Today's Message Index:
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1. 02:07 AM - Re: EXTERNAL PWR with MASTER/BATT SWITCH? (supik)
2. 11:38 AM - Re: EXTERNAL PWR with MASTER/BATT SWITCH? (Robert L. Nuckolls, III)
3. 01:44 PM - Re: EXTERNAL PWR with MASTER/BATT SWITCH? (supik)
4. 06:01 PM - Re: Re: EXTERNAL PWR with MASTER/BATT SWITCH? (Robert L. Nuckolls, III)
5. 06:55 PM - Re: EXTERNAL PWR operations (follow up) (Robert L. Nuckolls, III)
6. 08:11 PM - Re: EXTERNAL PWR operations (follow up to follow up) (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: EXTERNAL PWR with MASTER/BATT SWITCH? |
..battery protection?
--------
Igor
RV10 in progress
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=496420#496420
Message 2
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| Subject: | Re: EXTERNAL PWR with MASTER/BATT SWITCH? |
At 08:54 PM 5/16/2020, you wrote:
>
>What's the reason, most of the certified aiplanes do not allow
>connecting the External Power Src without turning the MASTER /
>BATTERY contactor ON?
That's a kinda long story . . . but the pick-em-up
truck is off the jacks and running, so I guess I'll
offer up the long version:
"Ground Power" is a somewhat nebulous term like
"automobile", "train" or "boat". It alludes to
a function but without specifics as to quality
or safeguards.
I've seen some pretty creative pieces of hardware
for getting an airplane started. I was sold a
cold-weather assist in Brainerd, MN one really cold
morning where the energy source was a converted
engine driven welder that some electron-herder
had figured out a way to re-regulate down to
aircraft system voltages.
On another, not quite so cold morning in Kansas City,
I got a start from a WWII era engine driven
ground power unit wherein the line boy shot
28v to my 14v airplane and had his back to me
all the time I was waving at him to shut-it-off!
That experience prompted me to add ov protection
to recommended ground power systems.
During the final phases of aircraft certification
there's a Function and Reliability test phase
where a carefully carded and monitored series
of flights totalling 150 hours are carried out.
During cert on the Premier I at RAC/Beech, I was
called to ride the test aircraft on one of the
ICT-SLC-TUC-ICT round-robins to investigate
the reason that a ground power cart in Tucson
wasn't allowed to connect to the aircraft. The
airplane was fitted with some sort of power
quality monitor that would allow application
of ground power only after certain 'quality'
issues were addressed.
The ICT-SLC-TUC-ICT loop was flown at night.
Piled all my test gear in the baggage
compartment and sat in the back waiting a chance
to look at the TUC conditions.
In what was a rare rain storm, I stood on
the ramp, head in the baggage compartment
staring at my DAS screen. The airplane accepted
the GPU. I didn't see anything amiss! We
cranked up and I rode home in wet shoes.
At the de-brief it was discovered that, "Oh,
you wanted to check that 'other FBO GPU."
When you're flying two or three crews 24/7
to rack up hours on the a/c, it's a bit of
a chore to keep all the activities tightly
coordinated. So . . . back on the airplane
at 2100 and we took off for SLC.
This time TUC weather was clear and I got
some really good data. Seems that particular GPU was
a plug-in-the-wall device that used silicon
controlled rectifiers to deliver regulated
energy to the aircraft. But before being allowed
to connect to the ship's systems, the non-
loaded GPU had a terrible output waveform.
The power quality gizmo said 'no-go'. Now,
had the power quality monitor not been
present, the system would have connected.
With a battery on line all would have
been right with the universe.
So, THAT particular GPU's features required
the ship's battery to be present for operation.
If the GPU's designers had included some
fat capacitors in the output filters of their
product, I would have been cheated out of two
rides in the B390. BTW, after the first night's
experience I was compelled to wear earplugs
during climb out. The noise levels were pretty
much okay a cruise but really irritating during
climb out.
