In Plain English
AIM 5-3-5 addresses how pilots should execute course changes while flying airways or published routes. Pilots are required to operate along the centerline of the airway (per 14 CFR 91.181), and turns must be planned so the aircraft stays within route boundaries.
If you wait until you cross the fix to begin your turn, you'll likely overshoot the airway, especially at high speed. The fix is to lead the turn — start it early using available cockpit tools like DME.
Variables that affect how much to lead the turn include:
- Turn radius and bank angle
- Wind direction and velocity
- True airspeed (TAS)
- Degree of course change
- Type of turn fix (DME, overhead VOR, or intersection)
Example from the AIM: an aircraft at 17,000 ft, 400 KTAS, 25° bank, turning more than 40° will exceed the 4 NM half-width of the airway if it doesn't lead the turn. Above 290 KTAS, the risk of busting boundaries grows significantly. The FAA expects pilots to lead turns and use good technique to stay as close to centerline as possible.
AIM Source Text
FAA AIM ¶ 5-3-55-3-5. 5-3-5. Airway or Route Course Changes
Pilots of aircraft are required to adhere to airways or routes being flown. Special attention must be given to this requirement during course changes. Each course change consists of variables that make the technique applicable in each case a matter only the pilot can resolve. Some variables which must be considered are turn radius, wind effect, airspeed, degree of turn, and cockpit instrumentation. An early turn, as illustrated below, is one method of adhering to airways or routes. The use of any available cockpit instrumentation, such as Distance Measuring Equipment, may be used by the pilot to lead the turn when making course changes. This is consistent with the intent of 14 CFR section 91.181, which requires pilots to operate along the centerline of an airway and along the direct course between navigational aids or fixes. Turns which begin at or after fix passage may exceed airway or route boundaries. FIG 5-3-1 contains an example flight track depicting this, together with an example of an early turn. Without such actions as leading a turn, aircraft operating in excess of 290 knots true air speed (TAS) can exceed the normal airway or route boundaries depending on the amount of course change required, wind direction and velocity, the character of the turn fix (DME, overhead navigation aid, or intersection), and the pilot's technique in making a course change. For example, a flight operating at 17,000 feet MSL with a TAS of 400 knots, a 25 degree bank, and a course change of more than 40 degrees would exceed the width of the airway or route; i.e., 4 nautical miles each side of centerline. However, in the airspace below 18,000 feet MSL, operations in excess of 290 knots TAS are not prevalent and the provision of additional IFR separation in all course change situations for the occasional aircraft making a turn in excess of 290 knots TAS creates an unacceptable waste of airspace and imposes a penalty upon the preponderance of traffic which operate at low speeds. Consequently, the FAA expects pilots to lead turns and take other actions they consider necessary during course changes to adhere as closely as possible to the airways or route being flown.