LATAM B773 near Belo Horizonte on Dec 20th 2018, electrical failures, RAT deployed

Last Update: July 27, 2021 / 08:13:04 GMT/Zulu time

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Incident Facts

Date of incident
Dec 20, 2018


Flight number

Aircraft Registration

Aircraft Type
Boeing 777-300

ICAO Type Designator

Airport ICAO Code

A LATAM Boeing 777-300, registration PT-MUG performing flight LA-8084 from Sao Paulo Guarulhos,SP (Brazil) to London Heathrow,EN (UK) with 341 passengers and 16 crew, was enroute at FL290 about 90nm southwest of Belo Horizonte,MG (Brazil) when the crew decided to divert to Belo Horizonte due to electrical problems, the RAT (Ram Air Turbine) deployed. The crew performed an overweight emergency landing on Belo Horizonte's runway 16 at 01:43L (03:43Z) and became disabled on the runway due to all 12 main tyres having deflated.

The airport reported the airport will be closed until 19:00L.

The airline reported the aircraft diverted due to technical reasons, during landing the tyres of the aircraft were damaged and will need to be replaced before the aircraft can be removed from the runway. All passengers disembarked safely. The removal of the aircraft however has been prohibited until at least 19:00L.

Passengers reported there was a strong odour in the cabin, subsequently the captain announced there was a serious electrical fault prompting the diversion to Belo Horizonte.

Another passenger reported that suddenly all lights and the inflight entertainment system went out, only emergency lights remained. Shortly afterwards the aircraft began the descent towards Belo Horizonte, the crew announced that they had lost all electrical systems and were unable to dump fuel. The landing was hard, emergency services foamed the landing gear. About one hour after landing they were able to disembark via stairs. In the terminal it was being said, that the aircraft had lost all navigation equipment, too, the commander flew the aircraft with the assistance of ATC on the remaining radio for a safe landing. When the crew finally showed up in the terminal lounge, the crew was received with lots of applause, the captain took and answered a lot of questions.

Azul Linhas Aereas ATR-72-212A registrations PR-AKJ and PR-AKD have been dispatched to take about 7 tons of equipment, including the needed hydraulic jack and replacement tyres, to Belo Horizonte.

On Dec 20th 2018 Brazil's CENIPA reported electrical failures occurred in cruise flight, which also compromised other aircraft systems. The crew decided to perform a precautionary diversion to Belo Horizonte for an overweight landing. As result the brakes overheated causing the fuse plugs of the tyres to open and the tyres to deflate. There were no injuries, the aircraft sustained minor damage.

Translated transcript (taken from audio, not the text on the video below) of Portugese communication between Aircraft (AC) and ATC (CTL) when crew indicates the nature and extent of their problems and checks available runway length:

AC: 8084, control
CTL: Radio check
AC: Please confirm runway, it's length is 3 thousand meters?
AC: Please, are we... control? Confirm?
-- unreadable --
CTL: TAM 8084, it's three thousand meters, the runway length
AC: Three thousand meters?
CTL: Three thousand meters ratified, plus 600 paved, 3600 meters in total
AC: OK, pal, thank you. Just, please, remember to call the firefighters. OK, buddy?
AC: The aircraft is very heavy, no way to jettison
AC: Asking for your help. OK, CTL?
CTL: Firefighters, copy
AC: Positive. We're very heavy and the jettison system isn't working either
AC: We're practically without any electrical system working correctly, OK?
AC: So, we have this problem, [which is] a bit serious, OK?
AC: So, please, leave the firefighters prepared.
AC: We're trying here to do the procedure, trying to alleviate the aircraft weight as much as possible
AC: But it's difficult. We're proceeding to the segment and [will] land, OK?
AC: If you could close the runway, that would help us. Thank you.
CTL: Runway is closed already, awaiting your aircraft to land.
CTL: And firefighter support on the ground was requested already.
CTL: Please confirm to Control if you will proceed to land now or if you will do some more holds to burn fuel
AC: No, no. We'll proceed to land, sir.
CTL: Roger and authorized. Procedure ILS Kilo. Report established, TAM 8084.
AC: We don't have any extra information, sir. You inform altitude and vectors, OK?
CTL: Roger. TAM 8084, can you make a right turn to heading 065?
AC: I prefer maintaining this heading for some more time in order to increase the distance, sir.
CTL: Roger. TAM 8084 autorized to maintain the current heading in descent to 7000 feet
AC: Roger, 7000 feet, 8084.

