System Reset - Use with Caution

A system reset is not always the quick fix that it may seem. Performing an inappropriate manual system reset in flight can seriously impair the safety of the flight. Multiple system resets on the ground without performing the necessary troubleshooting actions can also have serious consequences. This article, developed by Airbus, addresses when system resets are applicable and how to perform them correctly. Read the full article on Airbus Safety First here. 

What is a system reset?

A system reset is the action of switching off a system and then switching it back on again with the objective to retrieve normal system behavior or recover a previously lost function. It is different from re-engaging a tripped Circuit Breaker (C/B). 

Tripped circuit breaker

A C/B will trip when there is an overload of electrical current detected in the circuit. This is to protect from overheating or a short-circuit condition in the wiring that could lead to further damage or fire. Management of tripped C/Bs is not covered by this article. In this article, the term “reset” describes the action of switching off a system and switching it back on again. This action can also be called a “cycle”.

Automatic reset vs. manual reset

Certain avionic systems, such as the Flight Management System (FMS), have an automatic reset function. The reset action is completely managed by the system that has an automatic failure detection mode. Maintenance or flight crews perform a manual reset by using the cockpit control for the system, a circuit breaker, or a dedicated reset button (also called a reset switch). This article focuses only on these types of manual resets.

Manual reset using system controls

For specific systems, such as the flight control system, the maintenance or flight crews can perform a system reset from the cockpit using pushbutton-switches available on the overhead panel.

Manual reset using a circuit breaker

Pulling a system C/B and then pushing it back in will trigger a system reset because this will isolate and then restore the power supply to all parts of the system. It will also cause the software of the system to reload. This is considered as a “hard system reset”. There are two types of C/Bs: traditional C/Bs and electronic C/Bs. The traditional C/B is manually opened and closed. The electronic C/B, also called Solid State Power Controller (SSPC), is controlled by a remote interface (on A220/A380/A350). Various system C/Bs are located in the cockpit of Airbus A220/ A300/A310/A320 aircraft, the avionics bay, the cabin, and the cargo compartments. There are no C/Bs in the cockpit of Airbus A330/ A340/ A350/ A380 aircraft. They are replaced by system reset buttons on the overhead panel. 

Manual reset using a reset button

Pulling a system reset button (fig.3) then pushing it back in the cockpit will only reset the system software part (only available on Airbus A330/A340/A350/A380 aircraft). This is known as a “soft reset” because the system will remain powered.

Inappropriate system resets can have serious consequences

Past events have highlighted how some system resets can have irreversible consequences. One example is where a system cannot be recovered after an inappropriate system reset in flight. Another example is where a reset of flight control computers is unduly performed. Depending on the system malfunction encountered, this can cause unexpected movements of the flight control surfaces, which may lead to serious consequences if performed in flight. Avionics systems are interconnected systems, therefore, a system reset of one system can have significant consequences for the other systems that rely on its data. Inappropriate system resets can have unexpected side effects and hide deteriorating conditions of the system. In combination with a failure of another system, the safety of the flight can be impaired. Therefore, it is important that maintenance personnel and flight crews only perform system resets in accordance with the guidance in the relevant procedures, as for the cases described in this article.

System resets by flight crew and maintenance personnel.

There are only some very specific situations where flight crew or maintenance personnel can perform system resets, for full information refer to the original Airbus Safety First article or the full article on the EASA Air Ops Community Site.

Summary

Unauthorized resets of an aircraft system can hide a deteriorating condition of the system. What may seem to be a "quick-fix" on the ground to dispatch the aircraft can lead to a system fault reappearing in flight that may even affect the safety of the flight. 

If not specifically requested in an ECAM/OEB/FCOM/QRH procedure, the flight crew can only consider attempting a reset to recover the operation of an affected system if it is listed in the System Reset table of the FCOM/QRH. If there is no reset procedure available in the System Reset table of the FCOM/QRH, which is associated with the malfunction or ECAM alert encountered, then the flight crew must NOT attempt to reset the system. Any system reset performed by the flight crew needs to be reported to maintenance personnel and must be recorded in the aircraft technical logbook, including the number of attempts and outcomes. For A320 aircraft only, in some circumstances, due to possible electrical transients, the flight crew cannot perform on-ground resets that are not listed in the reset table.

Maintenance system resets are only performed in accordance with specific TSM/AFI tasks. Troubleshooting can start with resets but should not end there. The appropriate troubleshooting actions or at least recording actions should always follow. 

For A320 aircraft only, the same on-ground resets from the System Reset table of the QRH are available in the A320 TSM and can be used to manage intermittent faults and ease the aircraft dispatch. In this case, it is possible to perform system resets that are not specifically listed in the TSM. 

Manual system resets performed by flight crew or maintenance personnel are not a way to fix repetitive faults. Multiple and unreported resets can hide degraded system conditions. The fault could reappear later and have significant consequences during a flight. An efficient system for reporting and managing system resets is crucial for monitoring the health of all aircraft systems, which is key to maintaining safe aircraft operations.