Part 91 Series — Alcohol, Oxygen, Seatbelts & Lights: The Human Side of Legal Flying

Most pilots spend a lot of time thinking about the airplane.

Fuel.
Weather.
Runway length.
Performance charts.

But Part 91 doesn’t just regulate the machine — it regulates you.

Can you fly the morning after a late-night party?
When are passengers required to wear seatbelts?
At what altitude is supplemental oxygen legally required?
Do you really need those lights on during the day?

In this sixth installment of our 8-part Part 91 series, we’re shifting focus from metal and instruments to the human element of aviation — the rules that govern pilot fitness, passenger safety, and required use of aircraft systems.

Because sometimes the most important compliance decisions aren’t about the airplane.

They’re about the person in the left seat.

The Core Human-Factor Regulations of Part 91

In this article, we’re covering:

• §91.17 — Alcohol or drugs

• §91.105 — Flight crewmembers at stations

• §91.107 — Use of safety belts and child restraint systems

• §91.209 — Aircraft lights

• §91.211 — Supplemental oxygen

These regulations define how pilots and passengers must behave — and what physiological limits apply.

§91.17 — Alcohol and Drugs

This regulation is commonly summarized as:

“8 hours, bottle to throttle.”

Under Part 91, no person may act as a crewmember:

• Within 8 hours after consuming alcohol

• While under the influence of alcohol

• While using any drug that affects safety

• With a blood alcohol concentration (BAC) of 0.04% or greater

Important distinction:

The legal driving limit in many states is 0.08%.
For pilots, it’s half that.

And even if you meet the 8-hour rule and are under 0.04%, you may still violate the regulation if you are “under the influence.”

Fatigue, dehydration, and hangover effects can impair performance even when technically legal.

§91.105 — Flight Crewmembers at Stations

During takeoff and landing, required flight crewmembers must:

• Be at their assigned stations

• Have seatbelts fastened

While en route, a pilot may leave the controls if:

• The aircraft is equipped with autopilot, or

• Another qualified pilot is at the controls

For most small-aircraft operations, this means:

Stay strapped in during critical phases of flight.

§91.107 — Safety Belts & Passenger Briefings

No pilot may take off unless:

• Each person on board has been briefed on seatbelt use.

During taxi, takeoff, and landing:

• Each person must occupy an approved seat.

• Seatbelts must be fastened.

Shoulder harnesses (if installed) must also be worn during takeoff and landing by crewmembers.

For small aircraft carrying family or friends, this regulation makes the PIC responsible for ensuring compliance.

Passenger briefings are not optional.

§91.209 — Aircraft Lights

From sunset to sunrise:

• Position lights must be on.

Anti-collision lights:

• Must be on at all times unless the PIC determines they would create a safety hazard (for example, in fog or while taxiing near other aircraft at night).

Even in daytime operations, many pilots leave anti-collision lights on to enhance visibility.

§91.211 — Supplemental Oxygen

This regulation addresses physiological limitations at altitude.

Required oxygen use:

• Above 12,500 feet MSL up to and including 14,000 feet MSL:
  Required flight crew must use oxygen if exceeding 30 minutes at those altitudes.

• Above 14,000 feet MSL:
  Required flight crew must use oxygen continuously.

• Above 15,000 feet MSL:
  Oxygen must be provided to each occupant.

For many small-aircraft pilots flying in mountainous regions, this rule becomes very practical.

Hypoxia can occur before you realize it.

At this level, you now understand the baseline human-factor rules under Part 91.

But as with many regulations, the deeper implications matter most.

Judgment, Physiology & Real-World Risk Management

Let’s go beyond the minimums.

“8 Hours” Is a Minimum — Not a Recommendation

Alcohol metabolism varies by:

• Body weight

• Hydration

• Food intake

• Sleep

A pilot may be legally under 0.04% and still cognitively impaired.

Professional standard in general aviation often exceeds regulatory minimums.

If you have to ask whether you’re fit to fly, you probably shouldn’t.

Oxygen Use Before It’s Required

While oxygen is legally required above 12,500 feet for extended periods, performance degradation can begin much lower.

Night vision begins to degrade around 5,000 feet MSL.
Cognitive performance may decline before noticeable symptoms appear.

Many experienced GA pilots use supplemental oxygen well before the regulatory threshold.

Passenger Briefings — More Than a Seatbelt Reminder

A proper passenger briefing may include:

• Sterile cockpit expectations

• Emergency exits

• Fire extinguisher location

• No-touch policy for controls

• Use of headsets

The regulation requires a seatbelt briefing — but professionalism suggests more.

Lights and Collision Avoidance

Anti-collision lights enhance visibility — especially in busy training areas.

However, in clouds or fog at night, strobes may impair vision.

The regulation gives PIC discretion — use it wisely.

Why These Rules Exist

Human factors account for a significant percentage of general aviation accidents.

Impairment, hypoxia, distraction, and passenger management are recurring themes.

Part 91 codifies physiological realities into regulatory safeguards.

It’s not about limiting freedom.

It’s about protecting decision-making capacity at 5,000 feet.

Looking Ahead

In Article 7, we’ll move into:

• IFR operational requirements

• Lost communications procedures

• VOR equipment checks

• Takeoff and landing under IFR

In other words: what changes when you fly in the system rather than just under it.

Because once you enter the clouds, the structure tightens.

Until then:

Fly sober.
Brief your passengers.
Use oxygen wisely.
And respect the human limits — they matter more than horsepower. 🛩️