Beyond hypoxia, the high-altitude pilot faces evolved-gas and trapped-gas disorders, plus the mechanics of getting the airplane down quickly and safely.
At reduced pressure, nitrogen dissolved in body tissues can come out of solution and form bubbles — the same mechanism as a diver's "the bends." Symptoms include joint pain (bends), skin itching/mottling (creeps), respiratory distress (chokes), and in severe cases neurological signs (staggers). Risk rises with altitude (generally a concern above ~18,000–25,000 ft on prolonged unpressurized exposure), with recent SCUBA diving (do not fly for 24 hours after diving requiring decompression stops, and at least 12–24 hours otherwise), with dehydration, fatigue, obesity, and age. Treatment: descend, administer 100% oxygen, and seek hyperbaric treatment for serious cases — even after a normal landing.
Gas trapped in body cavities expands as pressure decreases on climb and contracts on descent. The most common problems occur on descent, when contracting gas in the middle ear (barotitis/ear block) or sinuses (sinus block) cannot equalize, especially with a head cold. Gastrointestinal gas expansion on climb causes discomfort. Pilots manage these with the Valsalva maneuver, swallowing, and by not flying with congestion.
When a decompression or other emergency requires losing altitude fast, the goal is maximum rate of descent without exceeding structural or speed limits. A typical technique: confirm oxygen, then reduce thrust to idle, extend speedbrakes/spoilers, and roll into a bank (often 30–45°) to increase descent rate while holding Vmo/Mmo (or the published emergency descent speed). Some procedures lower the landing gear for additional drag below gear limit speed. Throughout, the crew uses checklists and CRM, levels off at a safe altitude (typically 10,000 ft cabin/airplane or the MSA), and declares an emergency. Speed and altitude awareness prevent trading one emergency for an overspeed or terrain conflict.