AT72, vicinity Itaituba Brazil, 2019
AT72, vicinity Itaituba Brazil, 2019
On 16 September 2019, an ATR 72-200 diverted to Itaituba when landing at its intended destination Manaus was prevented by its unexpected closure due to an aircraft accident. During this diversion, intermittent indications of low fuel quantity were annunciated and one engine subsequently ran down on final approach and the other whilst backtracking after landing. It was found that due to a series of undetected faults in the aircraft’s fuel quantity sensing system, the flight deck indications of fuel tank contents were over reading and the low fuel indication system was also malfunctioning for the same reason.
Description
On 16 September 2019, an ATR 72-200 (PR-MPY) being operated by MAP Linhas Aéreas on a scheduled domestic passenger flight from Belem to Manaus with en-route stops at Altamira and Itaituba had to return to Itaituba when the final destination airport at Manaus was unexpectedly closed due to an aircraft accident there. After intermittent indications of low fuel contents, the left engine ran down due to fuel exhaustion during final approach in day VMC. A single engine landing was completed but soon after this, the right engine stopped for the same reason but was successfully restarted and this enabled the aircraft to taxi to its assigned parking position.
Investigation
A Serious Incident Investigation was carried out by the CENIPA, the Brazilian Aeronautical Accident Investigation and Prevention Centre. Relevant flight data was successfully downloaded from both the FDR and the CVR and data was also recovered from the QAR.
It was noted that the Captain, who had been acting as PM for the flight under investigation had a total of 13,324 hours flying experience of which 3,200 hours were on type. He had been employed by the airline for almost five years and had previous experience at a similar regional airline. The First Officer had a total of 8,448 hours flying experience which included 4,169 hours on type and had been employed by the airline for seven years in what was his first airline job. The two pilots had operated the same flights the previous day as the ones they were rostered for on the day of the investigated event.
What Happened
Having been accommodated in a hotel in Belem overnight, both pilots reported for duty at Belem airport in mid-morning. They were to operate scheduled passenger flights from Belem to Manaus and back to Belem with intermediate stops in both directions at Altamira and Itaituba. Sufficient fuel was loaded for the Belem to Altamira sector and the flight was completed without event. Once there, having been presented with and signed the flight despatch document prescribing the minimum departure fuel required for the next sector, the Captain arranged for and supervised the uplift of what he believed would be sufficient fuel to reach Manaus without further refuelling at Itaituba. He was aware that although fuel was available at Itaituba if required, there were no ground personnel there to support for any requested refuelling.
Once on the ground at Itaituba, it was found that the fuel used for the Altamira-Itaituba sector had been 20% greater than expected - 778kg instead of the forecast 644kg which left a recorded 2,022 kg in tanks. The flight despatch document for the Itaituba departure, which the Captain was presented with and signed, showed a minimum fuel required of 2,116 kg but no fuel uplift occurred. It was not possible to determine whether the pilots had “not noticed or disregarded the difference between the prescribed minimum departure fuel and the indicated fuel on board”.
The flight departed for Manaus but whilst en-route, ATC advised that Manaus had been closed because of an aircraft accident there with no estimate of when it might reopen. The Captain initially instructed the First Officer to reduce speed and remain at FL 180, but a few minutes later, when the presence of icing conditions was suspected, a descent to FL 160 was made. When the lack of information on any re-opening of Manaus continued, the decision was made to divert. Fuel Remaining shown on the FQIS indicated that sufficient fuel was available to divert to the designated alternate, Tefé, but the Captain considered that returning to Itaituba might be a better option since it was not only slightly nearer (253 nm instead of 281nm) but the weather conditions were known to the crew whereas proceeding to Tefé would mean continuing westwards beyond Manaus and after contacting company operations, this was approved and after 25 minutes holding, the diversion was commenced by climbing to FL190.
The pilots’ recollection was that the CAP FUEL caution, which is activated when the fuel remaining in a main (wing) fuel tank according to the FQIS is less than 160 kg and a low fuel light is illuminated, was present for “some time” during the flight back to Itaituba before going out. However, recorded data showed that this had actually occurred whilst in a turn during the Manaus hold at FL 160. The same data source also showed that three other brief CAP FUEL activations had then occurred whilst the flight was still level at FL190 during the return to Itaituba, the first two within six minutes of each other midway between Manaus and Itaituba at FL190 and the third ten minutes later with 85nm to go.
Twenty one minutes later when only 3.3 nm from Itaituba, a fifth CAP FUEL activation occurred as the left engine power dropped and 15 seconds later, this engine failed at a recorded 1,446 feet agl. The First Officer continued visual positioning via left base to final for runway 05 and a single engine touchdown was completed without further event. As the speed on the landing roll decreased through 70 knots, the Captain took control to prepare for the 180° turn and backtrack necessary to clear the runway but whilst this was occurring, the right engine also ran down. A restart attempt of this engine was successful and enabled the Captain to complete the taxi in to the allocated parking position where the engine was shut down and the passengers disembarked. The pilots stated that only when the successful restart of the right engine had occurred did the right side ‘LO LVL’ light on the FQIS illuminate (see the illustration below).
The flight deck fuel quantity gauge and LO LVL lights after the aircraft arrived at Itaituba. [Reproduced from the Official Report]
The (external) MFLIs were used to check the actual useable fuel in tanks and this was found to be zero in both cases despite the FQIS indication that 410 kg remained.
