Azman B735 at Port Harcourt on Jan 3rd 2019, engine problem on approach
Last Update: May 27, 2022 / 16:15:54 GMT/Zulu time
A passenger reported the right hand engine emitted a loud bang and streaks of flames, possibly as result of a bird strike, the aircraft tumbled rolling right and left, then the aircraft climbed out. The crew attempted two more approaches, but went around again and landed on the fourth approach about one hour after the first approach (editorial note: the Mode-S receiver lost contact with the aircraft during the first go around, thus we are unable to verify how many go arounds were done).
The airline reported there was unusual noise from the right hand engine, the crew initiated emergency procedures and brought the issue under control. The aircraft landed safely, all passengers disembarked unharmed. Reports circulating in Nigerian media (who provide the tail number as 5N-NAS which is unknown) about an engine explosion or loss of engine are false.
Nigeria's CAA opened an investigation into the occurrence reporting the aircraft suffered an engine issue but was able to land safely. A team of inspectors was dispatched on site to determine what led to the incident.
On Feb 4th 2019 Nigeria's AIB released their preliminary report reporting that departure and climb out of Lagos were uneventful, the captain ( 43, ATPL, 3,724 hours total, 92 hours on type, in words: ninety two hours) was pilot flying, the first officer (24, CPL, 629 hours total, 431 hours on type) was pilot monitoring. About 20-25 minutes into the flight, while the aircraft was enroute at FL290, the crew heard a bang and noticed a right yaw which lasted for about 5 seconds. When the crew looked up the engine parameters, all parameters were normal. The purser reported there was nothing abnormal noticed from the cabin except for the bang sound. The crew continued the flight normally.
On approach to Port Harcourt's runway 21, established on the localizer and glideslope runway 21 and after extending flaps 15 and lowering the gear, the crew disconnected the autopilot when the aircraft yawed severly to the right, severe vibrations were noticed and thrust asymmetry was determined. The left hand engine indicated about 3 to 3.5 units of vibrations and it's N1 was at 65%, the right hand engine indicated 35% N1 and the oil bypass filter light had illuminated. The crew decided to shut the right hand engine down.
The captain handed controls to the first officer in order to work the related procedures and to figure out what had happened. While executing the severe engine damage checklist, the approach became instable prompting a go around. The crew completed the checklists, were vectored onto another approach to runway 21 but came in high. The crew declared emergency and went around a second time. The aircraft landed safely on their third approach.
Nigeria's AIB released an immediate safety recommendation: "NCAA should ensure Azman Air Services Limited immediately takes further necessary steps to ensure that it review the training of the incident flight crew in order to be able to understand and recognise engine failure/malfunctions and its effect (s) at every phase of flight before they are allowed to resume flight duties."
On May 25th 2022 the AIB released their final report concluding the probable causes of the serious incident were:
The failure of number 4 and 5 bearings of engine number 2 leading to loss of power during approach.
- The failure to recognise the abnormal engine conditions (surge) during cruise phase and hence, not making appropriate decision. This might have been connected to the loss of situational awareness.
- Non implementation of the Flight Data Monitoring programme in accordance with section 220.127.116.11 of Azman Air Safety Management System Manual.
- Non rectification of the number two engine vibration anomalies recorded over a period of 8 months.
- Inadequate regulatory oversight of the Azman Air Safety Management
The AIB analysed:
Flight crew qualification and composition
Azman Air Services Operations Manual Part A chapter 6 qualification requirements states that, captains newly employed by Azman Air are expected to have a minimum of 500 hours Pilot-In-Command time on type.
However, the Pilot had 92 hours on aircraft type. Therefore, the Pilot did not meet the minimum Azman Air qualification requirement to be a Pilot-In-Command. In addition, the flight crew pairing for the flight, exhibited a weakness of the flight crew experience level on the aircraft type (B737); with the Pilot-in-command having 92 hours on type and the Co-pilot having 431 hours on type respectively.
Azman Air Operations Manual Part A section 5.1.3 (Crew Scheduling) states that a flight crew member is prohibited from operating an aircraft if not qualified for duty in accordance with the requirements specified in Chapter 6 of the airline’s Operations Manual. Azman Air crew assignment process includes crew composition requirements and inexperienced flight crew composition limitations amongst others.
However, the operations manual does not specify the criteria for determining the experience level required for crew composition.
Therefore, the investigation could not establish the criteria used in the crew composition as the Azman Air Operations Manual Part A was not specific.
Conduct of the flight
At 10:10 h, AZM2316 departed DNMM for DNPO. About six minutes after airborne, the number two engine indicated turbine vibration of 5.26 units (vibration >3 is considered abnormal and >4 is considered critical) over a period of 40 seconds. It decreased and stabilised at 3 units over a period of 90 seconds which was not noticed by the crew. Five minutes later, AZM2316 levelled off at FL290 with both engines operating normally at 85% N1.
FDR information showed that at about 18 minutes after take-off, the number two engine N1 decreased to 68% and then increased to 85% over a period of 5 seconds uncommanded. This corresponds to the point at which the crew heard a loud bang with airframe vibration and a yaw to the right. There was enough time for good instrument scan to spot any abnormal indications for making appropriate decision or taking necessary actions immediately. However, the scan was not enough to detect the abnormal indications immediately from the take-off phase.
The investigation believed that the crew did not maintain continuous scan on the engine instruments from take-off to the time they heard the bang from the number two engine.
