On 14th December 2018, a Boeing 737-300, 5N-BUO with flight number APK7100 was operated as a scheduled passenger flight from Murtala Muhammed International Airport, Lagos to Akanu Ibiam International Airport, Enugu on Instrument Flight Rules (IFR) Flight Plan. The service was initially intended to be operated by another Boeing 737 with nationality and registration marks 5N-BUK in the operator’s fleet, but due to Engine cowl valve light illumination and the unserviceable wing Flap system, the aircraft was changed.
Thereafter, passengers were relocated to another aircraft (5N-BUO) for the flight. While performing the Pre-flight inspection on 5N-BUO, the Captain noticed the pressurization mode controller was in the alternate position and the left engine bleed switch on the air conditioning control panel was placarded ‘INOP’. The Captain said he invited the Engineer into the cockpit who verbally explained that the No.1 engine supplies bleed at 18 psi and that it was working regardless of the placard placed on it.
At 10:27 h, 5N-BUO departed Lagos for Enugu with 136 persons on board and about 9,000 kg fuel. The First Officer was the Pilot Flying while the Captain was the Pilot Monitoring. The take-off/climb phase was normal. The flight crew stated that the pressurization system was monitored until reaching FL 310. At cruising level of 31,000 ft above mean sea level, the cabin pressure altitude was normal for about thirteen minutes. However, at about 20 miles to Top of Descent (TOD), while the first-officer was briefing the Captain for descent; a bleed trip-off on the right Engine was observed as indicated by illumination of the bleed trip-off light, and the crew carried out the Quick Reference Handbook (QRH) procedure.
According to the First-Officer, some minutes after the checklist procedure was carried out, the cabin altitude started climbing at the rate of 3,500 ft per minute. The crew requested descent and were cleared to FL 110 by the Lagos ATC although they did not disclose any emergency or operational difficulty to the ATC. According to the flight crew, during the descent, the cabin altitude warning horn sounded at FL 165 following which the rapid depressurisation checklist was accomplished as the cabin altitude remained uncontrollable. The crew then embarked on emergency descent but at FL 120, the crew and passengers’ oxygen mask had deployed automatically and the cabin altitude remained at 14,000 ft. The crew later had contact with Enugu ATC and were cleared to descend to FL 65. The cabin altitude however became controllable at FL 80.
The first landing attempt on runway 26 with the First-Officer flying was unsuccessful as the aircraft came high on the approach and was not on the Localizer. The Captain took control of the aircraft and asked the first officer to request ATC for an orbit. The Captain executed the orbit and positioned on final Approach. Approach and Landing Checklists were completed and the aircraft touched down at 11:32 h.
The crew and passengers disembarked normally without any injury.
The incident occurred in daylight and Visual Meteorological Conditions (VMC) prevailed.
The aircraft later departed Enugu at 17:43 h and landed at 18:34 h as a ferry flight to Lagos on FL 100.