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