Post #5 Aircraft Mishaps due to NDI results negatively and possitively

In effort in providing examples why NDT/NDI is critical in the aviation maintenance process, the following  are a couple of examples from the National Transportation Board that are a primary reason why it’s import for the inspections but to follow proper procedures in recording periodic maintenance or conduct modifications. This first accident was caused due to failure of not recording data of implementing Manufacture modifications and NDI/NDT technician’s failure to conduct inspections. 

On May 18, 2011, about 1727 Pacific daylight time, a modified Boeing 707, registration N707AR, operating as Omega Aerial Refueling Services (Omega) flight 70 crashed on takeoff from runway 21 at Point Mugu Naval Air Station, California (KNTD). The airplane collided with a marsh area to the left side beyond the departure end of the runway and was substantially damaged by postimpact fire. The three flight crew members sustained minor injuries. The flight was conducted under the provisions of a contract between Omega and the US Naval Air Systems Command (NAVAIR) to provide aerial refueling of Navy F/A-18s in offshore warning area airspace. According to the Federal Aviation Administration (FAA), Omega, and the US Navy, the airplane was operating as a nonmilitary public aircraft under the provisions of 49 United States Code Sections 40102 and 40125.

Shortly after liftoff, when the airplane was about 20 feet above the runway and about 7,000 feet down the runway, all three crewmembers heard a loud noise and observed the thrust lever for the No. 2 (left inboard) engine rapidly retard to the aft limit of the throttle quadrant. The captain stated that he applied full right rudder and near full right aileron to maintain directional control and level the wings, but the airplane continued to drift to the left. The captain reported that he perceived the airplane would not continue to climb and decided to "put it back on the ground."Injuries to Persons Impact Information section his brief for more information the three crew members sustained minor injuries. Damage to Aircraft The airplane sustained substantial damage due to impact forces and was partially consumed by post fire.

 Figure 1. Diagram of a Boeing 707 engine nacelle strut, showing the fracture area on the midspar fitting

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Service Bulletins and Airworthiness Directives

To address the midspar cracking issue, a series of Boeing service bulletins (SB) and FAA airworthiness directives (AD) were published between 1975 and 1993, beginning with Boeing SB 707-3183 (dated June 27, 1975). Subsequently revised and updated (revision 1, dated May 13, 1977), SB 707-3183 called for an initial inspection of inboard and outboard midspar fittings on the No. 2 and No. 3 engines, followed by repetitive close visual inspections at varying flight cycles (depending on airplane configuration) and eventual replacement of the fittings with an improved design that incorporated larger radii of 1.0 inch in critical areas. Replacing the fittings was a terminating action for the repetitive inspections. The bulletin also included instructions to enlarge the pylon access cover over the fitting for better access (for both inspection and cleaning). Revision 2 of SB 707 3183 (dated January 28, 1988) incorporated SB 707-3377 (dated November 21, 1979), which gave instructions for the installation of nacelle droop stripes to facilitate visual detection of broken nacelle support structures, such as midspar or overwing fittings, by indicating a misalignment between the nacelle strut skin and the fairing skin. The FAA required the actions recommended in SB 707-3183 and its revisions via ADs 77 09-03, 88 24-10, 92-19-15[4], and 93 11-02.

Omega records indicated that the company conducted the first visual inspection in 1996, shortly after the conversion. Omega observed that the AD list for the airplane indicated that AD 93-11-02 was completed, but inspections per the Boeing SB were entered into the maintenance program and continued until 2003, when a records review found that, in 1983, a previous owner/operator had marked the compliance status of the AD in effect at the time (77 09-03) with "C" (meaning complete)[5]. This status reconfirmed to Omega that the records showed the fittings had been replaced and inspections were no longer necessary, and Omega deleted the inspection requirement from its maintenance plan.[6] Following the accident involving Omega flight 70, an examination of the No. 2 and 3 nacelle struts confirmed that both the inboard and outboard midspar fittings were of the older design and had not been replaced with the improved design in accordance with the AD. The examination also noted that droop stripes had been installed on the accident airplane nacelles, as required by the AD.

Tests and Research

Metallurgy

The Nos. 1 and 2 engine pylon fitting components were brought to the NTSB Materials Laboratory for examination. Metallurgical examination revealed that the No. 1 pylon-to-engine bolts, the No. 1 engine pylon-to-wing fittings, and the No. 2 engine pylon-to-wing fittings all failed in an overload event with the exception of the upper and lower tangs of the inboard midspar fitting on the No 2. pylon, which failed due to fatigue.

The upper tang of the No. 2 pylon inboard midspar fitting failed in the reduced section between the lug where the drag support fitting was normally attached and the chromium-coated radius, with the fatigue initiating at its upper inboard corner and occupying approximately 15 percent of the fracture surface. Corrosion product covered the fatigue fracture surface, consistent with it being exposed to the atmosphere for a significant time. Chemical cleaning of the fatigue fracture surface revealed that mechanical damage had obliterated any fatigue fracture features that may have been generated in the upper inboard corner and the corrosion product had obliterated any fine fatigue features, such as striations, leaving only vestiges of crack arrest marks. The lack of striations prevented a striation count analysis. The cleaning procedure also revealed surface fissures on the fatigue fracture surface that were oriented parallel to arc-shaped crack arrest marks and are consistent with high stress, low cycle fatigue propagation.

Chromium electroplated coating had been applied to the upper tang radii, and machining marks in the coating adjacent to the fracture face indicated that a machining operation had been performed after the electroplating. It is probable that the machining operation was intended to remove any excess coating that might have interfered with the fit of the lug in the drag support fitting. The examination noted that machining marks would have intersected with the inner edge of the fracture face at the inboard upper corner and may have been the fatigue initiator, but mechanical damage in the corner prevented a determination.

The lower tang of the No. 2 pylon inboard midspar fitting failed in the inboard chromium plated radius with the fatigue initiating at multiple locations in the upper portion of the inboard edge and occupying approximately 1 percent of the fracture face.

The plated radii in the No. 2 pylon midspar fittings were measured at a nominal 0.38 inch, identifying them as the older style fittings that should have been replaced in accordance with the effective AD. The new midspar fittings have radii of 1.0 inch.  (2013, January 2). 

Reference

NTSB. (2013, January 2). Omega Aerial Refueling Services flight crashed on takeoff. Retrieved from http://www.ntsb.gov/investigations/fulltext/AAB1301.html

The next example are images of stress and heat fractures or broken of due to fatigue cracking with in a aircraft engine.

Figure 1.  This is a good compression blade that will be loaded into a compression fan blade ring assembly.

Fig 2.  This an example of a compression blade that came in contact with a small rock, or a very small piece of metal such as tiny piece of wire clipping left over from a mechanic doing maintenance while safety wiring 2 bolts together.

Fig 3. This is an example of figure 2. debris as tiny as it was, now shows secondary impact into this compression ring set of blades.

Fig 4. As the compression and combustion mix as you can see this would be results in seeing a flame blow out.  

Fig.6  A turboprop cutaway to show all internal parts of how air travels through the chamber. NDI can either have all these components delivered to a lab for a individual penetrant inspection looking for heat stress in the below image fig 6.

This is what NDI is looking for, in avoiding engine disasters. 

Fig 6.