On Dec 22nd 2018 The Aviation Herald learned that the electrical problems started with the failure of the right hand backup generator, as result both transfer and both converter circuit breakers popped leaving all electrical busses unpowered although left and right main generators as well as their generator controls, the APU and the left backup generator remained operative (see ELEC page below, the lines to the L MAIN and R MAIN should be green not white and these symbols should be green, too). As result almost all systems except a few connected to the standby busses lost power, only systems remaining powered are: stand by busses (powering one VHF radio) and emergency lighting, the left inboard and outboard and upper center displays as well as the left CDU remain also powered (according to FCOM via the batteries and/or RAT - with the RAT operating also the right hand - first officer's - displays are being powered).

On Dec 23rd 2018 the airline reported that it was not a failure of power generation but a failure of power distribution. The causes of that failure are now being investigated by CENIPA. After landing, when the crew shut the engines down, power returned, as result the cameras came back online and the crew could see that there was no fire on the landing gear or excessive heat on the brakes, hence the crew queried the need for an emergency evacuation as suggested by emergency services. The captain wanted to avoid an emergency evacuation unless really necessary as the emergency evacuation could have resulted in injuries.

On May 24th 2019 Brazil's CENIPA released a brief update that they found evidence of a short on connector D7232 which connects the right hand backup generator to the right hand converter. The backup converter responded by opening 4 connectors feeding the transfer busses (two transfer bus breakers and two converter circuit breakers).

In July 2021 Brazil's CENIPA released their final report concluding the probable causes of the serious incident were:

The deconfiguration of the TBB (Transfer Bus Breaker) and CBB (Converter Circuit Breaker: editorial note: CBB is only used in the conclusions, in the rest of the report CCB is used), regardless of whether it was caused by the failures in the monitoring circuit of the contactors' position or due to a combination of these with previous failures of the oil level monitoring circuit, made the Main and Backup Electrical Systems unable to power the aircraft, even with power generation.

Even though it was not possible to determine how the short-circuit started in the connector or even to indicate the dynamics of the CBB and TBB deconfiguration, it is a fact that the failure generated a series of as-designed system reconfigurations, preventing the supply of the Main Electrical Systems and Backup, despite the generation of energy.

The loss of the Main and Backup System, due to the occurrence of a short-circuit in a connector and its splits, causing a deconfiguration of the TBB and CCB contactors, to the point of having their risk assessed as Class II, Hazardous, based on the exposure to the next critical fault, which would be the loss of RAT, could indicate a weakness in the aircraft's electrical system design.

CENIPA analysed:

The aircraft was climbing to the FL290 with all pilots in the cabin and after approximately 10 minutes of flight, the crew noticed the alert messages ELEC BACKUP LEFT and ELEC BACKUP RIGHT generating the message ELEC BACKUP SYS.

The alert messages that were presented to the crew through the EICAS could be associated with the following faults recorded in the Backup Converter's NVM:
- Right Generator DP Trip - Conv Fail, recorded at 02h42min10s (UTC);
- DC Content Trip - Conv Fail, recorded at 02h42min11s (UTC); and
- Left Low Oil Pressure Trip - Low Oil Pressure, recorded at 02:42min42s (UTC).

The Backup Converter monitored the Backup Generator oil level for a period of 30 to 35 seconds, using a pressure switch. Thus, although there was a difference of just over 30 seconds between the first two messages and the third, it was possible to associate that the three messages originated at the same time.

The messages recorded on the Backup Convertor NVM were consistent with a short-circuit between pins 2, 9 and 10, as shown by the x-ray of the connector's cross section.

In the condition shown in Figure 33, the overcurrent, originating from pin 9, passing through pin 10 and being directed to the energy monitoring circuit generated by the Backup Generator would be consistent with the messages Right Generator DP Trip and DC Content Trip and the damage observed in the Backup Converter.


