Ryanair B738 near Stockholm on Apr 25th 2011, instrument failure, multiple electrical problems
Last Update: November 22, 2012 / 17:08:06 GMT/Zulu time
Incident Facts
Date of incident
Apr 25, 2011
Classification
Incident
Airline
Ryanair
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
The incident was caused by the system logic for the Generator Control Unit (GCU) and the Bus Power Control Unit (BPCU) enabling erroneous status signals from the breaker (Generator Control Breaker, GCB) to lead to a transfer bus losing power.
The captain (35, ATPL, 7,011 hours total, 6,200 on type) was pilot flying, the first officer (38, CPL, 7,400 hours, 6,100 hours on type) was pilot monitoring, when the aircraft was accelerating through about 110 KIAS on takeoff from Skavsta Airport and a warning indication illuminated. The first officer acknowledged and extinguished the indication, the takeoff was continued. Climbing through 400 feet AGL the crew began to investigate what warning had occurred and discovered that master caution and a "right side source off" light had illuminated indicating that one of the power sources normally supplying one of the main transfer busses had disconnected. As there were no memory items for that type of fault, the crew continued the climb to minimum safe altitude at 1000 feet, processed the normal takeoff checklists and thereafter the relevant checklists for the source off indication. The checklists required, with one power source missing, that the engine generator be selected on, upon selecting the right hand generator on, a new warning was illuminated indicating the transfer bus #2 was not receiving power from any of the generators and a number of additional warning indications appeared as well as the first officer's instruments dropped offline and the autopilot disconnected. The transponder also lost altitude reporting capability, ATC was no longer able to see the altitude of the aircraft.
The crew powered the APU up and upon connecting the generator it did not connect to the transfer bus. Instead, a warning light illuminated indicating the onboard batteries were being discharged.
The pilots reported both autopilots became inoperative, the automatic pitch trim ceased, the electrical trim ceased, PFD and ND on the first officer's side went blank, the transponder's altitude reporting capability was lost, nose wheel pedal steering was lost, and the indicators for the trailing edge flap positions failed.
Warning indications occurred for battery discharge, master caution, right hand source off, right hand transfer bus off, mach trim fail, auto slat fail, fuel pump 2 fwd, fuel pump 1 aft, electrical hydraulic pump #2, probe heat B, engine EEC alternate, zone temperature.
Air Traffic Control queried the aircraft after losing altitude information, the crew attempted to engage the other transponder, the altitude reporting however remained lost.
The crew read the emergency checklist for transfer bus off, as the items were the same as the previous checklists that had caused the loss of the transfer bus, the crew decided to not action the checklist and instead return to Skavsta Airport.
In order to reduce workload of the commander, that could not be estimated in the existing environment especially with failed automatic and electrical trim, the crew requested radar vectors back to Skavsta, ATC initially wanted them to use a NDB but began providing heading vectors after the crew explained again they had instrument problems and declared PAN.
The crew aborted the first ILS approach to Skavsta's runway 26 due to still being in discussion with their maintenance base over the incident and the commander wanting the first officer to inspect the flaps visually from the cabin to see whether they had reached their commanded position. A second ILS approach to runway 26 was completed with a safe landing. The cockpit voice recorder's circuit breaker was immediately pulled after landing.
The aircraft had accumulated 28,847 hours in 18,833 cycles since its manufacture in 2002, the 50 cycle inspection had been carried out immediately prior to the incident flight. There were no deferred maintenance items.
After landing maintenance immediately began with the approved fault isolation but the fault had cleared at that time and could not be reproduced anymore. The generator control unit 2 (GCU2) showed a BTB fault indication, after a BITE test the indication cleared and GCU2 pass came on. As a safety measure both BTB2 and GCU2 were replaced.
Another BTB fault had been indicated by GCU2 six days prior to the incident, at that time the GCU2 and BTB2 had been replaced too.
SHK looked into the status of the aircraft and found several occasions where a "source off" indication had occurred and a power source could not be connected to the transfer bus #2, each of those faults had cleared after the GCU2 or BTB2 or other components like cabling had been replaced.
Following numerous extensive fault isolation tests Ryanair mechanics were finally able to trace down the primary cause of the faults as a short between the phases in the feeder cable from IDG2. The cause of the transfer bus not connecting during the incident flight however was never identified.
The SHK enagaged in an extensive analysis of the fault with the focus on why the two transfer busses did not interconnect. The fault isolation procedures were insufficient, a much deeper analysis was needed after the SHK learned of many more similiar incidents.
The SHK thus went into analysis of the system logic and identified a possible scenario that could explain the sequence of events. The SHK attempted to reproduce these events on a B738 full flight simualator but were unable to inject the faults as the simulator had not been designed to permit injection of such faults.
The SHK analysed based on all information available, that the GCU2 had received an erroneous signal, that IDG2 had been connected to the transfer bus thus locking out the possibility to connect APU or other power sources like the left transfer bus #1 to the transfer bus #2. However, as IDG2 in fact was not connected, transfer bus 2 lost power. The erroneous indication must have originated at the GCB2 (unit connecting IDG2 to bus #2) itsself indicating the switch had closed although it had not moved.
The SHK reported that they attempted numerous time to get hold of the relevant GCB2 installed during the incident or other GCB2 with the same part number through out the investigation without success however.
The SHK verified that with a signal "breaker not open" although the breaker was open the aircraft systems would react in the very same way as during the incident. The only way to force a correct signal again would be to select the IDG off first, then on again like it is normal in case of a relay fault.
The SHK analysed that this scenario most likely was not covered in the fault tree analysis during system design, and was not covered by regulations.
The SHK concluded the analysis stating: "The guidance text in FAR-part 25 and JAR-25 shows that this incident can be classified as on the borderline of a major failure condition."
Two safety recommendations were issued to Boeing, first to ensure the busses would not lose power upon an erroneous signal and second to introduce a procedure permitting the reconnection of the IDG in case of an erroneous GCB signal.
Incident Facts
Date of incident
Apr 25, 2011
Classification
Incident
Airline
Ryanair
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
This article is published under license from Avherald.com. © of text by Avherald.com.
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