elevation.
The bottom line is that any flight that takes us to high altitudes should be considered and planned carefully. When we mix such a flight with an underwater excursion, we have a whole other set of factors to consider. In this case, the better option might be to delay flight until that excess nitrogen has dissipated.
Robert N. Rossier, EAA 472091, has been flying for more than 40 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.