Among the aeronautical skills that all pilots must have is the ability to plot courses in flight, to alternate destinations when, for one reason or another, continuation of the flight to the original destination is impracticable. This may be accomplished by means of pilotage, dead reckoning, or radio navigation aids.
Course, time, speed, and distance computations in flight require the same basic procedures as those used in preflight planning. Because of the limitations in cockpit space and available equipment, however, and because the pilot's attention must be divided between solving the problem and operating the airplane, advantage must be taken of all possible shortcuts and rule of thumb computations.
It is rarely practical while in flight to actually plot a course line on the chart used, and to mark checkpoints and distance as is usually done on preflight planning. Because the alternate airport selected in an emergency is usually not very far from the original course and known position, such actual plotting is seldom necessary.
Courses to alternates can be measured accurately with a protractor or plotter, but they can also be measured with reasonable accuracy using a straightedge and the compass roses shown at VOR stations on the chart. The VOR radials and airway courses (already oriented to magnetic direction) printed on the chart can be used satisfactorily for approximation of magnetic bearings during VFR flights. This approximation can be made on the basis of the radial of a nearby VOR or airway that most closely parallels the course to the station. The pilot must remember that the VOR radial or printed airway direction is outbound from the station. To find the course to the station, it may be necessary to determine the reciprocal of the parallel radial or airway. Distances can be determined by using the measurements on a plotter, or by placing a finger at the appropriate place on the straight edge of a piece of paper and then measuring the approximate distance on the mileage scale at the bottom of the chart.
If radio aids are used to divert to an alternate, the pilot should select the appropriate facility, tune to the proper frequency, and determine the course or radial to intercept or follow.
In the event the diversion to an alternate airport results from an emergency, it is important for the pilot to divert to the new course as early as possible. Before changing course, first consider the relative distance to all suitable alternates, select the one most appropriate to the emergency at hand, and then determine the magnetic course to the alternate selected. The heading should be changed to establish this new course immediately, and then later, the wind correction, actual distance, and estimated time and fuel required can be computed accurately while the airplane is proceeding toward the alternate.
To complete all plotting, measuring, and computation involved before diverting to the alternate airport may only aggravate an actual emergency. It is important to develop the ability to orient one's self immediately with a chart showing the terrain and landmarks over which the flight is made, and to make rapid and reasonably accurate computations of headings and arrival time estimates.