Flying Lesson # 22 – Compass Turns

June 30, 2012

Leaving the confines of the circuit was like dropping out of that nest and gliding for the first time!

“Archer tower, Cessna 150, juliet oscar oscar, eastern apron for departure to the east, dual, request taxi”.

At the point in which we would normally turn base, we kept flying! After reaching Garden City to the east, we started our climb to 2000 feet; an altitude I haven’t experienced for almost 6 weeks! Tracking towards, and then over Mount Cotton, we headed out toward the bay!

Today I flew with Steve as Robert is on holidays for 2 weeks. At this point, changing instructors for a couple of weeks doesn’t make too much of a difference as most of the ‘hard’ stuff is out of the way. Now that constant circuits are nothing more than a distant memory, we’re starting to cover the more advanced elements of flying. Namely; compass turns, side slipping, precautionary search and landing, as well as forced landings – all in preparation for GFPT (General Flight Progress Test).

The plan for today was to try and cover compass turns, slide slipping and precautionary search and landing. Due to my complete and utter incompetence with using a compass, however, the entire lesson was spent trying to get my head around the concept. For some reason, in the heat of the moment, I just couldn’t decipher the compass headings and know when to roll out. Now, before you write me off completely, here’s an overview of the theory involved and what was required of me:

The attraction of the compass poles to the earth’s magnetic poles, introduces forces that affect the stability of the float assembly in a magnetic compass. These forces will act in opposite direction depending on proximity of the nearest magnetic poles. This results in a false turn indication. This effect is most prominent at either pole extreme ie. north or south. One rule of thumb to help correct this error is to overshoot or undershoot the turn by 20°. In the southern hemisphere (which we are), a southerly turn should be stopped prior to arrival at the desired heading and in a northerly turn, the compass card, or float assembly, should be allowed to pass the desired heading prior to stopping the turn. The easiest way to remember this is with the acronym ONUS – Overshoot North, Undershoot South. 

Now, lets say I’m heading west and want to turn on to a heading of north. Instead of rolling out at 360° or 0°, I need to overshoot north by 20° and instead roll out at 020°. If I was heading east and want to turn to a heading of south, I need to undershoot and roll out at 160° instead of 180°. Now try and work out every possible roll out point in your head. I’m confusing myself just writing this!

This same affect occurs when heading east or west and accelerating or decelerating. Acceleration results in a southerly turn indication and deceleration results in a northerly turn indication. The acronym used here is SAND – South Accelerate, North Decelerate.

I’m sure by now I’ve either confused the hell out of you or possibly lost half my readership. The moral of the story is that it’s a lot to compute whilst also flying the plane. Thankfully, by the end of the lesson, I was all over it and quite comfortable with the roll out headings.

As compass turns consumed the entire lesson, we didn’t have time for the other stuff and so turned around and headed home. Steve showed me what to look for to track towards and locate Archerfield. It was all a bit of a whirlwind, but I’ll have a go at remembering what we looked for. If I’m incorrect, I’m sure Robert will correct me on his return from holidays!

The first point of reference is Mount Cotton, which has a few prominent radio towers on top of it, and then from there, we locate and track toward target which is about 5° north of IKEA along the Pacific Motorway. IKEA is a massive blue and yellow building which is hard to miss and when over flying Target, it literally has a massive target painted on the roof. When over Target, we report our position and intention to ATC ‘Archer tower, Cessna 150, juliet oscar oscar, over Target, one thousand five hundred, received alpha, inbound’. ATC then asks for us to report our 2 mile final. After tracking over Target, Archerfield Airport sticks out like a sore thumb so we start to line up for the relevant runway. When we get level with Garden City at Upper Mount Gravatt, we descend to 1000 feet and then once abeam QEII Stadium at Macgregor we report that we are 2 miles out. From that point everything starts to look very familiar as we join the circuit for landing. Even with a slight cross-wind, my landing was near perfect. I was elated to get a “good landing” from Steve.

Next week, I’ll again be flying with Steve on the Saturday as Robert doesn’t return until the following week. We should be covering side slipping and precautionary search and landing.

Can’t wait!


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