Tuesday, January 3, 2012

Winds Aloft

Text Based Winds and Temps Data from NOAA
"Winds and temps aloft" are things you learn about as you start doing cross country flight planning. I did a good job on my cross country flight planning in training and flying my plan. However, now, I don't think I really understood in my gut the full extent of what these winds do, or how I may use them.

In my recent flying experience I've gained a greater awareness of these winds beyond simply using that data to calculate ground speed and heading. I finally started to understand the other uses of those winds. I am also starting to understand how winds that you aren't expecting can cause major problems for a long flight, even if they don't cause turbulence.

An excellent case in point is the flight my husband and I did last night. I got a weather briefing that told me the winds at 6000 feet (only 500 feet over the planned cruise altitude to our destination) would be a 22 knot headwind. Not only a 22 knot headwind, but the pireps the briefer had said there was moderate turbulence all the way down to 6000 feet - and that was a big jet reporting moderate turbulence, not just a little plane like we fly. However, the winds at 3000 feet were forecast to be light and variable. On the basis of that information we decided to fly southeast at 3500 feet instead of 5500 feet.

On the flight out the winds were as forecast at 3500 feet, light and not a factor at all. On the way back we had a couple options, we could climb to 4500 feet as a legal VFR cruise altitude for a northwest route and stay in the calm air, or we could go up to 6500 feet. 6500 was also a legal altitude but that would put us into the 22 knot winds. My husband decided to go on up to 6500 feet to take advantage of the winds if he could without encountering moderate turbulence. It worked. We gained 15 MPH ground speed from the winds. A nice example of how the winds can help and how knowing what the winds are above and below your planned cruise altitude is very helpful. You can avoid winds slowing you down and use winds pushing you along sometimes just by climbing or descending 2000 feet.

On the flip side, if the headwinds are stronger than planned and you do not change your cruise altitude to other levels that better match the winds you planned (sometimes you can, sometimes you can't), you will take longer to get where you're going than you planned. While in a car its not a huge issue if it takes you 1 or 2 hours to travel those 60 miles, in a plane it can be a life or death matter. Cars measure fuel burn in MPG (miles per gallon) the distance you travel is the largest factor in determining if you will get where you're going on the fuel you have. You will burn a bit more fuel for the same distance if you are sitting in traffic, but most of the time the difference is not significant. In planes the situation is different.

In a plane your fuel burn is measured in GPH (gallons per hour). You know approximately how many gallons your plane will burn per hour of cruise flight. You know how much fuel you're carrying and you know how long your flight should take. You are even required to carry reserve fuel (30 minutes for day, 45 minutes for night). However, if you take off with just enough fuel to get where you're going plus a 30 minute reserve AND you run into a stronger than expected headwind for an extended portion of your flight, you could easily run out of fuel. This is considered a very Bad Thing in aviation. That mistake doesn't strand you on the side of the road thumbing for a ride. That mistake results in a forced landing at best and a crumpled airplane and loss of life or limb at worst.

Some obvious things come into mind to prevent this situation... for one thing, carry a lot more than 30 minutes of extra fuel. Many pilots always plan to land with an hour of extra fuel. Another thing is to carefully observe the winds as you fly. Determine if they seem to be stronger or lighter than forecast for each major section of your flight. If they are stronger than forecast/planned, land and get extra fuel before you run out of fuel and options. Or find a different altitude with more favorable winds, a tail wind or a lighter headwind.

For myself, I understood all of the above intellectually a long time ago. Now, however, I am starting to understand this in my gut. This understanding is changing how I think about cross country flight planning and the sort of information I want to have available and consider not only before I take off but en route. This is good. It will help me be a better, more efficient and safer pilot.

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