jet2 B752 near Kavala on Sep 7th 2011, electrical problems

Last Update: June 14, 2012 / 12:48:29 GMT/Zulu time

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Photo of Jet2.com SU-GBG, Boeing 757-200 — Jet2.com SU-GBG, Boeing 757-200 (Photo credit: Aero Icarus / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Sep 7, 2011

Cause
Electrical problems

Airline
Jet2.com

Aircraft Registration
SU-GBG

Aircraft Type
Boeing 757-200

ICAO Type Designator
B752

The United Kingdom's Air Accident Investigation Board (AAIB) released their bulletin reporting the aircraft was enroute in Bulgarian Airspace when the crew received "L AC BUS OFF" and "L GEN OFF" messages followed by a number of systems failing. The crew worked the relevant checklists which briefly restored the left AC bus power, however the power failed again this time with a thin haze of smoke and a burning electrical smell in the cockpit. The crew donned their oxygen masks and goggles and declared emergency. The crew subsequently powered the APU up which supplied the left AC bus for about 17 seconds before the left AC bus failed again. On final approach to Kavala it was apparent that the fumes had dissipated, the passengers disembarked normally after safe landing.

The AAIB reported the aircraft had experienced a "L AC BUS OFF" message 13 flights prior to the incident flight. Maintenance had performed troubleshooting, however with inconclusive results, the left hand Integrated Drive Generator (IDG) was disconnected and put onto the deferred defects list under minimum equipment list requirements.

On the day of the incident flight, prior to the flight, maintenance performed another go at the defect and found an open circuit at a connector between the left hand engine pylon and the left hand IDG, the terminal crimp pin was found corroded. The wiring loom between the left engine pylon and IDG was replaced, a successful operational check followed and the left hand IDG was returned to service with the left AC bus system declared serviceable.

Following the incident flight a post flight visual inspection found no cause and no origin of the smoke, the circuit breakers and the bus ties were found without defects. Error codes in the Bus Power Control Unit (BPGU) indicated that the current demanded by the loads on the left AC bus were not in balance with the output by the IDG. Maintenance replaced the left generator control unit and the left GBPU, ground run the engines for 45 minutes with both AC busses operating normally without any unusual smells. The aircraft was subsequently released to return to Manchester.

During the return flight to Manchester, about 140 minutes into the flight, the crew received again "L AC BUS OFF" and "L GEN OFF" messages without any associated smells, the crew successfully powered the left AC bus via the APU and completed a normal landing in Manchester.

After landing the crew attempted to reconnect the left IDG and was successful.

Maintenance again found the error codes in the BPGU suggesting that the load had not been in balance with the power generated by the IDG.

Further maintenance actions followed which identified pin 12 at the connector at the left engine pylon was open, the same pin that had already been identified in maintenance action prior to the incident. The connector was disassembled and it was found that the crimp terminals at pin 11 and pin 12 had parted from their wiring and the connector backshell was loose. Pin 11 connects an earth shield, while pin 12 connects a winding around the A phase output of the IDG to the left generator transformer for fault detection.

All crimp terminals were replaced, a new backshell mounted and electric continuity verified. Following this maintenance activity no further fault occurred. The faulty crimp terminals as well as the faulty backshell were sent to the AAIB for further testing.

The AAIB analysed that the backshell was loose due to stripped threads probably because of overtightening. This defect permitted moisture to enter the connector causing corrosion on the internal components. In addition to permitting moisture into the connector the loose backshell also removed support of the wiring loom permitting the loom to vibrate, which promoted mechanical damage of the individual wires at their point of attachment to the crimp terminals. The loss of connectivity on pin 12 erroneously meant that the A-phase produced zero current and prompted the GCU via logic from the BPGU to disconnect the left IDG.

The AAIB further analysed the intermittent nature of the defect, due to corrosion with the crimp terminals, made it difficult to isolate the fault. There was no evidence of overheating at the connector however, and only during the incident flight there was smell and haze associated with the "L GEN OFF" messages. It is therefore probable that the presence of an additional factor was necessary to produce the smell and smoke. The AAIB continued: "However, loss of electrical continuity at pin 11 of the D1114J connector, as determined from examination of the crimp terminal, disconnected the earth shield from the left IDGÂ’s exciter field power supply wiring. It is therefore possible that electromagnetic interference could have affected the exciter field voltage and, in turn, the left IDG output AC voltage. The aircraft manufacturer confirmed that a reduction in AC voltage can cause fuselage-mounted electrical motors and transformers to overheat, resulting in a hot electrical smell and possibly light smoke, but without leaving any visible evidence once these components have subsequently cooled."