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On 9 April 2021 Boeing issued the following press statement:

Recommendation for Potential Issue for Some 737 MAX Jets

SEATTLE, April 9, 2021 —Boeing has recommended to 16 customers that they address a potential electrical issue in a specific group of 737 MAX airplanes prior to further operations. The recommendation is being made to allow for verification that a sufficient ground path exists for a component of the electrical power system.

We are working closely with the U.S. Federal Aviation Administration on this production issue. We are also informing our customers of specific tail numbers affected and we will provide direction on appropriate corrective actions.

120 MAX airframes were affected from line numbers 7399 through 8082 inclusive, approximately one-fifth of the global MAX fleet.

The AD (2021-09-08) was eventually issued on 30 Apr 2021 and required modifcation of the electrical bonding of the P6 panel assembly, including the mounting tray for the standby power control unit (SPCU), located behind the first officer. Also modifcation of the electrical bonding of the main instrument panel (MIP) assembly located in front of and between the captain and first officer.

Boeing submitted a Service Bulletin (SB) in late April to the FAA for approval. The SB contained the necessary steps to modifiy the aircraft to fix the grounding problem. It proposed adding a bonding strap or cable screwed onto two different surfaces creating a grounding path.

The FAA approved the fix for the known problem areas (the SPCU and MIP) but has also asked for additional analysis over whether other areas/systems could be affected by the grounding issue before they would approve the SB. This took Boeing some time to compile and for the FAA to approve which delayed the issuance of the SB and keept the affected MAX fleet grounded.

On 12 May 2021 The FAA approved Boeings analysis and the SB was issued so rectification work on the aircraft could begin. FAA Administrator, Steve Dickson, told the transportation subcommittee of the House Appropriations Committee in a virtual hearing in Washington that fixing improperly installed electrical components will be “pretty straightforward,” and that the FAA is attempting to determine the “root cause” that led to the flaw.

On 19 May Boeing resumed deliveries of the MAX following approval of the fix by the FAA.

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*** Updated 23 Nov 2020 ***

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Electriclal Bonding

The issue is one of poor electrical bonding due to flaws that were traced to a manufacturing change made in early 2019 involving fasteners and a coating.

The problem is unrelated to MCAS or any other previous MAX problem but has probably made the headlines because of this recent history.

In some situations, a coating such as paint, may affect the electrical grounding path, which could cause the component to malfunction.

The manufacturing change involved a change of supplier for the fasteners. Since the change was considered so insignificant the change did not have to be approved by the FAA.

 

Affected Areas

The first component identified as affected is the Standby Power Control Unit. This unit, new to the NG and also present on the MAX, controls the standby power distribution and carries some very high currents hence the need for good electrical bonding.

It is located on the P6 panel behind the F/O’s seat.

It has since emerged that there are two other areas of concern; the storage rack that holds the SPCU and the main instrument panel.

SPCU

Bonding Fasteners

It is not yet known what type of fasteners or coating are causing the problem.

Fasteners used on the 737 include CherryMax, Hi-Lok, Zeus, rivets and screws.

All units and panels are bonded to the trays or racks they are housed in, which are in turn bonded to the airframe. For the following reasons:

FAA AC 43.13-1B - Chapter 11 Aircraft Electrical Systems states:

“11-185. GENERAL. One of the more important factors in the design and maintenance of aircraft electrical systems is proper bonding and grounding. Inadequate bonding or grounding can lead to unreliable operation of systems, e.g., EMI, electrostatic discharge damage to sensitive electronics, personnel shock hazard, or damage from lightning strike.”

 

The Fix

On 30 April 2021 the FAA issued AD 2021-09-08

SUMMARY:

The FAA is adopting a new airworthiness directive (AD) for certain The Boeing Company Model 737-8 and 737-9 airplanes. This AD was prompted by manufacturing design changes to certain metallic support panel assemblies installed in the flight deck, which resulted in insufficient electrical bonding of the panels and consequent insufficient electrical grounding of installed equipment. This AD requires modification of the electrical bonding of these assemblies to provide sufficient electrical grounding for equipment installed in the flight deck. The FAA is issuing this AD to address the unsafe condition on these products.

SUPPLEMENTARY INFORMATION:

Background

The FAA has received a report of an electrical bonding and grounding issue that was discovered during testing of a newly manufactured Boeing Model 737-8 airplane. During standard production testing by Boeing, electrical power systems did not perform as expected. Investigation identified insufficient bonding of certain metallic support panel assemblies installed in two areas of the flight deck, which affects the electrical grounding of installed equipment. The reported event occurred prior to delivery of that airplane. Investigation identified design changes to the flight deck support panel assemblies, which affected the dedicated bonding and grounding paths that existed prior to the changes. The affected areas are the P6 panel assembly, including the mounting tray for the standby power control unit (SPCU), located behind the first officer, and the main instrument panel (MIP) assembly located in front of and between the captain and first officer. The issue affects certain Boeing Model 737-8 and 737-9 airplanes manufactured after the design changes were implemented. All affected in-service airplanes passed all testing prior to delivery, and there have been no reported in-service failures due to this condition. However, without dedicated grounding paths implemented by design, there is a potential for degradation or loss of the existing uncontrolled ground paths on those airplanes over time.

Degradation of bonds essential for the electrical grounding of equipment, if not addressed, could affect the operation of certain systems, including engine ice protection, and result in loss of critical functions and/or multiple simultaneous flight deck effects, which may prevent continued safe flight and landing. The FAA is issuing this AD to address the unsafe condition on these products.

All affected airplanes, both in the U.S. and worldwide, have been removed from service, pending development and implementation of approved corrective action that will address the unsafe condition.

AD Requirements

This AD requires modifying the electrical bonding of certain support panel assemblies installed in the flight deck to provide sufficient electrical grounding for equipment installed in the flight deck.

The manufacturer is currently developing service information for a modification that will address the unsafe condition identified in this AD. Once this service information is developed, approved, and available, the FAA intends to approve that service information as a method of compliance for the requirements of this AD.

Costs of Compliance

The FAA estimates that modification of the flightdeck panels will take 24 work-hours per aircraft.

Applicability

This AD applies to The Boeing Company Model 737-8 and 737-9 airplanes, certificated in any category, line numbers 7399 through 8082 inclusive, with an original airworthiness certificate or original export certificate of airworthiness issued on or before April 9, 2021.

Modification

Before further flight, modify the electrical bonding of the support panel assemblies installed in the flight deck to provide sufficient electrical grounding for equipment installed in the flight deck, as specified below, as applicable, in accordance with a method approved by the Manager, Seattle ACO Branch, FAA.
(1) Modify the electrical bonding of the P6 panel assembly, including the mounting tray for the standby power control unit (SPCU), located behind the first officer.
(2) Modify the electrical bonding of the main instrument panel (MIP) assembly located in front of and between the captain and first officer.

 

Electrical Bonding Video

This video discusses the recent news story about the 737 MAX electrical bonding issue that has lead to the grounding of 120 MAX aircraft in April 2021.

 

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