At the time of the recording Right Low Oil Pressure Trip, the configuration of the contactors for powering the aircraft's electrical system was: TBB closed and CCB open.

The current leakage, through the ANAGND, in the monitoring circuit of the position of the contactors made the Backup Converter identify the positions of the TBB and CCB differently from their effective positions, generating four messages recorded simultaneously at 02h53min57s.

The occurrence of the four failures may have caused a deconfiguration of the aircraft's electrical system. This situation would have been maintained until the engines were shutdown, since the position of the contactors would remain locked after any monitoring failure occurred.
Another possibility would be that, according to the Backup Converter's protection logic, due to the CCB Commanded Open but Sensed Closed message or due to the TBB Commanded Closed but Sensed Open message, the Backup Converter would command the TBBs to the open position and the CCBs to the closed position.

However, even if the CCBs are in the closed position, the Backup Generators would not be available for the electricity supply. The fact that messages related to the Backup system for the generation of electric power have already occurred, generating the ELEC BACKUP SYS fault, made the Backup Generators (right and left) not available for powering the aircraft's electrical system.

Thus, the primary (IDG) and secondary (Backup Generators) energy sources were not available for the aircraft's power supply, due to the successive short-circuits occurring at the connector.

After a few seconds, with no action from the crewmembers, the PFD and the ND of the PF and of the PM were restored. The EICAS screens came back, approximately, thirty seconds later. This can be attributed to the use of the aircraft's battery.

The crewmembers activated the APU in an attempt to have a source providing power for the aircraft systems, however, the action did not change the power condition, since the TBBs were open.

That said, it is possible to conclude that the transfer buses were not being electrically powered and, consequently, the RAT was activated.
Even after the aircraft started to use the Standby system, the volume of fault messages that appeared in the EICAS hindered the total understanding of what was happening, on the part of the crew, as new warnings were constantly appearing and with priority over the previous ones.

This dynamics may have made it difficult for the pilots to identify how the aircraft's electrical power was being distributed, since it was attempted to jettison fuel to avoid overweight landing, even though this was not possible, as the pumps were powered by the main DC bus on their respective side that did not receive power in the Standby power configuration.

Despite the difficulty of correctly identifying how the aircraft was being powered, through the communications and recordings that the Investigation Team had access to, it was clear that all the resources available to the crewmembers were used and that the tasks were divided to not generate work overload for the cabin occupants.

It was also possible to deduce, based on the data collected, that, despite experiencing a situation in flight unprecedented to all, there was no change in the emotional state of the crewmembers to the point of interfering in the operational performance.

The good interaction they maintained favored both the maintenance of spirits in the cabin and the assessment of the situation for making safe and necessary decisions, considering the scenario faced.

The overweight landing caused the brakes to overheat, making the tires to deflate through thermal fuses and a beginning of fire, which was fought by the Aerodrome firefighters.

After the total stop of the aircraft, the crew shut the engines down, thus interrupting the operation of the PMG. At that moment, the power supply to the Backup Converter would have been interrupted. Thus, the contactors returned to their non-energized positions, that is, TBB closed and CCB open.

With this, the aircraft started to be powered by the APU (through TBB) that had been turned on, after the loss of power by the main and backup mode, and kept in this condition during the rest of the flight.

The reconfiguration of the contactors and, consequently, the restoration of the aircraft's electrical power through the APU allowed the crew to monitor, from the cabin, the action of the firefighters and the situation of the landing gear through the cockpit display that showed the landing gear images.

Communications between the crewmembers and the TWR-CF presented difficulty in understanding the difference between the terms evacuation and disembarkation by the TWR controllers at the Aerodrome, which was confirmed in the interviews.

Communications indicated that the two terms were used synonymously, prompting the crewmembers to ask several questions about the need to command the emergency evacuation or wait for a landing using the stairs.

Additionally, the congestion of the phone in the communications of the emergency teams at the airport made it difficult for the TWR-CF to obtain the information requested by the aircraft, particularly in relation to the need of performing an evacuation or disembarkation, since there was no direct communication between the aircraft and the firefighters.

