On 19 February 2021, an ATR72-500 (EC-LYJ) being operated by Swiftair completed an apparently uneventful scheduled domestic cargo flight from Palma de Mallorca to Madrid but a post flight maintenance inspection found that the inner left main landing gear wheel tyre tread was missing. The Madrid airport authority were advised and an inspection of the landing runway used and taxi route in was carried out but nothing was found. The following morning, only after a flight which departed from the same runway at Palma as the Swiftair aircraft had done reported having seen tyre debris on the runway was the missing tyre tread subsequently found.
After a delay, notification of the event was received by the Spanish Commission for the Investigation of Accidents and Incidents (CIAIAC) and an Investigation was carried out. The notification delay meant that no useful recorded flight data was available from the aircraft CVR or FDR.
It was noted that the flight crew, the only persons on board, consisted of a 43 year-old Captain with a total of 7,920 hours flying experience of which all but 200 hours were on type and a 33 year-old First Officer with a total of 409 hours flying experience of which 176 hours had been on the ATR42/72 since joining Swiftair.
The flight had originated in Ibiza and, after being on the ground at Palma for an hour during which cargo was loaded, departed for its final destination, Madrid, from runway 24R. On arrival, the crew stated that they had been unaware of problems at any time during the 90 minute flight including whilst taxiing in after landing on runway 32R half an hour before midnight.
Whilst the main finding post flight was the missing tyre tread (see the illustration below), the wheel itself, a flexible hose from the main landing gear actuator, the wheel-speed transfer assembly, the left main landing gear folding door and an inspection panel had also been damaged.
The failed tyre minus its tread. [Reproduced from the Official Report]
When no evidence of any tyre debris was found during the search at Madrid, it was agreed with Swiftair that they would advise the authorities at Palma of the situation although this did not then happen which was a matter of concern for the Investigation.
Between the departure of the flight from Palma and the departure of the next morning which reported debris, five flights departed from the same runway but none reported noticing any FOD on it. Two routine inspections of the 3,280 metres long runway 24R were carried out at Palma between the takeoff of the Swiftair ATR72 at 2215 LT and the takeoff of the flight which reported seeing debris the following morning at 0800 LT - between 2300L and 2315L by one technician and around sunrise the following morning between 0727 and 0735 by two technicians. No FOD was observed on either of them. The view was expressed by the Palma Wildlife Control Service that “similarity between the tyre colour and the asphalt runway surface made it difficult for the inspection to spot debris on the runway at night”.
Following the report of debris seen by the 0800 departure, the TWR controller requested an unscheduled inspection of runway 24R which took place between 0815 and 0824 and found debris between N3 and N5. Between 30% and 40% of the detached tread was recovered but no non-wheel debris was found. The failure of both scheduled runway inspections to find any FOD was a further matter of concern. During the Investigation, the Palma airport authorities carried out trials to see whether their personnel were able to find various objects, but despite knowing for sure that objects were there, not all were located. It was concluded that the visual inspection methods are not a reliable way to ensure small objects will be detected, even in daylight. However, although difficult to detect, the sort of small metallic objects most likely to be hazardous account for only a small percentage of all the FOD found on runways and in 80% of cases there are no consequences for aircraft safety.
Once it was clear that tyre failure had occurred during departure, the Investigation sought to establish why this had happened. A detailed analysis of the fragments identified the presence of a cut in the tread “whose origin was compatible with the tyre having rolled over a foreign object, initially puncturing the tread and subsequently causing it to detach completely”. It was considered the failure to recover any such causal foreign object from the runway did not rule out the possibility that this may have caused the tyre to fail since tyres may not necessarily fail immediately after puncture due to the circumstances of the event such as impact energy, depth of perforation and the profile characteristics of the FOD since these are the factors which determine whether any internal damage is sustained and if so how long tyre integrity will be maintained. In some previous cases, it was noted that tyre failure may not occur until after several landings and takeoffs after damage is caused.
Signs of a possible FOD origin for the tyre failure/ [Reproduced from the Official Report]
The history of the wheel and tyre was reviewed. The failed tyre had undergone its first retread in November 2019 and the wheel-rim assembly had accumulated 309 flight cycles and 489 hours since installation. No evidence was found to suggest that the wheel and its tyre had not been fully serviceable prior to the taxi out and takeoff from Palma.
The Probable Cause of the investigated event was formally documented as “the perforation of tyre number 2 on the left main gear by an external object (FOD), which caused the tread to detach”.
Two Safety Recommendations were made as a result of the findings of the Investigation as follows:
- that AENA, as the provider of the Runway and Apron Service at Palma de Mallorca Airport, study the possibility of improving the detection of FOD on the runway. [REC08/21]
- that Swiftair implement the measures necessary to ensure that all incidents are monitored through to closure. [REC09/21]
The Final Report of the Investigation was approved on 30 March 2022 and subsequently published simultaneously in both the definitive Spanish language version and in an English language translation on 13 June 2022.