The flight crew did not recognise that there was a surge in the number two engine, even with the loud bang, airframe vibration and subsequent yaw of the aircraft. This might have been connected to loss of situational awareness. Had the flight crew utilised available technical resources on-board, the condition could have been fixed by referring to appropriate (Engine limits or surge or stall) checklist prescribed in the company’s Quick Reference Handbook (QRH) pages 7.2 through 7.5 in which an engine shutdown was recommended if the problem could not be fixed and eventually landing was imminent as soon as practicable. In this case, a return to departure airport was recommended because the aerodrome of departure is the first alternate aerodrome as AZM2316 was still in communication with Lagos Area Control at the time.
As the flight progressed to DNPO, AZM 2316 commenced a right turn for finals RWY 21, both engines were stable and symmetrical at approximately 57% N1. At about 4.5 NM, AZM2316 was configured for landing (gears down, flap 15°, 150 kts). About a minute later number two engine N1 decreased to 47% within a period of 5 seconds causing asymmetric engine power and the turbine vibration to increase. At this point, the crew heard a loud deafening bang accompanied by severe vibration and a yaw to the right. Four seconds later, the number two engine turbine vibration showed severe vibration of 9.90 units, the autopilot disengaged as the TO/GA engaged accompanied by autopilot warning. About 25 seconds later, the number two engine N1 further decreased to 30%, its fan vibration increased to 4 units. The crew then executed a missed approach and were vectored to fly heading 350° by the ATC.
During the Go-Around, as the flight crew executed the “ENGINE FIRE or Engine Severe Damage or Separation” checklist, the number two engine Oil Filter Bypass Light illuminated which led the crew to abandon the ENGINE FIRE or Engine Severe Damage or Separation” checklist and carried out a precautionary shutdown of number two engine in accordance with the ENGINE OIL FILTER BYPASS checklist. A good airmanship dictates to always go for the most appropriate checklist and accomplish it to the end before switching to another checklist. Had the flight crew accomplished the “Engine Limit Surge or Stall” checklist, it would have directed them to shut down the number two engine without necessarily going through the Oil Filter Bypass procedures, thereby saving their time, easing the tensions in the cockpit at that time and also prepare them for landing through the appropriate landing and GoAround checklists.
During the second landing attempt, the crew were preoccupied by the limitation of 10° bank, the aircraft failed to intercept the runway centerline passing through 500 ft RADALT (Radio Altimeter) while experiencing high vibrations in number one engine. Subsequently, the aircraft crossed the localizer extremely late at 150 ft AGL (Above Ground Level) and one dot high. As the approach became unstable the crew executed another missed approach. About 40 minutes after the first approach, AZM2316 landed on RWY 21 on the third attempt.
Number two engine vibration, engine surge and oil filter by-pass
Abnormal engine vibration, sudden or progressive, is a positive indication of engine malfunction. Abnormal vibration can be caused by compressor or turbine blade damage, rotor imbalance, or other problems. Early warning of engine malfunction permits corrective action before extensive damage results.
According to the B737 Airplane Maintenance Manual (AMM) 71-00-47/101, severe vibration of the engine core (compressor or turbine) will eventually lead to the removal of the engine from service to save it from severe damage.
In this case, the Azman Air Flight Data Monitoring (FDM) reports showed trends of severe levels of Compressor/Turbine vibration guide occurring over a period of eight months prior to the date of this occurrence. The operator did not provide any evidence of taking the recommended corrective actions when FDM anomalies were found in accordance with the company’s FDM programme.
As the operator continued operation of the number two engine without carrying out the necessary corrective actions to save the engine, on this incident flight, about six minutes after take-off, the number two engine experienced severe turbine vibration of 5.6 units. About 12 minutes later, while the aircraft already in cruise phase, there was a compressor surge or stall of the number two engine which was occasioned by a momentary drop of N1 from 85% to 68% accompanied by a loud bang, yaw to the right and airframe vibration. The N1 returned to normal five seconds later and the flight continued to Port Harcourt. However, as the aircraft was configured for landing runway 21, the number two engine N1 momentarily dropped again, this time from 57% to 47% causing asymmetric engine power, accompanied by another loud bang, severe vibrations and yaw to the right. At this time, the number two engine’s turbine vibration began rising. About 25 seconds later, the number 2 engine N1 further decrease to 30%, its Fan vibration reached 4.0 units. The number two engine at this point was losing power as the N1 continued decreasing. As the flight progressed into a Go-Around, not later than five minutes, the number two engine oil filter bypass light came on. This was an indication of a clogged scavenge oil filter element and an impending filter bypass. The crew followed relevant QRH and shut the number two engine in flight.
As there was severe turbine vibration, it is probable that, the number 4 and number 5 bearings, which are located in the aft sump of the engine, gave in. Consequently, pieces of particles from the damaged bearings or seals clogged the aft scavenge oil filter leading to the illumination of the oil filter bypass warning light in flight.
Azman’s FDM programme reports had already revealed the trend of the status of 5NAIS engine number 2 but not all the recommended maintenance actions in the early stages were implemented. The implementation of these would have saved the number two engine from the severe damage.
The post occurrence Borescope Inspection carried out on the engine suspected possible damage of the number 4 and number 5 bearings. The post-occurrence inspection of the Magnetic Chip Detector located in the aft sump also indicated trapped metallic particles. This could be an indication of bearing failure in the aft sump of the engine. The initial number 2 engine disassembly report confirmed that the number 4 bearing was damaged and there were signs of rubbing of the HPC and HPT blade tips on the engine casing. Consequently, the engine suffered severe vibration leading to its malfunction during the approach phase of the incident flight.
This article is published under license from Avherald.com. © of text by Avherald.com.
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