The American agency in charge of the investigation into the spectacular failure of an engine of a Boeing 777 confirmed this Friday that one of the fractured blades appeared to have suffered from
"metal fatigue"
while stressing that the next required inspection of this was still a long way off when the incident occurred.
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A United Airlines plane had taken off from Denver airport on February 20 when the right engine caught fire.
A rain of debris fell on a residential area as the aircraft turned around to land in an emergency, without causing any injuries.
It was subsequently shown that two engine fan blades broke.
In an interim report, the United States National Transportation and Safety Board (NTSB) reiterated, as it had done two days after the fact, that examination of the fractured blade near of its base was "compatible" with
"metal fatigue"
.
This physical phenomenon implies that the repeated use of a material can cause its deformation and then its rupture.
According to the maintenance file, the last inspection of this blade had been carried out 2,979 cycles (the equivalent of a takeoff and a landing) earlier.
It was therefore still a long way from the next inspection required by the US aviation authority, the FAA, which recommends one every 6,500 cycles.
This blade was also the subject of a thermo-acoustic imaging examination in 2014 and 2016. The data from the 2016 examination was re-examined in 2018 after a previous incident on a similar engine, the PW4077 manufactured. by Pratt & Whitney for the Boeing 777s.
No final conclusion yet
The NTSB points out that the investigation is continuing and that it has not yet reached any definitive conclusion on the cause of the engine damage.
It still needs to take a closer look at inspection reports "to examine the presence and disposition of any anomalies" near the breaking point on thermoacoustic imaging examinations.
A metallurgy specialist should also inspect broken blades.
The FAA had ordered, three days after the incident, a thermo-acoustic imaging inspection of the titanium blades of all engines of the same type to detect any cracks that are difficult to see with the naked eye.
Pratt & Whitney had specified that it would carry out the tests in its workshops in Connecticut, in the northeast of the United States.