EASA AD 26-121 — Equipment / Furnishings – System Interface Unit – Replacement

Summary

EASA Airworthiness Directive Proposal 26-121 is a regulatory document addressing the replacement of the System Interface Unit (SIU) on Airbus Helicopters Deutschland GmbH MBB-BK117 D-2 (except D-2m variant) and D-3 helicopters. The directive applies to SIU Manufacturer Part Number 503-24-6-G installed on helicopters with serial numbers 20001 to 20360 and 21001 to 21999. It aims to ensure continued airworthiness by mandating replacement of affected SIUs based on their time since manufacture or last overhaul.

What Changed

This new proposed Airworthiness Directive introduces a requirement to replace the affected System Interface Unit (SIU) due to a discrepancy in the Time Between Overhaul (TBO) definition between maintenance manuals. It clarifies that the correct reference for overhaul timing is the time since manufacturing or last overhaul, and mandates replacement of SIUs exceeding 9 years and 6 months or approaching 10 years since manufacture or last overhaul. It also sets conditions for installation of SIUs to ensure only serviceable parts are used.

Why It Matters

This proposal is critical for aviation professionals because overdue SIU overhauls could lead to failure in deploying emergency beacons during survivable accidents, potentially delaying rescue operations. Operators and maintenance organizations must ensure compliance to maintain safety and regulatory conformity. The directive helps prevent unsafe conditions that could impact crew and passenger survivability in emergencies.

What To Do

Operators of affected MBB-BK117 D-2 and D-3 helicopters must replace any installed affected SIU that has accumulated more than 9 years and 6 months since manufacture or last overhaul within six months of the directive's effective date. Installation of affected SIUs is only permitted if the part is serviceable, and subsequent replacement must follow the directive's requirements. Comments on the proposal must be submitted by 22 October 2026.