Peace B733 near Enugu on Dec 14th 2018, loss of cabin pressure
Last Update: January 2, 2023 / 18:01:20 GMT/Zulu time
The aircraft ferried back to Lagos at maximum FL100 departing Enugu about 6:15 hours after landing.
Nigeria's AIB released their preliminary report complaining they learned of the occurrence through social media before being notified by the operator.
The AIB reported two passengers received minor injuries as result of the depressurization, the passengers suffered asthmatic attacks.
The flight had originally been assigned to be performed by Boeing 737-300 5N-BUK, however, due to a engine cowl light illuminating and the flaps system being unserviceable the aircraft was switched to 5N-BUO. Upon entering 5N-BUO the captain noticed the switch for the pressurization mode controller was in the alternate position and the left bleed air switch had been placared "INOP". The captain consulted with the engineer who told the captain, the left bleed air system was operating normally at 18psi despite.
The flight was uneventful until 20nm before the top of descent when the right hand bleed air system tripped offline and the cabin altitude began to climb at 3500 fpm. Descending through FL165 the cabin altitude horn sounded, descending through FL120 the passenger oxygen masks dropped automatically and the cabin altitude remained at 14000 feet. The aircraft was cleared to FL065, at FL080 the cabin altitude became controllable again.
The AIB wrote:
Engine No.1 bleed switch on the air-conditioning panel was placarded ‘INOP’. Pressure on the left duct was 18 psi and this was entered in the Deferred Defect Log book on 12th December, 2018 with a limit of 22nd December, 2018. Action on the low bleed on engine no.1 was raised in accordance with MEL (36-05 CAT C).
Engine No. 2 with serial number 724992 was installed on the 11th of December, 2018. The aircraft was operated from 12th to 14th of December, the day of the incident. On the 13th of December 2018, the RIGHTSIDE bleed trip OFF LIGHT illuminated during descent. QRH procedure was followed and flight continued to destination (Lagos). The engine installation task was recorded in the engine logbook and signed on 14th December, 2018.
On Jan 2nd 2023 the NAIB released their final report concluding the probable cause of this serious incident was:
Non-adherence of the cockpit crew to the one air-conditioning pack dispatch limitations and requirements in accordance with the MEL (Minimum Equipment List).
1. Ineffective monitoring of the air conditioning and pressurization system.
2. Inadequate Crew Resource Management.
The NAIB analysed:
During cockpit crew pre-flight checks, it was noticed that left bleed switch was in the OFF position and placarded “INOP” (inoperative) with pressurization mode selector knob in “STBY” (standby) position. The Pilot invited the engineer to the cockpit, who reset the system, selected pressurization mode to “AUTO” and advised crew to monitor during the flight. After engine start No. 1 engine bleed air indication was 18 psi.
The minimum requirement for bleed air duct pressure for air conditioning pack operation is 18 psi (with Anti-ice OFF) (Aircraft Maintenance manual Boeing 737-300/400/500 Section 36-11-05 pages 112, 514 and 515), with one bleed switched OFF, placarded and hence considered inoperative, maximum flight altitude is limited to 25,000 ft AMSL in accordance with MEL.
According to the cockpit crew statement, while cruising at FL310 about 20 miles to top of descent (TOD), the Co-pilot noticed BLEED TRIP OFF light on No. 2 engine, which is an indication of a failure in the bleed air system which could be due to excessive bleed air temperature, excessive bleed air pressure or under-pressure.
At high altitude with increased bleed air demand for air conditioning and pressurization, one bleed source is not recommended as standard practice to hold pressurization beyond 25,000 ft. Hence at FL310, the bleed air supply was inadequate and could not hold a cabin altitude of 5,600 ft according to pressurization schedule as compared with cabin altitude of 3,400 ft at FL250. Therefore strict adherence to limitations of the MEL under this conditions should have been applied.
The cabin altitude warning horn indicates cabin altitude of 10,000 ft, thus showing that cabin altitude on this flight exceeded 8,000 ft limit (Airplane Flight Manual ,Section 08-23-11), which trend was not monitored by the flight crew. The cabin climb rate of 3,500 ft/min observed by the crew on completion of the BLEED TRIP OFF checklist far exceeds normal cabin rate of climb and should have prompted the crew to initiate immediate rapid descent.
The Rapid Depressurization checklist also requires crew to deploy passenger oxygen masks when cabin altitude is uncontrollable. It is unlikely this was accomplished as the masks deployed automatically on descent past 12,000 ft. Since maintenance action had already commenced on the aircraft prior to the arrival of AIB-N investigators, the position of the PASS OXYGEN switch on landing is uncertain.
This article is published under license from Avherald.com. © of text by Avherald.com.
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