crack is the first sign of impending failure
Anyone can do it
Anyone can look for cracks with the unaided eye. A small flashlight and mirror are all you need. There are three crack requirements:- Crack must be at the surface
- Crack must be large enough to be visible, and
- Surface must not be covered with films that hide the crack.
- Proper Light
- Experience
- Knowledge
Crack Requirements: You cannot detect cracks on surfaces that have a thick, flexible, powdercoat finish. That beautiful “automotive style” finish that is foreign to aircraft hides cracks. A less attractive, traditional aircraft performance finish is easier to inspect. By performance I mean designed for function rather than cosmetic appearance. Each part has a role to play; performance paint films protect from corrosion without hiding defects.
Condition Requirements: If I placed a new employee in our NDT department and showed him the process of detecting cracks in aircraft parts I found that this was not enough training. There needed to be knowledge beyond the process steps. What a crack looks like, where they typically occur, what some false indications look like. A more experienced employee was better at finding cracks than a less experienced employee. Work on improving these three condition requirements.
Crack - The Definition
Where is the end of this crack? Is it hidden under the paint? Is it longer on the other side of the skin? Is it
If you don’t know where the end of the crack is, then how can you stop-drill a hole at the end?
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| Cracks are always longer than they appear to the eye |
If you don’t know where the end of the crack is, then how can you stop-drill a hole at the end?
Look and Understand
“The evidence of impending doom was clearly visible”
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| Figure 1. Aloha 243. Crack was seen by passenger boarding aircraft on last flight. |
During my years analyzing engine failures one thing that struck me: How sad when you see a catastrophic failure and you realize that it could have been prevented if someone had “looked” at the part.
Aloha 243 illustrates several crack detection concepts:
- Some cracks can be detected with the unaided eye before advanced NDT (non-destructive testing) methods.
- One must understand what one is seeing.
As one Aloha 243 passenger boarded the aircraft she spotted a crack but didn’t understand what it was. Why didn’t the pilot and stewardesses see the crack? Evidently they didn’t look. Why wasn’t it spotted by NDT professionals? They were not there at the right time. Visible crack detection with the unaided eye remains the primary inspection tool used to detect cracks, corrosion, and other deterioration on aircraft. It’s not only the responsibility of the mechanic, but also the pilot. Now is the time to include crack detection in your preflight and as part of your routine inspection program.
Learn the Warning Signs
One must understand what one sees. Sometimes you can’t see the crack but you can see the evidence. Figure 2 shows combustion gas stains on the side of a cylinder. Gas is leaking from the cylinder. Is this important? Is this something I should be looking for during a preflight inspection?
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| Figure 2. Lycoming aircraft cyinder with exhaust staining on side. |
Stains and Leaks are typical Crack Warning Signs:
Exhaust stains -- can be dark or white
Fluid stains - oil, hydraulic fluid, fuel
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| Figure 3. The rest of the cylinder. Warning signs were not seen or ignored. |
Warning Signs -- Oil and Gas Stains
Oil leaks and gas staining may be a sign that something’s cracked. Therefore it’s important to keep the equipment clean and investigate any signs of staining. Cylinder head-to-barrel separation as shown in Figure 3 is rare but does occur. In my cylinder shop we would reject four or five cylinders a year out of possibly a thousand. It should be noted that the best method of inspection for this type of problem is gas staining. We were never able to detect this type of crack using pressure tests. Therefore, we always wanted the cylinders to arrive at our shop dirty. One thinks of clean and then inspect...stop and take a look at stuff before you clean. It’s often better to inspect an aircraft before it is cleaned rather than after.
Know Where to look?
- Cracks often start at boundaries such as holes, edges, radiuses.
- Cracks follow grain boundaries
- stress + corrosion=cracks
- Cracks follow the line of greatest stress
- Cracks usually grow larger
An aircraft cylinder is a pressure vessel for hot gas. If the pressure vessel cracks it does so at the walls. The walls are hidden by the fins. From this angle you will not see any cracks. Orient the cylinder so you can look between the fins at the walls.
Figure 4. No cracks visible from this view. Continental O-200 cylinder.
Same cylinder but different view. Now we can see some staining and possibly a horizontal line at the base.
Figure 6. Closeup of O-200 Cylinder showing crack.
Same cylinder with crack extending from inside to outside as evidenced by the oil stain. Digital cameras are great at seeing details. Compressed, hot gasses produce the force that turns your propeller or pops the head off of your cylinder--whichever is easier. Structural integrity of the cylinder head is the number one concern. Removing metal by porting can only weaken the structure.
Begin with Research
Inspection occurs at finite points on an infinite surface. Where are you going to inspect? Effective crack detection begins with research. Some aircraft have a well-known history of cracking at a particular location. Find out where these areas are. Review the manufacturer’s service instructions. A good resource is the FAA’s Service Difficulty Report (SDR). http://av-info.faa.gov/isdr/ Other resources are the various aircraft type associations that have a wealth of information.
Figure 7. Cracks in exhaust valve face.
