On 16 October 2015, one of the left engine fan cowl doors of an Airbus A320 (9V-TRH) powered by IAE V2500 engines and being operated by Tiger Air on a scheduled passenger flight and taking off from Singapore at night in normal visibility was observed to drop onto the runway during takeoff. ATC were advised of the loss of a large engine panel and although engine indications remained normal, of the intention to return to Singapore. However when an indication that the landing gear on the same side as the lost fan cowl doors was not locked down was observed even after an attempt at gravity extension, a MAYDAY was declared. After a low flypast during which engineers reported that the main gear “appeared to be down”, an uneventful landing followed. By the time the cowl door that had fallen from the aircraft during takeoff was recovered from the active runway, it had been reduced to small pieces.
An Investigation was commenced by the Singapore Air Accident Investigation Bureau (AAIB) and taken over and eventually completed and published by the Singapore Transport Safety Investigation Bureau (TSIB) which was formed whilst the Investigation was in progress.
It was found that the reason for the left main gear-not-locked-down indication was that some of the fan cowl debris had lodged in the corresponding gear door and damaged the gear proximity sensor which then produced a false indication of unlocked gear. As such the rest of the Investigation was focussed on loss of both fan cowl doors from the left engine and the potential external consequences of the delay in recovering the resultant runway debris from the outboard cowl door which had detached from the aircraft during takeoff.
The reconstructed outboard cowl debris recovered from the runway (left) and the inboard cowl recovered from the sea virtually intact (right). [Reproduced from the Official Report]
It was found that the loss of the fan cowl doors had caused buckling of the left engine forward pylon as illustrated below.
Left engine forward pylon damage caused by the cowl door detachment. [Reproduced from the Official Report]
It was established that the day before, the aircraft had arrived at Singapore in service and been parked on Bay 702 after which it was scheduled for routine Line Maintenance which was completed in the hours of darkness in a “sufficiently illuminated area” and included a Technician checking IDG oil level of both engines, a task which required opening the right side fan cowls of each engine in order to visually sight the oil level. The LAE responsible for signing off this Technician’s work reported having “performed a walk-round check” which he stated had included checking that “there were no gaps between the surfaces of the fan cowl and the engine nacelle” which, from his experience, would indicate an unfastened fan cowl. He added that although “he would normally also squat down and extend his hand to reach under the fan cowl to feel if the latches were secured”, he had not done so on this occasion. However, he did state that he had ended his inspection at the front of the aircraft near to the nose landing gear and, from a squatting position there had visually inspected both engines “without noticing any protruding unfastened latches”.
After remaining out of service on Bay 702 after completion of this Check, the aircraft was towed to the departure gate to prepare for passenger boarding and arrived there 45 minutes before the scheduled departure time, whereupon the waiting flight crew boarded. Fifteen minutes later, the LAE in charge of the departure check arrived and proceeded to perform an external check prior to releasing the aircraft for departure. This LAE stated that he had not squatted in order to sight the condition of the fan cowl latches but said that he had observed the engines from the vicinity of the nose landing gear and had “not noticed any protrusions at the bottom of the fan cowl which he said would indicate unfastened latches”.
As required by Company Procedures, a flight crew external inspection was also carried out by the First Officer who stated that he had visually inspected both engines whilst near the main landing gear (although not in the position prescribed by the aircraft manufacturer’s external inspection specification) and from the outboard side of each engine. In both positions, he reported having “looked downwards at the fan cowls but he did not bend down or squat to check” because he had been “taught during his training not to bend down or squat to check due to risk of possible injury from sharp edges (e.g. vent pipes on the fan cowl) when standing up”. He did not detect any latch protrusions from either position and stated that he had “also checked that the fan cowl surfaces were flush with that of the engine nacelle” and that there was no gap.
As the aircraft subsequently took off, the Senion Cabin Crew Member (SCCM) “was alerted by a passenger and informed that the left engine fan cowl had fallen”, an observation which was subsequently found to have been recorded by a “runway camera”. After a visual confirmation, the flight crew were immediately informed noting that “the interior of the engine was visible”. They confirmed that all engine indications were normal, although noted that a fault message from the No 2 Landing Gear Control Interface Unit (LGCIU) had appeared during takeoff.
The climb was stopped at 8,000 feet and the aircraft took up a hold so that the Captain could go into the cabin to assess the situation. He observed that the left engine fan cowls were missing but that there was no visible damage to the surrounding wing area and also that the right engine fan cowls were intact. He decided to return to Singapore but when the landing gear was selected, the left main gear did not lock down and the approach was discontinued and the aircraft was returned to the hold. An attempt to obtain a gear fully locked down indication using the manual gravity extension procedure was also unsuccessful and a MAYDAY was therefore declared to ATC whilst remaining in the hold to burn off sufficient fuel to achieve a landing weight not above the permitted MLW. After a subsequent low flypast to allow ground engineers to view the landing gear, the crew were advised that “the left main landing gear appeared to be down” and the subsequent approach culminated in an uneventful normal landing.
Inspection found that both the inner and outer fan cowls were missing from the left engine. Debris from the latter was found on the runway and the former was recovered substantially intact from the sea by a passing ship. Handling of the cowls and debris from them prior to investigators gaining access meant that the position of the latches when they detached could no longer be determined. However, all four latches (consisting of the hooks at the bottom edge of the inboard fan cowl and their corresponding keepers at the bottom edge of the outboard fan cowl) were recovered and apart from normal in-use wear, no damage was seen on any of the recovered latches.
