Description

On 3 January 2019, a Boeing 737-500 (5N-AIS) being operated by Azman Air on a scheduled domestic passenger flight from Lagos to Port Harcourt as AZM2316 experienced transient uncommanded right engine thrust variation accompanied by unexplained noises and indicated vibration increases en-route but these were not considered a reason to shut the engine down until it clearly and suddenly failed during an ILS approach at destination in day IMC. The now single engine approach was discontinued as was a subsequent one but a third approach was completed successfully half an hour after the second one. 

Investigation

The Nigerian Accident Investigation Bureau (AIB) carried out an Accident Investigation. The CVR and FDR were both removed and downloaded but whilst relevant data was obtained from the FDR, such data from the CVR was found to have been overwritten.

It was noted that the 43 year old Captain, was  PF for the accident flight had a total of 3,724 hours flying experience of which 92 hours were on type all acquired since joining the operator less than a month prior to the investigated event and including type conversion training time. His previous job had been in corporate aviation flying an HS125. The 24 year old First Officer had a total of 629 hours flying experience of which all but 198 hours were on type. 

What Happened

About six minutes after takeoff, the turbine vibration level indicated for the right engine began to gradually increase over a period of 40 seconds reaching a recorded/indicated maximum of 5.26 units. After a further 40 seconds, this level began gradually decreasing over the next 90 seconds before stabilising at approximately 3.0 units. The climb was continued and a couple of minutes later, the flight was transferred to Port Harcourt APP and advised to expect an ILS/DME approach to runway 21. Four minutes later, the aircraft was levelled at FL290 with both engines appearing to be operating normally at a thrust setting of 85% N1 with the AP and A/T engaged. 

However, after two minutes FDR data showed that the right engine N1 had, within five seconds, decreased to 68% before returning to 85%. This occurred at the same time as the crew reported having heard “a sudden loud bang from the right side of the aircraft which caused vibrations” which was accompanied by a brief yaw to the right. Having confirmed with the Purser that the noise had also been heard in the cabins, the flight crew “instructed the Purser to walk through the cabin and check if there was anything unusual” and subsequently received the response that “everything seemed normal”. The flight crew “then assumed the noise might be as a result of cargo shift”. Descent was then commenced to FL210 as previously released by Lagos ACC and before reaching, Port Harcourt APP re-cleared the flight to continue to FL110. 

Just over 20 minutes after beginning descent, a right turn to establish on the runway 21 ILS was commenced, the flaps were set to 15 and the landing gear was selected down. The indications for both engines were normal with the N1 at approximately 57%. Soon after establishing on the ILS LOC, with about 4.5nm to go, the crew reported hearing “a very loud deafening bang” as the right engine N1 decreased to 47% within 5 seconds and its indicated/recorded turbine vibration level increased to “severe”. As before, this was accompanied by a sudden yaw to the right but this time, the AP also disengaged although the A/T remained engaged despite significantly asymmetric thrust. 

The Captain took over manually and stabilised the aircraft before briefly handing over control to the First Officer in order to assess the situation. Having confirmed asymmetric thrust, he then took over again as PF and decided to go around. When TO/GA thrust was selected on both engines, the indicated/recorded turbine vibration on the right engine increased to severe - 9.90 units - followed by the right engine fan vibration increasing to 4 units. 

During the repositioning for another approach with ATC vectoring provided, the crew reported “reading the ENGINE FIRE or ENGINE SEVERE DAMAGE or SEPARATION Checklist in the B737 QRH” but did not action it, They then reported noticing that the right engine Oil Filter Bypass Light had illuminated and five minutes after attempting the two engine go-around, “a precautionary shutdown” of the right engine was carried out “in accordance with the ENGINE OIL FILTER BYPASS Checklist in the B737 QRH”. 

After a further 20 minutes of manoeuvring, ATC radar vectored the flight back onto the ILS LOC but “the crew stated that they got preoccupied by the limitation of 10° bank and map shift and did not align with the runway centreline”. An emergency was then declared to ATC and the airport RFFS was put on standby. After two further minutes, the crew had still not succeeded in establishing on the runway centreline and subsequently stated that they were now also seeing high turbine vibrations being indicated on the number one engine. At 150 feet agl, the aircraft was still not aligned with the runway centreline and was above the glideslope and showing a half scale fly down. The TO/GA switch was again pressed and a second go-around was initiated. 

During this second Go-Around, the crew reported having the field in sight and “decided to turn around and make a visual approach to runway 03”. However, “after turning left, they realised that they were far off to the right of the 03 extended centreline” and so requested instead to extend downwind and position for another ILS to runway 21. Positioning to this ILS approach followed, alignment with the LOC was achieved and the approach then continued to a successful landing 40 minutes after the first approach and almost 1½ hours after leaving Lagos. The aircraft was taxied to its assigned parking position and all 109 passengers disembarked unassisted and uninjured. The ground track of the flight showing the two go arounds and the third approach to a successful landing is shown below.

The ground track of the flight whilst in the vicinity of Port Harcourt. [Reproduced from the Official Report]

Why It Happened

Having compiled the account of the flight summarised above, the Investigation looked at the Airworthiness origin of the problem, the pilot’s response to it in relation to relevant procedures, the aircraft operators attitude to airworthiness in the presence of what was found to be a long history of malfunction affecting the right engine and the performance of the State Safety Regulator’s responsibility for overseeing the operational safety standards of AOC holders.

• Airworthiness

A boroscope examination of the right engine found that all its HPT nozzle guide vanes and blades were unserviceable, that there was evidence of oil spillage on the inner side of the turbine exhaust sleeve and the flame arrestor was missing. It was provisionally concluded that Number 4 and Number 5 bearings had failed. Metallic particles were found on the engine magnetic chip detector in the aft sump. A subsequent initial strip down of the engine at a UK MRO found that the extent of the damage meant that the engine was beyond economic repair. It was therefore decided that the high cost of a full strip down could not be justified in terms of value to the Investigation. However, it was clear that the condition of the engine had been progressively deteriorating over several months - see the first part of the section below on Operating Standards and Safety.

