A first-of-its-kind Boeing 777 uncommanded engine failure triggered by nose-gear tire debris has led the United Arab Emirates (UAE) General Civil Aviation Authority (GCAA) to recommend FAA evaluate whether Boeing needs to assess the risk such an incident poses to a dual-engine shutdown.
"There was significant potential for the No. 2 engine to ingest some pieces of debris, which could have resulted in a more hazardous situation than actually occurred during this incident," GCAA's Accidents Investigation Sector wrote in its recently released report on the Sept. 2016 incident. Boeing "had not performed a risk analysis of the possibility of nose wheel tire debris being ingested into one or both engines, potentially leading to engine failure," it added.
The incident involved an Etihad Airways 777-300ER at Abu Dhabi International Airport (AUH). According to investigators, the 777, carrying registration A6-ETL, experienced an uncommanded shutdown of its No. 1 (left) engine as the aircraft was departing on a scheduled flight to Sydney's Kingsford-Smith Airport.
"At rotation...with an indicated air speed of 196 kt, a loud bang was heard," GCAA's report explained. "This was followed by a 'L ENG FAILURE' message on the engine indicating and crew alerting system (EICAS) display, with an associated high No.1 engine exhaust gas temperature (EGT)." The engine then shut down automatically.
The flight crew continued the departure and, after a brief evaluation, declared an emergency and returned to AUH. After the crew and responding emergency-services providers determined there was no fire risk, passengers were evacuated using air stairs. The aircraft could not taxi because the aircraft landed overweight, causing the breaks to overheat. The heat activated the main-gear thermal fuses and deflated the tires. Total flight time was 33 min., 41 sec.
During the short flight, the pilots determined the aircraft had an unspecified nose landing gear (NLG) problem. They relayed their concerns to air traffic control, but declined an offer to fly past the tower so controllers could perform a visual assessment. AUH airport operations inspected the departure runway and discovered tire debris.
Following the aircraft's return, investigators determined that more than 90% of the aircraft's No. 1 (left) NLG tire tread "had separated and disintegrated," GCAA wrote. Pieces of the tread struck the fuselage aft of the NLG bay and the No. 2 engine's inboard fan cowling. Pieces also were ingested into the No. 1 engine, "which subsequently caused an uncommanded in-flight shutdown (IFSD) shortly after takeoff," GCAA wrote.
Tire-manufacturer Bridgestone told investigators that the incident was the first in which 777 tire debris caused engine damage. "The occurrence rate of tire tread detachment caused by [foreign object debris (FOD)] for the nose wheel tires, in the period from 2012 to 2016, was 165 failures per million flights," GCAA said.
Etihad said the incident was its first involving 777 nose-wheel tire tread separation.
Investigators determined the tire was cut "due to contact with a sharp foreign object," but were unable to determine where or when the damage occurred. A routine inspection of the tires on the day of the incident did not detect any damage. The runway inspection and a check of the aircraft's taxiway route did not find any FOD besides the tire debris. Investigators ruled out a manufacturing or maintenance deficiency.
One possible explanation for the damage: sharp edges found on several guidance lights embedded in the ramp near the gates. Investigators determined the lights were being damaged by towbarless tugs disengaging from aircraft while on or near the lights--an issue that the airport is addressing through training.
GCAA made several recommendations based on its probe. The primary one focuses on determining the risk that a similar incident could take place, potentially affecting both engines.
GCAA said FAA should "evaluate a requirement" for Boeing "to carry out a risk assessment of nose landing gear tire debris being ingested into both aircraft engines following tire failure or tread shedding." FAA said it is evaluating the recommendation. The agency's engine-certification requirements specify foreign-object ingestion tests for birds, ice, hail, and water.
"During initial airplane design, Boeing validates that there is no threat to safe continued flight and landing due to thrown tire tread," the company said. It does not model the specific combination of a NLG tire failure leading to FOD ingestion into multiple engines, nor is it required to do so.
GCAA also urged AUH to install an automated FOD detection system, noting that Dubai International Airport recently installed one. GCAA said it would ensure UAE airport operators are regularly inspecting in-ground lights for damage, implement procedures to ensure tugs are not damaging surface lights, and would analyze whether automated FOD detection systems are necessary. The agency also plans to "supervise" a study on whether airports need magnetic-equipped airfield sweepers.
AUH's inspection program calls for airfield visual inspections every four hours, but magnetic sweepers are not used. Each gate also has a FOD bin.