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On 13 July 2018, a 737-800 EI-ENM (35038/3528), Delivered Jan 2011, was cruising at FL370 when a rapid decompression in the cabin occurred due to miscalculation of one of the Cabin Pressure Controllers (CPC) which resulted in the opening of the Outflow Valve (OFV).

While the crew conducted an emergency descent with manually closed OFV, the cabin pressure increased to the maximum differential pressure. At 9,000 ft AMSL, the aircraft levelled off and the OFV was opened manually which resulted in a second rapid decompression.

Subsequently, the airplane landed at Frankfurt-Hahn Airport. According to the last statements, 33 persons on board suffered minor injuries due to pressure fluctuation.

The full report is published here

Cause

The occurrence was caused by a fully opened OFV commanded by the ECS during cruise flight at FL 370. The malfunction was caused by a Single Event Upset in one of the Cabin Pressure Controllers. In this case, the system redundancy of the cabin pressure control system was not suf-ficient to prevent rapid decompression.

The following extracts are of particular interest

Single Event Upset

It is highly likely that the cause of the depressurisation was the erroneous opening of the OFV caused by a damaged data record from a "Single Event Upset" (SEU) in the Cabin Pressure Controller. A "Single Event Upset" is is a so-called “soft error” in connection with the function of semiconductor components, which in aerospace are mostly caused by ionising radiation at great altitudes. Charged particles emit energy when passing through semi-conductor components, for example. This may cause the charge distribution in the component to change and may result in a so-called Bitflip (switch of the p-n transition). The results of calculations such a component may process at the time may be affected. The SEU does not cause any damage on the component and does affect it only at the time it occurs.

According to the statement of the aircraft manufacturer, the occurrence probability of SEU is 3.5x10-8 per flight hour, if the worst comes to the worst. This means one occur-rence per 28.4 million flight hours. In addition, they came to the conclusion that 2.7% of all rapid decompression occurrences are the result of CPC malfunctions caused by SEU. Accordingly, the manufacturer reckons with nine more similar occurrences during the expected service life of this entire fleet. Due to the high redundancy in ECS this failure probability corresponds with the valid certification requirements.

As far as the BFU is aware, within the last 10 years three similar occurrences happened where it is highly likely that CPC failures caused by SEU on board of a Boeing 737 occurred. Two of the CPCs involved featured an older software version and the calcu-lation parameters as critical were not identical to the one in the current case. Back then, the manufacturer provided software patches which implemented TMR proce-dures for the relevant parameters.
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EI-ENM Depressurisation

Managment of the Depressurisation

The flight crew donned their oxygen masks 7 seconds after the pressure loss. 28 seconds after the depressurisation was noticed they then closed the OFV manually and the cabin pressure began to increase again. 15 seconds after moving the OFV, an emergency descent with still manually closed OFV was conducted. At the time the emergency descent was initiated, cabin pressure altitude had de-creased by about 2,000 ft. The flight crew did not notice it. Verbal briefing in terms of coordinated decision making concerning the subsequent procedures did not occur.

During descent with closed OFV, cabin pressure increased up to the maximum differential pressure of 8.72 psi and both pressure relief valves were activated. The crew did not notice this. Positioning and design of the cabin pressure control system indication made the correct identification of the situation more difficult. The injuries the passengers suffered were most likely caused by this re-pressurisation phase between the two decompressions.

After levelling off after the emergency descent, the flight crew could not correctly iden-tify the status of the cabin pressure control system because they had not sufficiently monitored the ECS function in the manual operating mode and their mental image did not correspond with the real situation. After levelling off after the emergency descent, correct realisation of the situation taking into consideration all of the available parameters would have been possible. However, the crew would have had to reverse their working hypothesis and apply learned knowledge.

After levelling off from the emergency descent at 9,000ft, the flight crew opened the OFV completely and a second rapid decompression occurred 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 re-quired the flight crew to correctly interpret the situation.

The DCPCS Panel

The BFU states that "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."... "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." In other words, the DCPCS panel is confusing.

Depressurisation Training - Confirmation Bias

The pilots were trained in accordance with the requirements of the aircraft manufac-turer and the operator... Due to the limitations of accurately representing a loss of cabin pressure in a simulator and because of the absence of any of the associated physio-logical effects, the instructors were asked to play the scenario as unambiguously as possible by selecting only one of the rapid decompression simulator scenarios, which ensures, that the trainee clearly understands, that a loss of cabin pressure had occurred...

In addition, the documentation used by the pilots for preparation indicated that with such pressure loss it is highly likely that the airplane was structurally damaged and therefore pressure could not be restored. Accordingly, the simulator scenarios were exclusively trained in a way that emergency descent to a safe altitude was absolutely always necessary because permanent pressure loss had to be assumed.

Even though the training documentation indicated that most mistakes were made be-cause the necessary actions were taken too fast, this kind of drill-like and unambiguous training generated a certain expectation in terms of Confirmation Bias in the pilots concerning the trigger and further development of the situation. Only this can explain that certain parameters and indications contradicting the actual working theory such as the indication of the fully open OFV, the active fail and alternate lights or the rapidly decreasing cabin pressure altitude at increasing differential pressure after the OFV was closed during the descent, were not noticed.

Real World Events

The BFU analysis also shows that in only one of the 35 cases, involving cabin pressure loss and subsequent emergency descent, reported by several operators over the last 22 years, cabin pressure was restored and the emergency descent terminated. Even though a data set does not show if and/or how often it was attempted to unsuccessfully restore cabin pressure, it has to be assumed that flight crews with similar training react similarly in identical situations.

In addition, the survey of the 35 cases showed that none of the aircraft had suffered structural damage. The pressure loss had been caused by failure of individual ECS components. It has to be assumed that the structural damage propagated during training of rapid decompression scenarios does not correspond with statistical reality and is able to create false expectations in flight crews (Confirmation Bias), as the current investigation shows.
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