Wednesday, January 28, 2009

Just a Little Psychic...

That's what weather forecasters used to believe, that they were just a little psychic. But now they depend more and more on computer predictions. There are intrinsic limits to numerical weather forecasting, first detailed by Edward Lorenz in the ground-breaking paper "Deterministic non-periodic flow," [Journal of the Atmospheric Sciences, vol. 20, pages 130–141 (1963)]. Lorenz discovered that tiny changes in the beginning parameters of weather systems lead to large changes a little later, the so-called "Butterfly Effect". More poetically, the flap of a butterfly's wing in Australia might plausibly lead to a hurricane in New Brunswick.

I recently discovered a new weather product and have been pretty pleased with the results. First, what's wrong with what we have?

As you know, aviation terminal forecasts (TAFs) discuss wind, visibility, ceiling, and precipitation for 24 or 30 hours, depending on the location. They are only good within a short radius of the airport. There is no long-range forecast involved, and there is no mention of temperature or dewpoint. I've always been surprised by the decision to ignore temperature: it would be nice to know if there is going to be a density altitude problem or a need to have the airplane preheated.

Commercial providers like The Weather Channel, Weather Underground, and Accuweather provide long range forecasts and temperatures, but make little or no mention of ceiling or visibility.

What I would like would be a longer-range forecast including long-range ceiling, visibility, wind, and temperature. And I think I've found one: the National Ocean and Atmospheric Administration (NOAA) put out several such products called LAMP. LAMP stands for Localized Aviation MOS Program (trust the government to nest an acronym inside an acronym: MOS stands for Model Output Statistics).

My favorite is the NAM-MOS forecast, available here. Another acronym? NAM is the current North American Mesoscale model; you can read about other models in Wikipedia.

Here is a typical NAM-MOS forecast.

There's a lot of information there, and it's not the easiest thing to read. (Maybe this would be a good iPhone app?) The top says that it's the forecast for Boise on 28 January 2009. The columns on the left are, in order

  • HR: hour
  • N/X: maX and miN temperature
  • TMP: temp
  • DPT: dewpoint
  • CLD: cloud cover
  • WDR: wind direcion (nearest 10 degrees)
  • WSP: wind speed
  • P06, P12: precipitation probabilities
  • Q06, Q12: quantitative precipitation probabilities
  • T06, T12: thunderstorm probabilities
  • SNW: snowfall (categorical)
  • CIG: ceiling (categorical)
  • VIS: visibility (categorical)
  • OBV: obstruction to visibility

    I highlighted the ones I focus on in red. It's great to be able to forecast that, for example, density altitude might be very high in three days, and therefore decide not to invite a fourth friend along.

    The categorical outlooks are listed at the NOAS MOS website, but the short of it is that 5 or more means VFR. (Category 5 ceiling means between 2100 and 3000 feet AGL, category 6 is between 3100 and 6500, etc.)

    So now I know that the forecast for Saturday morning in Boise is for a temperature of 26 (no preheat required), light winds out of the southeast, and high (ceiling category 7) overcast clouds with no obstruction to visibility. Twin Fall is forecast to have similar weather, while Pocatello is forecast to have marginal VFR conditions in fog.

    You have to take this with a grain of salt, because it is a forecast, and there are plenty of butterflies in Patagonia. But it's much better than what I've had before. Give it a try.

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  • Wednesday, January 21, 2009

    Enunciate, please

    It seems that night currency time is the only time I get these days to work on improving my own flying. This evening, my son's Cub Scout meeting was cancelled, so I grabbed the opportunity to run out to the airport and do my night landings. It was a Moonless night, too, so night really meant night.

    The three landings make me legal, but I wanted to do something to keep me proficient. It was too late to call anyone to ride along as a safety pilot for instrument work; I decided to do a short cross-country and play a little game. So I got a DUATS briefing and headed out. [I know a lot of experienced pilots who don't think that they need to bother with a preflight briefing, although I have to say that I don't understand their reasoning. I mentioned last month the pilot who neglected the briefing and flew to an airport with an active Flight Restriction.]

    The game is this: you are alone in the airplane, but you can't do anything without asking for someone to do it, out loud, and saying "please". It's a variation on "Simon Says" or "Mother May I?" I developed it a few years ago when, under the wrong-headed impression that I wanted to be an airline pilot, I used it to practice for interviews.

    It's even more fun with a British accent, so I listened to the BBC in the car.

    So here's how it goes.

      "500 feet, landing light off, please."

      "1000 feet, fuel pump off, please, and mind the fuel pressure."

      "Set the heading bug to 016, please."

      "Tune the Idaho Falls VOR, please."

