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When a Fast-On terminal is mated for the first time
the material in the "lobes" is strained to the point of yielding,
generating a fixed maximum obtainable preload. Thermal cycling,
mechanical vibration and corrosion reduce that preload over time.
Moisture, drawn into the connector by capillary action, will facilitate
corrosion, especially where the stresses are highest, further reducing
mechanical clamping force. Repeated mate / demate cycles rapidly
deteriorate the connection integrity by wiping off any protective
plating, exposing the copper alloy base.
The assertion that a Fast-On is somehow "airtight" is false a priori,
unless encased in a protective covering such as heat shrink tubing with
a thermal adhesive, as the strained spring elements of the connection
form open ended tubes.
One reason for the failure of the Fast-On connection over time is that
the load path for any stress (static or dynamic) is through the
electrical path. Any connector designed for a rugged environment will
have a separate strain relief load path to protect and mechanically
isolate the electrical contacts and reduce contact strain.
By definition, connectors function by clamping two conductive elements
together so, in a very general way, all connectors are basically the
same. The differences come in the details of implementation. The card
edge connector, used in the tray on some avionics (e.g. King radios),
while not a Fast-On design, is another example where vibration induced
strain between the connector elements degrades connector integrity over
time. This is evidenced in that the first diagnostic step for a
recalcitrant radio is to remove and reinstall it into its tray.
I suspect the long term effects of vibration are not fully appreciated
by some builders. During certification vibration testing of the Chelton
Flight Logic system there was evidence of fretting even in a ARINC-600
(aerospace qualified) tray connector. Post test microscopic examination
showed that several of the 150 pins had worn through their 50 micron
gold plating.
IMHO, the "cost" to make an electrical connection is fixed so you will
either "pay now" for a quality connector or the labor to make a cheap
connector work or you will "pay later" in hotel rooms, rental cars ,
commercial flights to get home and the parts to repair after the nose
gear collapses because a Fast-On on the hydraulic pressure switch fails
at an inopportune time. This is not a hypothetical example, it
actually happened.
What would happen if a similar connection on a similar pressure
switch, say the door seal pressure switch on a IV-P, fails in the
flight levels and the cabin pressure gradually drops? Are you SURE you
would notice before you lost consciousness?
IMHO Fast-Ons have NO place in flight critical and flight safety
applications such as pressure switches, gear status switches, hydraulic
pump relays, ground connections, engine instrumentation, etc. Use them
on your Hobbs meter, cabin lights or cabin speaker if you must. I
believe in mitigating as many risks as practical before
leaving terra firma.
That is just my opinion.
Regards
Brent Regan
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