Human Factors

TUC is "the maximum time the pilot has to make rational, life-saving decisions and carry them out at a given altitude without supplemental oxygen." As altitude increases above 10,000 ft, hypoxia symptoms increase in severity and TUC drops rapidly.

Read the shape, not just the values: the fall from 30 minutes to under a minute happens across 15,000 ft, and above about FL350 you have less time than it takes to find a mask you did not pre-position.

• Hypoxic — insufficient oxygen partial pressure at altitude. The high-altitude case.
• Hypemic — the blood cannot carry oxygen: carbon monoxide, anemia, blood loss. The heater-muff case, and the smoker's baseline.
• Histotoxic — the cells cannot use the oxygen delivered. Caused by alcohol and other drugs; PHAK notes that drinking one ounce of alcohol can equate to an additional 2,000 ft of physiological altitude (PHAK ch. 17).
• Stagnant — poor circulation: G-loading, shock, a constricted artery, cold reducing circulation to extremities.

Symptoms build as euphoria and impaired judgment, cyanosis, tingling and numbness, headache, and a narrowing field of vision with difficulty interpreting instruments — all while the pilot has a false sense of security that everything is normal. That false confidence is the reason the regulatory altitudes exist, and the reason to use a pulse oximeter rather than self-assessment.

Because it is a physiology course, not a systems course:

• High-altitude aerodynamics and meteorology
• Respiration
• Effects, symptoms, and causes of hypoxia and other high-altitude sickness
• Duration of consciousness without supplemental oxygen
• Effects of prolonged use of supplemental oxygen
• Causes and effects of gas expansion and gas bubble formation
• Preventive measures for gas expansion, bubble formation, and high-altitude sickness
• Physical phenomena and incidents of decompression

Then flight training including normal cruise above 25,000 ft MSL, simulated rapid decompression, and emergency descent procedures. The endorsement applies to pressurized aircraft with a service ceiling or maximum operating altitude, whichever is lower, above 25,000 ft MSL.

Both are trapped gas expansion problems (PHAK ch. 17). Gas in body cavities expands and contracts with pressure; if it cannot escape or refill, pressure builds and you get pain.

Ear block: the Eustachian tube connects the middle ear to the back of the throat and normally opens with chewing, yawning, or swallowing. On climb, middle-ear pressure exceeds the outside and vents easily. On descent the reverse happens and the partial vacuum constricts the tube walls, which is why descent is the hard direction. Symptoms: fullness, pain, temporary hearing loss.

Sinus block: sinuses equalize through small openings into the nasal passages. A cold, sinusitis, or nasal allergy congests those openings; descent plugs them. Pain is excruciating over the frontal sinuses (above each eyebrow) or maxillary (upper cheek) — a maxillary block can also make the upper teeth ache, and bloody mucus may discharge.

Corrective actions: the Valsalva maneuver — pinch the nostrils shut, close the mouth, and blow gently — forces air up the Eustachian tube. Slow the descent rate. Do not fly with an upper respiratory infection; decongestant sprays are not adequate protection, and oral decongestants have side effects that impair pilot performance. If a block does not clear shortly after landing, see a physician.

Motion sickness comes from the brain receiving conflicting messages about the state of the body — the vestibular system and the eyes disagreeing. Anxiety and stress contribute, which is why it shows up in early training and usually fades. Symptoms: general discomfort, nausea, dizziness, paleness, sweating, vomiting. In flight: open fresh air vents, focus on objects outside the airplane, avoid unnecessary head movements. Medications like Dramamine work for passengers but are not recommended while flying — they cause drowsiness (PHAK ch. 17).

Dehydration is critical loss of body water. Causes: hot flight decks and flight lines, wind, humidity, and diuretic drinks — coffee, tea, alcohol, caffeinated soft drinks. Signs: headache, fatigue, cramps, sleepiness, dizziness. The first noticeable effect is fatigue, which is exactly what makes it a commercial problem — long hot legs and high altitudes both raise the rate of water loss. Prevention: two to four quarts of water every 24 hours, and carry it where you can reach it.

Nitrogen dissolved in tissue comes out of solution as pressure drops, forming bubbles. Where the bubbles lodge names the syndrome:

• Bends — mostly large joints (elbows, shoulders, hips, wrists, knees, ankles): localized deep pain from a mild "niggle" to excruciating, aggravated by joint motion, occurring at altitude, during descent, or many hours later
• Neurologic — brain: confusion or memory loss, headache, scotoma or tunnel or double or blurry vision, unexplained extreme fatigue or behavior change, seizures, vertigo, nausea, unconsciousness. Spinal cord: burning or tingling around the lower chest and back, symptoms spreading from the feet up with ascending weakness or paralysis, girdling abdominal or chest pain
• Chokes — lungs: burning deep chest pain under the sternum, aggravated by breathing, shortness of breath, dry constant cough
• Skin bends — itching around ears, face, neck, arms, upper torso; a crawling-insects sensation; mottled or marbled skin

Treatment is descent, 100 percent oxygen, land, and seek medical attention — and tell the physician you flew, because the symptoms mimic other conditions.

