First solo in the TB20 with an RNAV approach thrown in

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After completing differences training in the new TB20 share-o-plane at Gloucester, I wanted to make a short solo flight to consolidate what I’d learnt and become more comfortable in the pilot seat. With the aircraft free on Sunday afternoon, I booked a slot. Rich, one of the other part owners, happened to be at Gloucester airport and at short notice agreed to join me for the flight.

I had planned a simple local navex via Shobdon and Hanley, only expected to be airborne for half an hour or so. After the usual pre-flight, booking out etc., we hopped in and departed to the North West towards Shobdon. In the warm weather, it’s important to get the speed up to about 120 knots for air-cooling at the expense of a faster climb rate, so we clambered slowly. While I did get the gear up, Rich had to remind me to put the takeoff flap away. Perhaps it’s because of a change of habit from the Arrow and Warriors which only used flap for short field takeoffs.

Soon we were up at around 4,000 feet and on track.

“Lean Find” Demonstration

Rich demonstrated the Shadin fuel flow/temperature gauge and specifically how to use the “lean find” feature to set the mixture to the optimal flow rate. We were achieving 140 knots at about 45 litres/hour. To be honest, some of that went over my head because its not something I’ve fully read up yet and the sequence of buttons and knobs to press is quite complex. But the principle of selecting “lean find” mode and then adjusting the mixture until the lights flash was clear, so I now know what to look for.

Since we pay for the aircraft based on how much fuel we use, it’s to each pilot’s benefit to squeeze the best performance out of the engine without affecting it’s long term life.

Overhead Shobdon (at about 4000 feet), we turned and tracked east, then back south towards base.

Setting up for a GPS approach

On our return, I radioed up Gloucester, giving my position and altitude, requesting a rejoin via RNAV 09 UVNOP for a VFR GPS practice approach. Perhaps being quieter and being later in the day, this was granted and Rich helped me to punch in and activate the approach on the Garmin GTN650. He explained that the autopilot really has three modes – this is my current understanding and may not be 100% correct:

  1. Heading and/or Altitude tracking, where it simply flies the heading (on the heading bug) and/or maintains current altitude. You turn the right hand knob on the HSI to adjust your course which drives the orange heading bug and the autopilot will turn the airplane and follow it.
  2. NAV (Navigation mode), where it follows the Course Pointer (CP), a yellow pointer with deviation bar. This is set by turning the left hand knob and looks similar to (but not the same as) a VOR. The course deviation indicator (a yellow line normally inline with the yellow pointer) is driven by the GPS box.
  3. APP (Approach mode), where it also follows the Course Pointer, but to much tighter tolerances. This also arms the “Glide Slope” feature, which will acquire and fly down a vertical glide slope such as an ILS beam or GPS simulated vertical slope.

With the approach loaded and activated, we turned the CP towards the IAP (Initial Approach Fix) of UVNOP as directed by the GPS and the autopilot turned towards it. The distance was clearly shown on the corner of the GPS box. Inspecting the approach plate, we could see we were able to descend down to 2,500 feet (Altitude = QNH, not height = QFE) by that waypoint. We’d keep both altimeters on the QNH setting rather than using QFE because it means you would err on the side of caution (ie not think you were lower than you are). As we approached the IAP, I pressed APP to arm the approach mode and conducted the initial landing checks, but did not yet add flap or lower the gear or adjust the propeller speed. The mixture was gently increased to help with engine cooling and we slowed down from 140 to 110 knots.

We had also setup the ADF and identified the Gloucester NDB, which forms part of the “Missed Approach” procedure should we need to go-around rather than land.

As part of our briefing for the approach, we checked and calculated the Minimum Descent Altitude to be 650 feet, which included 50 feet reaction time. This is an altitude, so the altimeter would show about 100 feet on touchdown. Although GPS can be very precise, this approach is considered a Non-Precision Approach with advisory glideslope hence has higher minimums.

Flying the approach

At UVNOP, the Garmin status line on the bottom of the screen informed me to turn to a new course of 156 towards the Intermediate Fix and gave me a per-second count-down. Turning the CP, the autopilot again banked to capture the new course. With the mantra “turn, time, talk”, we reported our position and were told to report again at the intermediate fix. The plate showed we could now descend to 1700 feet, so I turned off the ALT-HOLD and pressed the down button a couple of times to achieve an appropriate descent rate, then pressed ALT-HOLD again when we almost got there.

We were still flying at about 110 knots – its important not to be too fast at this stage because if you exceed the gear extension speed, you may not be able to slow down enough to land in time. Reporting at the Intermediate Fix, we were told to continue the approach. There was one last turn onto the final approach course and the course deviation indicator showed we were well on track. In VFR conditions, the runway came into sight directly ahead, still about 10 miles/5 minutes away – long before our decision altitude of 480 feet (or 600 for me as an IR(R) pilot).

Final Approach

At the FAF (Final Approach Fix), the glide slope indicator aligned to show we were on glidepath and this was acquired by the autopilot. This is the time to check that the altitude was correct on the altimeter, cross check with DME then lower the gear, increase propeller speed to 2,500 and first stage of flaps, slowing the plane to around 100 knots with a descent rate of around 600-700 feet per minute. This helps the autopilot track the glideslope down nice and gently.

At about 2 miles out, it was time for the final stage of flaps (which needs a good push on the yoke to compensate), then double check reds/greens/blues (mixture full rich, gear fully down, propeller fully fine) and slowing further to around 75 knots. At our decision altitude of 650 feet, it was time to make the decision to land or not, so I pressed the big red button on the yoke to disengage and manually flared and landed. The autopilot will fly the aircraft into the ground if you didn’t touch it, and it’s up to the pilot how early or late he switches to manual.


There’s quite a lot of new aspects to me of this type of autopilot coupled approach (and new aeroplane), and I need to get my head fully around what the correct sequence of actions is throughout the approach. I’ve written this out in much greater length to review and cross check, but once I have practiced this a few times on my own, I think this should become more straightforward (I would never say routine). This should help if I need to do it in more challenging conditions.

A big advantage of GPS approach procedures is that you can join them from any direction, with at most a 90 degree turn at the IAP. The autopilot takes a lot of the workload and allows you to double check you are doing the right things, but of course you still need to be able to do these manually if it fails. It is also said that they can be more stable for an autopilot to track than an ILS, for a variety of technical reasons.

Big thanks to Rich for taking time out to fly with me and show me how the kit works.

There’s lots still to learn.

Total time today: 1:00
Total PIC: 164:50
Total Time: 267:15

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