FAA SAIB SW-08-38 - Sandel SN3308 Electronic Horizontal Situation Indicator (EHSI) configured as a Heading Repeater Display
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What to do
Operators of aircraft equipped with the Sandel SN3308 EHSI configured as a heading repeater and interfaced with directional gyros that flag heading invalid during manual slewing should install software version 2.11 or later per Sandel Service Bulletin 3308-07. They should also consider updating the Aircraft Flight Manual Supplement with FAA approval and perform the functional ground test procedure as specified in the SN3308 Navigation Display Installation Manual. No mandatory compliance deadline is specified.
Summary
FAA Special Airworthiness Information Bulletin SW-08-38 addresses an airworthiness concern with the Sandel SN3308 Electronic Horizontal Situation Indicator (EHSI) when configured as a heading repeater display, primarily affecting Eurocopter Deutschland MBB-BK 117 C-1 helicopters. The bulletin highlights issues with the compass rose blanking during manual slewing in free gyro mode, which may also apply to other rotorcraft and potentially fixed-wing aircraft with similar installations. EASA Safety Information Bulletin 2008-71 supports the FAA's advisory and informs European operators of the same concern.
What changed
This bulletin introduces awareness of a display issue where the compass rose disappears when the heading is flagged as invalid during manual slewing of the compass in free gyro mode. It recommends installing Sandel SN3308 software version 2.11 or later to allow continuous display of the compass rose in amber during manual heading adjustments. The bulletin also suggests possible revisions to the Aircraft Flight Manual Supplement and performing functional ground tests after software updates.
Why it matters
The issue affects pilot situational awareness by making it difficult to manually set the heading on the EHSI, potentially impacting safe navigation. Operators and maintenance personnel must address this to ensure reliable heading information is available, especially when operating near magnetic interference sources. Compliance with the recommended software update improves operational safety and reduces pilot workload during heading adjustments.
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