- The external circle represent distance in nm and speed in knots.
- The yellow outer circle indicates time.
- By aligning 1 hour on the yellow time scale with any speed on the white scale we get nm/hour or knots.
To Work Out Ground Speed
If we know how far we have flown and how long it took we can find groundspeed
- Find distance flown and on the outer white circle and set the time on the outer yellow circle against it.
- In the example below let’s say it took us 40 minutes to fly 40 nm. Set 40 against 40.
- We then look at the hour marker we can see its 60 knots per hour ground speed
- Let’s try another example below, using 45nm in 37 minutes
- Our groundspeed at the hour marker is 70 knots
To Work Out Time
When we are pre flight planning or adjusting our plans in flight, often we know our speed or expected speed and we know the distance we have to travel. We then need to estimate time so that we can calculate fuel .
- Find intended ground speed on the outer white circle and remember we are talking speed per hour.
- Set the 1 hour time on the outer yellow circle against the speed you will fly.
- We can now read off time against and distance
In the example below at 90 kt per hour let’s look for the time taken to fly 60nm. Follow the white distance circle to 60 nm and you will note on the yellow circle it say 40 minutes.
To Work Out Distance
Sometimes we may need to work out distance. For example if we were unsure of position and we knew we had been flying at 80 kt ground speed for 45 minutes we could estimate distance and use a ruler to locate on the map where we are.
Looking at the example below we have set speed to 80 kt per hour.Look to 45 minutes on the yellow scale and we can see that we have flown 60 nm.
We may also want to estimate distance to a waypoint or destination. If we know we have a ground speed of 80 kt and we have an interval of 70 nm we can see the total interval will take 52.5 minutes. If we have been flying this interval for 30 minutes already, then we have 22.5 minutes to the next waypoint or destination.
To Work Out Fuel
Every flight we have to estimate fuel both before the flight and when we have changes to our flight plan.
Below we have set the 1 hour marker to 20 L per hour on the outer scale.
You will note that we can now read off any time on the yellow scale and and know the fuel burn on the white scale.
Eg: In 3o min = 10L, 45 min = 15L, 40min = 13.25L
For the theory part of your XC exam you will only need to know the above 3 calculations.
To Work Out GS & Drift – How Much To Adjust Your Heading
The wind drift component below is needed for pre-flight planning in your practical test.
- The green section represents wind direction
- The 10, 20, 30 etc running in a cross pattern from the centre are wind speeds
- The outer circle is initially TAS, then later once you know your crosswind component, its wind speed.
- The 2nd outer scale is degrees to adjust for drift.
How to do it is in the video, but here are the steps.
- Position the black TAS triangle pointing at your intended cruising TAS
- Put the wind direction for that leg in line with black TAS triangle and 1C marker
- Circle the wind speed on the scale between the centre and the black TAS triangle
- Turn your green scale till the intended flight direction is at the top near the 1C marker
- Your circle you drew now represents the amount of crosswind and head or tail wind. If its in the top half it is a headwind. If its on the right of centre it is a crosswind from the right. Find your crosswind strength and headwind or tailwind strength by reading of the scale in the middle. Write them down.
- Adjust your GS by TAS minus headwind , or TAS plus tailwind.
- Now on the outer circle go around to the crosswind strength. The next scale in, will tell you in degree how much you need to adjust your heading to be able to stay on track.
Apologies for the stability, there was turbulence during the filming of this motion picture.