BOEING 757-222 near Denver, CO — 2011-09-26

Probable cause

**This probable cause was modified on 4/5/2012. Please see the public docket for this accident to view the original probable cause.** The initial damage to the fan blades was caused by the ingestion of a Red-Tailed Hawk that caused one or more fan blades to fracture, striking the fan case and causing it to bulge. The initial fan blade fragment release impacted and damaged other passing fan blades generating various sized blades fragments. Some of these blade fragments were propelled forward of the fan case by passing fan blades and were reingested, creating a cascading effect of collateral impact damage to the other fan blades, the fan case, and the inlet cowl.

NTSB narrative

On September 26, 2011, 1637 mountain daylight time, a Boeing B-757-222, registration number N526UA, operated by United Airlines (UAL) as flight 909, and powered by two Pratt & Whitney PW2037M turbofan engines, experienced a left engine (No. 1) bird strikeF following touchdown on runway 35R at the Denver International Airport (DEN), Denver, Colorado. According to the flightcrew, after initial touchdown and the application of full reverse thrust, two hawks were observed on the centerline of runway 35R and shortly thereafter impacts were felt on the fuselage. The airplane was towed to the gate using a tug where the passengers deplaned normally. The incident flight was a 14 CFR Part 121 domestic passenger flight from Chicago O’Hare Airport (ORD) to DEN. Examination of the airplane revealed only minor gouging of the fuselage while the No. 1 engine inlet cowl exhibited multiple impacts, gouges, and through-holes. Examination of the No. 1 engine revealed that all the fan blades were extensively damaged, three fractured transversely across the airfoil at or near the mid-span shroud, but no penetration or breaches were observed in any of the engine cases. The bird remains recovered within the No. 1 engine were identified as coming from a female Red-Tailed Hawk. An intact bird that struck the side of the No. 1 inlet was also identified as a Red-Tailed Hawk but was not sexed. Comparing the airplane and engine damage to the requirements for bird ingestion and engine debris containment at the time the engine and airplane were both certificated revealed that the engine complied with the bird ingestion and containment requirements set forth in Parts 33.77 and 33.19 and the airplane complied with the containment requirements set forth in Parts 25.903.

Analysis

Primary failure mode

Other

First missed decision gate

Avoiding the bird strike by altering approach or landing path.

NTSB coding

• Birdstrike · Landing-landing roll

Evidence available

• FDR / data
• Photos
• 10 docket documents

Docket documents10

• Flight Data Recorder 10 - Attachment 1 - Tabular Data - Flight - (CSV Format)fdr
• Flight Data Recorder 10 - Specialist's Factual Reportfdr
• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Release of Aircraft Wreckage, NTSB Form 6120.15 - Part 1form
• Release of Aircraft Wreckage, NTSB Form 6120.15 - Part 2form
• Approved Revised Probable Cause Statementinterview
• Current Part 33 Section 76 Bird Ingestion Regulationother
• Powerplants 8 - Attachment 1other
• Powerplants 8 - Attachment 2other
• Powerplants 8 - Factual Report of Group Chairman with 26 Picturesphotos