Attention:
Owners, operators and maintainers of De Havilland Aircraft of Canada Limited DHC-8-401 and -402 series aeroplanes
| File Classification No.: | Z 5000-35 |
|---|---|
| RDIMS No.: | 17119548 |
| Document No.: | CASA 2021-06 |
| Issue No.: | 01 |
| Effective Date: | 2021-04-15 |
Purpose:
The purpose of this Civil Aviation Safety Alert (CASA) is to raise awareness of electrical connector and backshell corrosion on Dash-8-401 and -402 series aeroplanes, and to provide information and recommendations to operators and maintainers of these aircraft.
Background:
There has been numerous reports of corrosion on the cable connectors and connector backshells on Dash-8-401 and -402 series aeroplanes. A high number of corroded connectors located within the nacelle/main landing gear (MLG) compartments, nose landing gear bay as well as wing and stabiliser trailing edges were identified, leading to an assumption that the corrosion may be a direct result of exposure to harsh operating environments including de-ice chemicals, water ingress and accumulation of grime. The majority of reports were associated with cable harnesses in the landing gear bays.
Lightning and high intensity radiated fields (L/HIRF) protection of aircraft electrical and electronic systems relies on a number of protection features including the integrity of the cable shielding. The cable shielding integrity (also termed shield transfer impedance) is comprised not only of the cable shield itself, but also of the electrical bonding of the cable shield through the backshell and connector to the airframe. Corrosion associated with the connector, backshell or shield termination can increase the electrical bonding resistance of the shield termination and adversely impact the transfer impedance of the cable shielding. An increase in transfer impedance of the cable shielding will result in higher levels of electromagnetic interference (EMI) coupling onto the internal wiring of electrical and electronic systems during an L/HIRF event, which can lead to damage or upset of electrical and electronic systems.
The shields must be electrically bonded to the aircraft structure at both ends of the cable harness for the shielding to be effective. Electrical bonding is achieved by terminating the wire shields to the strain relief of the backshell or to a ground module inside the backshell. Metal overbraid is electrically bonded to the backshell through a Tinel-Lock ring or a band clamp. The backshell is electrically bonded to the connector through the mating teeth and threads. The connector is electrically bonded to the line replaceable unit (LRU) or aircraft structure by removing the insulating finishes in the mating contact area.
Connector/backshell corrosion is an on-going finding on all aircraft types, just as structural corrosion will always be present in aviation. Metals associated with structures and systems, are subject to environmental attack. These metals are treated with protective finishes. There may be cases where these finishes were compromised over the life of the aircraft.
Recommended action:
Transport Canada (TC) recommends the following for all DHC-8-401 and -402 series aeroplanes:
The best means to address the ongoing threat of corrosion is to include comprehensive connector/backshell inspection/rectification instructions in the aircraft maintenance program. De Havilland Aircraft of Canada Ltd. (DHC) has revised the Aircraft Maintenance Manual (AMM) and the Maintenance Task Card Manual (MTCM) to address connector corrosion, and has published Service Letter DH8-400-SL-20-006B to make operators aware of the corrosion prevention procedures in the AMM for electrical connectors. Proper connector/backshell/shield care will improve the reliability of the aircraft and reduce nuisance warnings.
Contact office:
For more information concerning this issue, contact a Transport Canada Centre; or contact Joel Virtanen, Continuing Airworthiness in Ottawa, by telephone at 1-888-663-3639, facsimile at 613-996-9178 or by e-mail at cawwebfeedback@tc.gc.ca.
Original signed by
Rémy Knoerr
Chief, Continuing Airworthiness
National Aircraft Certification
The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.
Dash-8-401 and -402 Electrical Connector Corrosion
(PDF, 178 KB)