FAA Private Pilot Helicopter (PRH)Human FactorsEasy
A helicopter pilot is flying at night at 6,000 feet MSL without supplemental oxygen. Which symptom of hypoxia is likely to appear FIRST, even though the pilot may feel otherwise normal?
- ASevere uncontrollable muscle tremors
- BCyanosis (blue fingernails and lips)
- CDegraded night vision and dark adaptation
- DLoss of consciousness
Show answer & explanationAnswer & explanation
Correct answer: C. Degraded night vision and dark adaptation
The retina is extremely sensitive to reduced oxygen, so night vision begins to degrade at altitudes as low as 5,000 feet MSL, well before other hypoxia symptoms such as cyanosis or loss of consciousness occur.
Why the other options are wrong
- A. Muscle tremors are not a typical early hypoxia symptom.
- B. Cyanosis appears later as oxygen saturation drops further.
- D. Unconsciousness is a late, severe stage of hypoxia, not an early sign.
Hypoxic Hypoxia
Insufficient oxygen reaches the blood due to reduced atmospheric pressure at altitude, impairing brain and body function.
- Night vision degrades starting around 5,000 ft MSL
- Symptoms include euphoria, poor judgment, and blue lips/fingernails (cyanosis)
- Time of useful consciousness decreases rapidly above 25,000 ft
Memory trick: 'Night eyes go first' — vision fades before you even notice you're impaired.