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Monday, April 13, 2009

Totally Blind - but instrumented

Recently we had one of those rare weekend days in Dallas with low hanging cloud cover, but little vertical development (which is pilot talk for "not stormy"). The clouds just hung low over the runway at McKinney, occasionally spitting out a little rain. Perfect!

Since becoming an instrument pilot my view of weather perfection has changed. I used to like those clear blue days after a cold front had come and gone, when the air is crisp and you can see forever. I would get in a 'plane and climb as high as I could while fitting in a cross country (I was trying to build cross-country time in preparation for the instrument rating). The view was tremendous, and once I'd climbed past around 4,000 feet the air would become glassy calm.

Later I got my own aircraft, and the next year, an instrument rating. Now my idea of weather perfection is one where I can fly in actual cloud, shoot some approaches, but avoid too much turbulence, especially inside a cloud. When it rains in Texas, it tends to also storm, especially in the Springtime. Rain here tends to be an event of some violence, not something to mess around with in a lightplane.

So I launched into overcast skies, with 700 feet to the cloud base, on an IFR clearance direct KARLA, a GPS waypoint about 10 miles East of McKinney. Climbing to 2000 on a heading of 90 degrees, per ATC instructions, I swam up into clear air over a sea of white as far as the eye could see in every direction. These clouds where only 500 feet thick, and flat as a pancake.

Cleared to 3000ft, ATC gave me vectors to intercept the 212 radial from the Bonham VOR (BYP), and cleared me for the VOR-DME approach. I identified the radial using my #2 NAV and CDI, plus the DME, and also, just for grins, on my GPS. Fortunately they all agreed I was in the right place and heading in the right direction.

This pleased me. Turning in the clouds had given me what pilots call "the leans", which is vertigo. Pilots cannot trust their bodies senses in clouds, they must only trust the instruments. You turn right, and the body says you are climbing and turning left. When you can see on the panel that you are level and turning right, if you trust your sense of balance which says you climbing and turning left, you will move the controls in exactly the wrong way. That's one reason it takes so long to become instrument rated, and a reason why few private pilots complete it (almost all commercial pilots are instrument rated).

Once on the radial and 12 miles from BYP, I throttled back and descended into the murk at 2200 feet, then 7 minutes later at 22 miles on the DME down to 1400 feet, and clear of the clouds. I could see the airport dead ahead, and approach handed my over the tower. I told them I would end the approach and requested the ILS to runway 17. Cleared direct to the initial approach fix at FLUET, I recontacted DFW approach, and was cleared for the ILS. This time, I stayed at 2500 feet for the outbound leg and procedure turn, which meant I stayed mostly in the clouds, only occasionally popping out of the top at a particularly low area, or between cloud cells.

Once on the inbound leg, I intercepted the ILS glideslope, and setting up for 90 kts and 450 ft/min descent rate, came out of the soup at 1700 ft. The clouds were continuing to rise - now they were 1100 feet above the runway, making the field technically VFR. I canceled the approach and requested the GPS RNAV approach to runway 35 (the other end of the same single 7000 ft long runway at McKinney). ATC cleared me for the approach, and asked me to climb to 2500 feet, but them changed their mind and asked me to make the approach at 2000. I said "OK", because that would put me back inside the clouds, and because I could hear them working a Beechcraft King Air on the same approach. I suspected they want me to stay well below him.

At 2000 ft I kept occasionally catching sight of the ground, but by now I had got completely over my vertigo, and was flying entirely on instruments, which in the case of GPS on a real IFR flight simply meant following instructions until established on the purple line, and then descending along the GPS WAAS glideslope as indicated on the #1 CDI (just like an ILS). The main difference is that an ILS becomes more and more sensitive as you get closer to the transmitter, a GPS WAAS does not, making it very slightly easier to fly.

A GPS receiver uses satellites to fix its position in space, a constellation of 24 satellites controlled by the US military. For precision approaches, GPS uses a ground-based enhancement called "Wide Area Augmentation System", or "WAAS". WAAS approaches require an airport to have no ground facilities at all - so they are rapidly replacing NDB approaches at small airports, and even VOR/DME approaches will be a thing of the past soon.

It only took a few seconds to clear the clouds once on the WAAS glideslope - I could see the runway straight ahead about 5 miles away. I flew through a short, sharp rain shower, and landed. I didn't need the 3 approaches to legal currency, but the experience was useful, and knowing that I could do this for real was invaluable. It was also fun!

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