Post 6 NTSB NDT

Still researching. Will post tonight.

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.

Post #4 NDI and Fire Invesigation

In the past 14 years there have been an increase in-flight fires some of which resulted in crashes(Wood & Sweginnis, 2006). A fire accident usually can be broken down using the “Swiss cheese model method” meaning a chain of events leading up to an incident or accident crash. Fires in some cases is due to the better understanding of the in flight process. Other reasons involve aging or overload electrical systems and poor choices of insulating materials. The inability of the flight crew to identify the source of the fire (in so cases) do anything about it has also contributed (Wood & Sweginnis, 2006).

In avoiding an aircraft fire or a Fire Crash Investigation (FCI) it is critical again in the NDI field to detect stress areas in the metal prior to flight or to search for the clues that lead to the cause of the fire whether in flight or post crash fire. This is why according to manufacture specifications under certain conditions over the aircrafts life time, regardless of platform, need to follow protocol and or periodic maintenance scheduling in finding a stress build up in the aircraft structure.

In an investigation in looking for clues is what FCI calls Slipstream effect. Since pretty much all aircraft use Aluminum, the point at which sagging and bending occur is at 850 degrees F. Might see this on a post crash fire and think in-flight fire. At 1175 degrees F the alloy will melt completely and molten metal may slipstream into tiny droplets that might impinge on structure. Gravity droplets at the crash scene will be larger.

Below is a chart of various metals and their melting points, notice Aluminum is close to the bottom of the chart along with other common materials mostly used in current aircraft.

                Here are some illustrations found by NDI proving the exact area in the metal, (aluminum) that could very well lead to or did cause fires due to over stressed areas. These images are what NDI technicians’ are looking for after the part is manufactured before installed into service, and while in service in figures 1 and 2.

               

Fig. 1

 Fig.2

Figure 3. The highlighted area in yellow using Ultrasound method, being a very weak section in the weld on a hydraulic actuator. Hydraulic oils being the heat source, is compressing into this area therefore generating friction and  heat. If this weld is weak enough or corroded enough, its springs a leak. The temperature at which this oil comes out and (where) it comes out could determine if a fire could start in-flight or on the ground.  

Fig.3

Fig.4

Fig.4. Using Liquid Pentrant shown under black light exposing the indication (cracked/weakened) area.

As heat is placed on an area of metal, the metal will start to expand therefore now losing its strength.  How fast it loses its strength is determined by the source by which is originating (heat). Eutectic Melting is the lowest melting temperature of any alloy metals in the chart above, under a high stressed load the metal is subject to this point a phenomena accrues called the “Broomstraw Effect” within the grains of the metal becomes pronounced and delimitation happens between the layers/grains then one would see in a “green stick” fracture Fig. 5. This is considered highly indicative of in-flight fire, the assumption being that the heat occurred in flight and the stress occurred at impact.  This can also occur if the part is under high stress as it is normally used in the aircraft and then heated.  Thus a “broom straw” fracture is not a 100% guarantee of in flight fire.  The eutectic melting temperature of aluminum alloy is approximately 890 degree F (Wood & Sweginnis, 2006)

               

Figure 5.

                                                                                                         Fig.6                     

  Fig 6. Is an illustration as the dripping heat source impacts the structured air frame part, the heat expands  and alters the metal properties and weakens the metal thus effecting its purpose used in the aircraft. NDI found this part using Ultrasound. However Eddy Current, Magnetic, or X-ray methods could expose this defect. Question is the area, size, what type of defect is being sought will determine the method used and go from their.                                      

The breakdown of  questions in the chain of  fires / excess heat as follows; in methods and questions that are apart of the investigation that NDI/NDT will be considered in

 Ignition

i) Ignition source must raise the temp of the combustible vapors or mist

ii) Sparks from aluminum may not ignite jet fuel

iii) Must have sustained ignition to continue the fire

e) Solids

i) Materials may char or burn decomposing into other compounds, some hazardous

ii) All substances will be affected by heat and may have varying flash points and

potential for flashover

f) Aluminum alloys in aircraft

i) Prolonged heat causes structure weakening

(1) High heat / short time

(2) Low temp / long time

ii) Eutectic melting: Lowest melting point creating a “broom straw” effect if metal is

highly stressed. Stress along metal “grain marks.” Might be an indication of in-flight

fire as the heat weakened the metal and the impact caused the stress. Takes about

890 degrees (warning–if the part is under a high load then it might be an in-flight

issue).

iii) Slipstream effect: Sagging and bending occur at 850 degrees. Might see this on a

post crash fire and think in-flight fire. At 1175 degrees the alloy will melt completely

and molten metal may slipstream into tiny droplets that might impinge on structure.

Gravity droplets at the crash scene will be larger.

vi) Other clues to check

(1) Crumpled parts

(a) Check inside for fire evidence maybe in-flight or high intensity ground fire

(2) Buried parts show fire damage that maybe occurred in-flight

(3) Mud and soot should be on top of soot–maybe

(4) Molten metal-ground fire-puddle or pool in-flight will have a pattern

(5) Rivet holes if failed in-flight then should be clean–maybe

(6) Consistent fire damage. In-flight should show adjacent parts with same fire

pattern. Equipment by fire-then other parts should show pattern. (Lawin, 2014).

References

Wood, H., & Sweginnis, R. (2006). Aircraft accident investigation. Casper, Wy: Endeavor Books.

Lawin, R. (2014). Air detective tip 14. In R. Lawin (Ed.), SFTY 330 Aircraft Accident Investigation 0911Retrieved from https://erau.blackboard.com/bbcswebdav/pid-15077989-dt-content-rid-29820714_4/institution/Worldwide_Online/SFTY_UG_Courses/SFTY_330/Air_Detective_Tips/AirDetectiveTip14_0911.pdf