陈光/文
On October 28, 2016, an American Airlines plane equipped with two CF6-80C2B6 Boeing 767-300ER aircraft, executed by Chicago Flight AA383 to Miami has 161 passengers and 9 crew members on board.
When the passenger plane took off from Chicago O’Hare International Airport, the plane accelerated on the runway (the engine was at maximum take-off thrust) to 2000m, the plane’s taxiing speed had reached the take-off speed, and the pilot heard a bang There was a loud noise and the thrust of the right engine dropped sharply. The pilot felt that something was not good, and immediately terminated the take-off. The plane stopped on the runway after taxiing for 820m.
Figure 1. The fuel flowed out of the aircraft caused a large fire in the aircraft
Due to the fuel mains leading to the engine and the fuel tanks in the wings were thrown out of the high pressure of the engine The broken block of the second-stage turbine wheel was interrupted and pierced, and a large amount of fuel leaked and caused a fire (Figure 1), which burned the right wing and fuselage of the aircraft, as shown in Figure 2.
At this time, the crew immediately turned on the aircraft Emergency slides on the left side of the fuselage (2 on the left side of the fuselage and 1 on the front of the right side of the fuselage) to organize passengers to escape from the aircraft. Since the left engine was not stopped, the engine exhaust tail flow caused a passenger escaping from the slide Seriously injured, 170 people on board escaped the plane safely, of which 21 were slightly injured.
Figure 2. The burnt situation of the right wing and fuselage of the aircraft
The broken block of the turbine wheel that interrupted the fuel main is the right engine that is about to leave the ground. At that time, the high-pressure turbine 2-stage roulette was caused by a non-contained rupture failure. When the aircraft accelerates on the runway to a speed of 140km/h and is about to leave the ground, the engine is in a state of take-off thrust, and the second-stage high-pressure turbine wheel suddenly burst into two large pieces (Figure 3), of which the A block with a mass of 25.7kg is thrown away. After exiting the engine, the main pipe that supplies fuel to the combustion chamber was interrupted, and the fuel tank of the right wing of the aircraft was punched through, and two holes were punched in the right wing. Then it flew over 895m and penetrated the roof of the UPS warehouse, and finally fell into the warehouse. On the floor.
Figure 3. The high-pressure turbine 2-stage roulette broke into four pieces.
The other half of the roulette was thrown out of the engine and hit the runway and fell into three. Blocks (B, C, D) flew nearly 420m and fell onto different grasslands on the side of another runway. The masses of blocks B, C, and D were 37.8kg, 3kg and 0.9kg respectively.
The engine of the CF6-80C2B6 that was involved in the accident is composed of a 1-stage fan, a 4-stage supercharged compressor, a 14-stage high-pressure compressor, a 2-stage high-pressure turbine, and a 5-stage low-pressure turbine. Figure 4 shows its high-pressure turbine rotor. , The engine has worked 68785h and 10 984 cycles.
Observed the section of the roulette wreck, and found that there was an obvious black-gray area at the center of the front edge of the roulette, as shown in Figure 5, where the roulette broke. Electron microscope observation of the fractured area revealed that a 7.81mm×1027mm white area was surrounded by fine crystals under the surface of the roulette, as shown in Figure 6. There are multiple stripes and cracks in this area, which is called "discontinuous white spot area" ".
Figure 4. CF6-80C2 high pressure turbine rotor assembly
Further observation and analysis, the discontinuous white spots are caused by the presence of micron-level oxides in the material of. Because there are microcracks and streaks caused by impurities under the surface of the roulette, that is, inside the roulette, the cracks gradually grow until the roulette bursts after working under low cycle conditions for a long time. The broken roulette burst after 10 984 cycles of operation, and its design life was 15,000 cycles.
After the analysis and research of NTSB and GE, it is confirmed that there are micron-level oxides in the roulette material, which are not removed during the roulette material smelting process.In the material, and in the year of production and after being put into use, it cannot be detected.