    You get the idea.

    While it sounds silly, I have found that playing this game really makes my flying smoother. There must be some fancy explanation of the cognitive benefit of multi-modal expression in relation to the neural pathways. Whatever; it works.
    It really was a lovely night.

    After my third landing I let myself into the FBO (I have a key, but the lineman had left the door unlocked and the coffee pot on) and put my flight bag in its locker. I was looking forward to flying with my students tomorrow, but that short time alone talking to myself gave me some wanderlust. (This is ironic, since I have visited the East Coast twice already this month, but I want to be a pilot, not a passenger.) "I want to go somewhere," I said aloud, not impolitely but not saying "please", either. I looked at the flight tracker, counting arrivals strung out on a path from Lovelock, Nevada to San Francisco. I stood by the big map on the wall, pulled the string to San Francisco, and sighed: too far, for now. Maybe Reno? Rapid City? Bozeman? Alas, no big trip for me, at least for a while. So I turned off the coffee and locked the door and listened to the BBC on the way home.

    At least my mind could travel.

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    Tuesday, January 13, 2009

    Taking Advantage

    Most people learn to fly near home, and only experience one climate while training. My students learn to fly in a very dry climate (12"/30cm of rain a year). Carburetor ice is unusual; airframe ice is not. My King Air initial training was in East Texas, where airframe ice is unusual, while carburetor ice is common. Everyone gets a partial picture.

    This is a problem for instructors. We can't show our students every kind of weather situation, so we end up doing a lot of talking (" might see that if you fly to the Oregon coast..."). My instrument students practice circling approaches with unlimited visibility. This does not prepare them for the totally disorienting experience of trying to keep the airplane right-side up and the runway in sight at 400' above the ground in one mile visibility. No wonder people lose control while circling for real. For this kind of thing, showing is better than telling.

    Today I got to show. The ceiling was 1600 feet, but the visibility was unlimited. There was moisture in the air, too: the temperature was around 3C, and the snowmelt was evaporating. It was a great opportunity to do a short cross-country by pilotage. I grabbed my IFR charts, just in case.

    I noticed the RPMs falling while he copied the ATIS, and again while he checked the controls.

    "Hey," he said, "the RPMs keep falling."

    "Turn on the carburetor heat," I offered. The engine was at 1200 RPM. He pulled the knob, and in quick succession the engine coughed, dropped back to 1000 RPM, then quickly accelerated to 1400RPM.

    "That was carburetor ice."

    We followed the highway to the next airport, an uncontrolled field. We talked about the airspace classes along the route, and the cloud clearance requirements. But now they weren't abstract. We were flying under the VOR approach, and it was easy to picture a Citation dropping out of the overcast, coming at us at a combined speed of 250 knots. It made sense to leave 500 feet between us and the overcast above.

    He did a good landing at the uncontrolled field, and taxied back for takeoff. A hawk sat atop the windsock, facing into the cold wind. We stopped on the taxiway so I could take a picture.

    We followed the VOR approach course home, still well below the clouds. The overcast was breaking up, and there were some blue holes visible above. It seemed to get a little colder, and the wind got a little gusty. He did some short- and soft-field landings, flying the base and crosswind legs sideways. I tried to surprise him with a simulated engine failure on downwind, and he made the runway. We called it a day.

    Actually, we called it a good day.

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    Sunday, January 11, 2009

    Passenger Woes

    So there I was, sitting in a nice window seat of a Boeing 737, idly minding my business while the crew taxied. We would be using 34R out of Salt Lake City, a runway I have used many times as a pilot.

    "Folks, it looks like we're number one for departure. Flight attendants do your thing."

    We whip around onto the runway, and as we do so I notice that there is a loose latch on one of the nacelles. I'm not even remotely qualified in the 737 but it looks like an access door of some kind. It's one of those two-step latches on which you push a button, then fiddle to get it open. The picture is from a different flight on a different airline, but you can see the latch at the far right.

    What do you do?

    We were already rolling, and I quickly realized that whatever the right thing to do was, I would have no influence on the outcome. They're not gonna abort for a passenger calling an aircraft defect, and even if they were by the time the word got up front the airplane would be near V1 and it would be too dangerous.

    The inspection door stayed on through the flight. As we deplaned I told the FO about it, and even gave him an out ("it's on top, you wouldn't see it during a walk-around.")

    Would the NTSB have blamed me if it had come off in flight? I'm reminded of a scene many years ago, standing on the ramp with my jaw dropped watching my buddy land his Mooney gear-up with absolutely no way to stop the accident. But it's not about blame; I only wish that I could have helped.

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