MART Vol. II MO/MP - NESA - Civil Air Patrol

5.1 Navigation TermsIn order to effectively communicate with the pilot and ground teams, thescanner and observer must have a clear understanding of various terms that areused frequently when flying aboard CAP aircraft. These are not peculiar to searchand rescue, but are used by all civilian and military aviators.Course - The planned or actual path of the aircraft over the ground. Thecourse can be either true course or magnetic course depending upon whether it ismeasured by referencing true north or magnetic north. The magnetic north pole isnot located at the true North Pole on the actual axis of rotation, so there is usuallya difference between true course and magnetic course.Pilots measure true course against a meridian of longitude at the midpoint ofeach leg, and all of these meridians point to the true North Pole. However, sincethe aircraft compass can only point at the magnetic north pole you must applymagnetic variation to the true course to determine the magnetic direction youmust fly in order to follow the true course. East magnetic variation is subtractedfrom measured true courses and west variation is added.You can find magnetic variation factors in several places, and you will learnmore about this in the section concerning charts. Magnetic variation factors alsotake into account abnormalities in the earth's magnetic field due to the unevendistribution of iron ore and other minerals.Heading - The direction the aircraft is physically pointed. An airplane's trackover the ground doesn't always correspond with the direction they're pointed.This is due to the effect of wind. True heading is based on the true North Pole,and magnetic heading is based on the magnetic north pole. Most airplanecompasses can only reference magnetic north without resorting to advancedtechniques or equipment, so headings are almost always magnetic.Drift, or Drift Effect - The effect the wind has on an aircraft. The air mass anaircraft flies through rarely stands still. If you try to cross a river in a boat bypointing the bow straight across the river and maintaining that heading all the wayacross, you will impact the river bank downstream of your initial aim point due tothe effect of the river current. In an aircraft, any wind that is not from directly inthe front or rear of the aircraft has a similar affect. The motion of the airplanerelative to the surface of the earth depends upon the fact that the airplane ismoving relative to the air mass and the air mass is moving relative to the surfaceof the earth; adding these two gives the resultant vector of the airplane movingrelative to the surface of the earth. The angle between the heading and the actualground track is called the drift angle.Drift Correction - A number of degrees added to or subtracted from the aircraftheading intended to negate drift or drift effect. In the rowboat example, if you hadaimed at a point upstream of the intended destination, you would have crossed ina straighter line. The angle between the intended impact point and the upstreamaim point is analogous to drift correction.Ground Track - The actual path of the airplane over the surface of the earth.Nautical mile (nm) - Distances in air navigation are usually measured innautical miles, not statute miles. A nautical mile is about 6076 feet (sometimesrounded to 6080 feet), compared to 5280 feet for the statute mile. Mostexperienced aviators simply refer to a nautical mile as a mile. Aircrews shouldremain aware of this difference when communicating with ground search teams68