West Sweden ATP near East Midlands on May 3rd 2018, temporary total electrical DC failure

Last Update: April 25, 2019 / 17:09:19 GMT/Zulu time

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Incident Facts

Date of incident
May 3, 2018

Classification
Report

Flight number
PT-9G

Aircraft Registration
SE-MHF

ICAO Type Designator
ATP

A West Air Sweden British Aerospace ATP, registration SE-MHF performing flight PT-9G from East Midlands,EN to London Stansted,EN (UK) with 2 crew, was enroute at FL110 in night conditions when the aircraft suffered the total loss of DC-power resulting in the loss of a significant number of flight instruments and aircraft systems. The crew worked the related checklists and decided to return to East Midlands. The aircraft landed safely back in East Midlands about 45 minutes after departure. After vacating the runway DC power restored without any crew action.

The British AAIB released their bulletin concluding the probable causes of the serious incident were:

The aircraft experienced two separate, independent failures within the DC electrical power system during a cargo flight from East Midlands Airport to Stansted Airport, resulting in the loss of multiple flight deck instruments, lighting, left engine control and standby flying controls. The crew were able to return to East Midlands Airport where a normal landing was made, following which the DC electrical power was restored. The loss of electrical power experienced during the flight was consistent with a failure of the No 1 TRU or its contactor, followed by a subsequent failure of the DC essential busbar couple function.

The cause of both failures, which could not be repeated during subsequent testing, was probably intermittent and transitory so could not determined.

The AAIB described the sequence of events:

The aircraft was operating a cargo flight from East Midlands Airport to Stansted Airport and was established in the cruise at FL110 with the No 2 autopilot engaged. The co‑pilot was the pilot flying and the commander was the pilot monitoring. As the aircraft was approximately 8 nm west of Milton Keynes and, shortly before commencing the descent towards Stansted Airport, the master caution aural alert sounded and the TRU 1 and dc lo volts central warning panel (CWP) lights illuminated. This indicated that the No 1 TRU was no longer supplying 28 VDC voltage to the No 1 essential DC busbar.

The crew carried out Emergency and Abnormal Checklist (EAC) Card 49 ‘TRU failure or single DC busbar low voltage’ (Figure 1). The commander initially attempted to reset TRU 1, but this was not successful. He then switched the No 1 non-essential DC busbar off and selected the DC essential busbar couple to connect the No 1 and No 2 essential DC busbars together. Following these selections, the dc lo volts warning message extinguished, and the crew observed that the No 1 battery voltage indicator was in the green arc, indicating that the battery voltage was between 23 and 29 V. During this period, the commander was recorded on the CVR stating that he considered it was a busbar relay fault, rather than a TRU fault.

The crew conducted a DODAR1 decision-making exercise and decided to return to East Midlands Airport. Shortly after, the commander tried to reset the TRU 1 again but this was unsuccessful. En-route the crew confirmed that the load on the No 2 TRU was below 180 amperes (A) and that no circuit breakers in the cockpit had tripped.

Approximately 15 minutes after the initial loss of electrical power, the commander noticed that his flight director had failed. The crew confirmed that the DC essential busbars were coupled and the dc lo volts CWP caption had re-illuminated. The commander subsequently recalled that the No 1 inverter had failed and the No 1 battery voltage had reduced to 12V.

Seven minutes later, a master caution alert sounded and the gpws CWP caption illuminated. This was shortly followed by the commander’s electronic flight instrumentation system (EFIS) primary flight display (PFD) and navigation display screens becoming corrupted and unreadable and the autopilot disconnecting; the quick access recorder (QAR) recording also stopped. The commander then declared a PAN.

A few minutes later, as the aircraft was descending to 5,000 ft amsl, the No 1 engine control frozen indication and standby controls fail CWP caption illuminated, the flight deck lights flickered and a pulsing was heard on the radio by both crew. As a precaution, the crew advised ATC that the radios might stop operating. The FDR stopped recording a few seconds later. The commander selected the emergency busbar for his radio (the co-pilot’s was also supplied by the emergency busbar) but the pulsing sound continued.

The aircraft was on base leg for Runway 27 at East Midlands Airport when the flight management system (FMS) failed, along with the No 1 DME and the autopilot flight mode annunciator panel. When the aircraft was about 7 nm from the runway, the pulsing sound on the radios stopped. The aircraft was vectored to a visual approach to Runway 27, which was followed by an uneventful landing, flown by the co-pilot whose EFIS screens were operating normally. As the aircraft touched down, the pulsing sound on the radios briefly returned before stopping again.