• The Crew Response

It was found that the QRH required an engine to be shut down if the indicated vibration level exceeded 4.0 units whether or not any vibration was felt unless operating with reduced thrust resulted in the vibration level being kept below 4.0 units. No thrust lever movement on the affected engine was to be made until the A/T, if engaged which it was, had been disconnected. This drill also required setting the transponder to ‘TA’ so as to “prevent climb commands which can exceed reduced thrust performance”.

The QRH also required that an engine shut down must be carried out if “airframe vibrations with abnormal indications” occurred. Again this procedure commenced with the memory actions of disengaging the A/T followed by closing the affected engine thrust lever and discharging the first fire extinguisher shot into the affected engine. 

• Operating Standards and Safety at Azman Air Services

It was found that whilst the OFDM process was thoroughly documented as part of the SMS, there was no evidence that any use was being made of the data available from it. An examination of eight months data from the programme was found to show, by way of example only, that:

1. in a continuous 20 day period in two months prior to the event under investigation, the ‘do not exceed’ engine vibration limit for the right engine had been exceeded during all 66 flights operated.
2. in the five day period 14 - 18 December 2018, less than 2 weeks before the event under investigation, abnormal right engine turbine vibration occurred a total of 40 times spread over all the 23 flights in that period.
3. in the two month period immediately prior to the event under investigation, the fan vibration limit guide for the left engine exceeded the threshold for abnormal operation on 65 of the flights operated.
4. other recorded exceedances included “multiple ILS deviation warnings, Localiser deviation warnings, EGPWS warnings, EGPWS Sink Rate Cautions, High rate of descent below 2000 feet, Low power on approach, Speed brake not armed below 800 feet and Approach speed high below 500 feet”.

It was concluded that instances of engine vibration and other deviations found in OFDM data had become “a new normal” with neither flight crew nor airline management paying attention any more.

It was found that the OM ‘A’ stated that Captains newly employed by Azman Air are expected to have a minimum of 500 hours Pilot-In-Command time on type - the Captain involved had just 92 hours in command on type. It was also found in respect of crew pairing that whilst the OM ‘A’ included crew composition requirements and inexperienced flight crew composition limitations, but it did not specify any criteria for determining experience levels.

• Safety Regulator Oversight of Azman Air Services

The Investigation found “no evidence of any safety audit of the operator’s SMS to ascertain the level of its implementation and the operator’s compliance with the safety management requirements of the Nigeria Civil Aviation Regulations (CARs)”. It was also noted that a revision to the 2009 CARs in 2015 had omitted a previous provision that mandated an AOC holder operating aircraft with an MTOM exceeding 27 tonnes to include an OFDM programme as part of its SMS, a change which represented a reportable difference from the requirements contained in ICAO Annexes.    

The Cause of the loss of power on final approach was determined as “the failure of the number 4 and 5 bearings of engine number 2 leading to loss of power during approach”.

Four Contributory Factors were also identified:

1. The failure of the crew to recognise the abnormal engine conditions (surge) during cruise phase and hence not making an appropriate decision. This might have been connected to the loss of situational awareness. 
2. Non-implementation of the Flight Data Monitoring programme in accordance with section 2.2.5.1 of Azman Air Safety Management System Manual. 
3. Non-rectification of the number two engine vibration anomalies recorded over a period of 8 months. 
4. Inadequate regulatory oversight of the Azman Air Safety Management System.

One Interim Safety Recommendation was made on 4 February 2019 as follows:

• that the Nigerian Civil Aviation Authority should ensure that Azman Air Services immediately takes further necessary steps to ensure that it reviews 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. [No Reference Number]

Seven further Safety Recommendations were made on completion of the Investigation as follows:

• that Azman Air Services should implement fully, the Flight Data Monitoring programme as stipulated in 2.2.5 of the Azman Air Safety Management System Manual, including holding regular OFDM meetings, timely corrective actions on the anomalies identified in OFDM reports, distribution to all concerned personnel for timely corrective actions, entering the anomalies into the safety risk management process and presentation during the senior management review, if relevant. [2021-001]
• that Azman Air Services should review relevant portions of its Operations Manual, including 5.1.3 of Azman Air Ltd Operations Manual Part A to specify the criteria for determining the experience level required for crew composition. [2021-001]
• that Azman Air Services should ensure that all its flight crew possess the requisite qualification as contained in Azman Air Services Operations Manual Part A chapter 6 before they are assigned flight duties/responsibilities. 
• that Azman Air Services should develop strategies to implement its safety policy, including the propagation of safety culture in the organisation. 
• that Azman Air Services should include the one engine out manoeuvre during approach in its simulator curriculum. 
• that the Nigerian Civil Aviation Authority should carry out a safety audit of Azman Air Services to ensure all the elements of their Safety Management System are being effectively implemented. 
• that the Nigerian Civil Aviation Authority should review the Nigeria Civil Aviation Regulation (Nig. CARs) 2015 to ensure they meet the International Standards and Recommended Practices as contained in the Annexes to the Chicago Conventions, including 9.2.2.10 to incorporate the provisions that mandated an AOC holder that operates aircraft with a maximum certificated take-off mass of more than 27,000 kg to include a flight data monitoring programme as part of its safety management system.

The Final Report of the Investigation was released on 25 May 2022.

Related Articles

• Contained Engine Failure
• Airworthiness
• Aircraft Technical Log
• OFDM
• Safety Oversight
• SMS