The engineering answer to everything the last lesson described is the pressurised cabin, and this lesson covers the physiology wrapped around it: what the cabin protects, what happens when the protection fails, the sickness that dissolved nitrogen brews when pressure drops, why yesterday's dive changes today's flight, and, changing axis entirely, what acceleration does to the same circulation the course has been tracing.
The pressurised cabin
In cruise, a commercial aircraft's cabin is held at a pressure corresponding to an altitude of about 5,000 to 8,000 ft, whatever the aircraft's own level. The protective function reads straight off the previous lesson's ladders: at 8,000 ft or below the crew and passengers sit under the reaction threshold's serious effects, saturation stays around 93 per cent or better, no masks are needed, and the pressure differential across ears and sinuses during climb and descent stays manageable. The systems that achieve this belong to the airframe course's pressurisation lesson; here the cargo is what the machinery is protecting.
Losing pressurisation
The protection can fail fast or slowly. A rapid decompression follows structural failure or damage to the pressure hull: sudden hypoxia at aircraft altitude, expanding gases in every body cavity, and a TUC measured on the previous lesson's ladder. A slow decompression creeps in through a failed outflow valve or mis-set pressurisation controls, and is in its own way more dangerous, because insidious hypoxia arrives without a bang to announce it. Either way, passenger oxygen masks are released by a barometric trigger when the cabin altitude reaches about 10,000 to 14,000 ft.
The syllabus fixes the crew's vital actions when cabin pressurisation is lost, in order: oxygen mask on, emergency descent, land as soon as possible, and no further flight for the next minimum 24 hours. The last item is the one candidates forget, and its reason is the next section: decompression sickness symptoms can occur up to 24 hours later, so the day ends at the first landing even if everyone feels fine.
Decompression sickness
The blood and tissues carry nitrogen in solution, in equilibrium with the pressure around the body. When ambient pressure falls far and fast enough, the dissolved nitrogen comes out of solution as bubbles, and the bubbles are the disease: decompression sickness (DCS). In civil aviation it takes exposure to altitude, the risk growing above about 25,000 ft, and it is dramatically aggravated by anything that loaded extra nitrogen beforehand, diving above all. The classic symptom set has four nicknames, worth learning as a set:
The bends — deep, aching pain in and around the joints, the commonest form.
The creeps — tingling, itching, crawling sensations in the skin.
The chokes — chest pain, coughing and difficulty breathing from bubbles in the lungs' vessels; rare and serious.
The staggers — neurological effects: unsteadiness, visual disturbance, weakness; the most dangerous form.
Prevention is everything that keeps nitrogen in solution or out of the body: cabin pressurisation itself, limiting time at high cabin altitudes, denitrogenation by pre-breathing 100 per cent oxygen where operations demand it, and respecting the diving rules below. Treatment, if symptoms appear in flight: descend, breathe 100 per cent oxygen, land as soon as possible, and on the ground the definitive treatment is recompression in a hyperbaric chamber, which shrinks the bubbles and drives the nitrogen back into solution; medical follow-up is mandatory because symptoms can rebound.
Diving and flying
Scuba diving is DCS's force multiplier. At depth the body breathes air at well above sea-level pressure and loads extra nitrogen into solution; flying afterwards stacks an altitude decompression on top of the dive's, so bubbles can form at cabin altitudes that would never trouble a non-diver. The recommended minimum intervals between diving and flying:
12 hours — after non-decompression dives totalling less than 2 hours in the preceding 48 hours.
24 hours — after multi-day, unlimited diving.
24 to 48 hours — after any dive that required decompression stops.
Acceleration and G
Now the change of axis. Linear acceleration is a change in the speed of motion along a straight path: the take-off roll, a catapult, braking. Angular acceleration is a change in the rate or direction of rotation: entering a spin or a tightening turn. The body registers both through the organs of equilibrium (the next module's subject) and, when sustained, through the circulation.
Sustained positive acceleration along the spine, +Gz, multiplies the apparent weight of the blood. The column the heart must lift toward the head grows heavier, blood is displaced toward the lower body and pools in the legs and abdomen, and arterial pressure at head level falls while the cardiovascular reflexes fight back with a rising heart rate. Whether that fight is lost depends on three factors the syllabus names: the magnitude of the acceleration, its duration, and its onset rate: a rapid onset outruns the reflexes, which need seconds to respond.
For a relaxed subject the effects of positive acceleration arrive in a fixed sequence as the load grows: around 3.5 to 4 g, grey-out, the dimming and narrowing of peripheral vision as the eye's circulation fails first; around 4 to 5 g, blackout, vision lost entirely while hearing and consciousness persist; and beyond that, G-LOC, g-induced loss of consciousness, with a dangerous period of confusion after the load relaxes. The values shift with the person and the day, and straining manoeuvres and anti-g equipment raise them, but the sequence grey-out, blackout, unconsciousness is fixed, because it is the order in which the eye and brain lose their pressure margins.