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Auto-Heading Intercept: The Logic Behind It

Aug 15, 2018
Mike Michalski, Instructor - Customer Training

Has this ever happened to you?

You have just departed an airport with instructions to climb on runway heading to 1500 feet and then turn to join an airway segment or SID that is part of your flight plan. You know that when you couple the autopilot to long range navigation source, the FMS will automatically command a turn towards the active leg – From ABCDE To FGHIJ – without any requirement to put in a heading if none was assigned. However, the turn direction you would most likely fly manually ends up being the opposite of what the FMS actually does.

Let's examine how the FMS calculates this turn by setting up a hypothetical situation below.

Auto Heading Intercept

After departing runway 09 and climbing to 1500 feet we couple the autopilot to the FMS. Instead of turning to the left to join airway J-123, the FMS will calculate the shortest turn direction from the present heading of 090° to the airway course of 265° would be a right turn (indicated by the red lines) and that is what it does. However, once nearing the heading of 265°, the FMS logic, which is always trying to center your Course Deviation Indicator (XTK = 0.0nm), will continue the right hand turn (indicated by the green line) to set up an intercept with the airway of up to 45°. The actual intercept angle is calculated based on three factors; 1) the ground speed, 2) the distance offset from the desired track (XTK on the NAV page), and 3) the maximum angle of bank capability of your autopilot at the altitude you are at. One way to avoid turning the long way around would be to initiate a left turn with the Heading Mode of your flight director and select FMS navigation once the aircraft is assured to continue turning in the same direction.

Let's look at another example in the terminal area shown below. You are declaring missed approach with the intention of trying the approach one more time. Approach control clears you to proceed direct back to the holding fix but assumes you will turn left, away from the nearby mountainous terrain, as the missed approach procedure shows. Because the FMS had calculated a slight crab angle to compensate for the crosswind on final course, the shortest direction of turn is to the right (shown in red). To prevent this situation, when on the the DTO page press the Line Select Key 2R (labeled LEFT) before pressing ENTER for the waypoint selected. This will force a left turn in the desired direction.

Missing Approach Procedure

If you have ever been flying when the FMS made an unexpected turn direction to accomplish an input you just made with regard to the active leg, this digital logic is what may have caused you to ask, "What's it doing now?"

If you have any questions about this article, or other operational questions in general, please contact the UA Training department.

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