Intermittent ADS-B Out Dropouts Above FL180 on a King Air 350
An operator reports that ATC loses the ADS-B Out position report from a King Air 350 only above FL180. The aircraft has a Garmin GTX 335R transponder, a GTN 750Xi as the position source, and dual GPS antennas. The crew sees no transponder failure, and the GTX built-in test passes on the ground. A previous shop replaced the transponder with a known-good unit; the dropouts continued on the next two flights. The FAA ADS-B performance report shows SIL=3 but intermittent NIC/NACp degradation and missing position messages during climb and cruise. The aircraft is based at a humid coastal airport and has recent paint work around the upper fuselage. Maintenance control wants to defer the issue because the transponder still replies to secondary radar, and the next flight is a 2-hour IFR charter at FL200. The AME must decide what data to gather, how to isolate the fault, and whether the aircraft can be released.
1. What does the ADS-B performance report tell you, and what troubleshooting sequence would you follow? Explain why replacing the transponder did not fix the problem.
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The report separates integrity, accuracy, and position-source performance. SIL=3 indicates the transponder is declaring a high-integrity position, while intermittent NIC/NACp degradation and missing position messages point away from a simple transponder transmitter failure and toward the position source, antenna, or RF path. Because the fault appears only at altitude, suspect a coax/connector problem that changes with pressure, temperature, or vibration, a GPS antenna issue, or paint/contamination over an antenna. A transponder swap would not fix a GPS-side fault.
A logical sequence is:
- Download the GTX and GTN fault logs and compare failure times with the ADS-B report.
- Inspect both GPS antennas and the transponder antenna for paint, damage, corrosion, or loose connectors.
- Measure coax insertion loss and inspect connectors at the GPS antenna, avionics rack, and transponder.
- Substitute a known-good GPS antenna or position source, one at a time, and repeat a flight profile that reproduces the fault.
- Verify the final configuration with a new ADS-B performance report.
Record each step and result. Do not treat a successful ground BIT as proof that the altitude-dependent RF path is serviceable.
2. Can the aircraft be released for the FL200 IFR charter? Justify your decision using airworthiness and operational considerations.
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The aircraft should not be released for the planned flight until the ADS-B fault is resolved or a valid deferral is in place. ADS-B Out is required in the airspace and route structure where the charter will operate, and intermittent position reports are not equivalent to a serviceable system. The fact that secondary radar still paints the aircraft is not sufficient: ATC separation and surveillance services may depend on ADS-B performance.
Check the operator's MEL and the applicable airspace requirements. If ADS-B Out is required for the route and no relief is available, the flight cannot proceed. A ferry or positioning flight might be possible only under an appropriate special flight permit or authorization, with the limitation documented and accepted by the authority. Maintenance control's desire to defer is not, by itself, a valid basis for release.
The release decision should be made by certifying staff within the scope of their licence, after determining that the aircraft is airworthy for the intended operation. If the fault is unresolved, record the restriction and notify the operator in writing.
3. After the fault is corrected, what records and return-to-service entries should be completed?
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The records should identify the defect, the troubleshooting performed, the root cause, the corrective action, and the approved data used. If a component was replaced, record the part number and serial number and the removal/installation references. If a coax or antenna was repaired, record the measurements and the test method.
Before return to service, repeat the relevant system tests and obtain an ADS-B performance report showing the required parameters. The certification entry should state that the ADS-B system was tested and found satisfactory and should reference the performance report or test record. Any maintenance release must be signed by appropriately licensed certifying staff within the scope of the rating.
Also update the aircraft technical log and, where applicable, the operator's reliability or defect-tracking system. Do not simply write 'ADS-B checked, serviceable' without linking the entry to the objective evidence. If the fault was intermittent, note the flight profile used to verify the repair so later investigations can compare conditions.