As the aircraft vacated the runway, the dc lo volts and tru 1 CWP warning messages extinguished and power was restored to the flight deck lights, the commander’s EFIS screens and flight director. The crew checked the electrical load on TRU 2 and noted that it was more than 180 A, so they followed QRH Card 49 again. The crew selected the inverter transfer on and selected inverter No 2 and both non-essential DC busbars off, to reduce the electrical load, and the DC essential busbars were confirmed as being coupled.

After the aircraft was parked, the crew and two engineers from the operator’s maintenance organisation discussed the event and began fault-finding. As the No 1 battery busbar was connected to DC power, the No 1 battery overheat CWP caption briefly illuminated and one engineer noted that the No 1 battery was drawing over 300 A. Both batteries were switched off prior to further functional testing of the electrical power system.

The AAIB analysed:

The initial loss of DC electrical power was caused by a failure of either the No 1 TRU or its contactor providing connection of the TRU’s output to the No 1 DC essential busbar. This fault condition persisted until the aircraft landed, when the dc lo volts and tru1 warning captions extinguished.

The crew correctly followed EAC Card 49 resulting in the successful coupling of the No 1 and No 2 DC essential busbars. The further attempt to reset the No 1 TRU, following the busbar couple, deviated from the procedure contained in Card 49 although it did not affect the configuration of the DC electrical system at this stage as the No 1 TRU or its contactor remained in a failed condition.

The subsequent recurrence of the dc lo volts CWP caption and resulting loss of electrical services is consistent with a reduction in voltage of the No 1 essential DC busbar, caused by the failure of the couple between the No 1 and No 2 DC essential busbars. The busbar couple failure was consistent with one of the busbar tie contactors (PH7 or PH2) failing open. The No 1 battery continued to provide DC electrical power to the No 1 essential busbar until it was sufficiently discharged for electrical services to be lost.

The couple push button selector-indicator (PBSI) on the DC electrical control panel is unusual in that it has a two-part ‘mimic’ line, with the left and right halves illuminated by power from auxiliary connections to the PH7 and PH2 busbar tie contactors respectively. Therefore, should one contactor fail open, the busbar couple function will fail yet one half of the couple PBSI mimic line will remain lit. This may have led the crew to believe that the No 1 and No 2 DC essential busbars remained coupled, when they were not.

The tru1 CWP message extinguished after landing, coincident with the restoration of electrical power to those services that had been lost during the flight. It is possible that the airframe vibration from the landing was sufficient to clear the electrical fault that had caused the No 1 TRU to fail, and also to allow the DC busbars to couple once again, as confirmed by the crew after landing. Inspection of the aircraft’s DC electrical system following the event did not reveal any component defects that would have caused the electrical failures experienced during the flight.

The transient No 1 battery overheat CWP caption noticed by ground staff during post-flight fault-finding was caused by the discharged No 1 battery receiving a charging current in excess of 300 A for a sufficient period of time to allow it to reach the 60°C temperature threshold required to trigger the CWP warning.

The AAIU noticed that the flight data recorder data ahowed several gaps, parts of the flight had not been recorded. The AAUB analysed: "Inspection of the FDR found evidence of moisture within the electronics module. This most likely caused the intermittent operation of the magnetic-tape recording function. The moisture may have also prevented the correct operation of the BITE as no fault was noticed during the period of incorrect operation." The AAIB further analysed that 35% of all FDRs returned to BAe showed evidence of moisture within the electronics module, 22 of the FDRs had failed due to moisture damage. The AAIB wrote: "Discussions with engineers, and inspection of SE-MHF, indicate that rainwater can enter the cargo bay area during loading, which may then find its way into the rear equipment bay and the FDR. There was also some evidence that rainwater had dripped onto the FDR. Over time this will increase the probability of moisture entering the FDR and cause it to fail as corrosive products develop. Although tested for resistance to moisture ingress at certification, the PV1584 is not hermetically sealed and therefore moisture and liquids can easily enter the unit. Unlike later generation solid-state recorders, the unit was not required to be tested for its waterproofness or the potential effects of dripping water."
Incident Facts

Date of incident
May 3, 2018

Classification
Report

Flight number
PT-9G

Aircraft Registration
SE-MHF

ICAO Type Designator
ATP

This article is published under license from Avherald.com. © of text by Avherald.com.
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