It was noted that to fasten the latch which is situated at the bottom of the top-hinged fan cowls, the hook is placed into the keeper and the latch handle is closed flush with the surface - see the illustration below - and in the event that the latch is not closed, the latch handle will clearly protrude from the surface.
The cowl latching mechanism. [Reproduced from the Official Report]
It was found that in the late afternoon prior to the investigated departure, TigerAir had conducted an airside orientation tour for a group of interns during which they had visited the aircraft and some photographs of it had been taken which showed that at least three of the four fan cowl latches on the left engine were unfastened - one of these photographs is shown below.
The left engine showing the three open latches. [Reproduced from the Official Report]
It was noted that because of similar previous incidents on this aircraft type, Airbus had issued a Service Bulletin to make a hole in the fan cowl hold-open device (HOD) which could receive a pin attached to a red warning flag to ensure that unfastened fan cowls would be more noticeable. This modification (see below) had been incorporated on the incident aircraft.
The red flagged pin showing its insertion in the HOD to indicate an unfastened fan cowl. [Reproduced from the Official Report]
It was found that AMM procedures prescribed the both flight deck warning notices and insertion of the red flagged pin into the HOD whenever the fan cowls were unlatched and that any opening and closing of fan cowls must be recorded in the Aircraft Technical Log. However, there was no mention in the AMM of the need to crouch down to check the latches, although the TigerAir contracted maintenance provider had, in 2009, issued a ‘Quality Notice’ to inform its personnel of the need to bend down or crouch to confirm fan cowl latches were fastened and in May 2015 had issued another ‘Quality Notice’ on the need to record fan cowl opening and closing in the Aircraft Technical Log and the need to positively confirm that fan cowl latches were fastened. It was noted that such notices were seen by the LAEs who signed off routine tasks completed by Technicians but not by the (unlicensed) Technicians. It was found that the LAE supervising the IDG oil level check had not made the required Technical Log entries and that the Technician was unaware of any of the task-relevant content in the AMM.
It was found that the FCOM detailing the flight crew external inspection made mention of the need to crouch down in order to check correct cowl latch closure although in May 2015, Airbus had highlighted the relevant fan cowl procedures in an ‘Operators Information Transmission’ (OIT).
The Investigation noted that whilst the fan cowl closing procedure on modified aircraft such as the one involved, required the depressing of the HOD before the fan cowls could be fully closed and the four securing latches fastened, there was little difference in flushness between the fan and nose cowls whether latches were unfastened or fastened - see the illustrations below.
A comparison of the fan/nose cowl flushness with the fan cowl latches unfastened (left) and fastened (right). [Reproduced from the Official Report]
It was also found that unfastened latches “can barely be seen from a distance to the side of an engine and cannot be seen at all when standing next to engine”. It was also noted that “while it might be possible to notice latches that were unfastened from the front of the aircraft at positions around the nose landing gear (i.e. some distance away from the engine), the silhouette of the engine drain mast might obscure the protruding latches”.
The potential risk to other aircraft from undetected runway FOD was considered and it was noted that at Singapore, regular runway surface inspections by vehicle and response to any ad hoc reports of runway FOD have been replaced by an automated system which uses multiple panning cameras along the length of each runway to capture images of any possible debris and alerts the duty operator to view it and initiate an appropriate response if its validity is confirmed. It was found that this system almost immediately (in less than a minute) detected the fan cowl debris but “the limitation of the image resolution was such that the duty operator interpreted the image as that of a runway ground light and took no action”. The system gave four more alerts of the same debris but the duty operator continued to determine that there was no FOD present. Finally, a sixth alert almost an hour after the takeoff “provided a better image” and this time ATC was advised and an aerodrome maintenance vehicle was dispatched to recover what turned out to be fan cowl debris from the incident aircraft. It was concluded that “the resolution of the cameras at the time of the incident did not support effectively the task of interpreting camera images for the purpose of ascertaining the presence of FOD”.
Safety Action taken during the conduct of and known to the Investigation included the following:
- The European Aviation Safety Agency (EASA) issued an Airworthiness Directive (AD 2016-0053) on 14 March 2016 to modify the fan cowls such that a special key had to be used to unlatch the fan cowls, the key cannot be removed unless the fan cowl front latch is safely closed.
- TigerAir issued a Flight Staff Instruction to emphasise the requirement for flight crews to bend or squat down when inspecting latches from either side of the engine.
- The Singapore Airport Operator has committed to and commenced an upgrade of its FOD detection system to incorporate higher definition cameras so that better quality images will be produced and more accurate interpretation by the duty operator enabled. This action was expected to be completed in September 2017.
Three Safety Recommendations were made as a result of the Investigation as follows:
- that TigerAir reminds its flight crew that fan cowl flushness with the nacelle is not a reliable method for checking that fan cowl latches are fastened. [RA-2017-028]
- that TigerAir’s Maintenance Contractor reminds its maintenance personnel that fan cowl flushness with the nacelle is not a reliable method for checking that fan cowl latches are fastened. [RA-2017-029]
- that Airbus emphasise in its maintenance documentation on the external walk round check the need for inspection personnel to bend down or crouch to bring (their) eye level low enough to confirm (that fan cowl) latches are fastened. [R-2017-030]
The Final Report of the Investigation was issued on 11 August 2017.