Comair B738 at Johannesburg on Dec 16th 2019, cabin did not pressurize

Last Update: August 13, 2020 / 15:54:50 GMT/Zulu time

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Photo of Comair ZS-ZWT, Boeing 737-800 — Comair ZS-ZWT, Boeing 737-800 (Photo credit: Asylumkid / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Dec 16, 2019

Classification
Report

Cause
Cabin did not pressurize

Airline
Comair

Flight number
MN-601

Departure
Johannesburg, South Africa

Destination
Durban, South Africa

Aircraft Registration
ZS-ZWT

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Comair Boeing 737-800, registration ZS-ZWT performing flight MN-601 from Johannesburg to Durban (South Africa) with 183 passengers and 6 crew, was climbing through about FL150 out of Johannesburg when the crew received a cabin altitude warning. The crew descended the aircraft to FL100, worked the related checklist and attempted to climb again, however, needed to stop the climb at FL130 and descend to FL100 again. The crew then declared PAN PAN and returned to Johannesburg for a safe landing on runway 21R about 40 minutes after departure.

The SACAA released their final report concluding the probable cause of the incident was:

During the climb phase, the air cycle machine failed, causing reduced air inflow from engine number 2 air conditioning pack, which resulted in cabin pressurisation system being unable to maintain the required pressure level for a safe environment of the passengers and the crew members.

Contributory Factors

- The lack of availability of serviceable spares contributed to the inability to rectify the previously identified faults in the aircraft air conditioning and pressurisation system prior to the serious incident.

- High fuselage leakage on forward entry door hinges.

The SACAA analysed:

The last maintenance inspection that was carried out on the aircraft prior to the serious incident flight was an A-check inspection on 18 November 2019 at 67 503.034 airframe hours. At the time of the serious incident, the aircraft had flown 142.68 hours. The A-check inspection does not cover inspection of the bleed air system and associated components.

...

Although both the last external and internal zonal inspections on the FWD entry door were carried out on 3 August 2018, this inspection does not require inspection of the condition of the doors’ seal. This is because seals are ‘on condition’ items, which means that they are changed only when required (that is, when damaged or worn).

According to the workpack information, provided records of snags (faults) relating to the air conditioning and pressurisation systems on ZS-ZWT had been logged since 3 December 2019 (13 days) prior to the day of the cabin altitude serious incident flight on 16 December 2019.

According to the AMM, pressurisation requires proper operation of the pneumatic (engine bleed) system, the air conditioning packs and the cabin pressure control system. The pneumatic system supplies air to the air conditioning packs. The air conditioning packs regulate the air flow into the fuselage. The cabin pressure control system regulates air escaping from the fuselage to control cabin pressure. Air leaking from the fuselage must be within specified limits for the packs to operate efficiently. Unusual pressurisation occurrences are typically associated with low air inflow, high fuselage leakage, or a combination of the two.

Although correct fault isolation procedures were followed for troubleshooting the air conditioning and pressurisation systems, the faults found during the troubleshooting were pre-existing faults which were identified prior to the cabin altitude serious incident flight on 16 December 2019. Attempts to resolve all faults on ZS-ZWT prior to and after the serious incident had been unsuccessful despite carrying out correct fault isolation procedures during troubleshooting maintenance due to lack of availability of serviceable spares as indicated by robbing of components from other aircraft, in paragraph 1.16.1.7.

From the troubleshooting described in paragraph 1.16, the primary cause of the cabin pressurisation failure during climb on 16 December 2019 was likely due to a reduced air inflow from the No.2 engine’s air conditioning pack, which was caused by failure of the air cycle machine. This was exacerbated by a high fuselage leakage through the forward entry door hinges, as a result, the aircraft’s pressurisation system could not maintain a safe environment for the passengers and the cabin crew members.

According to Flight and Defect Folio records, from 27 December 2019 until 28 January 2020 the faults associated with the No.1 engine pack and TCV were still occurring and the aircraft had been dispatched IAW MEL 21-33B. This is an indication that troubleshooting maintenance was unsuccessful in resolving ZS-ZWT’s air conditioning system failures.