Monday, September 15, 2008

Nav7: from Archerfield to Goondiwindi

Soon after taking off from runway 28L I left the controlled airspace of Archerfield aerodrome, on my way to Goondiwindi. This marked the start of second leg of Nav7. The first one was recounted here. The last words I heard from the tower controller were "cleared for take off 28L". No notification of leaving controlled airspace, no "frequency change approved" and no requirement to make an airborne report either. That's because Archerfield is a GAAP airport, an idiosyncrasy of the Australian airspace system. A GAAP airport is a non-radar towered airport with simplified procedures designed to catter for high-density General Aviation traffic. And there's no taxi clearance either.

I was now in Class G airspace, but only a few minutes away from entering the Class C controlled airspace of the Amberley RAAF base. Most week-ends this airspace is de-active and reverts to Class G airspace where CTAF rules apply. It's just like any other non-towered aerodrome, just with much much longer and wider runways. The airspace however can become active on very short notice. The consequences of busting controlled airspace, and especially military controlled airspace being dire, I double-checked the status.

I tried to get the status from the Amberley ATIS on the NDB frequency but I couldn't hear much of the recorded voice message over the morse code and the static. So I called Brisbane radar and requested the status of Amberley, which the controller said was de-active. Good. That meant I could track direct to Gatton at 2500ft instead of having to request a clearance and possibly being told to go the long way around the aerodrome via the westbound VFR route. 

I identified the Goodna VFR entry point for Archerfield on my left and kept my eyes out for any incoming traffic. I could already see Ipswich, the Amberley aerodrome and the Swanbank Power House. I kept listening to the CTAF frequency for Amberley and made a few position reports. There was only one other plane on the frequency, a Cessna 172 doing circuits at Amberley. I could now see the township of Marburg right ahead of me in the foothills of the Mt Grandchester range. I climbed to 4500ft and found some light turbulences, a harbinger of things to come.

I kept tracking west to Gatton, which was easy to identify thanks to Lake Clarendon to the north and the highway and the railway both running west. I turned south and started climbing to 8500ft. I had initially planned to stay at 6500ft but later decided to buy myself a bit more safety and gliding distance in the event of an engine failure. 2000ft extra mean about 3 more minutes at the descent rate obtained for the best glide airspeed of 68 knots, which in turn means about 3 extra nautical miles in the glide. Over a mountain range with peaks at 3700ft, this could make  the difference between landing in a paddock in one of the valleys below or having no other place to land than a heavily timbered area on a mountain slope. Did I mention that one of the highest peaks in this area is named Mt Mistake, at 3582ft?

Turbulences were present throughout the climb but ceased suddenly as I passed 7000ft. It must have been the altitude where the temperature inversion was sitting and prevented air heated up by contact with the ground from rising any higher. My best guess is that this was a subsidence inversion caused by the air being pushed downward in a high pressure system and warmed up by adiabatic compression before spreading out parallel to the ground. Yes, I've been reading the CPL Meteorology book recently in preparation for the PPL Theory exam. Anyway, whatever the reason, the visibility above the inversion was just amazing. I could see all the way to Clifton, about 30 nautical miles away.

I was far from any CTAF so I was listening to the area frequency, which was very quiet on this Sunday afternoon. There was a lot of static though, and I couldn't make it go away by using the squelch knob. I made sure the radio was not in the test position which bypasses the squelch. I was tempted to turn the volume down but then I couldn't hear what was being said on the frequency, which is bad airmanship. Then suddenly it dawned on me. I tried using the COM2 radio instead of COM1 and the problem disappeared.

The air coming into the cabin through the ventilation duct started to get a bit colder, reminding me of the temperature when I left home earlier that day. The Outside Air Temperature (OAT) gauge indicated only 2 degrees C. That's the LCD display in the top-left corner of the instrument panel, left from the airspeed indicator. With the rule of thumb of loosing about 2 degrees per 1000ft, this was consistent with a ground temperature of about 20 degrees.

Approaching Warwick I gave a call to the CTAF, even though at this altitude I couldn't interfere with traffic in the pattern. Better safe than sorry. Over Warwick I turned right and started tracking to the south-west and Goondiwindi via Inglewood. 

Lake Leslie appeared just a few miles to my left. I looked at my map and realised the planned track was supposed to take me right over the lake. I was offtrack to the north by a few miles. I did a one-in-sixty calculation which gave me a new heading for Inglewood. By the time I had finished the calculation and turned onto the revised heading I could already see Inglewood in the distance, more than 20 miles away. Damn visibility. Makes dead reckoning navigation too easy, as Australians like to say.

I tried not to look at the GPS at all on this nav, but I still had it on just in case. I had a quick glance. My groundspeed was 81 knots. My true airspeed was around 115 knots, so that's a headwind component of 35 knots, a lot more than the forecasted 25 knots. The problem was that fuel planning in Nav7 is critical. If I had taken off from Redcliffe with full tanks, I would have had barely enough fuel to complete the flight within the legal limits. That's why I refuelled in Archerfield, which increased my fuel margin by 11 litres, or 16 minutes in the air. Now the increased headwind was robbing me of my fuel margin. Not good.

I decided to descent to 6500ft hoping that the winds were weaker there. They indeed were, and my groundspeed increased to 97 knots, 16 knots more than what I was doing 2000ft higher. The increased groundspeed came at a price though. I had crossed the 7000ft level where turbulences had stopped on climb, and they came back with a vengeance on descent. It was very very bumpy. It was like someone was slapping the airplane from the outside. Maintaining altitude within a hundred feet of 6500ft required all my attention and I was not terribly successful at that. I decided to stay at this level anyway since I only had about 20 miles to run before starting my descent into Goondiwindi.

I found the aerodrome right where I expected it, although later than expected. Maybe I should have delayed my descent a little, which would have given me a steeper descent angle and therefore a better view of the area. The fact that the runway 04/22 was nearly parallel with my track of 247 degrees didn't help spot the runway early either.

No-one could be heard on the CTAF frequency. I overflew the runway, looked at the windsock which was favoring runway 22 with a strong crosswind from the right. I joined the circuit and made a descent crosswind landing on this sealed runway. When I turned on final I was surprised by how narrow the runway looked. I looked it up later and realised it is the exact same width as the runway I am used to at Redcliffe. Only that it is double the length, hence the optical illusion that it is narrower. I had just experienced a textbook optical illusion.

I taxied to the apron, chose a parking spot not too far from the terminal and shut down. I tiddied up the airplane only to found that my empty water bottle had halved in size since I last drank from it at 8500ft. The elevation at Goondiwindi is 714 feet, which means the atmospheric pressure is about 250 hPa higher than at 8500ft, an increase of one-third. One more practical experience that confirms what the book says. One thing I love about aviation is that science is never far if one knows where to look.

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