An Iran Air Fokker 100, registration EP-CFM performing flight IR-455 from Tehran Mehrabad to Tabriz (Iran) with 101 passengers and 9 crew, was enroute at FL300 about 100nm before Tabriz when the aircraft was struck by lightning about 10nm from a thunderstorm cell, which caused the loss of AC Essential Bus which lost further systems like weather radar, transponder, TCAS, captain side instruments and loss of automatic cabin pressure control. The captain handed aircraft control to the first officer, cabin pressure was maintained in manual control. Due to the weather at Tabriz the crew decided to return to Tehran, however, the cabin altitude began to climb at 2000 fpm and could no longer be controlled prompting the crew to perform an emergency descent and divert to Ardabil (Iran). Aradabil at that time was not operational and was made operational due to the emergency. After entering a hold waiting for the airport staff to arrive the aircraft landed on Ardabil's runway 15 without further incident.
Iran's CAO released their final report concluding the probable causes of the serious incident were:
Due to ESS AC BUS CB tripping, aircraft lost some system including automatic cabin pressurization system.
Contributing Factor
High flight crew workload resulting from ESS AC BUS FAULT can be considered as a contributing factor for difficulties arose in controlling cabin altitude in manual mode.
The CAO analysed:
Before approaching to the destination airport, aircraft experienced ESS AC BUS fault on MFDU, followed by loss of some systems including WX RADAR, TCAS, ATC Transponder, left EFIS and automatic cabin pressurization system. A few minutes later, aircraft descended for approaching to OITT in manual cabin pressure control mode, but due to cockpit crew workload (after ESS AC BUS trip), they did not have interaction with manual cabin pressurization system during descent. At FL240 by considering the meteorological condition on OITT and loss of some aircraft systems, the pilot decided to return to the departure airport (Tehran) but during the climb toward the Tehran Mehrabad airport, aircraft experienced excessive cabin altitude, so, he declared MAYDAY and had emergency landing at Ardabil airport (OITL) as the nearest suitable airport.
Subsequently after the serious incident, AAIB investigator removed flight recorders and downloaded the data, but since the flight recorders was powered by AC Essential Bus, there was no any data or voice recorded after the loss of AC Essential Bus; therefore, it can be assumed that the AC Essential Bus had been remained off for the rest of the flight. The last aircraft altitude recorded on FDR indicated FL300 while aircraft was in cruise phase before descending for approach to the OITT.
The pilot opinion was that the AC Essential bus was lost due to lightning strike near CB clouds, but the inspections on aircraft after landing in OITL and also in detailed by maintenance expert in Tehran base maintenance, did not show any clear sign of lightning strike on the fuselage, bonding wires and discharge wicks, and there were no any other reports from cabin crew or passengers that support this scenario. Therefore, it seems that aircraft probably was not affected by lightning strike to cause problem for AC Essential Bus.
The AC Essential Bus CB was removed from aircraft and was tested in the electrical shop. The result was out of limit that indicated by the manufacturers table and curve.
The test results indicate that the voltage drop across all three phases exceeded the limits specified by the manufacturer, which could have been a factor in the tripping of the CB.
According to the cockpit crew reports, they lost the left PFD and ND (along with loss of some other systems), when the ESS AC Bus fault appeared on MFDU, but the design feature for power supply of left EFIS (PFD and ND) indicates that the loss of the AC Essential Bus should not result in loss of left EFIS. The left EFIS is powered by the AC Emergency Bus. This bus is normally powered by the AC Essential Bus. When the AC Essential Bus is lost, the AC Emergency Bus should be powered by the emergency inverter, so the test of the AC Emergency Bus was proposed by Dutch Safety Board for proper functioning of Emergency Inverter and Transfer Relay (Task 24-26-00-710-815-A).
The aforementioned test was conducted and the system's proper operation was confirmed. However, based on the test results, the root cause of left EFIS loss could not be determined.
...
In the investigation team meetings held with the flight crew and the F100 instructor pilot, it was determined that due to the high workload resulting from the loss of ESS AC BUS, and subsequently the loss of systems that powered by ESS AC BUS, the pilots did not have enough time to have proper interact with the manual cabin pressurization control system (through below QRH procedure), and this factor could have possibly been a contributing factor for initiating manual cabin pressure control difficulties.
Iran's CAO released their final report concluding the probable causes of the serious incident were:
Due to ESS AC BUS CB tripping, aircraft lost some system including automatic cabin pressurization system.
Contributing Factor
High flight crew workload resulting from ESS AC BUS FAULT can be considered as a contributing factor for difficulties arose in controlling cabin altitude in manual mode.
The CAO analysed:
Before approaching to the destination airport, aircraft experienced ESS AC BUS fault on MFDU, followed by loss of some systems including WX RADAR, TCAS, ATC Transponder, left EFIS and automatic cabin pressurization system. A few minutes later, aircraft descended for approaching to OITT in manual cabin pressure control mode, but due to cockpit crew workload (after ESS AC BUS trip), they did not have interaction with manual cabin pressurization system during descent. At FL240 by considering the meteorological condition on OITT and loss of some aircraft systems, the pilot decided to return to the departure airport (Tehran) but during the climb toward the Tehran Mehrabad airport, aircraft experienced excessive cabin altitude, so, he declared MAYDAY and had emergency landing at Ardabil airport (OITL) as the nearest suitable airport.
Subsequently after the serious incident, AAIB investigator removed flight recorders and downloaded the data, but since the flight recorders was powered by AC Essential Bus, there was no any data or voice recorded after the loss of AC Essential Bus; therefore, it can be assumed that the AC Essential Bus had been remained off for the rest of the flight. The last aircraft altitude recorded on FDR indicated FL300 while aircraft was in cruise phase before descending for approach to the OITT.
The pilot opinion was that the AC Essential bus was lost due to lightning strike near CB clouds, but the inspections on aircraft after landing in OITL and also in detailed by maintenance expert in Tehran base maintenance, did not show any clear sign of lightning strike on the fuselage, bonding wires and discharge wicks, and there were no any other reports from cabin crew or passengers that support this scenario. Therefore, it seems that aircraft probably was not affected by lightning strike to cause problem for AC Essential Bus.
The AC Essential Bus CB was removed from aircraft and was tested in the electrical shop. The result was out of limit that indicated by the manufacturers table and curve.
The test results indicate that the voltage drop across all three phases exceeded the limits specified by the manufacturer, which could have been a factor in the tripping of the CB.
According to the cockpit crew reports, they lost the left PFD and ND (along with loss of some other systems), when the ESS AC Bus fault appeared on MFDU, but the design feature for power supply of left EFIS (PFD and ND) indicates that the loss of the AC Essential Bus should not result in loss of left EFIS. The left EFIS is powered by the AC Emergency Bus. This bus is normally powered by the AC Essential Bus. When the AC Essential Bus is lost, the AC Emergency Bus should be powered by the emergency inverter, so the test of the AC Emergency Bus was proposed by Dutch Safety Board for proper functioning of Emergency Inverter and Transfer Relay (Task 24-26-00-710-815-A).
The aforementioned test was conducted and the system's proper operation was confirmed. However, based on the test results, the root cause of left EFIS loss could not be determined.
...
In the investigation team meetings held with the flight crew and the F100 instructor pilot, it was determined that due to the high workload resulting from the loss of ESS AC BUS, and subsequently the loss of systems that powered by ESS AC BUS, the pilots did not have enough time to have proper interact with the manual cabin pressurization control system (through below QRH procedure), and this factor could have possibly been a contributing factor for initiating manual cabin pressure control difficulties.
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