Flying Training

Lesson 15: Circuits

Saturday 14 January 2006 at 8.00am with Kerry Scott in Citabria VH-WKM

Weather: cool; scattered cloud at 1000 feet, clearing.

WKM. Click to enlarge

Arrived 7.30am for an 8am flight. RRW was already in the circuit so I had WKM, the older Citabria. There are a few differences: it has gyro instruments such as an Artifical Horizon (AH), Direction Indicator (DI, or gyrocompass) and Automatic Direction Finder (ADF); and it has a coarser pitch prop. Most interestingly, though, it is prone to tailwheel shimmy, which manifests itself as juddering from the back on landing, and Kerry briefed me on how to stop this by pushing the stick forward for 2-3 seconds before lowering the tail again, slowly, to the tarmac.

Because of the cloud base we had to do low-level circuits, ie at a height of 800 feet rather than 1300, and this made for a quick succession of very busy circuits. Apart from the tailwheel shimmy, Kerry's preflight briefing included 'good neighbour' considerations, like not flying under 500 feet AGL, and the technique of judging the height on the flare by picking an 'in focus' point at around 20° either side of the line of flight. There was also some refinement of the downwind 'BUMFISH' checks, ie Brakes means checking them by depressing the pedals before returning your heels to the floor, and M means not only Mixture but also Magnetos (Switches then becomes landing light if required). Also HSHS in Kerry's briefing notes is Height, Speed, Heading, Spacing. Niall has Heading first [which I prefer, for what it's worth].

We started up at the fuel bowser, and taxiied to runway 24 initially, but it turned out that other aircraft were planning circuits on 06 so we did an aborted takeoff down to the other end of the runway and lined up. After takeoff we climbed at 70 knots (best rate of climb) to 700 feet, then by the time I'd looked around for other traffic it was 800 feet and time to turn. With these low-level circuits it's almost immediately time to turn again onto the downwind leg, and then the checks are no sooner complete than it's time to turn onto base.

The individual landings are becoming a blur, though I do remember being asked by ATC to come to a full stop after one landing, and hold on the runway for some reason connected with another aircraft's IFR flight. We also executed one go-around, at Kerry's request (not because our approach was wrong, but because it's part of the lesson). I'm afraid my flying wasn't accurate or precise enough for Kerry - I was either diverging or converging on the downwind leg, and she took control on one base leg when I let the height go below 500 feet AGL. At the end of the lesson I asked what I had to do to improve, and she was good enough to go through a detailed debrief which emphasised the importance of keeping the aircraft in trim and flying a precise circuit so as to make a good landing.

Key Points

Here are the points Kerry emphasised:

  1. Aviate is the first priority for the student - Navigation and Communication are subsidiary (though obviously of increasing importance).
  2. Don't forget to trim! Trim for level flight on the downwind leg, and for a descent at 75 knots on base.
  3. Throttle setting is 2000-2300 rpm on downwind (in WKM at any rate).
  4. Don't overshoot the final turn (critical at airports with parallel runways, eg Bankstown).
  5. Small attitude and power changes on final.
  6. Aim point is the taxiway rather than the threshold (allows for undershoot).
  7. The inertia of the plane will carry it down to the runway even if the attitude is changing (eg to allow for wind drift).
  8. On a go-around, make sure the nose is on the horizon - you must establish a positive rate of climb.

And turning to navigation:

  1. Check for wind drift on downwind, but don't overcorrect. Be established on height, speed and heading before checking the spacing. Make sure you're flying parallel to the runway - don't diverge or converge.
  2. On low-level circuits the base leg is level - don't drop below 500 feet above ground level!
  3. In a crosswind, adopt a crabbed approach down to the flare and touch-down.

Lastly, communication:

  1. Downwind call when on tower-controlled circuits. Consider specifying early, mid or late downwind.
  2. In CTAF (Common Terminal Advisory Frequency), announce the base turn. Downwind and final calls seem to be optional. [Later - They are not. But flying comes first.]

(Note: Kerry asked me to set 230 feet (Field Elevation) rather than QNH on the altimeter - I need to know in what circumstances the pilot needs to do this. When there's no ATIS? Or just so that you're almost correct when you hear the QNH? [Later: See Pre-solo air law exam for the answer])

Tailwheel shimmy references:

IAC Discussion Summary

Monty the Answer Man

From TALESPINNERS - November, 2002

Got the Shimmies? by Lee Jewell

I had been having a severe shimmy problem with the Scott 3200 tailwheel on my Citabria for several months and finally decided something had to be done about it. Some years ago, when we were having the same problem with the Precision Flying Club Cessna 170A, Howard Wells told me what he had learned about tailwheel shimmy while watching, of all things, grocery carts do the same thing. The vertical pivot axis around which the tailwheel swivels MUST be exactly vertical, or even slant forward from top to bottom or the wheel will tend to shimmy. If that angle slopes back from top to bottom you are in for a rough ride. With that in mind I planned to install a wedge-shaped shim between the tailwheel mechanism and the leaf spring to correct that angle. But, I quickly discovered the bolt was not long enough to have the required number of threads past the nut.

While I was looking at this, Dave Ballard, an A&P who parks next to me at Blue Ash, said, "Let's take the tailwheel apart". Several parts were worn and I replaced them, but that didn't cure the shimmy. My friend, Kathie Doyle, said the tailwheel was not aligned with the fuselage, something that I couldn't, or didn't want, to see. When I visited with Forest Barber, in Alliance, a week later, the tailwheel shimmied all the way down the runway. Forest thought the leaf spring was twisted and asked if the plane had been ground-looped. It had, though certainly not by me. Forest suggested having the leaf spring re-bent but I decided to just get a new spring from Aircraft Spruce. I knew some Citabrias had a 1 3/4 inch spring while mine had a 1 1/2 incher. The girl at Spruce didn't know the width, so I figured if their catalog said 'Citabria spring' it would be the right one. WRONG!! Too wide for my plane. Return that spring to Aircraft Spruce and call Univair. The first thing the Univair guy asked was, "Is it a one and one half inch or one and three quarter inch spring". Bingo!!! I knew I was in the right place. That new spring cured my shimmy problem!!!

With my newly gained education and experience I began to check all the Scott tailwheels I could find. Gary's 170B, our club 170A, Larry Wolfe's 170B and others all showed signs of the tailwheel tilting one way or the other, a sign of a twisted spring, or the spring relaxing its shape allowing the swivel angle to go to a trailing position. All were experiencing tailwheel shimmy. I relayed this info to Jeff Schaber who is the maintenance officer in our 170 flying club. Jeff went on the web and asked for ideas with many responses saying the same things I had learned from Howard. Confirmation enough! Jeff and I added two shims to the 170A to correct the swivel angle and, lo and behold, the shimmy was gone! This is a temporary fix for the club plane though, as the membership voted at the next meeting to order a new leaf spring for our 51 year old Cessna.

So, if you taildragger types out there are experiencing that dreaded tailwheel tango, take a good look at your leaf spring, correct that swivel angle, and get some relief that isn't spelled R-O-L-A-I-D-S.