• 12 hours after a dive that has not required a controlled ascent (no-decompression stop diving), before flying to cabin altitudes up to 8,000 ft
• 24 hours after a dive that required a controlled ascent
• 24 hours after any dive, before flying above 8,000 ft (AIM 8-1-2)

The physiology: the dive forces extra nitrogen into tissue; the climb is a second, larger pressure reduction on top of the ascent from depth. As a commercial pilot the practical issue is scheduling — a diving passenger or a diving day off has to be planned around a flight two days later, not the morning after.

Part 91 requires that blood alcohol level be less than 0.04 percent and that 8 hours pass between drinking alcohol and piloting an aircraft.

The interaction the examiner wants: a pilot with a BAC of 0.04 percent or greater after 8 hours cannot fly until it falls below that amount — and even with a BAC well below 0.04, a pilot cannot fly sooner than 8 hours after drinking. Both conditions, not either.

Two more facts worth stating: while hungover, a pilot is still under the influence — considerable alcohol can remain in the body for over 16 hours. And alcohol produces histotoxic hypoxia, so combined with altitude, two drinks may have the effect of three or four. As PHAK puts it: the regulations are specific, but it is a good idea to be more conservative than the regulations.

61.53 prohibits acting as PIC or as a required flight crewmember while that person knows or has reason to know of any medical condition that would make them unable to meet the medical certificate requirements for the operation, or is taking medication or receiving other treatment for a medical condition that results in that same inability (PHAK ch. 17).

This is the temporary disqualification rule, and it is self-executing. You do not need a denied medical to be grounded; a valid second-class in your pocket does nothing if you are sick, injured, or medicated today. Commercially, the pressure to fly through this is exactly the risk being tested.

The FAA has no specific medication regulation, but PHAK gives a working rule: wait at least five maximal dosing intervals — the time between recommended or prescribed doses — before flying after any medication with potentially adverse side effects such as sedation or dizziness. A 5- to 6-hour dosing interval therefore requires a 30-hour wait.

Note the caveat PHAK adds: observing the interval does not eliminate the risk of adverse side effects. And the underlying condition matters as much as the drug — the symptoms of a common cold suppress the desire to fly for good reason, and treating them with a drug that has adverse effects only compounds the problem. Diphenhydramine (Benadryl) is singled out for drowsiness and a prolonged half-life that extends the window of impairment.

CO bonds to hemoglobin far more readily than oxygen does, so even a trace of exhaust in the cabin progressively crowds oxygen off the red blood cells — hypemic hypoxia. A small crack in the heater muff is enough, and smoking loads CO into the blood before you ever take off.

Symptoms: headache, dizziness, drowsiness, and impaired judgment, appearing gradually enough that you attribute them to something else. Action: heater off, fresh air vents open, 100 percent oxygen if available, and land. Carry a CO detector; the ones that change color are cheap and the electronic ones give you a number.

Confirmation bias — seeking and weighting information that supports the conclusion you have already reached, and discounting what contradicts it. The classic: you have decided to go, so every improving trend in the forecast is meaningful and every deteriorating one is "probably conservative."

Expectation bias — perceiving what you expect to perceive rather than what is there. You expect a clearance to a familiar altitude, so you hear it. You expect the runway you always use, so you line up on it. You expect three green because you moved the gear handle.

Defeats:

• Read back and verify against a written source, not against memory
• Make someone or something argue the other side — a passenger, a checklist item, a written go/no-go rule with numbers in it
• Look for the disconfirming evidence deliberately: "what would have to be true for this to be a bad idea, and can I see it?"
• Verify the state, don't infer it — three green is the verification; the handle position is not

SRM is the single-pilot version: managing all available resources — instruments, avionics, autopilot, checklists, passengers, ATC, Flight Service, flight following — to reduce workload and maintain safety. CRM applies when there is a crew, and it adds the interpersonal machinery: briefings, task allocation, monitoring and cross-checking, and an explicit obligation to speak up and an explicit obligation to listen.

The commercial delta: you may now be the pilot flying with someone else in the seat, or the pilot whose passengers are also crew on an aerial-work flight. Say what you would brief: who flies the airplane, who works the radio, what "my controls / your controls" sounds like, and what the sterile-cockpit rule is and when it applies.

Part 91 imposes no duty limits, which is precisely the hazard — the constraint has to be yours.

