On 26 September 2011, a Boeing 757-200 being operated by United Airlines on a scheduled passenger flight from Chicago to Denver experienced a left engine bird strike during deceleration after landing on runway 35R at destination in normal day visibility. The affected engine ran down as the aircraft cleared the runway and was shut down after a report of smoke being emitted from it. The aircraft was stopped and the remaining engine also shut down prior to a tow to the assigned terminal gate for passenger disembarkation. None of the 185 occupants were injured but the affected engine was severely damaged and there was visible evidence that some debris from it had impacted the aircraft fuselage.
An Investigation was carried out by the National Transportation Safety Board (USA) (NTSB). The flight crew reported that after initial touchdown and the application of full reverse thrust, two hawks had been seen on the centreline of runway 35R and soon afterwards impacts had been felt on the fuselage followed by a ‘hot’ odour in the flight deck. Shortly after clearing the runway, an indication of low left hand engine oil pressure had occurred.
An intact bird subsequently identified as a juvenile Red-Tailed Hawk and the remains of a second bird of the same species were recovered from the runway. The remains of the second bird, along with remains collected from the No. 1 engine, were eventually identified as coming from a female Red-Tailed Hawk. It was noted that the approximate live weight range for this species is 0.7kg to 1.5kg with the female larger in size.
A detailed examination of the aircraft disclosed minor gouging of the fuselage and the underside of the left wing, superficial damage to one passenger window, damage to the left-hand main landing gear forward right tyre and considerable damage to the affected engine inlet cowl including two holes through the outer skin of the structure. All the fan blades of the affected engine were found to be extensively damaged with three blades fractured transversely across the airfoil at or near the mid-span shroud. No evidence of uncontained failure was found but there were several bulges in the fan case corresponding to hard impacts. Examination of the right hand engine disclosed that several of the fan blades had minor leading edge impact damage primarily located outboard of the mid-span shroud.
The Investigation considered the damage caused to the left engine by the ingestion against the applicable engine certification criteria. It was noted that the PW2037 engine involved had been certified at a time when the requirement was that ingestion if a 1.8kg bird may not cause the engine to catch fire, lead to an uncontained failure, generate more than a specified imbalance loading attributable to sustained damage or be unable to be shut down. It was noted that these criteria had all been satisfied as would be expected after ingestion of a bird of less than the maximum weight specified.
In respect of the debris containment requirements, it was noted that whilst these had been met, they do not currently require engine inlets and nacelles to contain engine debris. However, aircraft manufacturers remain responsible for the overall safety of the aircraft and do have overall engine debris containment responsibility. It was noted that for transport category aircraft, Part 25.903 subsection (d)(1) requires aircraft manufacturers to incorporate “design precautions” which minimise hazards to an aircraft in the event of an engine rotor failure or a fire originating inside the engine which burns through the engine case as detailed in FAA AC 20-128A which requires design precautions based on both service experience and tests.
No reference was made in the Investigation report to the arrangements for control or notification of bird hazards at Denver in particular or such requirements of this type which apply more generally.
The Probable Cause was determined by the NTSB as:
“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 re-ingested, creating a cascading effect of collateral impact damage to the other fan blades, the fan case, and the inlet cowl”.
The Final Report ENG11IA051 was approved on 5 April 2012. No Safety Recommendations were made.
AC 20-128A - Design Considerations for Minimizing Hazards caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor Failure (external link)