What is the SDF Approach?
The SDF (Simplified Directional Facility) represents a non-precision instrument approach that provides lateral guidance to aircraft navigating through challenging conditions.
Unlike an Instrument Landing System (ILS), it lacks a vertical glideslope. This means pilots shoulder the responsibility of managing their own descent profile. While it shares similarities with a Localizer Type Directional Aid (LDA), an SDF operates with notably less precision.
Pilots must rely on alternative instruments for critical altitude information. This makes the SDF particularly valuable at airports where installing more sophisticated systems would be impractical or cost-prohibitive.
Technical Specifications of SDF Approaches
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Guidance Type: Provides lateral (directional) guidance only and does not include a vertical glideslope, classifying it as a non-precision approach.
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Course Width: The signal has a fixed width of either 6 or 12 degrees, which is wider and less precise than a standard localizer.
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Alignment: The course may be offset by up to 3 degrees from the runway centerline, requiring a visual correction before landing.
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Signal Range: Usable off-course indications are limited to 35 degrees on either side of the course centerline.
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Frequency: Operates within designated frequency bands to prevent interference and ensure reliable signal transmission.
Current Status of SDF Approaches in the USA
The landscape of SDF approaches across the United States has changed dramatically. By 2023, only one remains operational: the approach serving Runway 05 at Moore-Murrell Airport (AMOR) in Morristown, Tennessee.
This decline accelerated in 2018 when the Federal Aviation Administration (FAA) decommissioned the SDF at Marsh field Municipal Airport (KOFI). The decision reflected the agency’s broader shift toward advanced navigation technologies.
This transformation mirrors aviation’s move from older, maintenance-heavy systems toward precision satellite-based alternatives like RNA and GPS. Today’s lone surviving SDF stands as both a working navigation tool and a living piece of aviation history.
SDF Performance Standards and Maintenance
SDF systems require careful maintenance protocols. Regular ground checks are essential, verifying signal strength, course alignment, and other performance-critical parameters.
These installations feature sophisticated alarm systems that continuously monitor performance around the clock. Any deviation triggers immediate alerts to maintenance crews, ensuring the system maintains operational readiness.
As legacy technology, the final operational SDF creates significant maintenance challenges. Sourcing replacement parts becomes increasingly difficult. Retaining specialized technical expertise grows more complex each year. These factors accelerated the widespread abandonment of SDF systems nationwide.
Executing an SDF approach requires adaptation from pilots. Without vertical guidance, they must craft their own descent strategy while maintaining lateral course tracking.
Pilots face an additional complexity: the course may deviate up to 3 degrees from the runway centerline. When combined with the characteristically wide 6 or 12-degree course width, this demands exceptional situational awareness—far beyond what a standard ILS requires.
Course sensitivity presents another critical consideration. Usable indications exist only within a 35-degree corridor on either side of the centerline. Pilots must exercise constant vigilance to remain within this narrow window, particularly when weather conditions deteriorate.
Vertical navigation becomes an exercise in precision timing and altitude management. Without glideslope assistance, pilots must manage their descent carefully. They typically maintain gradients no steeper than 400 feet per nautical mile—a crucial safety margin for obstacle clearance.
The approach culminates at the Minimum Descent Altitude (MDA), where pilots level off and maintain altitude until visual contact with the runway environment. Given the SDF’s inherently wide course, breaking through the cloud deck often reveals the need for immediate visual corrections to achieve proper runway alignment.