11.5 14 CFR 91.167 Fuel Requirements, Preferred Routes & Flight Planning Calculations

14 CFR 91.167 Fuel Requirements, Preferred Routes & Flight Planning Calculations

Quick Answer: Under 14 CFR § 91.167(a), an IFR flight must carry fuel to (1) reach the first airport of intended landing, (2) fly from there to the alternate, and (3) fly 45 more minutes at normal cruising speed (30 minutes for helicopters). Paragraph (b) deletes only leg (2) — never the 45 minutes — and only when the destination has a published standard instrument approach and the forecast from 1 hour before to 1 hour after the ETA shows a ceiling at least 2,000 feet above airport elevation and visibility at least 3 statute miles. That is the same weather test as the 1-2-3 rule in § 91.169(b). Standard alternate minimums under § 91.169(c) are 600-2 (precision) and 800-2 (non-precision). Route planning uses published preferred IFR routes, TEC routes, and CDRs; the arithmetic is GS = TAS ± wind component, time = distance ÷ GS, fuel = time × fuel flow, and ETA = departure + ETE, expressed in UTC.

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1. IFR Fuel Requirements: 14 CFR § 91.167

This is one of the most frequently missed regulation questions on the IRA knowledge test, because candidates confuse the fuel rule (§ 91.167) with the alternate-filing rule (§ 91.169). They share a weather test, but they are different regulations answering different questions.

+-----------------------------------------------------------------------------+
|                14 CFR 91.167(a) - IFR FUEL REQUIREMENT                      |
|                                                                             |
|   No person may operate a civil aircraft in IFR conditions unless it        |
|   carries enough fuel (considering weather reports and forecasts and        |
|   weather conditions) to:                                                   |
|                                                                             |
|     [1]  Complete the flight to the FIRST AIRPORT OF INTENDED LANDING;      |
|            +                                                               |
|     [2]  Fly from that airport to the ALTERNATE AIRPORT;                    |
|            +                                                               |
|     [3]  Fly after that for 45 MINUTES at normal cruising speed             |
|          (30 minutes for helicopters).                                      |
+-----------------------------------------------------------------------------+

The § 91.167(b) Exception — What It Actually Removes

Paragraph (b) states that item [2] — the alternate leg — does not apply when both of the following are true:

1. Part 97 prescribes a standard instrument approach procedure to the first airport of intended landing (or a special IAP has been issued to the operator for it); and
2. For at least 1 hour before through 1 hour after the estimated time of arrival, the appropriate weather reports or forecasts indicate a ceiling at least 2,000 feet above the airport elevation and visibility at least 3 statute miles.

[!WARNING] The 45 minutes never goes away. Removing the alternate from the flight plan removes the alternate leg from the fuel calculation. It does not shorten or delete the 45-minute reserve, and it does not convert the flight into a VFR-reserve operation. The three-part structure of § 91.167(a) always applies to IFR flight; only item [2] can drop out.

Fuel Rule vs. Alternate Rule — Side by Side

Standard Alternate Minimums: § 91.169(c)

Once an alternate is required, the airport chosen must be forecast, at the ETA at that alternate, to meet:

These are the standard values. An approach chart carrying the inverted-triangle A symbol has non-standard alternate minimums published in the front of the Terminal Procedures Publication, and an "A NA" notation means the airport may not be used as an alternate at all. A WAAS-equipped aircraft may plan an RNAV (GPS) approach at both destination and alternate, but must use LNAV or circling minima, not LPV minima, when checking alternate weather.

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2. Preferred IFR Routes, TEC, and CDRs

ATC route amendments are the single most common source of a rushed, mis-copied clearance. The FAA publishes the routing it wants to see, and filing it is the cheapest workload reduction available to an instrument pilot.

Planning rule: if a preferred route exists between your city pair, file it. Filing direct between two major terminals almost guarantees a full-route amendment read at high speed by a busy clearance delivery controller — exactly the situation in which altitude and transponder errors are made.

Route planning also requires cross-checking the same publications the ACS lists as route-planning inputs: available navigational facilities, special use airspace, preferred routes, primary and alternate airports, en route charts, the Chart Supplement, NOTAMs, and the Terminal Procedures Publication.

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3. The Core Flight Planning Calculations

Groundspeed, Time, Distance, and Fuel

$\text{Groundspeed (kt)} = \text{True Airspeed} \pm \text{Wind Component}$ $\text{Time (hr)} = \frac{\text{Distance (NM)}}{\text{Groundspeed (kt)}} \qquad \text{Fuel (gal)} = \text{Time (hr)} \times \text{Fuel Flow (gph)}$

A useful cruise rule of thumb: true airspeed increases roughly 2% of indicated airspeed per 1,000 feet of altitude, so 120 KIAS at 8,000 feet is approximately 139 KTAS before temperature corrections.

Worked Example: A Complete § 91.167 Fuel Computation

1. Groundspeed: $120 - 15 = \mathbf{105\text{ kt}}$
2. Time to destination: $180 \div 105 = 1.714\text{ hr} = \mathbf{1{:}43}$
3. Time to alternate: $45 \div 105 = 0.429\text{ hr} = \mathbf{0{:}26}$
4. Reserve: $\mathbf{0{:}45}$ at normal cruising speed
5. Total required flying time: $1{:}43 + 0{:}26 + 0{:}45 = \mathbf{2{:}54}$ (2.90 hours)
6. Fuel required: $2.90 \times 9.5 = 27.6\text{ gal}$, plus 1.5 gal taxi/climb allowance $= \mathbf{29.1\text{ gallons}}$

If the destination met the § 91.167(b) test, step 3 would drop out and the requirement would fall to $1{:}43 + 0{:}45 = 2{:}28$ (2.47 hours), or about 23.5 gallons plus the taxi allowance — but the 45 minutes stays.

Estimated Time of Arrival and UTC Conversion

Flight plans, clearances, and every weather product are expressed in Coordinated Universal Time (UTC / Zulu). Convert by adding the offset for the local time zone:

Worked example: Departing at 1:15 p.m. CDT with an ETE of 1:43. Departure in UTC is $1315 + 0500 = \mathbf{1815Z}$; the ETA is $1815 + 0143 = \mathbf{1958Z}$. That ETA is the anchor for the § 91.167(b) and § 91.169(b) weather windows — evaluate the destination forecast from 1858Z to 2058Z.

Climb and Descent Planning

• Climb rate required for a published gradient: $\text{FPM} = \left(\dfrac{\text{Groundspeed}}{60}\right) \times \text{Gradient (ft/NM)}$
• Descent distance (3:1 rule): 3 NM for each 1,000 feet to lose
• Descent rate on a 3° path: $\text{FPM} = \text{Groundspeed} \times 5$

[!TIP] Run the numbers against the worst case, not the flight plan. Compute the fuel requirement using the forecast headwind at your planned altitude, and recompute if ATC assigns a lower altitude, a reroute, or a hold. The regulation is written in terms of what the aircraft actually carries, not what the original plan assumed.