Monday, October 6, 2008

Nav7: Back to Redcliffe

Before starting up in Goondiwindi for the last leg of Nav7 I didn't forget to turn the GPS logger back on, so this post will be illustrated with the actual track I flew. Hopefully this will compensate for the lack of photos. GPS tracking is the aviation equivalent of reality TV: no details will be spared, and all embarassing navigation mistakes will be exposed.


The plan was to fly from Goondiwindi (YGDI) back to Redcliffe (YRED) via Millmerran (YMMN), Toowoomba (YTWB) and the township of Esk. As can be seen below, I departed Goondiwindi on runway 22, flew a short crosswind leg, turned downwind and kept climbing for an overhead departure.


I could have made my life simpler by departing on downwind rather than overhead since my outbound track was nearly parallel to the downwind leg. Another thing I could have done better would have been to make sure the extended outbound track pointed at the airfield instead of being about one mile to the north of it.

I kept climbing to 5500ft while maintaining a heading of 033 degrees to Millmerran. The track to Millmerran as drawn on the map was actually 041, but given the forecasted westerly winds at 25 knots I had computed before the flight that the nose of the airplane had to be pointed 8 degrees left of track to compensate for the expected drift. The idea was to keep flying the planned heading and check later if I was where I expected to be or if any adjustements had to be made. In a nutshell, that's the navigation technique known as dead reckoning.

About two thirds of the way to Millmerran I could see on the left-hand side a power line oriented north-south crossing a road oriented south-west north-east. From the WAC chart I could easily tell where I was, which was good, except this was not where I was supposed to be!

I was actually about 5 miles north of the track. I had already flown 45 miles from Goondiwindi, with 20 more to go before Millmerran. Maybe the wind was not as strong as forecasted or had assumed a different direction, resulting in my heading over-correcting the expected drift and taking me further to the left of track. Or maybe I misaligned the compass and the DG. Finding why it happened was anyway secondary to fixing the problem.


That's when the navigation technique known as one-in-sixty comes in handy. It is based on the fact that being off track by 1 mile after 60 miles flown means an angular error of 1 degree. 2 miles after 60 miles mean 2 degrees, and so on. It works great, at least for small angles. From a mathematical point of view, this relies on the fact that the function tan(x) can be approximated as x for small values of x. But let's not complicate things, since the beauty of the method is that it can be done in the cockpit while flying.

It goes a little bit like this: I am off track by about 5 miles after about 45 miles flown. Let's say 50 instead of 45 because it makes calculations easier. 5 in 50 is like 6 in 60. So I was over-correcting drift by 6 degrees. If I turn right 6 degrees now, I will fly parallel to my intended track. However, if I do that I will still be 5 miles off-track. So I need another one-in-sixty calculation to determine the closing angle that will take me straight to my destination. I still have 20 miles to run till Millmerran, 5 in 20 is like 15 in 60, so if I add an extra 15 degrees to the 6 degrees found previously, this means I need to turn right 21 degrees to be on a track to Millmerran. New heading is therefore 012.

And it works, as can be seen in the picture above: after turning onto the new heading my new track was pointing at Millmerran. Near Millmerran I went a bit right of track in order to keep the aerodrome on my left, simply out of curiosity. Then I turned onto a new heading and tracked for the Toowoomba aerodrome. I used the drift correction from the one-in-sixty exercise conducted a few minutes before to compute a new heading.

I picked up the ATIS for Oakey on their VOR frequency. The sequence letter for the information was Zulu, meaning the military control zone was not active and had reverted to a CTAF. My track only clipped the CTR by a mile or so, but I would nevertheless have needed to request a clearance if it had been active. No need to upset the military. I prefer to see fighter jets and big black helicopters at airshows rather than at the end of my left wingtip.

Toowoomba is blessed with an NDB navaid, so I tuned it in and the needle on the ADF was pointing straight ahead. The ADF needle points at the navaid in relation to the airframe, not in relation to the track as is the case with VOR navaids, so one has to take drift into account. However, since I was no longer offsetting a lot of drift, the nose of the airplane was pretty much pointing where the airplane was going, and therefore the needle pointing straight ahead was good news.

I made a couple of calls on the Toowoomba CTAF to let everyone know I was intending to overfly the field at 5500ft. At this altitude I couldn't really have conflicted with traffic in the circuit, but it's good practice. The GPS logger can be cold and cruel at times, pointing mistakes that would otherwise go unnoticed, but in the present case I'm happy to report that my track took me right overhead the airfield.


After Toowoomba I tracked for Esk. Under my left wing were the Cressbrook and Perseverance Creek reservoirs that I had overflown on Nav3 six months ago on my way to Oakey. The hilly scenery was really beautiful in the low light near the end of this winter afternoon. I found Esk and confirmed it with Mount Esk, the racetrack and the red rooves of the Esk Hospital. I descended to 3500ft and once clear of the ranges kept descending to 1500ft to remain under the CTA steps of Brisbane International. 

The GPS logger got its revenge when I joined the circuit at Redcliffe. I was coming in from the west at 1500ft and my track was taking me straight to the airfield. I turned right a little so as to be on the south side of the extended centerline for runway 07/25.

The dead side at Redcliffe is always to the south because all circuits are over the water, i.e. left-hand circuits for 07 and right-hand circuits for 25. That's both for reasons of not overflying populated areas, and avoiding any controlled airspace violation. The Brisbane CTA step over the Redcliffe aerodrome is at 1500ft, and there's a 1000ft step just a couple of miles south of the field.

I knew from listening to the CTAF frequency that runway 07 was in use, which I confirmed by looking at the windsock as I was descending to circuit height over the dead side. The winds had changed since earlier in the day when I took off on 25.

I joined crosswind above the opposite end of the runway. I turned downwind a bit too early so I widened the leg a bit. This is why the track does not look anything like the nice figures in flying training books. I landed on 07, taxied out and refuelled the airplane. I cancelled SARTIME by phone then sat down a few minutes to jot down some notes about the flight so that I could blog about it later. Total flying time today 4.3 hours, at $185 an hour plus $29 for landing at Archerfield. Oops. Looking forward to being able to take passengers with me to share the cost.

So that's it. The navigation part of the PPL is now over. It's a bit of a weird feeling. Next step, PPL pre-test and then test. And hopefully many more adventures after that. And I need to blog about the CSU endorsement. And the PPL theory test. And the airshow in Singapore. And write a guest post for Plastic Pilot. Too many blog topics, not enough time.

2 comments:

PlasticPilot said...

Brilliant ! A lesson about the good old navigation means. You're brave not taking a look at the GPS in flight to simply "reset" your navigation error.

Julien said...

There won't be any GPS allowed in the PPL flight test in two weeks, so I thought I'd better practice in real conditions. Following the magenta line on the GPS display is not really a challenge :-)