United 328's fan blade cracked at an internal radius — the 2016 inspection missed it

On February 20, 2021, United Airlines Flight 328, a Boeing 777-222 with 239 people aboard, suffered a fan blade separation in the right Pratt & Whitney PW4077 engine about five minutes after takeoff from Denver, at roughly 12,500 feet. The engine burned through the flight; the crew returned and landed safely at Denver, and nobody on board or on the ground was injured, although debris damaged a vehicle and a residence.

The NTSB found the blade failed by fatigue from a crack that began at the surface of an internal radius in a hollow cavity. Manufacturing left surface carbon contamination and a geometric discontinuity that together cut the blade's fatigue life; at 15,262 cycles since new it failed at less than a quarter of expected life. A 2016 thermal acoustic imaging inspection had shown low-level indications at the crack origin area, but they were read as sensor noise and never re-inspected or team-reviewed.

Contributing factors were the inadequate inspection of blades and the manufacturer's insufficiently frequent inspection intervals, which let low-level cracks grow undetected. Fire severity was worsened by inlet design and a K-flange failure that let hot gases reach the nacelle and thrust reversers. The FAA's 2022 airworthiness directives mandated tighter inspections and inlet modifications.

No injuries, but debris damaged a vehicle and a home; the FAA cut inspections from 6,500 to 1,000 cycles and added ultrasonic checks every 275.

Fatigue crack started at an internal radius that the required inspection method did not reliably see.

A 2016 inspection image showing indications at the crack site was never re-reviewed.

The 6,500-cycle overhaul interval let low-level cracks propagate undetected.

Manufacturing anomalies — carbon contamination and a geometric discontinuity — cut the blade's fatigue life.

Inlet design and a K-flange failure turned the blade failure into an engine fire.

Treat a low-level inspection indication as a finding, not noise: the 2016 image showed marks at the future crack site but was never re-reviewed. Shorten intervals when parts fail early.

Pratt & Whitney immediately inspected the PW4000 fleet, cut thermal-acoustic inspection from 6,500 to 1,000 cycles, and introduced ultrasonic blade inspections every 275 cycles; the FAA made them mandatory in April 2022. Boeing modified inlets and cowls. By January 2023, 17 cracked blades had been found in the wider program since 2004, validating the tighter inspection regime.