Alternate Airport Fuel Calculator

Calculating Essential Alternate Airport Fuel Requirements

Precise alternate fuel planning is a fundamental element of safe flight operations.

Under Instrument Flight Rules (IFR), pilots must ensure that an aircraft carries enough usable fuel to fly to the intended destination, execute a approach or missed approach, proceed to the designated alternate airport, and continue flying for a specified reserve period (typically 45 minutes at normal cruise speed).

This calculator determines total fuel requirements by factoring in true airspeed, headwind and crosswind components, leg distance, fuel burn rate, and required reserve margins.

Mathematical & Trigonometric Formulas

1. Wind Components

Given wind speed ($V_w$) and the relative wind angle ($\theta$) to the flight path:

$$\text{Headwind Component} = V_w \times \cos(\theta)$$ $$\text{Crosswind Component} = V_w \times \sin(\theta)$$

2. Ground Speed (GS)

$$\text{Ground Speed (kt)} = \text{True Airspeed (TAS)} - \text{Headwind Component}$$

3. Wind Correction Angle (WCA)

$$\text{WCA (°)} = \arctan\left(\frac{\text{Crosswind Component}}{\text{TAS}}\right) \times \frac{180}{\pi}$$

4. Leg Time & Fuel Breakdown

$$\text{Leg Time (hours)} = \frac{\text{Leg Distance (NM)}}{\text{Ground Speed (kt)}}$$ $$\text{Trip Fuel (gal)} = \text{Leg Time (hours)} \times \text{Fuel Flow (gph)}$$ $$\text{Reserve Fuel (gal)} = \left(\frac{\text{Reserve Time (min)}}{60}\right) \times \text{Fuel Flow (gph)}$$ $$\text{Total Fuel Required (gal)} = \text{Trip Fuel} + \text{Reserve Fuel}$$

Step-by-Step Worked Example

Consider an aircraft flying an alternate leg under the following parameters:

• True Airspeed (TAS): 140 kt
• Wind Velocity: 22 kt at an angle of 40° to the flight path
• Leg Distance: 180 NM
• Fuel Flow: 10 gal/hr
• Reserve Duration: 45 minutes

Step 1: Calculate Wind Components

• $\text{Headwind} = 22 \times \cos(40°) = 22 \times 0.7660 = 16.85 \text{ kt}$
• $\text{Crosswind} = 22 \times \sin(40°) = 22 \times 0.6428 = 14.14 \text{ kt}$

Step 2: Compute Ground Speed

• $\text{Ground Speed} = 140 - 16.85 = 123.15 \text{ kt} \approx 123.1 \text{ kt}$

Step 3: Compute Wind Correction Angle

• $\text{WCA} = \arctan(14.14 / 140) \times 57.2958 = 5.77° \approx 5.8°$

Step 4: Calculate Leg Flight Time

• $\text{Flight Time} = 180 / 123.15 = 1.4617 \text{ hours} \approx 88 \text{ minutes}$

Step 5: Compute Trip Fuel & Reserve Fuel

• $\text{Trip Fuel} = 1.4617 \times 10 = 14.62 \text{ gal} \approx 14.6 \text{ gal}$
• $\text{Reserve Fuel} = (45 / 60) \times 10 = 7.5 \text{ gal}$
• $\text{Total Fuel Required} = 14.62 + 7.5 = 22.12 \text{ gal} \approx 22.1 \text{ gal}$

Regulatory Requirements & Safety Margin Summary

Accurate fuel planning prevents fuel starvation incidents during diversions, providing adequate extra capacity for traffic holds, revised vectoring, and ground delay.