Nowadays, I believe there is a TSO describing
the qualities of the best we know how to do in
GPUs . . . I wouldn't be surprised to see notes
in operational documents for some ships to strongly
recommend hooking up to GPUs qualified to "TSO-
something-or-another.
But on a cold rainy night when you're trying
to launch out of Hog Waller International and
Bubba pulls up to your airplane with his pride
and joy ground power unit, the thought processes
involving the go/no-go decision may fall
victim to a bit of ground fog.
The short answer is: It probably wouldn't matter
but it MIGHT . . . and in any case it NEVER hurts.
Depending on where the current limiters were
set on that KC GPU, the battery just might
have saved some electro-whizzies from being
toasted in the 172XP I was flying.
Bob . . .
Message 3
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| Subject: | Re: EXTERNAL PWR with MASTER/BATT SWITCH? |
Bob, thank you for the long story. Actually I used your scheme with the OV protection
and the GPU is wired to be plugged in directly on to the battery through
the EXT PWR Contactor.
If I am not wrong, Marc Ausmann's scheme connects the GPU to the battery as well.
This is what I am planning for..
But with your scheme there is no need to turn on the MASTER SWITCH..
C172 and SR22 -both require to turn on the MASTER to close the BATTERY and EXT
PWR CONTACTORS at the same time. Thus you are not able to charge your battery
unless you leave the MASTER ON (stupid?) or you connect your charger directly
with the battery poles or take the battery out..
Cirrus POH says NOT to recharge the battery through the EXT Power socket. The only
thing which comes to my mind is if one would leave the battery on a GPU overnight
which would eventually overcharge it..?
------------------------------------------------------------------------------------
Cirrus POH:
Application of External Power
An external power receptacle, located just aft of the cowl on the left side of
the airplane, permits the use of an external power unit for cold weather
starting and maintenance procedures.
CAUTION
In accordance with the manufacturers recommendation, external
power should not be used to start the airplane with a dead battery
or to charge a dead or weak battery in the airplane. The battery
must be removed from the airplane and battery maintenance
performed in accordance with the appropriate AMM procedures.
----------------------------------------------------------------------------------
Dead battery is clear but why not charge a weak battery if your charger is safe..?
Or is it that the intelligent charger might not provide enough amps to close the
EXT PWR Contactor when plugged in through the EXT PWR Socket?
--------
Igor
RV10 in progress
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=496426#496426
Message 4
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| Subject: | Re: EXTERNAL PWR with MASTER/BATT SWITCH? |
At 03:41 PM 5/17/2020, you wrote:
>
>Bob, thank you for the long story. Actually I
>used your scheme with the OV protection and the
>GPU is wired to be plugged in directly on to the
>battery through the EXT PWR Contactor.
>If I am not wrong, Marc Ausmann's scheme
>connects the GPU to the battery as well. This is what I am planning for..
>
>But with your scheme there is no need to turn on the MASTER SWITCH..
Yes. I do that so that the external power
can also be used to charge a battery
in-situ without powering up the rest of
the airplane.
>C172 and SR22 -both require to turn on the
>MASTER to close the BATTERY and EXT PWR
>CONTACTORS at the same time. Thus you are not
>able to charge your battery unless you leave the
>MASTER ON (stupid?) or you connect your charger
>directly with the battery poles or take the battery out..
Yeah, that theme goes back quit a ways. See
exemplar 1974 wiring attached. 1962 C185
does about the same thing. With the battery
master off, you can crank the airplane but
you cannot charge the battery. In the 74
172 drawing we see the avionics bus
disconnect relay (item 4) which unhooks
radios while cranking and/or when ground
power is hooked up.
Hinky ground power sources have been around
since day-one but the philosophy behind their
use is not an industry wide consensus.