Why It Happened
Data from the QAR showed that the flight from and back to Itaituba had used approximately 1,463 kg of fuel and that the actual fuel on departure had been 1,500kg rather than the 2,002 kg indicated on the FQIS and recorded in the aircraft Technical Log as the fuel quantity on board.
On the second day following the occurrence under investigation, 2,357 litres of fuel, equivalent to 1,936 kg at the appropriate density was uploaded to the aircraft after which the FQIS indicated that there was 1,190 kg in the left wing tank and 1,200 kg in the right wing tank, i.e. a total of 2,390 kg. However, when the Mechanical Fuel Level Indicators (MFLI) were checked, the actual fuel on board was 1,350 litres in the left tank and 1,550 litres in the right tank making a total of 2,233 kg, and highlighting a difference of 157 kg between the FQIS indication and that from the MLFIs.
After regulatory authorisation, the aircraft was then ferried to Manaus where, during tests of higher accuracy involving transfers of fuel between the wings to verify the behaviour of the FQIS, a series of non-conformities were identified:
- with a left tank contents indication on the FQIS of 310 kg, the MFLIs for that wing indicated zero fuel
- even after removing the 75 litres of unusable fuel from the left tank using the drain located near the wing root, the FQIS indication was still showing 230 kg of useable fuel in the tank
- the FQIS LO FUEL light for the right wing tank illuminated when the FQIS quantity indication was 170 kg
- even when there was no longer any drainable fuel in the left tank, the FQIS left tank LO FUEL light was not illuminated
It was concluded that these findings corroborated the situation found after the flight under investigation arrived at Itaituba with no useable fuel at all but 410 kg indicated as available on the FQIS.
A detailed investigation of the FQIS found the following:
- All six FQIS fuel tank sensors (3 per tank) were subjected to functional testing and three were found out of acceptable tolerance and required replacement.
- The cabling for the number 3 fuel sensor in the right wing tank was damaged and required replacement.
- A fault in the number 1 sensor in the left wing tank was identified as the reason for the lack of a functioning low fuel light for that tank and was replaced.
After replacement of these FQIS components, for which a cause of failure could not be determined, the system was restored to full serviceability and the aircraft was returned to service. Despite a review or the aircraft’s fuel use relative to past flights, it was not possible to identify when the FQIS had begun to malfunction.
The Investigation noted that the maintenance of the operator’s ATR42/72 fleet was contracted to another company in the same ownership as the airline and that the airline did not have authorisation to carry out any maintenance activities itself. It was considered that this situation “may have led to failures in the supervision, or even, in the design of important organisational processes for maintaining operational safety”. It was also concluded that the airline appeared to be “performing technical monitoring of the services provided” by its maintenance contractor to an extent which exceeded the corresponding authority provided by its Operating Specifications. Clear evidence was also found of the “sharing of facilities and professionals involved in the supervision and execution of maintenance tasks, scheduled or otherwise (which made) it difficult to define clearly the levels of responsibility of each company for maintaining the airworthiness condition of the airline’s entire ATR 42 and ATR 72 fleet”.
Three Contributory Factors were identified, in summary, as follows:
- Aircraft Maintenance - After the occurrence of the serious incident and completion of the subsequent repairs to the FQIS, it was restored to its correct function, showing that the replaced FQIS components had faults that had interfered directly with the flight deck fuel contents indication. There had therefore been non-conformity in the maintenance services previously performed on the aircraft, independently of having been preventive or corrective, since the aircraft had continued in service with a system failure that contributed to the serious incident.
- Managerial Oversight - In view of the various non-conformities observed in the components of the FQIS after the occurrence, it could be inferred that there were failures in the monitoring and oversight of the maintenance services on which the airline relied.
- Crew Resource Management - There was inefficiency in the utilisation of the human resources available for the operation of the aircraft, due to an inappropriate management of the tasks assigned to each pilot, as evidenced by their departure from Itaituba with an indicated fuel on board which was less than the minimum fuel required.
Three more potential Contributory Factors were considered but the Investigation was not able to show that they had been relevant.
Two Safety Recommendations were made based on the Findings of the Investigation as follows:
- that the Brazil National Civil Aviation Agency (ANAC) review its own internal processes so as to verify the establishment of conditions and circumstances under which a regulated entity, pursuant to the RBAC-121 requirements, has to address and solve specific deficiencies in an internal fashion, attesting, before the Agency, the effectiveness of the corrective measures adopted for the re-establishment of the minimum acceptable conditions, as prescribed in the document “Guidelines for Regulatory Quality”. [IG-133/CENIPA/2019 - 01]
- that the Brazil National Civil Aviation Agency (ANAC) reassess its own internal safety oversight processes, in order to make sure that they are capable of identifying, as applicable, the degradation of a regulated company’s technical and financial conditions required to guarantee the safety of the activities performed by such company, as established in the Article 27 of the “Specific Safety Program (PSOE-ANAC)”. [IG-133/CENIPA/2019 - 02]
The Final Report was completed on 29 December 2023 and published in January 2024. In commenting on the final report prior to issue, it was noted that ATR had advised that they were intending to publish a Service Bulletin in respect of the secondary detection of low fuel level sometime in 2024.