What FAR 91.167 Actually Requires
FAR 91.167 governs IFR fuel requirements for civil aircraft operating under Part 91. The rule is straightforward but regularly trips up candidates who confuse it with VFR reserves or Part 135 requirements.
Under 91.167, you must carry enough fuel to:
- Complete the flight to the first airport of intended landing
- Fly from that airport to the alternate (if one is required under 91.169)
- After reaching the alternate, fly for 45 minutes at normal cruising speed
That 45-minute reserve is calculated at normal cruising speed — not best-endurance, not best-range. Interviewers will press on this distinction.
The Alternate Requirement and the 1-2-3 Rule
91.167 and 91.169 are paired regulations. When no alternate is required, your fuel math changes significantly — you're not computing a leg to a second airport.
Under 91.169, you do not need to file an alternate if the destination airport has at least one standard or special instrument approach procedure and the weather forecast from 1 hour before to 1 hour after your estimated time of arrival shows a ceiling of at least 2,000 feet AGL and visibility of at least 3 statute miles. This is the 1-2-3 rule: ±1 hour, 2,000 ceiling, 3 SM visibility.
You're filing IFR to an airport with a published ILS. TAF shows OVC015 at your ETA. You file an alternate. Interviewer asks: "Why did you need an alternate?" — Ceiling of 1,500 feet falls below the 2,000-foot threshold, so the 1-2-3 exception doesn't apply. Alternate required, fuel planning must include that leg plus 45 minutes.
Part 91 vs. Part 121/135: Know the Difference
Airline interviewers frequently ask candidates to compare fuel requirements across regulatory parts. Mixing these up signals shallow regulatory knowledge.
- Part 91: Destination fuel + alternate fuel (if required) + 45 minutes at normal cruise
- Part 135 turbine: Destination fuel + alternate fuel + 45 minutes at normal cruise (similar structure, but weather minimums for alternates differ under 135.223)
- Part 121 domestic: Destination + alternate + 45 minutes — but fuel burn methodology, dispatcher involvement, and dispatch release requirements create a more complex system
When you're interviewing at a regional or major carrier, the expectation is that you understand Part 91 cold and can articulate how Part 121 builds on that foundation. Operators like United Airlines and Delta Air Lines want to see regulatory fluency, not memorized definitions.
Common Interview Questions on 91.167
Based on recent pilot gouge, these phrasings appear most often:
- "Walk me through IFR fuel requirements under Part 91."
- "Your destination is below minimums when you arrive. You have alternate fuel plus 45 minutes. Are you legal?"
- "What changes about your fuel planning when no alternate is required?"
- "How does Part 121 fuel planning differ from what you've been doing under Part 91?"
Under 91.167, I need enough fuel to reach my destination, then fly to my filed alternate if one is required, and from there fly 45 minutes at normal cruising speed. Whether I need an alternate is determined by 91.169 — if my destination has a published approach and the forecast shows at least 2,000 and 3 within an hour either side of my ETA, I can depart without an alternate. If not, the alternate leg is part of the fuel calculation. The 45-minute reserve is always at normal cruise, not a reduced speed.
If you're preparing for an airline technical interview, practicing these regulatory explanations aloud matters as much as knowing the content. An AI voice coach can score your answers and surface the gaps before you're sitting across from a chief pilot.
Key Numbers to Have Cold
- 45 minutes — IFR reserve at normal cruise (Part 91)
- 2,000 feet / 3 SM — alternate exception weather minimums (1-2-3 rule)
- ±1 hour — the forecast window around ETA for the alternate exception
- 1 hour before / 1 hour after — same window, stated differently; some interviewers will phrase it both ways to see if you know it's identical
Regulations questions at airlines are about demonstrating you've internalized the rules, not recited them. If you want to see exactly how these questions appear in real airline oral boards, the airline oral exam preparation guide breaks down the regulatory questioning format used at major carriers.