When everything is controlled by electronics, the electrical connector becomes the weak link. Air Force F-16 on short final for landing and the computer shuts down the engine. No software fault, just a corroded electrical connector that controls the main fuel shutoff valve.
Obviously, you cannot wait until the F-16 engine shuts down and then fix the problem. A preemptive maintenance approach is necessary to prevent electrical connector faults from occurring, especially as computers start to control our vehicles. For the mechanic, the past approach to electrical connectors was to "leave them alone if they are working." For the engineer (who didn't want the mechanic fooling with the electrical connections) preventative maintenance instructions are now necessary and need to be added to the maintenance manual.
The Air Force solution is to maintain electrical connectors by treating them with MIL-L-87177A Grade B lubricant. Before you think that any lubricant will work. An Air Force study found that some lubricants either didn't work or actually caused corrosion.
The Air Force Tech Order/Manual 1-1-689A, Avionics Cleaning and Corrosion Prevention/Control" has added MIL-L-87177 Grade B as a direct replacement for MIL-C-81309 Type II and Type III in all cases.
The Navy also has electrical contact corrosion problems and has also addressed it in their 2004 DoD Corrosion Prevention and Mitigation Strategic Plan "MIL-L-87177 coating for corrosion protection of electrical connectors."
Diagnosing Electrical Contacts Failures
Typically electrical contacts fail intermittently because corrosion deposits increase the electrical resistance. Faults are often classified as:
1. CND - Cannot Duplicate
2. RETOK - Retest OK
A CND or RETOX type fault is a red flag warning of an electrical contact problem.
Maintenance of Tin Coated Electrical Contacts
The majority of electrical contacts are the shinny tin coated connectors.
1. Minimize movement between the contacts.
Contacts need to be mechanically stable. Tin is soft and even the slightest rubbing disrupts the protective surface layer. Oxides and debris builds-up between the surfaces leading to a rapid and dramatic increase in contact resistance. Gold contacts do not oxidize and are more immune to rubbing and fretting damage. Make sure wire bundles and connectors are properly secured to prevent pulling and vibration.
2. Maximize contact pressure.
Higher contact forces (a minimum of 3.53 ounces) reduce the relative movement between contacts. The sliding action during insertion also wipes the contact surfaces and helps establish metal-to-metal contact. Loose contacts need to be replaced.
3. Tin coated contacts need lubrication.
Slight movements between contact surfaces caused by vibrations and thermal expansion increases electrical resistance in tin coated contacts. Lubrication:
a. reduces friction and the generation of wear particles
b. prevents oxidation of contact surfaces
c. seals the contacts from moisture
d. lubrication permits higher contact pressures without using excessive insertion pressures.
4. Tin coated contacts are unreliable over 212 F (100 c). Tin looses mechanical strength causing the metal to creep (run away from the contact stress). The contact pressure gradually decreases causing CND and RETOK problems. Be suspicious of tin connectors used in hot areas.
5. Beware of inexpensive connectors. The tin coating needs to be at least 100 micro-inches thick. Purchasing mil-spec connectors is a good method of assuring proper thickness.
6. Mating with gold connectors is not recommended. Tin rubs on the gold causing tin oxide build-up. Lubricants are not good at preventing this. Tin connected to silver does not have this problem and is OK.
7. Tin should not be used to make or break current. Tin and tin alloys are not arc resistance and have a relatively low melting temperature.
8. Direct water away from the connector. Water and condensation travels down the wire. If the connector is the low point then the connector needs to be water-tight or the wire needs to be relocated.
References:
The Tin Commandments: Guidelines For the Use of Tin On Connector Contacts" AMP Technical Report, Tyco Electronics Corp. Note: the original "Tin Commandments" are from J.H. Whitley, published in Plating and Surface Finishing, October 1981.
Effects of Corrosion Inhibitive Lubricants on Electronics Reliability, Scott L. Dotson, 752nd Combat Support Squadron (CSS).
Bought this software to help me troubleshoot an alternator problem on my airplane.. I have to say it was very helpful and led me down the right path.. So here is what I found:
The Airplane was having an issue with the amperage dropping in flight and the system dropping to battery (hence the Alternator stopped putting out power), once on the ground the alternator would kick back in. After pulling the cowl I found one of the spade connectors on the field wire (just behind the forward fire wall) was loose on the wire.
A visual inspection would not have told me this I had to physically pick up the wire and give a very slight tug on the connector and wire to feel the disconnect (Consequently the wire pulled clean out of the connector). I replaced the spade connector and went for a test flight, everything worked great after that.
Diagnosis: on the ground the alternator would work because the loose spade connector would make contact with its mate, once I took off the now pressurized engine compartment would force the spade to separate from its mate just enough to disconnect the field wire and therefore shut down the alternator. Upon landing, the spade connector would slip back into place. Replacing the connector insured a good solid connection and the problem went away.
Thanks,
Drew
