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<< Lancair Builders' Mail List >>
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<< Solder is technically unacceptable for aircraft usage due to the fatigue
problem >>
>From my years in avionics (military, GA, commercial carrier) and over 300
full panel retrofits from '78 to '87 as well as my time on CH-53s, B737s and
BAe 146s in both repair and overhaul, I did far more soldering than crimping.
Solder unacceptable? Without strain relief, perhaps. With strain relief,
solder connections are used in a large percentage of connectors on your
avionics and instruments. Some of these are mil spec, some are not, all are
either mil spec or FAA certificated and all depend on correct and adequate
strain relief. It is only in the last two decades that crimp pins have fully
displaced the solder cup pins in mil spec connectors for light wire gauges.
They are far from fool proof, and STILL require correct and adequate strain
relief. I have lost count of the number of failed crimp connections I have
repaired and they far outnumbered the failed solder connections both as a
percentage and overall. Many of those crimp connections that I repaired were
properly installed, they failed anyway. I may have seen a failed solder
connection when it was properly strain relieved, but I don't remember it.
If you use crimp connections, you must use the correct racheting crimp tool,
the correct pin size for the wire, strip the insulation to the exact
dimensions specified, then provide strain relief external to the pin. Why
all of this concern?
First, toss that smash-jaw-home-depot-flymart set of pliers in the trash.
The ones that have broad crimping jaws will only match well to one or two
sizes of pins and you have no quality control vs. over and under crimping.
Under crimping has the obvious disadvantage of poor contact and possible wire
pullout. Over crimping is far more common (a little crimp is good, more must
be better, right?) and causes far more problems. An over crimped connection
can cause stress and failure of the pin itself, bending of the pin which
prevents proper seating in the connector, and (worst of all) cutting of the
wire. The last exhibits an intermittent connection; they are oh-so-fun to
troubleshoot. The "blade jaw" pliers are even worse, since an over crimp
literally cuts the wire inside the connection. The worst case scenario is a
correct crimp on the insulation area and an over crimp on the conductor.
Guaranteed intermittent and it won't come loose!
Second, too small of a pin requires you to either crimp the wire on the
insulation (strain relief) section of the pin, or cut a few strands out to
make the wire gauge fit the pin. High resistance and poor strain relief are
the result. Your life depends on the correct contact area of the crimp
connector making good contact with the correct amount of wire; too little
and high resistance and heat are the result. The failure may not show up for
a year or two and Murphy's Law stipulates that it will be in a totally
inaccessible area of the airframe and the connector will be nicely melted
when you find it, ruining many good connections and other systems that were
properly installed. <LOL> Too large of a pin and you have to over crimp just
to keep the wire from falling out (see previous paragraph). Please do not
stuff more small gauge wire into a large pin to make a small wire crimp
properly.
Third, crimp pins and splices have two crimp areas: a small diameter for the
conductor and a larger one for the insulation. There is a VERY narrow range
of stripping dimensions that put the correct amount of wire into the
conductor area and leave enough insulation to be properly crimped in the
second (strain relief) area. Under strip and you deprive the conductor area
of the required contact (see previous paragraph on high resistance). Over
strip and the strain relief area has to crimp on wire instead of insulation.
If there is motion that the strain relief was supposed to take care of, this
motion is now focused on an under crimped hunk of wire instead of a correctly
crimped area of insulation. The only bright side is that when it fails it
may fall off and be readily visible upon inspection. Soldering a crimp
connector pin or lug melts the insulation that should be providing strain
relief and forces a stress riser where the solder ends as it wicks up the
wire. I have seen a lot of these fail. If you are going to use a crimp
connector, crimp it properly and trust it. If it is in a dirty area, adding
a piece of heat shrink or self vulcanizing tape insulation is perfectly
acceptable since their effect is OUTSIDE the connection. Solder was never
planned for by the engineer who designed and certificated that crimp.
Last, the strain relief of the insulation area crimp to the wire insulation
is NOT capable of very much motion or fatigue resistance. Let's face it,
we're looking at a very thin metal sleeve pressed up against some PVC or PTFE
insulation; bend it and it cuts. The connection must have external strain
relief. For a pin connector this comes in the form of the back shell and
clamp provided. The wires should be bundled and clamped such that any
tension on the bundle pulls only on the clamp and none of the wires from the
clamp to the pins are under strain. Especially in homebuilts, I can't count
the number of connectors I see every year with no back shell ("too much
trouble" is the most common excuse...) On a wiring harness, the strain must
be distributed to the other wires in the bundle; and that is a whole can of
worms on its own.
<< But in any event, do not splice where the wires can flex. Anchor both
sides of the splice down, eliminate all motion at the splice, or expect a
failure, sooner or later.>>
Sound advice. A crimp splice in a wire bundle will abrade any other wire it
comes in contact with. For that matter, so will wires that are crossed
rather than combed (all wires in a bundle running parallel). This issue is
nonlinear with respect to the frequency of vibration in the airframe; i.e.,
it becomes critical in turbine aircraft. All Walter users out there need to
be more diligent towards their wiring workmanship than the piston users.
Solder is great stuff. It takes a discontinuous surface of stranded wire and
forms a continuous conductor to the properly designed-for-solder connector
pin or another wire. It is almost impossible to have higher resistance in a
good solder connection than in the wire itself. Heat shrink or self
vulcanizing tape insulation work well for coverage and minimal strain relief
IF care has been taken to eliminate all angle from the connection and sharp
edges from the solder and wire. In connectors, I highly prefer mil spec
soldered pins; though I rank the skill required to rework these without over
temping them right up there with TIG welding. If used correctly, mil spec
crimp pin connectors properly stripped, crimped with a calibrated racheting
crimp tool, and installed with correct back shell strain relief are the
fastest and most reliable connection available. The trouble is, almost no
one does them correctly.
On a butt splice in a wiring harness, there is no contest. Proper solder
joints beat crimp splices for piston aircraft. Since neither is acceptable
for high frequency due to the rubbing problem on adjacent wires, what do we
do if we have to repair a wire on a turbine aircraft? We replace the whole
wire, end to end. We sometimes got away with doing a splice and covering it
with enough self vulcanizing tape to prevent adjacent wire abrasion.
Remember all of this before you bury your wiring in inaccessible and/or
uninspectable locations. The V-22 program is having real fun with this right
now.
Eric Ahlstrom
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LML website: http://www.olsusa.com/mkaye/maillist.html
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Please send your photos and drawings to marvkaye@olsusa.com.
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