CF6-80C2 high-pressure turbine 2-stage roulette is made of In718 nickel-based alloy, and the failed roulette was produced in 1997. Firstly, the American ATI Specialty Materials Company (ATI SM) used the triple smelting method (vacuum induction melting, electroslag remelting and vacuum arc remelting) to smelt In718 ingots according to the requirements of GE's technical documents.
uses triple smelting to eliminate impurities in the material and ensure that the various indicators of the material meet the requirements. The cast ingot has a diameter of 254mm and is handed over to Wyman Gordon for further thermal processing. First, the top and bottom of the ingot are cut off, and then cut into 9 blanks, each of which passes through three directions multiple times After hammer forging and the corresponding heat treatment, a forging blank for machining is formed. On August 8, 1997, Wyman Gordon delivered the forging blank that met the requirements of the specification to the German MTU company.
MTU company processed the final roulette on this forging blank. In each manufacturing process, according to GE's requirements, the raw material and the processed roulette were tested for multiple flaws with an ultrasonic detector, but no micro gap that caused this failure was found. The ultrasonic detection technology used by ATI SM for flaw detection in 2017 was the same as that used by the company in 2017, and the flaw detection equipment used was also made according to the same design.
In February 1998, after MTU completed the final inspection of the failed roulette, it issued a certificate that met the requirements and sent it to GE. In March of the same year, the roulette was installed on the failed CF6-80C2B engine. , The engine serial number is ESN 690-373. On April 30, 1998, Boeing delivered the 767 passenger plane equipped with the malfunctioning engine to American Airlines (AA).
Figure 5. Roulette section
During the 18-year service period of the ESN 690-373 engine, the high-pressure turbine 2-stage roulette has been carried out in accordance with the FAA airworthiness directive. Two mandatory inspections, namely January 15, 2011 and March 25, 2017, the inspection methods were in compliance with the relevant FAA directives, the results showed that the quality of the roulette was no problem, that is, no hidden roulette was found Micro gap.
As the nickel-based alloy is smelted, the micron-level oxides are not cleaned, and the formed defects cannot be detected. They are hidden inside the roulette. After more than 10,000 low-cycle loading and unloading, The fissure gradually expanded and developed, eventually leading to the burst of the high-pressure turbine 2-stage wheel.
verified by GE, there are 36 high-pressure turbine 2-stage roulettes produced in the same period (1997) as the faulty part, of which 8 are still in use. GE has performed high-resolution ultrasound on these 8 The detector checked and found no storage problems. In the
CF6-80C2 engine, the high-pressure turbine level 1 roulette has experienced several non-contained burst failures. For example: on September 22, 2000, when the American Airlines 767-2B7 (ER) was conducting ground maintenance test, the first-stage high-pressure turbine wheel of the No. 1 engine burst. The engine was damaged; on December 28, 2002, Air New Zealand’s 767-219(ER) climbed to 3400m when the high-pressure turbine stage 1, the wheel burst, the broken block penetrated the engine and nacelle, and damaged the leading edge of the left wing of the aircraft ; On June 2, 2006, when American Airlines 767-223 (ER) was conducting ground adjustment test, the high-pressure turbine 1 stage disc burst, the fragments damaged the aircraft in many places and caused fire; there were also 5 similar failures.
Figure 6 Fracture zone under the electron microscope
On January 31, 2008, NTSB had multiple non-contained bursts on the CF6-80C2 high-pressure turbine 1st stage wheel. The possible reasons can be summarized as follows: the high-pressure turbine wheel disc is fatigue fracture, which is caused by an intercrystalline crack. The reason is the imperfect design of the high-pressure turbine wheel disc, resulting in a high stress area at the bottom of the tongue and groove. , Where the stress is close to or equal toThe allowable stress of the material, so as long as there is a small defect, it may cause cracking. After the crack develops, it will cause the roulette to break. For this reason, GE has modified the design of the rear fillet of the roulette tongue and groove, and avoided man-made scars on the fillet during processing, assembly and use. No similar failures have occurred since then.
The flaws hidden for many years in the high-pressure turbine wheel discs caused the 767 passenger aircraft worth 200 million dollars to burn, and the design flaws caused multiple high-pressure turbine level 1 wheel discs to burst uncontained and other severe examples. In the engine development, For high-speed rotating parts, especially turbine disks at high temperatures, we must take it seriously, starting from the source of the material, not only with careful design, rigorous testing, but also meticulous in the process of cold and hot processing, so that the products produced Can withstand the test of long-term operation.
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