Ryanair B738 near Frankfurt on Jul 13th 2018, loss of cabin pressure, 33 passengers feeling unwell
Last Update: August 10, 2022 / 18:53:11 GMT/Zulu time
Germany's Police reported that passengers reported there had been a bang sound, not all too loud, then the passenger oxygen masks came down and the crew initiated a descent. The passengers complained about pain in their ears and nausea. 33 passengers were treated by medical staff attending to the aircraft and were taken to hospitals.
A passenger reported following a bang sound, the release of the oxygen masks and the descent passengers were feeling unwell with ear pain and nausea, a number of passengers were even bleeding out of their ears. One woman was taken to an ambulance on a stretcher.
The airport reported several occupants on board of the aircraft complained about health issues and were taken care of by doctors and paramedics. Several ambulances took passengers to hospitals.
The airline reported an inflight depressurization, in line with standard operating procedures the crew released the passenger oxygen masks and initiated a controlled descent. The aircraft landed safely, the passengers disembarked, a small number of passengers received medical attention as precaution. Hotel accomodation was authorized, however, there was shortage of hotel rooms available. The passengers are to be taken to Zadar by a replacement aircraft the following day.
A replacement Boeing 737-800 registration EI-EVD reached Zadar the following day with a delay of about 13 hours.
On Jan 18th 2019 Germany's BFU reported in their July Bulletin that the aircraft suffered a rapid loss of cabin pressure causing minor injuries to 33 passengers. According to the FDR the outflow valve opened from 18 degrees to a fully open posiion at 104 degrees with 8 seconds. The cabin altitude was 7925 feet initially, 2 seconds later the cabin altitude warning activated, the crew donned their oxygen masks and began working the memory items for cabin pressure loss, the master caution activated, the crew observed the cabin altitude climbing at 4000 feet per minute. The crew switched the outflow valve into manual control and commanded it to a closed position of 9.3 degrees when the cabin climbed through 14,830 feet 32 seconds after the outflow valve opened (averate rate of climb thus was 12,950 feet per minute), the crew initiated an emergency descent. During the descent, while still working the related checklists, the crew commanded the outflow valve to completely close. Descending through FL156 the first officer, pilot monitoring, observed the cabin altitude at 25,000 feet, later 24,000 feet. After levelling off at 9000 feet the captain handed controls to the first officer and found the cabin altitude indicating 33,000 feet and stated "it's not working, we have to open the outflow valve completely to depressurize". The BFU mentioned that according to the FDR the cabin altitude at that time was 7000 feet below mean sea level (-7000 feet MSL), the cabin pressure differential had reached the maximum of 8.72 PSI for 4:20 minutes. After the outflow valve was opened the cabin altitude settled at 5000 feet MSL.
Cabin crew reported there was one passenger who had bleeding out of the ear. The crew requested an ambulance to meet the aircraft upon arrival. After landing on runway 03 two more passengers reported not feeling well. However, at the airport 15 passengers received immediate treatment and 28 were taken to hospitals for ambulant treatment, a total of 33 passengers required medical assistance.
On Jan 22nd 2019 Germany's BFU released their interim report.
Checks of the aircraft and outflow valve did not establish any evidence of a mechanical malfunction of the outflow valve. The cabin pressure controller #1 revealed a message "NO AUTO FAIL".
On Aug 10th 2022 the German BFU released their final report concluding the probable causes of the occurrence were:
The occurrence was caused by a fully opened OFV (OutFlow Valve) commanded by the ECS (Environmental Control System) during cruise flight at FL 370.
The malfunction was caused by a Single Event Upset in one of the Cabin Pressure Controllers.
The BFU analysed:
In cruise flight, at FL370, an incorrect calculation by the CPC 2 resulted in an incorrect calculation of the reference value of the OFV position which caused the opening of the OFV and subsequent rapid decompression. The flight crew was forced to use their oxygen masks. They closed the OFV manually and the cabin pressure began to increase again. Subsequently, an emergency descent with still manually closed OFV was conducted. For several minutes, the cabin was pressurised with the maximum allowable differential pressure. After levelling off from the emergency descent, the flight crew opened the OFV completely and a second rapid decompression occurred. The injuries the passengers suffered were most likely caused by the re-pressurisation phase between the two decompressions.
Cabin Pressure Controller
The incorrect calculation of the CPC 2 was most likely caused by a Soft Error, a socalled Single Event Upset (SEU). The CPC manufacturer came to this conclusion by exclusion because the CPC neither showed any functional irregularities nor any hardware damage. According to the manufacturer, the probability of a SEU event with such consequences was at most 3.5x10-8 events per flight hour. It, therefore, corresponded with the certification requirements of CS-25 Large Aeroplanes concerning the failure probability of the entire system.
