A student pilot is planning a cross-country flight from Hannibal Regional Airport in Missouri to Iowa's Washington Municipal Airport. Adding a wind correction angle to the true course as taught, the trainee determines the true heading, and then proceeds to check the compass card for deviation before determining the compass heading to be flown.
Hold on—didn't we skip a step? What about adjusting for magnetic variation, calculating a magnetic heading, and applying the mnemonic " East is least and west is best?"
Good catch—but this is a special case. Check the chart, and see that both the departure and destination airports are located almost astride the agonic line, where magnetic variation is zero. That is, on the agonic line, true north and magnetic north coincide.
That may sound like a strange idea to a pilot who learned to fly and navigate in Elma, Wash., where magnetic variation is 17 degrees east, or in Caribou, Maine, where variation is 17 degrees west.
Why is variation expressed as easterly on the West Coast, and as westerly on the East Coast?
"On the west coast of the United States, the compass needle points to the east of true north; on the east coast, the compass needle points to the west of true north," explains Chapter 15 of the Pilot's Handbook of Aeronautical Knowledge, which also provides an isogonic chart as an illustration. (Remember that the lines on your sectional chart that show the degrees of variation to apply in a given area are known as isogonic lines, or lines of equal variation.)
And between the zones where easterly and westerly variation applies sits the agonic line. A dictionary definition of agonic is "not forming an angle."
In the United States the agonic line runs approximately west of the Great Lakes, south through Wisconsin, Illinois, western Tennessee, and along the border of Mississippi and Alabama.
You have probably checked your own airport's variation often enough to have the value memorized. But if you compare it to the variation given on older editions of the same navigation chart, you may be surprised to see that it has changed with time.
That explains why older discussions of the agonic line in articles and texts place it in a different position; not too long ago it ran through the southeastern tip of Florida—where variation is now 6 degrees west!
Sporty's announces instrument training app
Sporty's Complete Instrument Training course, available in online and DVD formats, is now a dedicated app for iOS devices. Formatted specifically for the iPhone or iPad, the app includes 13 hours of real-world video and advanced graphics including 3-D animations to help explain difficult concepts, Sporty's said. Read more >>
Gleim offers LSA pilot training kit
Gleim Aviation has made available its Deluxe Sport Pilot Kit, an all-in-one program designed to expedite training for this certificate. The kit includes online ground school, FAA test prep, a flight bag, flight computer, FAR/AIM, a logbook, and training materials. The cost is $199.95.
Note: Products listed have not been evaluated by ePilot editors unless otherwise noted. AOPA assumes no responsibility for products or services listed or for claims or actions by manufacturers or vendors.
Question: What does V MC mean with regard to multiengine airplanes and how does it affect me?
Answer: V MC is a calibrated airspeed computed under the most unfavorable configuration conditions and will ensure the pilot is able to maintain "straight flight" or directional control of a multiengine aircraft should the critical engine fail. This airspeed does not ensure climb performance with the critical engine failed, but does ensure the pilot is able to maintain directional control. For FAR Part 23 certification, V MC speed is calculated under takeoff configuration which includes full power on the operating engine, gear up, flaps up, out of ground effect, propeller controls in the position recommended for approach when all engines are operating, no more than 5 degrees bank toward the operating engine, and no more than 150 pounds of rudder pressure toward the operating engine, to name a few of the requirements. FAR 23.149 is the resource for exactly what is required for V MC certification. Read more about flying multiengine airplanes here.
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