Wind and the TAF

Posted on November 12, 2005 by David Megginson

I just read this TAF for Watertown International Airport (KART):

KART 	121738Z 121818 19008KT P6SM SKC
FM0600 17006KT P6SM SCT250 WS015/23035KT
FM1400 19012KT P6SM BKN250 WS015/23045KT

The tricky parts are the phrases “WS015/23035KT” and “WS015/23045KT” — those might be common out in the prairies, but I don’t see that kind of thing often in TAFs around the Great Lakes. The “WS” stands for “wind shear”. The following number is the altitude of the shear layer above ground level (1,500 feet in both cases), followed by the wind direction and speed at that altitude.

So starting at 06:00z tomorrow morning (that’s 01:00 EST), the wind will be from 170 degrees true at 6 knots on the ground, but from 230 degrees true at 35 knots just 1,500 feet up; from 14:00z (09:00 EST), the wind will be from 190 at 12 knots on the ground, but from 230 at 45 knots 1,500 feet up.

What does that mean, practically speaking? As you approach to land on runway 25 at 10:00 am local time tomorrow morning, you’ll be facing a headwind of 43 knots until 1,500 feet AGL, at which point the headwind will drop abruptly to about 8 knots — that means that your airspeed will suddenly drop by 35 knots as you descend through the shear layer, until your plane has time to reestablish its trimmed airspeed. If you’re approaching at 80 knots calibrated airspeed, you’ll suddenly find yourself at 45 knots with your nose swinging hard towards the ground trying to make up the missing speed (you’ll probably also be in moderate-to-severe turbulence). In a light aircraft, you may have room to recover at 1,500 feet; in something heavier, like a commuter turboprop, I’m not so sure.

When you take off from runway 25, exactly the opposite will happen. As you climb through the shear layer (and turbulence), your airspeed will suddenly increase by 35 knots, and the nose will shoot up to the sky to try to regain the plane’s trimmed airspeed. For a brief time, the climb rate will be spectacular, but you’ll have to make sure that you get the nose down before the extra speed decays on you and leaves you nose-high and slow.

The exact effect will depend on how thin the transition layer is and how fast the plane is descending or climbing. A slow descent or climb, or a thicker transition layer, will give more time for a gradual adjustment.

Anyone for some touch-and-goes at Watertown tomorrow?

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Know your fuel consumption

Posted on November 9, 2005 by David Megginson

Update: WordPress tells me that this is my 100th post. Whoopie!

Update 2: I went for another test flight on Friday, and the problem is fixed.

When you land after a flight, do you know — within a gallon/a few liters — how much fuel your plane should take? Some people always take off with full tanks and limit their legs to 2-3 hours, so they figure they never have to worry.

On Tuesday, I took my Warrior for its second post-maintenance test flight. I started with full tanks, flew for 2.75 hours at 75% power, then filled up again. The plane took 146 liters of fuel, over 50% more than expected, indicating that I landed with less than 45 minutes of fuel remaining. Upon closer investigation, there was some blue staining on the wing and a bit of streaking coming from under the left side of the cowling. My new fuel pump was leaking, throwing fuel overboard as I flew. I probably leaked fuel on my first flight as well, but since I didn’t start with full tanks, it was harder to be certain (I mentioned my concern to my AME then, but we saw no evidence of leaks inside the cowling).

I’m glad that I insisted on a second test flight before making the 800 nm trip to Atlanta, but I’m also glad that I routinely track my fuel consumption and know what to expect at the pump — it’s as important as being able to read the panel instruments during flight. Unlike a Cessna (with its “both” fuel setting), my Piper would have warned me of a problem when the first tank ran dry, giving me a few minutes to land with the remaining tank, but fortunately it didn’t come to that. A new fuel pump will arrive by courier tomorrow (Thursday) morning from the engine shop.

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Five-day aviation prog charts

Posted on November 9, 2005 by David Megginson

Anyone who’s planned a serious cross-country knows not to pay attention to any weather forecast until about 12 hours before the flight, not to take any forecast seriously until 2 hours before the flight, and not to rely on a forecast, ever. Nevertheless, we have to plan flights days or weeks in advance, and passengers want to know if we’ll be able to make a trip.

