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AC Inst & Engine Fuel Control Inverter


NATOPS1
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OK...

Currently the ACI&EFC Inverter is in the Ess AC Bus position for takeoff (KC/C-130T) and all the other models I have inquired about... (Older aircraft)

Question is why not run the inverter and take advantage of the "auto switch" function if the inverter fails while preventing the need to "manually switch" the inverter ON if the Ess AC Bus fails.

I cannot find a reason to NOT use the inverter it just has always been this way.

So run the inverter until after takeoff and then switch it to the Ess AC position at some point in the climb/after takeoff checklist.

Pro

Cons

Need some good feedback on this please.

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From an old memory from 20 years ago... I recall that the -1 position of the inverters was a standardized approach for all engine starts, self-contained, normal, etc., where during self-contained starts there are times when you needed to shut down the ATM, and thus loose engine instruments if using the - - position, to use higher bleed air pressure from the GTC for the engine starter. The - - position after that reduces wear on the inverter, and reduces a little bit of the noise.

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Yea I understand the reason we turned them off but nowadays we have solid state or brushless inverters and the wear issue has been taken care of....No ATM to take, so...if we are running them on the grnd for start and taxi why not use them to avoid a situation that could cause problems... Abort with a loss of engine instruments due to failure of the Ess AC Bus....thats more of the idea

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It was standard ops. for the inverters to be in the -1 config. for eng. start. Failure during start the Engr. could reach up and swich to the bus position and save the start. The operation of this switch was critical and you can go to an electrical schematic to verify this, that improper movement of this switch could induce two sources of 110 voltage to the attitude indicators. I know of an FAA guy that burned the pilots and co-pilots indicators.

Edited by steve haigler
bad info
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The ADIs run off the copilot's AC inverter, not the AC Inst and FC inverter.

I don't see how it was powered by two different sources, but $hit does happen. It wouldn't be the switch causing it, it would most probably have to be a relay with an internal broken contact arm or some other FM cause.

No reason at all that they couldn't be on. Get yer book changed.

Edited by tinyclark
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Just got a copy of it... well this goes to show old ideas are not bad ideas....

I wonder if anyone has asked the question why not run the non solid state inverter during critical phases of flight… Takeoff/Landing. If it has been decided that it is that important then why not use the system.

I also agree with Tiny the ADI's will only have a single voltage input unless their is a failure. The power relays cannot be in a two voltage configuration.

However they can get TWO attitude inputs… INS and Gyro. That’s why the placards tell us to switch the attitude source to the gyro position to prevent the ADI from being damaged.

The INS will continue providing the attitude source as well as the gyro with the inverter in the DC position.

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OK...

Currently the ACI&EFC Inverter is in the Ess AC Bus position for takeoff (KC/C-130T) and all the other models I have inquired about... (Older aircraft)

Question is why not run the inverter and take advantage of the "auto switch" function if the inverter fails while preventing the need to "manually switch" the inverter ON if the Ess AC Bus fails.

I cannot find a reason to NOT use the inverter it just has always been this way.

So run the inverter until after takeoff and then switch it to the Ess AC position at some point in the climb/after takeoff checklist.

Pro

Cons

Need some good feedback on this please.

Has there been a rash of ESS AC BUS failures on take-off recently?

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  • 2 months later...
  • 2 weeks later...

The reason we went to having the inverter in the DC position was because of the instability of the K44 relay which is one of the things we discovered from the Baghdad incident. The big problem, however, is the susceptibility of the TD Amp to input voltage anomalies. We have accomplished some testing recently that served to nail down the susceptibility (the reference circuit w/i the amp) and to correct the problem. Now that we know what the problem is, we should be able to correct the discrepancy by either fixing the amps or providing some undervoltage protection for the amps.

Mike Fisher

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I would like to hear a little background on the K-44 relay thing.

K-44A or B?

K-44A I would think...but if the voltage (Ess AC) to energize the K-44A relay is bad (low) then it would mean any voltage applied to the ACI & EFC Bus would be low as well. So there was low voltage on the ess AC that caused this? This low voltage then caused the TD to correct based on a bad input?

Edited by NATOPS1
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When in the AC position both of the relays involved are deenergized.. When in the DC position one relay is energized sending power directly from the inverter to the ACI&EFC bus.

Another relay takes power away from the ACI&EFC bus when you turn it off. And it has to be energized to do that.

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________________________________________

agarrett, I agree..... Sorry about that Very bad choice of words!!

Ess AC power energizes relay 2 in the OFF position.

When energized, relay 2 removes the Ess AC Bus power from the ACI&EFC Bus.

In the Norm position the ground is removed and relay 2 deenergizes. This allows the Ess AC Bus to power the ACI&EFC Bus.

Normal (Ess AC) has no relay energized so I do not see the connection...Ess AC powers the ACI&EFC Bus with no way to change (chatter) Cycle, move, energize, de-energize nothing.

So the susceptibility is in the TD amp not the relays that determine the ACI&EFC Bus power source.

That is plausible, bad voltage input to a reference circuit equals a bad reference. So Null sounds like the safer option because you no longer have any voltage issue.

(You also get rid of the less than 94% RPM issue that would move the TD to start limiting....)

Edited by NATOPS1
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NULL is really the second best option. The advantage of NULL is that you eliminate the susceptibility of the fuel control system to input voltage anomalies but you always have the potential of overtemps and having to manage the TIT more closely.

The reason(s) we chose the use of the inverter with the system in AUTO is as follows:

1. You get the benefits associated with the TD system.

2. You cut the deenergized side of the K44 relay out (except as a back up).

3. The DC side of the relay provide an additional backup through the reverse current relays so that if there is a problem on the ESS AC bus, the TD Amp doesn't respond.

There is an effort to modify the TD Amps with solid state pots that will likely include a modification to the reference voltage circuit. We ran prototypes of the mod across both test benches and a test cell and the change precludes the TD Amp from responding to low voltage conditions down to about 30 volts where it cannot direct any changes to the TD valve.

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"NULL is really the second best option."

I would say this is the more desirable option.

The reason(s) we chose the use of the inverter with the system in AUTO is as follows:

1. You get the benefits associated with the TD system.

(AGREE) However if engine RPM drops below 94% (as three engines did) the TD becomes your worst enemy.(START LIMIT)

2. You cut the deenergized side of the K44 relay out (except as a back up).

(Still do not see the connection between a deenergized relay and the TD amp...)

The issue is the AC power source not a relay...

(The Inverter will supply power and have the ESS AC as a back up (energized relay, inverter power keeps the inverter "selected" as the power source, if the inverter fails Ess AC will be applied to the ACI&EFC Bus via the deenergized relay)

3. The DC side of the relay provide an additional backup through the reverse current relays so that if there is a problem on the ESS AC bus, the TD Amp doesn't respond.

If the thought is there should always be DC power I agree, but there has always been DC power.

I'm a proponent of running the inverter but not for this reason or thought process...

During TO roll with the inverter off encounter an ESS AC Bus failure and try and abort without engine Instruments. I’ve heard all you have to do is switch the inverter on.... one more thing to do as the end of the runway gets closer... and the power is instantly available, right?

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