• Know the difference between acute fatigue (one bad night, fixable with rest) and chronic fatigue (accumulated over weeks, not fixable with one night, and requiring a real break)
• Recognize the presentation: slowed reaction, fixation, degraded decision quality, and — most dangerous — not noticing that decision quality has degraded
• Set a written personal limit: maximum duty day, maximum flight hours, minimum rest, and a rule for the day-job-plus-evening-flight case
• Understand that the pressure to fly tired is external and the decision to fly tired is yours — which is where 61.53 and IMSAFE actually bite

The symptoms overlap almost completely — lightheadedness, tingling, visual impairment, and in both cases eventual unconsciousness. That is why the distinguishing test is circumstantial, not symptomatic.

Hypoxia is an altitude and oxygen-delivery problem: is the cabin altitude high, is the oxygen system flowing (check the green flow detector), is there any reason to suspect CO? Hyperventilation is a breathing-rate problem, usually driven by stress or anxiety, and it happens at any altitude — including on the ground.

The practical resolution: if oxygen is available, use it and check the flow rate. If symptoms persist with confirmed good oxygen flow, treat it as hyperventilation — slow the breathing rate deliberately, talk out loud, and breathe into a bag. Doing the oxygen step first is right because hypoxia is the one that kills you while you deliberate.

• Somatogravic — acceleration on takeoff feels like a nose-up pitch, tempting a push-over into the ground. The night-departure-over-water accident.
• False horizon — a sloping cloud deck, a lit shoreline, or scattered ground lights read as the horizon. Common on the same departures.
• Black hole approach — a lighted runway surrounded by unlit terrain removes every peripheral cue and drives a low, flat approach into terrain short of the field.
• Autokinesis — a single static light stared at for several seconds appears to move, and pilots maneuver to "avoid" it.
• Graveyard spiral — the vestibular system adapts to a prolonged turn, so rolling wings-level feels like turning the other way; the pilot re-enters the bank and pulls on a descending spiral.

The single defeat for all of them is the same and worth saying plainly: believe the instruments. The visual system is the most reliable of the three orientation systems (vestibular, somatosensory, visual) — but only when it has valid references. At night over dark terrain it does not.

Run IMSAFE honestly, and say the answers rather than the letters:

• Illness — anything that would trip 61.53, including the cold I am "getting over"
• Medication — anything within five maximal dosing intervals, prescription or over the counter
• Stress — what is competing for attention today, and can I set it down
• Alcohol — 8 hours and under 0.04, and honestly whether last night still has hold of me 16 hours later
• Fatigue — hours of sleep, hours awake, and whether this is acute or accumulating
• Emotion / Eating — recent life events, plus food and water, since dehydration degrades judgment before you notice thirst

Then the part that makes it a commercial self-assessment: state the personal minimums the answer measures against, and state who can override them. The correct answer to the last question is nobody — not the customer, not the operator, not the schedule.

The five attitudes and antidotes do not change:

• Anti-authority: follow the rules, they're usually right
• Impulsivity: not so fast, think first
• Invulnerability: it could happen to me
• Macho: taking chances is foolish
• Resignation: I'm not helpless, I can make a difference

What changes is the fuel supply. Each attitude now has an external sponsor:

• Macho gets fed by the customer who is impressed you flew it
• Invulnerability gets fed by the hours you have accumulated since the private checkride
• Anti-authority gets fed by the operator whose informal culture treats the rule as optional
• Impulsivity gets fed by a schedule with no slack in it
• Resignation gets fed by feeling like the schedule, not you, is flying the airplane

The mitigation is structural rather than attitudinal: written personal minimums with numbers, a go/no-go decided against them, and a stated rule that the decision belongs to the PIC alone.

ADM is a systematic, structured approach to consistently determining the best course of action for a given set of circumstances. Its two defining elements are hazard (a condition with the potential to cause harm) and risk (the likelihood and severity of the hazard's consequence).

The models are tools for different moments:

• PAVE — before the flight, to enumerate the hazards
• IMSAFE — before the flight, for the P in PAVE
• 5 Ps (Plan, Plane, Pilot, Passengers, Programming) — at scheduled checkpoints in flight
• DECIDE (Detect, Estimate, Choose, Identify, Do, Evaluate) — when something has already changed and you need a loop, including the Evaluate step people skip
• 3 Ps (Perceive, Process, Perform) — the fast version for an in-flight decision under time pressure

Pick one and use it consistently. Knowing five acronyms and applying none is the failure mode.

Controlled flight into terrain: an airworthy airplane under the pilot's control flown into terrain, usually from lost situational awareness, low visibility, or night operations over rising terrain.

It is a commercial problem because the accident chain runs through schedule: a departure accepted later than planned, a route that saves time over terrain, a descent begun before positive position knowledge to make an arrival time.

Mitigations that fit commercial flying: plan a minimum safe altitude for every leg and treat it as a hard floor; use terrain awareness displays but do not navigate by them; never descend below a planned altitude without positive position knowledge; and give yourself a written night-over-terrain rule — the altitude, the route, and the conditions under which you simply do not go.