EASA Proposes A350 Emergency Electrical Network Unit Modification After Logic Fault

The European Union Aviation Safety Agency (EASA) has proposed an airworthiness directive for the Airbus A350 that would require modification of the Emergency Electrical Network Management Unit (EENMU), after a control-logic fault was found in the emergency bus tie contactor. Proposed AD (PAD) 26-120 was issued on 6 October 2026, and comments close on 3 November 2026.

Aircraft and Parts Affected

The PAD applies to A350-941 and A350-1041 aeroplanes of all manufacturer serial numbers, except those on which Airbus modification 120968 was embodied in production. It sits in ATA chapter 24, electrical power.

The affected parts are EENMUs with part number 543CE06L18Y08 (amendment A, B, C or no amendment) and 543CE06L18Y09 (amendment A or no amendment). A serviceable part is any EENMU eligible for installation under Airbus-approved instructions that is not an affected part.

The Fault

EASA says the incorrect logic was found during flight tests and subsequent laboratory investigations. It concerns the Emergency Bus Tie Contactor (EBTC) in the electrical power distribution system. Under specific conditions, including a loss of main electrical supply or a total engine flame-out occurring after lockout protection activation, the faulty logic could lead to the loss of all AC busbars on one emergency side.

The root cause was traced to the EENMU firmware at the field programmable gate array (FPGA) function level. If it were left uncorrected, EASA says, power transfer between emergency AC busbars could be prevented, reducing electrical availability to essential systems and potentially compromising safe flight and landing.

What Would Be Required

The proposal would require operators to replace each affected part with a serviceable one, in line with approved Airbus instructions, within 78 months after the AD’s effective date. Airbus Service Bulletins A350-24-P165 to P171, as applicable, provide acceptable instructions. Depending on aircraft configuration, operators would first or concurrently have to apply Service Bulletin A350-24-P028 or P046.

After the modification, no affected EENMU may be installed on any aeroplane. The standard effective date would be 14 days after the AD is issued.

Timeline and Documents

The referenced Airbus service bulletins were first issued between January 2019 and June 2026: P046 on 31 January 2019, P028 on 16 December 2019, and the P165-P171 series on 5 June 2026. Revision 01 of P167 followed on 1 August 2026.

The fault needs specific conditions to arise, namely a loss of main electrical supply or total engine flame-out after lockout protection has activated. The 78-month window gives operators time to schedule the unit change. Operators and MROs will want to check serial-numbered unit records, since the clock starts only once the final AD is issued.

What This Means for Operators

A proposed AD is open to public comment before EASA decides on the final text, so details may change, but the substance, a unit replacement tied to specific part numbers and a service bulletin, is unlikely to alter much. Because the fix is delivered through part replacement rather than a software-only update, operators will need to plan for unit availability and check the part numbers fitted to each of their A350-900 and A350-1000 aircraft against the list.

Aircraft built with Airbus modification 120968 are excluded, so newer deliveries may already carry a corrected unit.