Jeju B738 at Seoul on Dec 24th 2015, loss of cabin pressure and subsequent excess pressure
Last Update: September 24, 2018 / 15:54:16 GMT/Zulu time
South Korea's Ministry of Transport reported passengers suffered from ear pains and anxiety when the cabin altitude climbed above 8000 feet. The cabin pressure controller is being examined.
South Korea's ARAIB opened an investigation.
On Sep 24th 2018 the ARAIB released their final report in Korean (Editorial Note: to serve the purpose of global prevention of repeat a release in English would be necessary and possible as every investigator is able to speak/write English, a Korean only release does not achieve this purpose as set by ICAO annex 13 and just forces many readers to waste much more time and effort each in trying to understand the report) concluding the probable cause of the accident was:
the engine bleed switch was in the position off although the flight crew had checked its position three times while working the various checklists. The checks based on the checklist were performed improperly due to the lack of understanding of pressurization system.
Contributing factor was lack of training of the flight crew in theoretical and practical understanding of the pressurization system.
The ARAIB reported that the crew performed the pre flight checklist which included the check of the bleed air switch, the crew failed to notice the switch was in the off position though. The crew performed the pre-takeoff checklist which again checked the position of the bleed air switch and again the crew missed that the switch was in the off position. When the aircraft climbed through 10,000 feet the 10,000 feet checklist was executed again checking the bleed air switch but again the crew missed the switch was off. When the aircraft climbed through 13,317 feet the cabin pressure warning activated, the crew descended the aircraft to 10,000 feet. During the descent the captain discovered that both engine bleed switches were off and the switches were turned on. While working the cabin pressure checklist the crew followed the instructions and turned the cabin pressure control to manual and closed the outflow valve. The cabin began to pressurize. The captain informed ATC that they did have problems with the cabin pressure but now had solved the issue, the aircraft continued the climb. While the aircraft was climbing through 18000 feet the captain became concerned with the passenger health as the cabin altitude was decreasing rapidly and flight attendants reported passengers were complaining about ear and eye pain. Believing that the health issues were the result of lack of cabin pressure (rather than overpressure as existed) the crew again initiated an emergency descent to 10,000 feet, the cabin altitude continued to decrease and the passengers continued to suffer from ear and eye pain. The crew declared emergency and requested a high speed approach to Jeju. A technician on the ground advised that the outflow valve - in this stage of flight - should be fully open after discovering that the outflow valve was fully closed, the crew fully opened the outflow valve which resulted in a sharp depressurization of the cabin, the engineer again intervened advising the outflow valve should be opened gradually while monitoring the rate of cabin altitude. The flight crew closed the outflow valve again and opened it slowly in order to maintain a reasonable cabin altitude rate. A short time later the aircraft landed in Jeju, after ambient pressure and cabin pressure had equalized at 0 feet about 4 minutes after touch down the cabin doors were opened. (Editorial note: the translation of as well as the time line provided in the factual portion of the report suggests the aircraft was still in flight at 8000 feet MSL when the ground engineer instructed the crew to open the outflow valve, then to gradually open the outflow valve, however, translation of the analysis seems to suggest the outflow valve being fully closed was only discovered on the ground and the opening of the outflow valve happened after landing).
The ARAIB repored the captain later stated in his testimony the cabin altitude was indicated at 35000-40000 feet. Tests showed that if the cabin is being pressurized accordingly the cabin altitude indicator would rotate in reverse direction into high altitude indications above 36000 feet until the pressure relief valve opens as did happen during the accident flight. The ARAIB wrote: "in other words, at HL8049 with the engine bleed systems activated and the outflow valve fully closed the cabin altitude is increasingly reduced (cabin pressure increasingly increased) and the cabin altimeter rolls into reverse indications, it was confirmed this indication was abnormal."
The ARAIB analysed that the pressurization system on HL8049 worked normally with no technical malfunction. The aircraft departed without the engine bleed switches being turned on causing the cabin to gradually lose pressure as the aircraft climbed. After the altitude warning had occurred, the crew worked the related checklist and set the cabin altitude control to manual and fully closed the outflow valve in accordance with the checklist. The captain discovered that both engine bleed switches were off and turned them on. The cabin altitude indicator now moved counter clockwise, went through 0 feet and still rotating counter clockwise (indicating a cabin pressure increase and cabin altitude reduction) reached an indication of 36000 feet. There was no mechanism to stop the needle at 0 feet nor was there any information in the documentation/AOM that the needle could move counterclockwise beyond the 0 feet position. As the flight crew did not constantly monitor the cabin altitude they missed the fact that the needle was moving counter clockwise. The aircraft performed an emergency descent to 10,000 feet, while at 10,000 feet the cabin pressure differential to the ambient pressure reached 8.9 psi and the cabin pressure relief valve opened.
The ARAIB analysed that at that point the ambient pressure at 10,000 feet was 10.92 psi, the cabin pressure thus was 19.82 psi which the passengers were exposed to. The atmospheric pressure on the ground at 0 feet was 14.7 psi. When the aircraft descended for landing the needle continued to rotate further counter clockwise and reached an indication of 25,000 feet. The cabin altitude was thus 18,000 feet below ground level.
There was cabin altitude changes in excess of 1090fpm, which caused the passengers feel severe pain and even nose bleed.
The ARAIB analysed that the flight crew regularly underwent training for the cabin pressurization on a mock simulator, however, no records of the training or assessment of the crew existed. The lack of understanding of the pressurization system as well as not knowing that the cabin altitude indicator would move counter clockwise if the cabin pressure increases, demonstrates the training was inadequate at the airline's crew training center. The information about the rotation of the cabin altitude needle was also not mentioned in the ground school documentation of the airline nor did the mock simulator implement the rotation of the cabin altitude needle below 0 feet properly as the training department assumed the conditions leading to a cabin pressurization problem would be resolved in time.
This article is published under license from Avherald.com. © of text by Avherald.com.
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