According to the certification requirements, the failure probability considered the redundancy of individual components of the entire system and included that flight crews as last resort reacted correctly to system failures in each situation. In this case, the redundancy was not sufficient to restore the cabin pressure altitude to normal levels in time.
After the CPC 2 failure, the cabin pressure altitude continued to increase. Due to swift OFV control switches between the remaining CPC and the pressure switch, OFV oscillation lasting for several seconds occurred. This unwanted type of control occurred because cabin pressure altitude did not exceed 15,000 ft and the limitation of the maximum cabin rate of descent, implemented in the CPC logic for reasons of convenience, resulted in repeated triggering of the pressure switch.
Rapid decompression caused by a large irreversible leakage would promptly result in a cabin pressure altitude of more than 15,000 ft, hence such an oscillating type of control would not be expected. This would have subsided after a few cycles due to damped oscillation. Based on the latency until stable system function is re-established, the BFU is of the opinion that such system behaviour does not seem suitable. On the one hand, cabin pressure altitude continued to increase in this phase by another 1,500 ft even though several alarm limits had already been passed. On the other hand, the system malfunction progressed long enough that finally the flight crew as last resort had to intervene by controlling the OFV manually.
According to the NNC “Cabin Altitude Warning or Rapid Depressurization”, they switched the OFV to manual operating mode and therefore had control over the OFV position. However, they were neither able to correctly determine the triggering system constellation nor to correctly implement the counteractions or effectively monitor the system behaviour of the ECS during the subsequent emergency descent.
The ECCAIRS data enquiry the BFU conducted showed that since 1998 a total of 35 cases with pressure loss on board of transport aircraft had occurred. All of them resulted in emergency descents. It was possible in one case only to re-establish cabin pressure. It has to be assumed that in the other 34 cases the ECS system redundancy had not been sufficient to prevent rapid decompression. Structural damage of the pressurized cabin was not the cause in any of the cases.
Digital Selector Panel
Mounting location and design of the Digital Selector Panel of the Digital Cabin Pressure Control Systems in the cockpit made it more difficult for the flight crew to correctly recognise the conditions of the cabin pressure altitude and monitor it.
The location at the right overhead panel made it necessary for both pilots to actively focus their attention upward in order to monitor the relevant instruments. The BFU is of the opinion that in the situation caused by the rapid decompression it is highly likely that instruments which were not in the direct line of sight of the pilots received less attention than the ones directly on the panel ahead of them. Partial condensation on the oxygen masks caused by the rapid decompression, made visual verification of the cabin pressure altitude more challenging.
It has to be assumed that the panel design with several combinations of analogue instruments did not contribute positively to the situational awareness of the pilots. It has to be assessed as critical that the design of the cabin pressure altitude indication allows the needle to migrate into the scale from high altitudes. Aircraft are commonly not operated with cabin pressure altitudes below MSL, but the cabin pressure altitude indication should not allow any misinterpretation.
Besides the described analogue combined instruments there are no acoustic or optical warning devices which could have alerted the crew to the following abnormal configurations of the pressurized cabin:
- Fully open OFV during cruise flight
- Cabin pressure altitude passing MSL during descent
- Maximum positive cabin differential pressure
The correct recognition of these parameters presumed an active attention of the pilots.
Pathomechanism of the Injuries
According to final information, 33 persons on board the aircraft were injured. The ENT medical specialist of the Zentrums für Luft- und Raumfahrtmedizin der Luftwaffe at Fürstenfeldbruck stated that it is highly likely the disorder and injuries of the middle ear were caused by the excessive pressure in the cabin lasting several minutes.
The nosebleed several patients suffered from is subject to a similar mechanism. The expert opinion is, however, that it can be caused by rapid decompression or by excessive pressure, whereas the latter is more probable.
The excessive pressure was caused by the re-pressurisation of the cabin during the descent while the OFV had been closed manually. This was not noticed and not adequately monitored. Therefore, the crew did not initiate suitable countermeasures.
The manual full opening of the OFV after levelling off following the emergency descent resulted in a second rapid decompression until the cabin pressure altitude had adjusted to the actual altitude of the aircraft. It could not be determined how many of the injuries had been caused by this manoeuvre. The second rapid decompression could have been prevented by opening the OFV slowly and gradually. This would have required the flight crew to correctly interpret the situation.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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