"...put orange torque putty on ours for the future 50hr inspections. if the putty ain't broke, it's not loose."
This won't work for the same reason the safety wire won't work - the fastener need not rotate for a gasketed joint to loosen.
Thousands of airplanes were grounded when emergency airworthiness directive (AD) 2008-06-51 was issued because the large hex nut in the center was loosening and disrupting fuel delivery to the engine. How can it come loose; isn't the safety wire there to prevent this?
Threaded plugs and fittings require either a thread sealant (for tapered pipe threads), or a gasket for straight threads. The plug threads will not seal because there is a spiral leak path through the threads. This hex nut uses a gasket that you cannot see in the picture.
The surface of the plug against the housing will not seal because these flanged surfaces are not perfect. Small imperfections cause the two surfaces to rest on their high spots. A softer material in the form of a gasket is inserted between the surfaces. Gaskets embed into depressions, valleys, scratches thereby making a leak-tight seal.
Here is the problem with gasketed joints:
A gasketed joint can loosen without any rotation of the fasteners. Because there is no fastener rotation, the safety wire or other locking devices do not prevent gasketed joints from becoming loose.
The gasket must always push back against the flange surfaces for the joint to remain tight. If the gasket is crushed, or the material creeps so the gasket thickness is reduced, or it takes a "set", and stops pushing back then the joint becomes loose. If over time the gasket material flows out of the joint then its just as if we didn't fully tighten the plug. Many of you have experienced this with exhaust flanges that have to be tightened several times before they remain tight even though the nut never loosened. The technical term for this behavior is "long term creep relaxation" and appears to be the reason for leakage at the hex plug.
Spiral leak path in the clearance between the male and female thread. Leak path shown in red to the left.
Using a time-proven gasket material is what keeps this joint tight and leak free. Safety wire gives a false sense of security. Closely follow torque specifications as excessive tightening can crush the gasket material leading to creep, joint relaxation and leakage.
Fuel injection distribution "spider" valve on Lycoming engine.
Copper has a long proven history of providing satisfactory sealing for straight threads on fittings and spark plugs. Notice that spark plugs don't use lock tabs and safety wire and they don't rotate and fall out. In many cases the use of safety wire serves no purpose than to satisfy regulation dictates.
This lock tab will prevent loosening caused by fastener rotation but will not prevent loosening due to gasket creep. It's not unusual to see old carburetors with leaking bowl gaskets even though the lock tabs are intact. Lock tabs provide a false sense of security and are a hindrance to re-checking bolt tension.
Aluminum crush gaskets are also used on many fittings. All else being equal, the thinner the gasket the lesser the loss of gasket thickness under stress. Aluminum crush gaskets are comparatively thin and widely used in aircraft.
Orings are also used as a sealing gasket on fittings. Orings are soft and can fill in voids. They cannot be used for high pressures or high temperatures.
The stat-o-seal (incorrectly called a lock-o-seal by Cessna) combines the high crush strength of steel that allows high torque values with the conformability of an oring. The combination of hard and soft in a sealing gasket has several advantages:
1. The joint can be tightened to higher clamp loads.
2. The pliable oring is able to conform to surfaces making a better seal than a harder metal gasket.
When we say that the gasket leaks, this is incorrect as it is the joint that leaks. Instead of focusing on the gasket, examine the flange surfaces for damage, parallelism, and smoothness.
Exhaust flanges on aircraft engines are notoriously difficult to seal. Not only is there high temperatures, but the temperature changes results in different expansion rates. Gas leakage results in erosion of the flange surfaces, further making a seal almost impossible.
This flange has a "no blow" spiral wound gasket and has been further sealed with red RTV as a cauking. Although the RTV is not an approved use, it is used because the "approved" method is often unreliable. This type of usage, hard gasket and soft RTV, is not much different in concept as our lock-o-seal above which has both a hard and soft gasket surface. The irregularities that cannot be filled in with the hard "no blow" are filled in by the RTV. Hot gas cauterizes the RTV.
Exhaust flange with red RTV extruding out from between the flange surfaces.
