Arrow Cargo DC10 at Manaus on Mar 26th 2009, dropped parts of engine on houses
Last Update: October 20, 2015 / 22:31:40 GMT/Zulu time
In the meantime fire engines and emergency services were racing to a neighbourhood of Manaus (rua Humaita, Terra Nova), where parts of the engine had damaged 12 houses and a number of cars. Parts found so far include nacelle and engine parts, the largest part weighing around 250kg/550lbs.
Manaus Authorities reported, that the control tower of Manaus was notified of the debris on the ground and radioed the crew of the DC-10. The operation of the aircraft was not impaired, so that the crew continued to Colombia.
Arrow Cargo confirmed, that the airplane dropped parts of the engine. The airline will compensate for all damages incurred by the accident. The crew continued the flight to Bogota on two engines with the third engine shut down.
The provider for all air traffic control services in Brazil, the Forca Aerea Brasileira (FAB) reported, that the crew was contacted by Manaus control tower after a loud bang was heard by the controller. The crew reported no abnormalities. After the debris was established on the ground, the area control center Cindacta 4 (Manaus) queried the crew again, this time the crew reported problems with one engine but decided to continue the flight. The serious incident will be investigated by CENIPA, Brazil's Center for Research and Prevention of Accidents, collection of evidence has started. So far the diffuser of the exhaust pipe, the rear part of the exhaust pipe and several small internal components of the engine have been recovered on the ground.
On March 30th CENIPA announced, that they have delegated the investigation to the United States' National Transportation Safety Board according to ICAO regulations due to the operator being US based, the airplane being US registered, the airplane being outside Brazil and both aircraft and engine manufacturer being US based, too. CENIPA will participate as an accredited representative of Brazil in the investigation.
On Oct 20th 2015 the NTSB released their final report concluding the probable cause was:
Failure of the low pressure turbine stage 3 disk due to a design that is vulnerable to high pressure turbine unbalance-induced synchronous vibration that cannot be detected in flight, and the subsequent uncontained engine failure.
The NTSB reported that the aircraft received substantial structural damage including extensive tearing damage to the #2 engine pylon, aft engine support brackets and engine cowl doors, there were dents and perforations in the rudder, the upper and lower surfaces of both elevators and the right hand horizontal stabilizer. Liberated parts of the engine damaged 22 houses in the densly populated area near Manaus.
The NTSB reported that the aircraft was climbing through 8000 feet AGL out of Manaus when the crew observed the engine #2 oil pressure steadily decreasing. The captain shut the engine down, the aircraft diverted to Medellin.
The NTSB reported: "The engine was examined at Kelly Engine Center, San Antonio, Texas. An external inspection found no obvious engine damage forward of the turbine midframe (TMF); aft of the TMF there was localized crush damage and the LPT stator case was separated 360 degrees roughly in plane with the LPT S3 disk plane of rotation. The LPT S3 disk rim, web, bore, and aft spacer arm; 39 LPT S3 nozzle segments; all of the LPT stage 4 (S4) nozzle segments; the LPT S4 disk; and the turbine rear frame had liberated. The LPT S3 disk forward spacer arm was attached to the LPT forward shaft and was fractured circumferentially approximately 1.54 inches aft of the flange forward face, which is the approximate location of the fillet between the spacer arm and the disk rim. The engine was disassembled and a rotor check balance found that the total HPT rotor unbalance was 1,213 gram-inches; the CF6-50 Engine Manual HPT rotor unbalance limit is 40 gram-inches. A single HPT S1 blade was severely eroded, with approximately 85% of its airfoil material missing. The combustor positioning pins and bushings were severely worn, and combustor and HPT S1 nozzle components showed localized wear consistent with aftward shifting of the combustor during operation. The exhaust gas temperature (EGT) system was in poor condition, with the outer sheaths of several probes partially or completely missing. Metallurgical examination of the LPT S3 disk fracture surfaces revealed features consistent with a high-amplitude per-revolution stimulus that resulted in high cycle, high-amplitude fatigue (HAF) crack propagation over approximately 90 percent of the fracture. The remaining 10 percent of the fracture showed overstress features. The cracks propagated from initiation sites spaced about 0.1 inch to 0.2 inch apart around the inner circumference of the spacer arm and joined to form a single circumferential crack, leading to disk separation. The disk material met all specifications with the exception of a small deviation in the as-large-as (ALA) grain size requirement, which did not play a role in the fatigue fracture."
The NTSB continued: "According to GE, when the CF6-50 experiences a high level of HPT rotor unbalance, the resulting synchronous vibration forces can interact with the engine’s LPT rotor through a common bearing sump and excite a bladed-disk mode response in the LPT S3 disk. The resonant frequency experienced by the LPT S3 disk in this mode will result in forward spacer arm bending loads that can exceed the fatigue limit of the material and result in HAF crack initiation. The CF6 LPT S3 disk resonance response to HP rotor unbalance was first identified in the GE CF6-6 engine, which shares the CF6-45/-50 type certificate. The CF6-6 experienced four uncontained LPT S3 disk forward spacer arm separations between 1975 and 1978 due to HP rotor unbalance. As a result, GE redesigned the CF6-6 LPT S3 disk so that an HP rotor unbalance condition would not excite the LPT S3 disk and result in disk failure. The CF6-50 engine experienced 10 instances of LPT S3 disk forward spacer arm cracking between 1973 and 2009. Eight of the cracked CF6-50 disk forward spacer arms were discovered during shop-level inspections when LPTs were disassembled for unrelated reasons, such as engine model conversion or the replacement of life-limited parts. In the remaining two cases, disk cracks progressed to failure, leading to in-service uncontained engine failures."
As result of the occurrences the FAA approved a redesign of the LPT3 disk that introduced improved tolerance to High Pressure Turbine unbalanced forces in 2011. In 2012 Airworthiness Directives were issued reducing the life limit for the LPT stage 3 disk and introduced a plan to remove the old design from service.
SBEG 260700Z 00000KT 9999 BKN007 BKN100 23/22 Q1010
SBEG 260600Z 00000KT 9999 BKN007 BKN100 23/22 Q1010
SBEG 260517Z 00000KT 9999 BKN007 BKN100 23/22 Q1010
SBEG 260500Z 00000KT 9999 SCT007 BKN100 23/22 Q1010
SBEG 260400Z 00000KT 9999 SCT007 BKN100 23/22 Q1011
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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