Investigators believe a RwandAir Bombardier CRJ900 picked up a loose bolt during low-speed ground manoeuvring which was then thrown into its left-hand engine during take-off.
The aircraft suffered extensive damage to all 28 fan blades of the General Electric CF34 powerplant, discovered after the twinjet arrived at Kigali from Entebbe on 4 February last year.
Inspection of the engine found that the blade damage was "consistent with impact forces" from a bolt which was found lodged in the acoustic liner of the fan intake.
Examination of the six-point bolt by the US National Transportation Safety Board indicated that it did not match anything in the engine inlet – which was primarily fitted with 12-point bolts, with no six-point bolts located ahead of the fan.
"It would be very hard to determine where the bolt originated – it could have come from a number of non-aviation sources, fuel trucks, baggage carts, airstairs – but it looks like [it] certainly did not come from this airplane," the NTSB analysis adds.
No problems had been apparent during the pre-flight inspection at Entebbe.
The Rwandan ministry of infrastructure's aviation accident investigation division says that, since the bolt did not originate from the CRJ900 (9XR-WH), the "most likely" scenario was that it was picked up by the left outboard main-gear tyre while the jet taxied out.
During the take-off roll the increasing centripetal force from the rotating tyre dislodged the bolt, and it was thrown upwards and rearwards into the engine which was operating at a high power setting.
The inquiry points out that extensive works, relating to a broad airport expansion project, were in progress at Entebbe at the time.
"Despite several requests, no information from the Entebbe airport authorities on their [foreign object debris] prevention programme and runway inspection regime was received," says the inquiry.
Runway inspection at Kigali on the day had revealed nothing abnormal. After the inspection two departures and a landing took place prior to the CRJ900's arrival. Forty-eight occupants had been on board the jet.