Remove from Service
One method of avoiding failures from undetected cracks is to remove the component from service before the end of its fatigue life. Exhaust valves, for example, are replaced at overhaul regardless of their apparent condition. No such policy with Aloha 243; it had 89,680 takeoffs and landings. Maintenance inspection was assigned the task of keeping an aging relic in service. Many military, civilian aircraft (and the nation’s bridges) are operating beyond their designed life. Once they exceed their design life, inspection keeps them from failing. Inspection, no matter how rigorous is abbreviated; inspection a finite set of infinite points under sometimes difficult conditions and the problem becomes ruled by probabilities. No guarantees can be made to public users. Catastrophic failures will occur. Safety is up to the mechanic to look, detect, and report.
Figure 8. Cracked spark plug ceramic. Inspect unconventional locations.
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Figure 9. Cracks become fractures if not found.
one uses all of the senses when inspecting
Using Sound
One of the inspection methods used in road and rail tunnels is to hit structural elements with a hammer. “As a result of a hammer strike on the surface, the structural element will produce a sound that indicates if a hidden defect exists.” This is an old, but effective, tool that is forgotten in many industries. Before repairing aircraft cylinders I would take a plastic ball-point pen and run it down the fins. usually I would here a clear ringing sound. If there was a “thud” then I know I had a cracked cylinder. I saved myself the time of cleaning and penetrant detection. The same method can be used on cylinders on the aircraft.
Dye Penetrant System -- How it works
The simplest and least expensive crack detection system is dye penetrant. here’s how it works.Dye Penetrant consists of:
●Cleaner/Remover
●Penetrant
●Developer
Clean the part with the Cleaner/Remover. Spray on the Penetrant (use sparingly). Wipe off the excess and allow to soak in for 5-10 minutes. Completely remove the Penetrant with the Cleaner/Remover. Spray on the Developer and let it dry. Arrow shows crack indication.Cracks bleed out the penetrant. Use the Cleaner/Remover to clean up your part.
Apply Penetrant and let it sit
Use Cleaner/Remover to remove penetrant. Dry and then spray with Developer. Developer is just starting to dry in this picture.
Red dye has bleeded out of two cracks.
Remove from Service
One method of avoiding failures from undetected cracks is to remove the component from service before the end of its fatigue life. Exhaust valves, for example, are replaced at overhaul regardless of their apparent condition. No such policy with Aloha 243; it had 89,680 takeoffs and landings. Maintenance inspection was assigned the task of keeping an aging relic in service.Many military, civilian aircraft (and the nation’s bridges) are operating beyond their designed life. Once they exceed their design life, inspection keeps them from failing. Inspection, no matter how rigorous is abbreviated; inspection a finite set of infinite points under sometimes difficult conditions and the problem becomes ruled by probabilities. No guarantees can be made to public users. Catastrophic failures will occur. Safety is up to the mechanic to look, detect, and report.
Responsibilities
After inspection one should never declare the part “crack free”. Using some other process might detect cracks not visible or below the resolution of the method used. One is not guaranteeing that the part inspected is not cracked, that it will not crack, nor that it will remain “crack free.” What you guarantee is that the inspection was performed in accordance with the prescribed process and either passed such inspection or failed (and the reason for failure). “You have to assume for design purposes that your structure is crammed full of cracks that are just too short to be detected.”1.When my aircraft engine shop did NDT inspections we inspected some engine parts. No engine assembly, just NDT. Shortly afterward the owner called and said that the connecting rod had broken in half and had destroyed his engine and “what were we going to do about it?”
We didn’t assemble the engine, didn’t overhaul it. We didn’t cause it to break. But we had inspected it. Maybe we missed the crack? This is the liability of any NDT inspection; you cannot get around it. Understand the risks and charge accordingly.
Flight with Known Cracks
FAA Advisory Circular AC2-13A
"It is the Small Airplane Directorate's policy to not allow continued flight with known cracks in primary structure. An airplane with a known crack in the primary structure no longer meets its type design and may no longer possess (sic) its type design strength. This policy applies to all normal, utility, acrobatic, or commuter category airplanes, regardless of certification basis."
Shortly before the right wing broke off of a Turbo-Mallard (N2969) killing all 20 people aboard, at a meeting with pilots the following comment was made: "the maintenance supervisor told them that the skin-cracks in the airplanes were superficial." NTSB accident report NTSB/AAR-07/04.
Who's in Control?
This is the key item of importance to the mechanic--the mechanic must defer to engineering or appropriate maintenance instructions as to the proper course of action when a crack is discovered. It is not in the mechanic's (nor aircraft owner's) area of expertise to assess residual strength and crack growth rates.
What to do about it?
All detected cracks are a concern, regardless of their size. Leaving a crack in situ is rarely an option.
Many aircraft fly with known cracks if engineering demonstrates that the residual strength exceeds ultimate load; including crack growth until the next inspection. What to do about it is the decision of engineering presented through the aircraft's maintenance instructions. Most modern aircraft use a SEM (Structural Repair Manual).
What is NOT a repair
Stop drilling is not a repair; it's a temporary intervention. A stop drill hole does not properly restore the load-carrying capability of the structure and the underlying structural problem is still not properly addressed.
Additional Reading
1. Applied Mechanics for Solids. Chapter 9. “Modeling Material Failure”
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