Heading, ground speed and fuel calculation

Admin · December 19, 2011

Heading, ground speed and fuel calculation

We plan to cruise at 70 knots CAS, and using the rule of thumb that TAS is 1.5% greater than CAS for each 1000 feet of altitude amsl we will increase CAS by 6% to arrive at a TAS of 75 knots at 3500 feet. That calculation (or perhaps 2% per 1000 feet) is near enough for navigation purposes but it is not actually correct; see the section on density altitude.

To complete our flight plan we have to estimate the heading to fly, the ground speed and the flight time for each route segment. We will use the mental arithmetic method outlined in section 4.2.

1. Find the crosswind component of the forecast wind velocity by estimating the angle at which the wind meets each track, multiplying that by 1.5 and applying the result as a percentage of the wind speed:

(a) Segment 1: track = 094° true, w/v = 060/10 knots: angle = 34 × 1.5 = 51% of 10 = 5 knots crosswind.

(b) Segment 2: track = 061° true, w/v = 060/10 knots: angle = 1 × 1.5 = 0% of 10 = nil crosswind component.

2. Estimate the wind correction angle by dividing the crosswind component by the TAS and multiplying the result by 60:

(a) Segment 1: crosswind = 5 kn; TAS = 75 kn: 5/75 × 60 = 4° wind correction angle.

(b) Segment 2: crosswind = nil; = nil wind correction.

3. Calculate the true heading, remembering that the wind correction is applied in the direction the wind is coming from; so that:

(a) Segment 1: wind is from the left, correction angle is 4° and the heading is 094 minus 4 = 090° true.

(b) Segment 2: wind is from dead ahead, correction angle is 0° and the heading is 061 minus 0 = 061° true.

4. Convert the true headings to magnetic headings. There is an isogonal passing through the chart in the route planning module (the dashed purple line), which indicates the local variation is 10½°E. Applying that to our true bearing — remembering our aide-memoire "variation east, magnetic least" — and we have the following headings: 079°, 050° and 031° magnetic.

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5. Estimate the ground speed. Deduct the (acute) angle at which the wind meets the track from 115 (for angles up to 60°, use 105 for greater angles) and apply that as a percentage of the wind speed, to max 100%, subtract the result from TAS if wind coming from ahead to abeam, otherwise add; thus:

(a) Segment 1: track = 094° w/v = 060/10 kn: angle = 34; 115 –34 = 81% of 10 = 8 kn headwind and ground speed = 67 knots.

(b) Segment 2: track = 061° w/v = 060/10 kn: angle = 0; 115 –0 = 100% of 10 = 10 kn headwind and ground speed = 65 knots.

6. Estimate the time interval for each leg dividing the distance by the ground speed and converting to minutes; thus:

(a) Segment 1: distance = 74 nm; speed = 67 kn: 74/67 × 60 = 66 minutes.

(b) Segment 2: distance = 52 nm; speed = 65 kn: 52/65 × 60 = 48 minutes.

Sum the segment ETIs to produce the estimated time en route [ETE]: 144 minutes.

7. Calculate the total fuel needed including:

(a) The fuel consumed from start-up at the departure airfield to the set-heading point (10 minutes)

(b) Extra fuel consumed in the climb (two minutes per 1000 feet = 6 minutes, refer to fuel planning)

(d) Fuel consumed in the circuit and landing at the destination airfield, including an allowance for delays in the circuit (10 minutes)

(e) The fixed 30-minute reserve (see Note 1 below)

The total is 200 minutes — well within our full fuel endurance of 240 minutes and giving us 40 minutes margin on top of our fixed 30-minute reserve. We will use the fuel log during the flight.

(Note 1. The 30-minute absolute minimum reserve is for the average ultralight; if a particular RA-Aus aircraft has a fuel capacity and a forced landing capability more akin to that of a light GA aircraft then the standard 45-minute minimum reserve specified in the Civil Aviation Advisory Publication CAAP 234-1 should be employed.)