What happened
On 27 August 2022, an ATR 42-500, registration G-LMRC, was performing a scheduled passenger flight from Aberdeen to Sumburgh. While climbing through 10,000 feet, the co-pilot switched off the landing lights as part of standard procedures. Immediately, the crew heard a loud clunk from the electrical cabinet. Simultaneously, the primary flight displays (EADI and EHSI) for both the commander and co-pilot went blank, the autopilot disengaged, and the air conditioning packs lost power. The crew also noted a momentary deceleration and a drop in engine torque indications.
The power loss lasted approximately three seconds before the systems restored themselves. The crew elected to return to Aberdeen, where they landed without further incident. However, the same electrical failure recurred once on the ground when the commander switched off the landing lights during the after-landing checklist.
The investigation
AAIB investigators examined the aircraft's electrical network and components. They identified that power had been lost from the emergency electrical network, including the DC STBY, DC EMER, and DC ESS buses. There was also evidence of a potential undervoltage condition on DC BUS 2, which explained why the co-pilot's specific flight instruments failed.
Physical examination of the 1PA contactor revealed it was stuck in an intermediate position when energised. This prevented it from supplying backup power from DC BUS 1 to the emergency network. The investigation found that the contactor's plunger stop had shifted upwards by 0.4 mm, a displacement likely caused by impact damage to the coil casing. This specific failure mode was not previously envisaged by the manufacturer or the contactor producer, and standard maintenance tests were unable to detect it.
Findings
- The 1PA contactor was unable to provide backup power to the emergency electrical network because it was stuck in an intermediate position.
- The 1PA contactor failure was a contributory factor to the loss of the emergency electrical network.
- The simultaneous loss of the emergency network and DC BUS 2 could not be definitively determined, though a wiring issue in the DC Gen 2 connection was noted.
- The failure was triggered by the switching of the landing lights, which coincided with the electrical disturbance.
Safety action
Following the investigation, EASA issued an Airworthiness Directive (AD 20230181) requiring operators to test the 1PA contactor within 60 days and subsequently at intervals not exceeding 1,000 flight hours. The AAIB also issued a safety recommendation to the manufacturer to continue investigating the underlying causes of these electrical failures.