A One Air Boeing 747-400 freighter, registration G-ONEE performing flight HC-211 from London Heathrow,EN (UK) to Hong Kong (China), was climbing out of Heathrow's runway 09R when the crew stopped the climb at FL250, already over the North Sea reporting they had lost two of their four generators and requested radar vectors advising they had no lateral navigation left. The aircraft initially turned back towards England, then turned again and diverted to Amsterdam (Netherlands) advising they didn't need any fuel dump, and landed safely on Amsterdam's runway 06 about 85 minutes after departure.
The aircraft is still on the ground in Amsterdam about 7.5 hours after landing in Amsterdam.
On May 29th 2024 the AAIB reported the aircraft experienced failures on two electrical busses and lost all pitot heating. The aircraft diverted to Amsterdam due to marginal weather conditions at Heathrow. The occurrence has been rated a serious incident.
On Feb 10th 2025 the AAIB reported in an interim statement: "The aircraft departed in weather of rain, thick cloud, and icing conditions. Electrical failures resulted from water having leaked onto the aircrafts avionic equipment the effects of which included all pitot probe heaters becoming inoperative, and the cabin altitude increasing. The crew began descending the aircraft from FL250 and experienced unreliable flight instrument indications. They diverted the aircraft to Amsterdam Schipol Airport, which had clear weather." The investigation is still ongoing.
On Sep 3rd 2026 the AAIB released their final report concluding:
The aircraft suffered numerous electrical system failures that were caused by a loss of AC Bus 4, followed shortly thereafter by a loss of AC Bus 1. The loss of power was the result of water ingress into GCUs 1 and 4 which led to the GCUs commanding contactors to open to isolate AC Bus 1 and 4. Water had entered the main cargo deck prior to departure because the L1 door had been left open during rain, and sealant was missing from the corner of the L1 door mat. The investigation was unable to determine the exact path that water took from the cargo deck floor to the GCUs, but a number of weaknesses in the water ingress protection measures were found that could have caused or contributed to water ingress into the GCUs.
The system failures led to multiple EICAS messages and a highly complex situation, outside the scope of normal operations, creating a high workload for the flight crew. The crew managed these by using an effective TDODAR decision-making process supported by the presence of a third crew member. After the loss of the second AC bus, the crew appropriately declared a PAN and diverted to Schiphol Airport. There was no QRH procedure for the simultaneous loss of AC buses 1 and 4; however, the crew were aware that all pitot heating had been lost and that unreliable airspeed could occur in icing conditions. The rising cabin altitude led them to initiate a descent into icing conditions, and although they were aware of the ias disagree message, they decided to keep the autopilot engaged to reduce workload. This coincided with a temporary handover of flying control between the PF and PM for the crew to check correct routing in the FMC. They had a target pitch attitude of 0° in mind but allowed the autopilot to pitch the aircraft nose-down to -7.6° with an associated 6,240 ft/min descent rate. The crew were about to intervene when the autopilot began correcting the pitch as the aircraft exited icing conditions, and the correct IAS returned. The commander declared a MAYDAY, but the excessive descent rate led to an 18 kt overspeed. The flightcrew were able to manage the subsequent flap issues and landed safely.
The incident demonstrated the challenges posed by an unreliable airspeed situation when it is combined with multiple other failures. The crews training, structured decision-making, and use of all available crew resources were likely significant factors in preventing a more serious outcome.
There have been previous events of water ingress into GCUs and other events of water contamination of the electrical/electronic units in the E1/E2 rack on the 747-400, which can lead to the loss of flight critical systems. This has led to SBs and ADs to introduce measures such as installing a fibreglass reinforcing overcoat on the drip shield above the E1/E2 rack, but there was no requirement for a repetitive inspection of the drip shield. There was also no maintenance requirement to repetitively inspect in detail the condition of the floor panel seals, waterproof tape, power drive units and L1 door floor mat in the area above and near the Main Equipment Centre. Therefore, the AAIB has made four safety recommendations to address these maintenance concerns.
The aircraft is still on the ground in Amsterdam about 7.5 hours after landing in Amsterdam.
On May 29th 2024 the AAIB reported the aircraft experienced failures on two electrical busses and lost all pitot heating. The aircraft diverted to Amsterdam due to marginal weather conditions at Heathrow. The occurrence has been rated a serious incident.
On Feb 10th 2025 the AAIB reported in an interim statement: "The aircraft departed in weather of rain, thick cloud, and icing conditions. Electrical failures resulted from water having leaked onto the aircrafts avionic equipment the effects of which included all pitot probe heaters becoming inoperative, and the cabin altitude increasing. The crew began descending the aircraft from FL250 and experienced unreliable flight instrument indications. They diverted the aircraft to Amsterdam Schipol Airport, which had clear weather." The investigation is still ongoing.
On Sep 3rd 2026 the AAIB released their final report concluding:
The aircraft suffered numerous electrical system failures that were caused by a loss of AC Bus 4, followed shortly thereafter by a loss of AC Bus 1. The loss of power was the result of water ingress into GCUs 1 and 4 which led to the GCUs commanding contactors to open to isolate AC Bus 1 and 4. Water had entered the main cargo deck prior to departure because the L1 door had been left open during rain, and sealant was missing from the corner of the L1 door mat. The investigation was unable to determine the exact path that water took from the cargo deck floor to the GCUs, but a number of weaknesses in the water ingress protection measures were found that could have caused or contributed to water ingress into the GCUs.
The system failures led to multiple EICAS messages and a highly complex situation, outside the scope of normal operations, creating a high workload for the flight crew. The crew managed these by using an effective TDODAR decision-making process supported by the presence of a third crew member. After the loss of the second AC bus, the crew appropriately declared a PAN and diverted to Schiphol Airport. There was no QRH procedure for the simultaneous loss of AC buses 1 and 4; however, the crew were aware that all pitot heating had been lost and that unreliable airspeed could occur in icing conditions. The rising cabin altitude led them to initiate a descent into icing conditions, and although they were aware of the ias disagree message, they decided to keep the autopilot engaged to reduce workload. This coincided with a temporary handover of flying control between the PF and PM for the crew to check correct routing in the FMC. They had a target pitch attitude of 0° in mind but allowed the autopilot to pitch the aircraft nose-down to -7.6° with an associated 6,240 ft/min descent rate. The crew were about to intervene when the autopilot began correcting the pitch as the aircraft exited icing conditions, and the correct IAS returned. The commander declared a MAYDAY, but the excessive descent rate led to an 18 kt overspeed. The flightcrew were able to manage the subsequent flap issues and landed safely.
The incident demonstrated the challenges posed by an unreliable airspeed situation when it is combined with multiple other failures. The crews training, structured decision-making, and use of all available crew resources were likely significant factors in preventing a more serious outcome.
There have been previous events of water ingress into GCUs and other events of water contamination of the electrical/electronic units in the E1/E2 rack on the 747-400, which can lead to the loss of flight critical systems. This has led to SBs and ADs to introduce measures such as installing a fibreglass reinforcing overcoat on the drip shield above the E1/E2 rack, but there was no requirement for a repetitive inspection of the drip shield. There was also no maintenance requirement to repetitively inspect in detail the condition of the floor panel seals, waterproof tape, power drive units and L1 door floor mat in the area above and near the Main Equipment Centre. Therefore, the AAIB has made four safety recommendations to address these maintenance concerns.
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