West Atlantic AT72 at East Midlands on Jan 17th 2023, electrical issues

Last Update: May 16, 2024 / 16:59:25 GMT/Zulu time

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Photo of West Atlantic G-NPTF, ATR ATR-72-200 — West Atlantic G-NPTF, ATR ATR-72-200 (Photo credit: Alec BHX/KKC / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Jan 17, 2023

Classification
Incident

Airline
West Atlantic

Flight number
NPT-07C

Departure
Belfast International, United Kingdom

Destination
East Midlands, United Kingdom

Aircraft Registration
G-NPTF

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

A West Atlantic Avions de Transport Regional ATR-72-200, registration G-NPTF performing flight NPT-07C from Belfast International,NI to East Midlands,EN (UK), was on approach to East Midlands when the aircraft encountered significant electrical problems, the crew received many warnings and cautions as result prompting the crew to divert to Birmingham,EN (UK) where the aircraft landed without further incident.

The aircraft remained on the ground for a week and returned to service on Jan 25th 2023.

The British AAIB rated the occurrence a serious incident and opened an investigation.

On May 16th 2024 the AAIB released their final bulletin concluding the probable causes of the serious incident were:

A wiring defect on the DC Gen 2 speed sensor resulted in rapidly changing erroneous signals being sent to the GCU. This resulted in the rapid opening and closing of contactor 23PA in response to these inputs and, due to the rapidly fluctuating conditions, the BTC entered a self-protection mode and remained open for the remainder of the flight.

As a consequence of the above, the crew lost a significant number of instruments and systems during the final phases of a CAT II approach in reduced visibility at night. The power distribution anomalies also resulted in a number of spurious and potentially distracting EGPWS aural alerts. The crew conducted a go-around and, following a temporary loss of communications and permanent loss of autoflight capability, manually flew the aircraft to Birmingham Airport where it landed without further incident.

The wiring defect was probably caused by incorrect use of wire stripping tools at the third-party organisation that had overhauled the starter-generator.

The starter-generator manufacturer and the overhaul organisation have identified a number of safety actions they intend to take to prevent a reoccurrence.

The AAIB summarized the sequence of events:

The crew reported for duty at 2320 hrs at East Midlands Airport (EMA). The aircraft’s tech log contained an entry for an Acceptable Deferred Defect to the TCAS system but there were no other technical issues. The aircraft was de-iced due to frost conditions at EMA, and it departed at 0030 hrs. The flight to Belfast International (BFS) was without incident and the aircraft arrived at 0150 hrs. The crew conducted a routine turnaround and, due to low temperatures and active frost in BFS, the aircraft was again de-iced before its departure at 0310 hrs.

The departure from BFS and the cruise toward EMA were uneventful. Due to a forecast of freezing fog at EMA the crew prepared and briefed for a CAT II approach with the commander as PF. As they approached EMA, they were told to expect to hold due to delays caused by the Low Visibility Procedures in force at EMA. The aircraft entered the hold as directed and after approximately 10 minutes ATC gave radar vectors to intercept the ILS for Runway 27 at EMA.

The initial stages of the CAT II approach on the ILS proceeded normally. The aircraft achieved stable approach conditions at 1,300 ft amsl and, at 1,000 ft radio altitude, the crew saw the correct dual autopilot indications on the aircraft displays. At approximately 500 ft radio altitude the aircraft entered cloud. Shortly after that the elec caution on the Centralised Crew Alerting System (CCAS) illuminated. This indication directed the crew to check the electrical indications on the overhead panel where they noted that the dc gen2 fault amber light was flashing. The co-pilot’s Electronic Attitude and Direction Indicator (EADI) and Electronic Horizontal Situation Indicator (EHSI) were also flashing in time with the elec and dc gen2 captions. The autopilot (AP) disconnect horn sounded and the flight director (FD) modes on the EADI disappeared, although the FD guidance bars remained visible and appeared to give sensible indications. The crew decided to go around.

As power was applied for the go-around there were numerous audio and visual warnings including Enhanced Ground Proximity Warning System (EGPWS) warnings for “terrain ahead” and “too low gear”. The co-pilot’s EADI and EHSI continued to flash, and his ASI was cycling from maximum to minimum speed. The co-pilot made a positive rate of climb call and then, on the commander’s order, retracted the landing gear. The commander’s EADI and EHSI went blank for a few seconds but then recovered before blanking again for a few seconds approximately one minute later. The standby instruments continued to work normally.

During the climb, both pilots recalled hearing sounds that they believed were electrical relays cycling. The Autopilot Display Unit (ADU) was also flashing, and the crew were unable to reselect the AP. In the climb, the crew realised that the flap setting had remained at 30 and they retracted the flaps; flaps limiting speed was not exceeded. They carried out the published go-around procedure, levelled at 3,000 ft amsl and turned right toward the EME NDB (Figure 1).

Once level, the crew made a PAN call to ATC but heard no response. They then tried to contact ATC on the distress frequency 121.5 MHz but still received no response. They reselected the EMA Tower frequency, selected 7600 on the transponder, then broadcast a MAYDAY. Amidst audio conditions that the crew described as severe “static”, the pilots heard a faint message from ATC to contact EMA Radar. They changed to the appropriate frequency and from that point communications were restored. The crew requested radar vectors for a diversion to Birmingham International Airport (BHX).

As the dc gen2 fault amber light was flashing, the crew carried out QRH actions for a dc gen2 fault, which involves selecting that generator to off. Approximately seven minutes elapsed from the start of the event until the crew deselected DC Gen 2. When the DC generator 2 was switched off the audio and visual warnings stopped, the co-pilot’s screens stopped flashing and went blank, and a number of caution lights remained illuminated in the overhead panel. The commander recalled that the “Bus Tie Contactor (BTC) did not close” as would have been expected so the DC Bus 2 was unpowered. The crew consulted the QRH loss of equipment list for DC Bus 2 off (Figure 2).

In addition to the items on the Lost Equipment list, the commander also stated that there remained additional electrical failures to those on the list such as the loss of the environmental control system recirculation fan and the Aircraft Performance Monitoring System.

The crew recognised that the main battery was discharging and the Green hydraulic system which lowers the landing gear was unpowered. The commander was concerned for further electrical failures should the aircraft remain airborne for longer than the published 30-minute life of the main battery. The crew did the performance calculations for landing at BHX and used the Blue hydraulic system to pressurise the Green hydraulics to allow the landing gear to be lowered.

The left navigation receiver could be tuned to the ILS for Runway 33 at BHX, but all automatic flight functions remained unavailable. The commander flew a manual raw data ILS approach to Runway 33 and the aircraft landed without further incident and taxied to stand. The aircraft was taken out of service for detailed troubleshooting on the cause of the electrical issues experienced by the crew.