>CAUTION
>In accordance with the manufacturer=99s recommendation, external
>power should not be used to start the airplane with a dead battery
>or to charge a dead or weak battery in the airplane. The battery
>must be removed from the airplane and battery maintenance
>performed in accordance with the appropriate AMM procedures.
Yup, this is a legacy hang over from the
days of wild-n-wooly ground power hooked
to a dead and perhaps trashed flooded battery.
A lot of potential for spectacular
messes in the battery box. It's still generally
true . . . a totally discharged battery
should be recharged with a smart charger,
while preferably but not necessarily situated
outside the airplane. NOT by a 400A ground power cart
or 60A alternator.
>---------------------------------------------------------------------------
-------
>Dead battery is clear but why not charge a weak
>battery if your charger is safe..?
>Or is it that the intelligent charger might not
>provide enough amps to close the
>EXT PWR Contactor when plugged in through the EXT PWR Socket?
This is why YOU need to become the system
integrator and craft your own procedures
that meet design goals while minimizing
risk. What you suggest is perfectly doable
if you understand how NOT to let it devolve
into a bad day.
Remember that cookie-cutter airplanes are generally
owned and operated by individuals with just enough
knowledge to avoid doing something really stupid . . .
and even then it doesn't always work out. The
POH for most airplanes are crafted as much to minimize
lawsuits as opposed to educating the attentive
and thoughtful pilot.
Bob . . .
Message 5
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| Subject: | Re: EXTERNAL PWR operations (follow up) |
>>Or is it that the intelligent charger might not provide enough amps
>>to close the
>>EXT PWR Contactor when plugged in through the EXT PWR Socket?
You used some words that beg clarification.
"Chargers" come in all sizes from the diminutive
wall warts that are characteristic of Battery
Tender Jr and Battery Minder (1A or so output)
up to battery shop beasts capable of hundreds
of amps to charge batteries or crank engines.
To be sure, the wall warts are probably best
referred to as 'battery maintenance' devices.
While they WILL charge a battery given sufficient
time, they should not be expected to close
a ground power contactor -AND- charge a battery.
A 'real' charger is will replenish a deeply
discharged battery in a practical period of time,
say overnight at most. This class of charger is
in the 5A plus class.
Any modern charger of any size should be expected
to included automatic charge shut-off and possibly
maintenance modes of operation that would prevent
subsequent damage due to overcharge.
I don't recall seeing any TC light aircraft fitted
with a ground power contactor that was crew controlled.
All are wired to close the contactor as soon as ground
power is plugged in and energized. The third, 'pilot
pin' in the mil-std ground power jack makes contact
only after the two fat-pins are engaged. This
feature prevents burning the pins by holding current
flow at bay until the plug is mostly engaged
and contactor is closed.
I have always thought that the pilot, not the
ground crew, should have control over when ground
power is applied. Additionally, wiring that energizes
the ground power contactor should include defenses
against over voltage and reversed polarity. I think
I recall some Cessna SE products having a diode
in the ground power pilot lead that would prevent
contactor closure on a reverse polarity situation.
The ground power receptacle should be explored with
the same diligence to FMEA as the rest of the
airplane. This exercise goes to both your understanding
of how to best use the feature as well as reduce
risk to the airplane.
Bob . . .
Message 6
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| Subject: | Re: EXTERNAL PWR operations (follow up to follow up) |
A 'real' charger is will replenish a deeply
discharged battery in a practical period of time,
say overnight at most. This class of charger is
in the 5A plus class.
However, any charger with an auto shut down
feature will be thwarted by the parasitic drain
of the ground power contactor coil.
A smart charger expects the recharge current
to approach zero as full charge is achieved.
If it's powering a contactor too . . . the
current never drops low enough to terminate
the charge.
Hence, 'smart' chargers should be connected
to the battery via dedicated connections -OR-
the charger should be clipped to battery
terminals AFTER the ground power contactor
-OR- the battery should be removed from
the airplane for a deep replenishment charge.
Bob . . .
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