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32 AIRWORTHINESS Pinhole corrosion ‘Pinhole corrosion in one section of the system can be a warning sign of more extensive internal corrosion in other sections of the system and therefore may require replacement of the entire tubing network.’ As any LAME knows, inspecting corrosion in an airframe is exacting work; inspecting for corrosion on the inside of a small bore aluminium line as it meanders through the wing and fuselage is bordering on impossible, without hi-tech equipment. ‘The key issue is, how do you inspect for this? It is truly insidious, and inspecting for it requires technology at the cutting edge of inspection techniques. Very small borescopes, capable of operating over long distances and special electronic metal thickness detectors would be required. Then comes the question as to what data to be used,’ Alder says. ‘It would seem that pinhole corrosion could be mainly due to aircraft having low utilisation over many years. But there’s no telling when it could occur—a slight flaw in the anodising inside a pipe could be enough to precipitate it. It affects older aircraft more than newer ones, because the corrosion takes some years to bite its way from the inside to the outside of a pipe. On the basis of the number of reports received by CASA, pinhole corrosion appears to be age- rather than hours-related; although an aircraft which has been used regularly over a long time might still have the problem. Using an aircraft infrequently gives water time to collect and sit between flights. It also gives the special additives added to avgas at the time of manufacture time to evaporate. ‘While many aluminium pipes are rejected for a number of reasons, including external corrosion, these pipes can look perfectly normal on the outside until breached by pinhole corrosion from the inside,’ Alder says. ‘Remember that the pipe has been corroding from the inside out. The spot where the pinhole occurred is just the first place at which it broke through. You must also look at the matching component or section of tubing on the other side of the aircraft—it may have similar problems and be the next item to go.’ Alder says that considering existing manufacturers’ maintenance schedules, particularly those for light aircraft, which rarely (if ever) include an inspection for internal corrosion of the aluminium tubing, plus current inspection techniques and technology, pinhole corrosion in aircraft is looming as a potentially expensive problem to first of all find, and then fix. ‘Pinhole corrosion in one section of the system can be a warning sign of more extensive internal corrosion in other sections of the system and therefore may require replacement of the entire tubing network. ‘Some manufacturers permit splicing a replacement section into a pipe but others do not, citing changes to fluid flow and the creation of weak points where the splice is joined into the pipe.’ What can aircraft owners do? Store your aircraft in the best possible conditions—under cover in as dry an environment as possible. The major enemy is ingress of moisture and the main culprit appears to be moisture in the low points of the system. Replace tubing that has external corrosion. Go flying. In other words: use your aircraft. Every flight that generates heat from the engine, gets fuel and fluid flowing through the aircraft’s network of pipes, and puts the aircraft in a variety of attitudes, helps to reduce the likelihood of pinhole corrosion forming. Stay informed: If you hear of a case of pinhole corrosion in a similar aircraft to yours, then you should inspect or replace the corresponding part on your aircraft. You can email sdr@casa.gov.au to check pinhole corrosion occurrences for your aircraft type. The reports SDR510014540 Rigid hydraulic tubing located between LH wing root and engine nacelle-contained corrosion pitting through wall thickness resulting in loss of hydraulic fluid. Suspect caused by tubing contacting flexible hose in wing channel. Gulfstream 500S SDR510014546 Rigid aluminium fuel line from LH tank to fuel cock contained pinhole corrosion allowing fuel leakage into cockpit. Pilot returned to land after smelling avgas in cockpit. Investigation found corroded and leaking fuel gauge pressure tube assembly. Tube corroded in area of contact with black scat hose. Cessna 150 SDR 2011144 Defect description: Very difficult to determine exact cause. The defect was noted and rectified while the aircraft airconditioning system was being overhauled. When the cabin floor panels were removed to carry out reinstallation of
Flight Safety Australia Issue 87 July–August 2012 33 condensers it was noted that the insulation surrounding the cross feed tube was fuel soaked and causing a slow drip of avgas. The insulation was removed, exposing a very small pinhole leak. Component then removed and replaced. Fuel tube cut open to determine cause of leak. Corrosion (internal) found to be the cause. Opinion as to the cause of the defect: Corrosion and possible introduction of water to flush fuel system during EDA (ethylenediamine) cleaning. If water was introduced to fuel system by any means it would, during periods of non use, be liable to settle in the cross-feed tube and create the right environment to start the corrosion process. SDR 510010841 While performing engine ground runs strong smell of avgas evident in cabin. On removal of interior trim, soundproofing found saturated with fuel. Investigation results: Fuel feed line from R/H source found to be weeping avgas at the entry into the fuselage. The solid aluminium tube was holed at this point due corrosion. Replacement line installed and scoured, fuel replenished. No leaks evident. SDR 20020830 Defect description: corrosion due to water contamination. (Solid aluminium tube was leaking/pinhole.) Opinion as to the cause of the defect: corrosion and human factors. SDR 510009700 Pilot returned to land after smelling avgas in cockpit. Investigation found corroded and leaking fuel gauge pressure tube assembly. Tube corroded in area of contact with black scat hose. SDR 20020090 Defect description: Aircraft was found to leak while in our hangar for other defects. On further inspection the leak was found to be coming from the R/H inboard transfer line. This line was replaced. Inspection of the leaking tube revealed pitting in the walls due to corrosion. SDR 20000474 Fuel cross-feed RH tank to LH engine lower outboard fuel line at the rear of engine firewall corroded approximately one third of the way in from the end, causing fuel to leak into area rear of engine firewall. Hyd line assy left P/N: 31527-00 on flap operation system at sta 74.75 just forward of spar, inside of cabin
Page 1 and 2: 87 July-August 2012 Unmanned aircra
Page 3 and 4: CONTENTS Issue 87 | July-August 201
Page 5 and 6: Flight Safety Australia Issue 87 Ju
Page 11 and 12: ‘They’re being used now,’ she
Page 19 and 20: Check your flight notifications The
Page 21 and 22: Australian accidents/incidents 01 A
Page 33: Flight Safety Australia Issue 87 Ju
Page 55 and 56: If in doubt, don’t take-off ATSB
Page 57 and 58: Night flying-make sure you’re qua
Page 59 and 60: REPCON BRIEFS Australia’s volunta
Page 73 and 74: AUSTRALIAN Flight Safety Australia
Page 75 and 76: 2012 Have trouble finding aviation

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