NDI Process Failures Preceded Boeing 777 PW4077 Fan Blade Off (FBO) Event (United Airlines N7773UA)
On 13 February 2018 United Airlines Boeing 777-222 N773UA, suffered a fan blade off event and lost most of the inlet duct and both the left and right fan cowls of its No. 2 Pratt & Whitney PW4077 engine.
Two small punctures were found in the right-hand fuselage just below the window line. This occurred over the Pacific Ocean shortly before top of descent when en route to Honolulu (HNL), Hawaii. The aircraft made a safe landing without injuries to the 374 persons on board.
NTSB Safety Investigation
The PW4000 112-inch engine fan blade is a hollow core, wide chord airfoil made of a titanium alloy with 6 percent vanadium and 4 percent aluminum as alloying elements. The fan blade is about 40.5-inches long from the base of the blade root to the tip of the airfoil and about 12.5- and 22.25-inches wide at the blade root and blade tip, respectively. A fan blade weighs a maximum of 34.85 pounds.
During the accident flight while in cruise at flight level (FL) 360, the flight crew heard a loud bang, followed by a violent shaking and warnings of a compressor stall. The flight crew shut down the engine, declared an emergency, and proceeded to HNL without further incident. On examination of the engine it was found that…
The fan blade in position No. 11 was fractured transversely across the airfoil about 1.44-inches above the fairing at the leading edge and slightly below the surface of the fairing at the trailing edge.
There was a piece of fan blade found up against the leading edges of the fan exit guide vanes at about 4 o’clock. This piece of fan blade was about 15-inches wide chord wise, 23-inches long radially, and had a fracture surface on the inner end that corresponded to the fracture surface on blade No. 11.
Laboratory examination of fan blade No. 11 revealed a low cycle fatigue (LCF) fracture that…initiated from a subsurface origin in a region of micro texturing consisting mostly of primary alpha crystals on the interior surface of the hollow core fan blade.
The examination also revealed that the fan blade’s material conformed to the specified titanium alloy’s requirements.
…fan blade No. 10…the adjacent trailing blade, was fractured across the airfoil at about midspan.
The engine / fan history:
According to United Airlines’ maintenance records, the No. 2 engine had accumulated 77,593 hours time since new (TSN) and 13,921 cycles since new (CSN) and 8,579 hours and 1,464 cycles since the last overhaul. The engine was installed on the airplane on October 18, 2015 [and] had operated 8,579 hours and 1,464 cycles since it had been installed. The entire fan blade set, including fan blade No. 11 had last been overhauled by P&W’s Overhaul & Repair (O&R) facility in July 2015. As part of the overhaul process, the blades underwent a fluorescent penetrant inspection (FPI) and a thermal acoustic imaging (TAI) inspection.
P&W developed the TAI inspection process in about 2005 to be able to inspect the interior surfaces of the hollow core PW4000 fan blade. The records for the TAI inspection in July 2015 as well as an earlier TAI accomplished in March 2010 revealed a thermal indication in the same location as where the LCF crack occurred. The records for the fractured fan blade’s July 2015 TAI inspection was annotated ‘paint’ that, according to the inspector, was consistent with him accepting the indication because he thought it was an issue with the paint.
Flaking paint was a regular issue, affecting perhaps 25% of blades, requiring either a touch-up or a complete re-paint. However:
P&W in keeping with NDI industry practice when implementing a new inspection process classified the TAI as a new and emerging technology and therefore did not have to develop a formal program for initial and recurrent training, certify the TAI inspectors, or have a Level 3 inspector on staff, as is done in other established NDI techniques. The 1st shift inspector was trained by the engineers who developed the process and the 2nd shift inspector, who was the one who last inspected the United Airlines fan blade that fractured, was trained by the 1st shift inspector. Both inspectors stated that their training on the TAI was about 40 hours of on-the-job training. In comparison, the certification requirements for the commonly used eddy current and ultrasonic inspections are 40 hours of classroom training and 1,200 and 1,600 hours of practical experience, respectively.
One inspector stated:
The TAI training that he received never provided any reference material on what to look at. He was provided with some photos, but he said that they were very distorted. He did receive a study guide when he started the OJT that was about the steps to power up the machine and included what he said were fuzzy images of what the indications were supposed to look like.
…in 2015 [when the last inspection on the failed blade], and still in 2018 when the incident occurred, P&W was still categorizing the TAI as a new and emerging technology after having inspected over 9,000 fan blades.
At one point, P&W did provide [formal] training on the TAI, however, neither of the two inspectors were permitted to attend the training so that they could work to clear out a backlog of blades in the shop.
So clearing the backlog appears to have been more important than formal training to do the inspection! It is also noted that the backlog resulted in ‘”a lot of overtime”, up to an 4 extra hours a day and 12 hours work on Saturday, begging questions of personnel fatigue.
One inspector complained that the procedures for the inspection were “written for the lab rather than the shop” and had “lots of gaps”. They were on available electronically and it appears there was only one computer between two workstations. There were also facility issues:
The TAI is accomplished in an enclosed air-conditioned room within P&W’s overhaul and repair facility in East Hartford, Connecticut. The room has large windows that allow the afternoon summer sun to shine into the room. The TAI thermal scanners can detect the incremental temperature increase that occurs if there is a defect in the blade and the afternoon sun would cause ghost images on the thermal scans. Although the inside of the TAI room is air conditioned, the shop area is not air conditioned so the workers in the area would often go into the room to cool off in the summer. The TAI scanners would capture the thermal image of the worker. The TAI room air conditioner could not keep the room to within the required temperature limits for TAI in the summertime and they had to place an additional portable A/C unit in the room. After the incident, P&W put screens up and tinted the windows as well as installing flashing lights to alert the shop workers that a TAI is ongoing to preclude the false thermal images. In addition, P&W upgraded the room’s air conditioner that can maintain the temperature to the TAI’s requirements.
When they would reject a blade, it would go to an engineer for further evaluation. However, they [the inspectors] never got any feedback from the engineers if the rejection was a valid rejection or if it was a false positive.
Despite it being a critical inspection facility, only one inspector had seen the FAA visit the inspection facility and noted “they were around more frequently in the past than they are more recently”. The FAA’s Principal Maintenance Inspector (PMI) stated he visited the P&W site once or twice a month.
He said he conducts multiple audits throughout the year. He said that he has done audits in the hollow fan blade shop. He has been to the TAI area several times in the last eight years. He thought that P&W was very impressive and has brought other inspectors to P&W to show them around. He would show the inspectors from other small shops the technology.
After it was determined that the two previous TAIs of the fractured fan blade showed thermal indications at the location of the fatigue crack, P&W initiated an over-inspection of all of the digital images of the TAIs accomplished on PW4000 112-inch fan blades. On March 22, 2019, the FAA issued Airworthiness Directive 2019-03-01: Airworthiness Directives; Pratt&Whitney Division, Turbofan Engines to require an initial and recurring TAI inspection of PW4000 hollow core fan blades.
NTSB Probable Cause
…the fracture of a fan blade due to P&W’s continued classification of the TAI inspection process as a new and emerging technology that permitted them to continue accomplishing the inspection without having to develop a formal, defined initial and recurrent training program or an inspector certification program. The lack of training resulted in the inspector making an incorrect evaluation of an indication that resulted in a blade with a crack being returned to service where it eventually fractured. Contributing to the fracture of the fan blade was the lack of feedback from the process engineers on the fan blades the inspectors sent to the process engineers for evaluation of indications that they had found.
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