The regular, public forecasts have limited value for aviation. We need to know where the pressure systems and fronts will (supposedly) be, and what kind of moisture and winds we’ll be facing. The 48-hour U.S. prog charts (which also cover southern Canada) are a common stop, since they’re a lot easier for lay people to interpret than the Environment Canada GEM.

A 48-hour weather forecast is very likely to be completely wrong, but still, many pilots would like to torture themselves by looking even further ahead. For us, there is good (??) news: Unisys publishes U.S. (and southern Canadian) prog charts up to 5 days (yes, that’s 120 hours) ahead, not just for the surface but for higher altitudes like 700 mb (~10,000 feet) and 500 mb (~18,000 feet), with winds. So now you can agonize for five days about your flight — just don’t try to decide until two hours before, because the forecast will probably change.

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You can't, always

Posted on November 9, 2005 by David Megginson

The weather might be marginal somewhere along your route. You’re instrument rated, but you’re concerned that filing IFR will result in a much longer trip, or maybe you’re worried that you’ll hit ice at the required IFR altitudes. Assuming that you don’t cancel the trip (possibly the best choice), what do you do?

  1. file IFR, because you can always cancel and finish VFR if the weather’s not IMC; or
  2. file VFR, because you can always get a pop-up IFR clearance if the weather closes in.

I’ve seen both of these pieces of advice many times on aviation lists, and have sometimes heard them from other pilots, but I know from personal experience that they’re both wrong.

If you’re trapped under a lowering ceiling, you can’t always get a pop-up IFR clearance. First, you’re probably well below the minimum IFR altitude and have no way to climb VFR; even worse, you’re probably too low for ATC to see you on radar or even to hear your radio call. The only option is to cross your fingers and climb (illegally) through the cloud until you’re high enough to get into the system. That’s a scary option, especially near high terrain.

If you’re IFR, you might just as easily be trapped on top. Maybe there is a safe amount of VMC below you, but you have to have a way down to it before you cancel and continue VFR. Maybe you can shoot and approach and break off down low, depending on where you are, but it will frequently happen that once you start IFR, you find yourself effectively trapped in the system, maybe with ice-filled clouds below you.

So you can’t, always. As with much of flying, the pat answers aren’t that helpful, and pilots of small, piston planes are left with difficult choices that flight instructors (who rarely fly long cross-countries) and turbine pilots might not understand.

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Back in the air

Posted on November 7, 2005 by David Megginson

Today I took my Warrior for its first flight with the overhauled engine installed. I haven’t flown it since 13 July, and it was a nice feeling, despite high winds and a lot of low-level turbulence. I was supposed to test fly the plane last Monday, but there was still enough magnetism in the firewall (the one big part that didn’t get sent off for degaussing) to keep the magnetic compass from working.

You can read more about the lightning strike that grounded my plane in past postings.

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Flying and the Metric System

Posted on October 28, 2005 by David Megginson

The featured article of the day on Wikipedia for Saturday 29 October is Metrication, the process of converting a country to metric from various historical units of measure. Now that Ireland has switched, the only three countries left not officially using metric (or in the process of changing) are Liberia, Myanmar (Burma), and the United States, though many people still informally use older systems for some things — for example, I use metric for temperature and distance (on the ground) and for buying food, but not for weighing myself, measuring my height, or buying lumber.

Because Canada and Mexico are metric while the U.S. is not, we’ve come up with a funny mishmash for North American aviation. We use nautical miles for distance and knots for speed (even most Americans don’t know those); statute miles for visibility (or feet under conditions of very low visibility); feet for elevation, altitude, and runway dimensions; inches of mercury for air pressure; and Celsius for outside air temperature (but not for cylinder head or oil temperature). Got all that? That’s right, if you’re six miles from the airport and there’s six miles visibility, don’t expect to see the airport, because six statute miles of visibility is 9,656 meters, while six nautical miles of distance is 11,112 meters, about a kilometer and a half further. Even American pilots use Celsius for temperature: you can always tell which Americans visiting Canada are pilots, because they’re the only Americans who understand the temperature on the Canadian weather report.

