<19990720.165854.4214.27.kitfoxflyer@juno.com>
>Be aware that when you disconnect the battery from a vehicle or aircraft
>fitted with a alternator a sudden surge may occur. This can damage the
>alternator depending on the type of surge diodes fitted and can also in the
>case of motor vehicles damage electronic control units.
This rather generalized caveat has been floating around
in various forms for decades in transportation industries
where vehicles use battery/alternator DC power systems.
Many folk have interpreted it to have applicability under
all conditions, even when the engine is not running.
Others have enlarged the meaning to include the attachment
or disconnection of jumper cables between the vehicle's
power supply and that of another vehicle or exernal power
source.
I'd guess that the basis for the statement comes from what
we learned about alternator behavior when they first replaced
generators on airplanes back in the early 60's. While
a generator would willingly start up and provide stable,
useful power even when there was no battery on line, the
new fangled alternator would not. Depending on design
of the alternator/regulator combination, power supplied
by an alternator sans battery could be anything from
barely satisfactory to wildly hazardous to the health
of electro-goodies on the airplane.
This lays foundation for the birth of the split rocker,
battery master switch that found its way onto most of
the single engine airplanes flying today. The idea of
the split rocker was to prevent leaving an alternator
on line unless the battery was also on line. However,
it did allow leaving the alternator OFF until after
engine start and for battery-only ground ops. Of course,
it also allowed turning off the alternator in flight.
This last fact raised a new issue. 60 amp alternators
were standard equipment on most Cessnas . . . even the
lowly Day/VFR training ships like the C-150. As the
battery slid off toward oblivion, it's ability to
stabilize an alternator degraded too . . . especially
when the machine was a 60-amp, fire-breathing dragon.
Some folks experimenting with the alternator switch
in flight found that re-energizing the alternator at
cruise RPM, low system loads and a soggy battery produced
surge transients of wallet vacuuming proportions. Hence
the placard you see on many single engine certified
ships saying "DO NOT TURN ALTERNATOR OFF IN FLIGHT
EXCPET IN AN EMERGENCY".
Again, we find the certified side of the house "fixing"
a design problem with increased training and pilot workload.
It also shifts the blame for subsequent mishaps off onto
the pilot when the happless chap fails to observe the
placard. In conversations with a number of TC aircraft
owners, I've suggested that they superglue the halves
of their split rocker switches together if their airplane
has a pullable field breaker. This prevents inadvertent
operation of only the alternator side of the rocker switch
but still allows battery only ground ops and/or disabling
the alternator in flight should the situation warrant it.
Our recommended wiring diagrams for amateur built aircraft
show single operator, two pole switches for the DC power
master switch and a pullable breaker for the alternator
feeding the alternator field. Alternator and battery
come ON and OFF together.
Getting back to the original statement, we need to understand
also that as long as there is a battery of reasonably
good condition on the line (even if it's presently
discharged), there is no risk from adding or disconnecing
an external battery with or without the alternator on line
and/or engine running.
The risks associated with external power connection are
from inadvertent reversal of polarity and/or connection
of 28v ground power to a 14v airplane (unlike connectors
on the wall of your house for 120 versus 240 volts,
ground power connectors on airplanes are not mechanically
different for 14 versus 28v). The last risk associated
with ground power shows up on some TC aircraft where
the pilot has no control from his seat over the application
or removal of ground power from his aircraft's system.
All three of these gotchas have been addressed in the
recommended wiring we show for ground power jacks as
published on our website.
Bottom line is that there are valid reasons for people
to hand down these little bits of hangar wisdom. However
without an understanding of the physics and circumstances
behind the statement, it becomes more folklore than fact.
Educated pilots are much less likely to have a bad day -
in the air or on the ground. Education by sound byte
or excerpt can be worse than none at all. The politicians
and news anchors prove it every day.
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