A Tale of Two Compasses

Our aeroplane, Tweety, has two compasses. One is a magnetic compass and one is a directional gyro.


To understand them better I did some study to refresh my memory on the subject of direction from the navs course. I’ve written it down because maybe if I can explain it, I can understand it :)

Direction – true, magnetic and compass

As a pilot I need to know where I am and where I’m going. And I need to be able to let other pilots know where I am. Direction and expression of direction is based on geographic and magnetic poles of the Earth.

True Direction – geographic poles

Early astronomers named direction as north, south, east and west according to how these points related geographically. This is what we find on our maps and charts.

Magnetic Direction – magnetic poles

The earth acts like a large magnet in space. It’s constantly moving in a circle slowly around and close to, the geographic poles.

‘Magnetic’ poles are measured using a free swinging magnet and described by the angle they have away from true North.

The difference in magnetic and geographic poles is measured in degrees east or west. This measurement is called a variance and is indicated on maps and charts by isoganals.

The variance is constantly changing. That’s why it’s important to use current charts with up to date isoganals.

Directions on our maps are true directions but directions in our aircraft are shown by a magnetic compass.

So we need to convert true directions to magnetic directions and then we can use our magnetic compass to steer ourselves.

Depending on the direction East or West, the conversion is:

Direction (T) minus Variation (E) = Direction (M)

Direction (T) plus Variation (W) = Direction (M)

A good way to remember this is to think:

‘East is Least and West is Best’


  • Draw a line between two points on a map
  • Measure the distance in NM
  • Add or subtract the variance shown by the isoganals to convert true direction to magnetic direction

Compass Direction

The magnetic compass in our airplane is affected by electrical, magnetic and metallic interference. This can cause the compass to line up in a direction different from magnetic north.

This is called a deviation.

Engineers adjust the magnetic compass (the compass is ‘swung’) to minimise the deviation.

The deviation is measured again and a deviation correction card is kept in the cockpit so the pilot can check it.

In practise, deviation can be ignored if it’s small.

Variance, however, can never be ignored. In Australia, variation can be from 5° west to 15° east. Ignoring variation can result in a significant navigational error.

Tweety's Instruments

Two Magnetic Compasses?

A pilot uses a compass to establish a heading.

A magnetic compass will be affected by the slope of the Earth’s magnetic field (called the ‘dip’). This will cause it to read incorrectly when the aircraft is in a bank or during acceleration. It will work well when flying straight and level.

A directional gyro (or heading indicator) is a gyroscopic flight instrument not affected by the ‘dip’ when the aircraft is banking or during acceleration. It should be set every 15 minutes using the heading shown on the magnetic compass.


South Coast Recreational Flying Club Navigation Course notes

Ground Training Manual by Jan and Val Dyson- Holland


Fly high; Fly safe,



Tweety’s Homecoming

In a whirlwind couple of weeks, we’ve been from Dalmeny in the south for a week of R+R followed by a road trip to Leeton (via Temora) in the west.

We had opportunity to fly at Moruya Airstrip in their club’s Gazelle while we were away. What a joy – we surely live on the most beautiful coastline! And what a great little aeroplane.

I was also able to get a flying fix in a Foxbat with Lizzie when we called in briefly at the air show in Temora. Coastline is replaced by patchwork paddocks as far as the eye can see out there – very different, equally amazing! The show itself was quiet and we wondered about the wisdom of having it on the Easter weekend?

A few days at home and the weather looked good enough to bring Tweety back from Mornington Peninsula. Greg and Andy flew down to Melbourne last Wednesday on a commercial bird and trained, planed and automobiled it to Tyabb Airfield – former home of Tweety Bird. To Jack and the team at Peninsula Aero Club – many thanks for all your help :)

Weather was good to fly the Little Yellow Bird home on Thursday. After a circuit and some final adjustments, they cleared Sale tower and were off. They stopped briefly for refuel (she carries just over 50 litres) at Bairnsdale and Merimbula  and then back to Jaspers by the late afternoon.

We excitedly followed their progress all day through SAR times and sms messages and were able to be there when Tweety flew in on runway 24 at Jaspers Brush – what a moment, one I won’t forget in a hurry:)  Greg and Andy literally fell out of the doors when they arrived – they were pretzeled in the shape of  a Tweety Bird seat lol – she’s not built for long distance travel.The flight took about five and half hours.

What an amazing experience. I still can’t believe she is ours! To Andy, Alex and Liz – many thanks, you dudes – you guys are the best :) :)

Tweety on the ground at Jaspers!


Wecome home Tweety

Oh Happy Day!

Oh happy day – we have just bought our first aeroplane – yay!!!!!

She is a Skyfox Gazelle CA25N with a 912A  Rotax engine and has just retired from being on-line at Peninsula Aero Club Flying School.

We flew a gazelle in Moruya, a  very forgiving aeroplane for newbie pilots – just a lovely little aeroplane to fly – cruise speed of 70 KIAS and very easy to handle.

The wings fold back for trailering  and the doors can be taken off completely,  if you want to that is. And the visibility in these aeroplanes is amazing.

It hardly seems real, we almost can’t believe it  - what a privilege and what a responsibility ! And what fun :)

Here are some pics of Tweety from the aero club –  isn’t she a beaut? I’ll keep you posted on our next phase of the adventure – bringing Tweety Bird home!