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AC Instrument and fuel control BUS power


poptart
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If I recall, one of the odd setups on the aircraft; auto switching from Inverter to ESS AC. Heard a story at the Sim that the Israelis changed it. The other Sim story I heard was it was a left-over function from the A model days.

Read portions of an A model dash one at Keesler FTD Crew Chief School. A very different config from the Bs.

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Dan Wilson wrote:

What I have always been told (and it makes sense) is they changed it from the inverter position to the Ess AC position for normal ops to save some money, inverters wear out but the AC bus doesn\'t.

Dan

Dan, this is the way I also understand it. But the boys here in the \'box do everything on the inverters, supposedly because the inverters are a more stable power supply

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The inverter is only a source of bus power when AC is not available or in case they lose AC on start.

Now, on the newer configurations, the switch positions are changing to put all switches in the fore/aft postition to standardize the switches. The -1 position is for AC and the horizontal position ( the dash position) is for inverter power. Of course, since we are losing the Co-pilot\'s inverter switch, there is only one to focus on.

How is that for keeping y\'all on your toes?

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What I was told from an old B model eng was the original \"brush type\" generators did not put out clean enough power for the instruments on those buses so the inverters were installed to provide the stable power needed. They ran the inverters all the time. The AC Inst bus was set up to switch from the \"Normal\", or inverter, position to \"Standby\", AC. Along comes \"brushless generators that made clean enough power to fly with the switches in the standby position thus saving inverters and, as Dan said, money.

Again, this is what I was told, I haven\'t seen any documentation as to why or when the change was actually made.

My neighbors\' father in law is a retired B model eng. I\'ll ask him next time I see him. Maybe he can corroborate the story.

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- 1 for the USMC KC-130\'s 1980 to present and some Navy T\'s...

Newer AC different Bus setup so...

AC INST EFC BUS Auto switches from inverter power to Ess AC power in case of inverter failure... Money was the reason or that is what I was told many years ago.... Turn it off and save the inverter... Loss of the Ess AC bus unlikely so...

Inverter on during engine start in case of ATM failure or loss of air pressure during start resulting in a Gen Out light and thus no power on the Ess AC Bus. Same for taxi... GTC or APU flame out...

Why not run the inverter until after takeoff???? Seems like a good idea if you loose the Ess AC during TO roll, ABORT with no instruments.

Inverter on, Loss of Inverter, only indication is a selected power out light.... Gotta wonder....

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Just curious if anyone has ever really experienced loss of Ess AC. I\'m just killing a cat here...

All of this noisy motor talk reminds me of the ARN14 VOR Dynamotors back by the receivers on the right hand side of the ramp. There were two azimuth indicators back there too.

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A number of years ago, mid 80\'s, we were on decent into an airport on the Alaska panhandle, can\'t remember which one. Anyway, just prior to going into the cloud cover the OFF flag for both ADI\'s popped out. AC immediately pulled nose up, I switched to the inverter, then looked at a bus off light. Headed back to EDF. Old memory can\'t recall much more than that.

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Many, many, many years ago, I was told by a Southern Air Transport FE that they had one Herc that had neither the AC Inst & Fuel Cont inverter nor the Co-pilots AC Inst inverter installed. Can any old SAT or Transamerica guys confirm that story? Or if you\'re really, really old, maybe it was an old Saturn Herc?

Don R.

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We did a modification on the Talon 1\'s (back in the 1970s), adding an electrical contactor that switched the Aux Pump from the Essential to the Main AC Bus once the Main came on line. If the Main dropped, the Aux Pump would revert to the Essential.

That was done primarily to keep the spikes and sags in power (associated with the Aux Pump start) off some of the sensitive electronics that ran on the Essential (at that time we had a Nav computer with \"core\" memory---it would occasionaly dump when the Aux Pump was turned on).

That mod should have been added to many, if not all C-130s.

John

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Thanks for all the replies. That Aux pump running off the MAIN while it is up sounds like a capitol idea. The TCTO shows 120 manhours, but it also shows 20 of that for hydraulics op chks. Seems like it may have been padded a bit.

The TCTO also ran 4 gauge wire back to FS795, right near the pump, and put a terminal board there. Very smart, I wonder if this was thought about for AMP.

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My guess is probably not for AMP. The general guidelines for AMP is focus on the aging avionics. I will ask and see if this is a consideration for AMP, but I doubt it.

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  • 2 weeks later...

NATOPS1 wrote:

Ever turn on the Aux pump before the synchrophaser was turned off???

Many times over the years, but I never had any problems arise from it at all. Maybe you just had one of those gremlin moments.

Now if you have a co that turns the aux pump off and tries to turn it back on before the accumulator discharges is almost a guarantee to open some breakers.

Dan

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Surely there\'s some old heads around that can answer the question about the inverters. I think the -1 position was from the very old days when the Herk was still a DC airplane. That way they had AC power for instruments when only DC was available.

FWIW the reason the H3\'s fly in a 11 position rather than -- was simply one of ergonomics. A cockpit working group suggested the inverter switches be in the same relative position as the generator switches. You can thank Leo Starvetsky personally for that.

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Seems to me, from many moons ago, that the inverters were put in the -1 position was for self-contained starts. If you\'ve ever done a self contained start and had to turn the ATM off because of low bleed air pressure then you\'ll understand why the you need the -1 position. Would suck to be in -- and loosing the ATM power - no engine instruments for the rest of the start on the first engine. You may not always have a power cart to start with, and the -1 position just standardized the start procedure.

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