Plane & Pilot
Tuesday, April 7, 2009

Weather In The Cockpit


XM Weather provides real-time information in the cockpit for pilots who are serious about their weather decisions


weather in the cockpitAsk most pilots what subject in aviation they wish they knew more about, and a majority will answer, “weather.” Indeed, while forecasters do occasionally still get it wrong, and even the best meteorologists acknowledge that we still have much to learn, the science of weather prognostication improves each year.
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Using a Garmin 396 with XM Aviator coverage, Bill Cox ferried a Marchetti SF.260 across the North Atlantic
While most of my international flying is done on IFR flight plans, the first 3,000 nm of this trip was diagonally across the States from Southern California to Bangor, Maine, and on to Goose Bay, Labrador. The weather was great for the first 1,800 miles, so I flew VFR, with help from the XM Weather wind page. (Later, when the weather turned sour and I was forced to follow a set route, I used the wind pages for updated flight planning.)

XM’s wind information is graduated in 3,000-foot increments all the way to 42,000 feet, and that helps you answer a major question. You’re no longer required to invest the time and fuel to climb to a higher altitude in the hope that the wind will be more favorable. If it isn’t, you may have lost time and expensive avgas in the climb only to have to descend back to your original height. With XM, it’s no great trick to check wind at all higher altitudes right up to the airplane’s service ceiling, even prior to the flight on the ground.

In the case of the Marchetti, the optimum height for cruise is about 8,000 feet, close enough to the 9,000-foot wind chart. Before takeoff, I checked the winds at all heights to estimate the best altitude to optimize speed and minimize fuel burn. In some instances, higher was better, even if it was slightly slower, because fuel burn was disproportionately reduced. I knew I’d realize significantly better nautical mpg. I was on a fixed-price contract, of course, so fuel economy was more important, a simple matter of profit and loss.

Perhaps even more useful than the pure altitude pages, however, is the fact that if your trip is a long one, as mine was, you can go to the larger scale and determine if you should use different altitudes for different sections of the trip. Too many pilots are in the habit of choosing a specific altitude and sticking with it for the entire trip. That may be convenient, but it won’t always optimize the airplane’s performance. You obviously want to avoid pogoing through the sky as you travel, but there’s no sin in choosing the most efficient height as conditions change.

Another possible VFR benefit of the wind pages is that you can actually use them to change your route slightly to take advantage of pressure patterns. At the higher ranges on a Garmin 396 and 496, and especially on the new, larger 696, you can display wind trends well to the left or right of course, allowing you to preplan a slight deviation if it will reduce the time en route. This won’t have much application on short trips, but if the leg is longer than, say, 600 nm, you may be flying adjacent to a number of weather systems, and a deviation may make sense.

Remember that the shortest distance between two points is less important than the shortest time between those points. The Marchetti burns about 14 gph at max cruise down at 8,000 feet, but there were times when it became obvious the better choice was to climb to 11,000 or even 13,000 feet and fly slower at 65% or 55% power to realize better winds and significantly improved fuel burn.

There was a time when limited weather information was only available on the ground before a flight. Then, the industry transitioned to offer availability of atmospheric conditions through radio communications with the ground. Today, satellite technology has allowed XM Weather to bring a variety of products right into the cockpit, so a pilot can analyze and interpret conditions ahead and make informed decisions on where and when to go...or not. The bottom line is safety, and the satellite information offered by XM helps ensure a safer flight.



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