In Europe and most of the rest of the world, I know that they give runway dimensions in meters and air pressure in hectopascals (millibars), but I’m not sure if they use kilometers for distance, and I’m pretty sure they don’t use meters for altitude (or else standard altitudes wouldn’t mesh up). If the U.S. were finally to give in and go metric, would we switch to metric for all of aviation? It would certainly make things simpler for someone building a new plane or learning to fly from scratch, but there would be a lot of gauges to recalibrate, a lot of weight-and-balance to recalculate, and probably a lot of accidents caused by unit confusion until we straightened everything out. Remember that the Gimli Glider was, mainly, a result of confusion during metrication at Air Canada.

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XML 2005 Conference

Posted on October 13, 2005 by David Megginson

My Warrior’s engine is finally back from overhaul in Halifax after its lighting strike, and I hope to be back in the air in a bit over a week. Next month, I’m planning a long (7+ hour) flight from Ottawa down to Atlanta to speak at the XML 2005 conference. If you are one of the large group of people who share an interest in both technology and flying, I’d love to see you there: I don’t plan to talk about flying from the podium, but would love to chat about it in the halls or over a drink or meal.

This will be the longest single-direction flight I’ve done (though not the most hours I’ve flown in a single day). Since it will be icing season, I’m going to schedule in a lot of flexibility, and will use the airlines if nothing else works out.

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FedEx flying spares

Posted on October 10, 2005 by David Megginson

According to this New York Times article (via Kottke), FedEx launches five mostly-empty jets every night — from Las Vegas, Duluth, Laredo, Fort Myers, and Portland (ME) — all heading for Memphis through various meandering routes. These flying spares often end up diverting mid-flight to deal with package overflows at hubs or with equipment failures on other FedEx flights. Constant diversion sounds like an interesting challenge both for the flight crews and for the dispatchers.

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Vintage airport postcards

Posted on September 27, 2005 by David Megginson

Ron Dupas has an online collection of postcards, including a large collection with photos of airports. There are some great ones here, including the now-vanished Cartierville airport near Montreal, the TCA hangar in Vancouver in the early 60’s (my first memory of flying is looking at the pictures on the safety card in the seat pocket of a Vickers Viscount), and a shot from inside the control tower at Blatchford field in Edmonton from what looks like the late 1950s (included here).

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"VFR not recommended" not recommended

Posted on September 24, 2005 by David Megginson

Like most Canadian pilots, I’m running pretty late updating my AIP, so I just noticed the contents of Aeronautical Information Circular 10/05 from last April. It’s worth quoting in full:

Removal of the phrase “VFR flight not recommended” in pilot briefing

Until now, a flight service specialist was required to state the phrase “VFR FLIGHT NOT RECOMMENDED” at the beginning of a pilot briefing for a VFR flight when extensive instrument meteorological conditions (IMC) or conditions that may affect the safety of the flight were reported or forecast to occur along the planned route of flight. The phrase was advisory in nature and the conditions that prompted the use of the phrase were then to be stated and the pilot was to be asked if a briefing was still required.

Pilots have requested that the phrase no longer [be] used in briefings. It was reported that flights were cancelled because the phrase was used even though the flights could have been conducted.

NAV CANADA recognizes that the responsibility for determining if a flight should be conducted or not rests solely with the pilot. The requirement for flight service specialists to use the phrase “VFR FLIGHT NOT RECOMMENDED” is discontinued and the phrase will no longer be used at the beginning of a pilot briefing. Significant meteorological information that could influence the pilot to alter or cancel the proposed flight will continue to be provided at the beginning of the briefing in accordance with current practice.

Kathleen Fox, Vice-President, Operations

Most U.S. pilots I’ve talked to hate the phrase “VFR not recommended”, but I don’t remember ever having heard it in Canada, even before this circular went around. This is similar to the attitude towards icing — Canadian forecasts mention icing only when there is a strong possibility of moderate-to-severe, while the U.S. puts out a standard icing NOTAM if there is even a small chance of trace-to-light. I’m not sure which side is safer to err on.

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