Serbia AT72 near Sofia on Nov 26th 2017, noise and vibrations felt from airframe

Last Update: April 9, 2019 / 18:38:19 GMT/Zulu time

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Photo of Air Serbia YU-ALO, ATR ATR-72-200 — Air Serbia YU-ALO, ATR ATR-72-200 (Photo credit: cvtperson / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Nov 26, 2017

Classification
Incident

Airline
Air Serbia

Flight number
JU-123

Departure
Sofia, Bulgaria

Destination
Belgrade, Serbia

Aircraft Registration
YU-ALO

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

An Air Serbia Avions de Transport Regional ATR-72-212A, registration YU-ALO performing flight JU-123 from Sofia (Bulgaria) to Belgrade (Serbia) with 36 passengers and 4 crew, was climbing out of Sofia when the crew stopped the climb at FL100 due to noise from the back of the aircraft and subsequent vibrations of the airframe. The crew subsequently decided to return to Sofia and landed safely on runway 27 about 20 minutes after departure.

Bulgaria's AAIU reported the aircraft dropped a part of the aircraft structure in flight. The occurrence was rated a serious incident and is being investigated.

On Apr 9th 2019 Bulgaria's AAIU released their final report concluding:

On the grounds of the analysis conducted, the commission hereby states that this serious incident occurred as a result of:

The incorrect screwing up of the bolts used for the 291 BL panel attachment during planned technical servicing performed by the staff maintaining the aircraft.

The AAIU reported:

During the initial climb, at a height between 8000 and 10000 ft, the crew heard a loud clattering noise and felt vibrations as a result. The senior flight attendant informed the flight crew that she had also heard this unusual noise and had also felt shaking in the upper section of the cabin of the aircraft. At the time of the incident, the commander was the pilot flying and the aircraft was in a clean configuration. At 16:06 h, the crew made a request to ATC to stop climbing and to be vectored back to Sofia Airport. The crew assumed that there was a detached panel or problems with the nose landing gear, without having any indications of that. At about 16:12 h, the crew requested a longer approach since they had doubts regarding the condition of the nose landing gear. The airport emergency services were alerted. The aircraft was guided by vectors to intercept the final approach 15 nm from the runway threshold. At 16:17 h, the crew reported safe landing gear down indication, including the nose landing gear and the aircraft touched down normally on Runway 27 at 16:21. There were no injuries to the passengers and the crew. The aircraft was parked on a parking stand. After an inspection of the plane, conducted at the parking stand, it was found that a panel from the central part of the wing to fuselage fairing was missing.

A search for the panel remained without success.

The AAIU analysed (quoted in its entirety):

Considering the information in chapter 1, we can draw the conclusion that the emergency landing of the aircraft at Sofia Airport was caused by the detachment of a panel from the central part of the wing to fuselage fairing of the aircraft.

There is a strong likelihood that the vibrations on the pedals of the aircraft were not caused by a collision of the detached panel with the tail of the aircraft. The vibrations were caused by the generation of turbulent airflow passing through the missing panel located in the central section in front of the tail of the aircraft. This is confirmed by the fact that there are no damages on the fuselage, the vertical and the horizontal stabilizers and the respective flight control surfaces of the aircraft as a result of the detachment of panel 291 BL from the central section of the aircraft. After landing at Sofia Airport, an inspection of the aircraft was made by a team of Swissport and MCC Aero Technic BG, stipulated in paragraph 2.17.

Considering the fact that there are no structural damages at the points of attachment of the missing panel, we can conclude that it was had been incorrectly mounted. Owing to the fact that there is no evidence of any broken bolts or twisted points of their fixing, we can draw the conclusion that not all the bolts had been tightened and some of them were missing.