Based on the information available on the aircraft's cameras and because they realized that communications regarding the need for emergency evacuation were not consistent with the images they received, the crewmembers considered it safer and chose not to command the evacuation. They considered that the unnecessary use of slides would pose an additional risk to aircraft occupants.


According to the research carried out, it was possible to rule out that the origin of the short-circuit was due to its manufacturing process, since the time between the occurrence and the manufacture of the aircraft was approximately 26 thousand flight hours. Nor were found any environmental conditions external to the connector that could have contributed to the occurrence of the short-circuit.

In addition, no evidence was found in the Backup Generator that could be associated with the origin of the short-circuit, with the imbalance of the PMG phases and the damage observed in its stator related to the consequences of failures in the connector.

The connector region was checked in a scheduled inspection (2,250 days check) of the aircraft that took place between 13MAY2018 and 10JUN2018. However, the maintenance task performed did not previse the opening or manipulation of the D7232 connector.

On 12JUL2018, approximately one month after the scheduled inspection, a maintenance intervention was performed motivated by the failure message “Backup Gen Remote Oil Level Sense/Low Oil Pres Circuit (R) is failed”.

The maintenance task was performed according to the FIM manual 24-25-00-810-804 and prevised the verification of the Backup Generator oil level sensors, the measurement of the wiring resistance of the pins 1, 2 and 4 that passed connector D7232 and, finally, the replacement of the Backup Converter.

Thus, the replacement of the Backup Converter (BUC SN 2238 removed and BUC SN 2246 installed) indicated that the resistance of pins 1, 2 and 4 was within the expected values and, as the measurement of the other pins was not required, there was an indication of a possible connector failure that could result in the short-circuit.

On 18DEC2018, a new Backup Converter replacement was performed (BUC SN 2246 removed and BUC SN 2238 installed), this time motivated by the Elec Backup Gen R failure; being performed the failure research task related to the FIM 24-25-09-00-810-802.

During the first stage of the failure investigation, after replacing the Backup Converter, the aircraft was considered ready for return to service. That said, it was not required to continue the failure investigation, and several wiring pins were not checked between the Backup Converter and the Backup Generator, including the PMG pins.

After the installation of the Backup Converter SN 2238, the aircraft performed two flights between the cities of São Paulo and Miami, totaling approximately 15 flight hours and 2 cycles, with no record of new failures. The next flight, on 20DEC2018, was the one in which the failures related to this serious incident occurred.

Thus, despite the examinations, tests and research carried out, it was not possible to identify the origin of the short-circuit in the connector, since they were not identified: foreign material inside; environmental factors that could have contributed to the short-circuit; or maintenance procedures that could be associated with incorrect handling of the wiring harness and connector.

Related NOTAMs:
J0992/18 NOTAMN
Q) SBBS/QMTCM/IV/NBO/A/000/999/1937S04358W005
B) 1812201255 C) 1812202130

J0990/18 NOTAMR J0989/18
Q) SBBS/QMRLC/IV/NBO/A/000/999/1937S04358W005
B) 1812200617 C) 1812201400

J0989/18 NOTAMN
Q) SBBS/QMRLC/IV/NBO/A/000/999/1937S04358W005
B) 1812200539 C) 1812201000

SBCF 200500Z 13008KT 9999 SCT040 22/17 Q1016=
SBCF 200400Z 14004KT 9999 FEW040 22/17 Q1016=
SBCF 200300Z 09009KT 9999 SCT035 22/17 Q1016=
SBCF 200200Z 18002KT 9999 VCTS SCT040 FEW045CB 22/16 Q1017 RETS=
SBCF 200100Z 13008KT 8000 -TSRA BKN035 FEW045CB BKN070 20/16 Q1 018=
SBCF 200040Z 15011KT 4000 TSRA BKN040 FEW045CB BKN070 23/16 Q10 18=
SBCF 200025Z 16010KT 9999 TS BKN040 FEW045CB BKN070 25/18 Q1018=
SBCF 200000Z 19002KT 9999 VCTS SCT040 FEW050CB 26/17 Q1017=
Incident Facts

Date of incident
Dec 20, 2018


Flight number

Aircraft Registration

Aircraft Type
Boeing 777-300

ICAO Type Designator

Airport ICAO Code

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