It is not always possible to repair a system failure before the next flight. The MEL permits the dispatch of an aircraft with inoperative equipment or functions for a limited period of time, and under specific conditions, while maintaining an acceptable level of safety.

Current in-service data shows an increase in the number of requests for MEL extension indicating an increase of departures with multiple open MEL items. Even if dispatch under MEL always guarantees an acceptable level of safety, it increases the risk of exposure to multiple failures with their inherent operational consequences.

This article recalls the importance of fixing MEL items at the earliest opportunity to reduce this risk of exposure to multiple failures, and provides best practices where MEL extensions are necessary to maintain the highest possible margin of safety.

Event Description

An A330 aircraft was dispatched on a non-ETOPS flight under MEL item 36-11-01A ENGINE BLEED AIR SUPPLY SYSTEM on engine 1. Before departure, ENG 1 BLEED push button switch was set to OFF, the X BLEED selector set to OPEN and APU and APU BLEED were confirmed as operative, as per the dispatch condition of the MEL item (fig.1).

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(fig.1) MEL item 36-11-01 Engine Bleed Air System

During the cruise phase at FL390, the AIR ENG 1+2 BLEED FAULT ECAM alert triggered. The flight crew followed the ECAM instructions and managed to reset the ENG 2 bleed system. 20 seconds later, the AIR ENG 1+2 BLEED FAULT ECAM alert triggered again. The flight crew attempted to reset the ENG 2 bleed system but the reset was unsuccessful.

The flight crew then declared a PAN PAN situation to ATC and requested to descend to FL100. The CAB PR EXCESS CAB ALT ECAM alert triggered. The flight crew put on their oxygen masks and performed an emergency descent. Passenger oxygen masks automatically deployed during the descent. At FL190, the flight crew started the APU and set APU BLEED to ON. The APU bleed remained ON until the end of the flight. At FL100, the flight crew again attempted a reset of the ENG 2 bleed system, which was successful.

A decision to divert was taken and the aircraft climbed to FL170, then performed an approach and landed without further events.

Event Analysis

Significant operational impact

With the MEL applied for the inoperative ENG 1 bleed air supply system, the failure of the ENG 2 bleed air supply system during the flight led to the loss of cabin pressure at cruise altitude requiring an emergency descent and diversion. This had a notable operational impact, including the impression on the passengers who were aware of the sudden descent and observed the deployment of the oxygen masks in the cabin.

An acceptable margin of safety

Despite the significant operational impact, an acceptable margin of safety was maintained throughout this event thanks to the correct application of the MEL item dispatch conditions and operational procedures by the flight crew.

The impacts of one inoperative engine bleed system, combined with a subsequent failure of the other engine bleed system, are described and addressed in the dispatch conditions and operational procedures of the MEL item. The condition of an operative APU BLEED, and the likelihood of passenger oxygen masks deploying when at cruise altitude above 37 400 ft, are also described.

POSSIBLE BUT TEMPORARY SOLUTIONS TO FACE DISPATCH CHALLENGES

Raise of Dispatch Challenges

An increase of the operational pressure to fly is observed. In addition, various operational constraints such as supply chain disruption affecting parts availability or the lack of qualified maintenance personnel may make it difficult to repair or replace the equipment within the time frame allowed by the MEL. This leads to an increase of the number of aircraft dispatched with open MEL items and therefore an increased exposure to multiple failure situations as described in the case study.

In-service reports also show that the number of incorrectly applied MEL items is increasing.