Two AI’s which to keep

[manac]

182P primary AI is vacuum, secondary is electric.
Electric HSI.
Putting in a GFC500 w/275.

Keeping the vacuum AI would give me redundant AI’s on redundant power sources.

Keeping the electric AI would give me the redundant AI’s only, but let me get rid of the vacuum system.

Originally leaning towards keeping the vacuum AI but now thinking it’s overkill.

 

[tspear]

As long as you have two "power" sources I'm which either can easily get to land, dump the vacuum system.
Personally I would suggest dump the vacuum and put a backup alternator on that accessory pad.

Tim

Sent from my HD1907 using Tapatalk

 

[Jim K]

That's my plan, backup alternator on the vacuum pump pad. Between the g500 (assuming replacing a legacy a/p) and the vacuum system you should gain 30 lbs or so useful. Saving my pennies...

The 275 has an hour battery on top of however long your ships battery lasts. That should plenty of time to land or exit imc.

 

[GRG55]

As others have said, makes no sense to spend the money on solid state gyros and then keep the vacuum system.
Standby alternator and instrument battery back up should be plenty of redundancy for a 182.

 

[MBDiagMan]

Do you really believe that two electrical systems offer the same redundancy as a totally different and separate vacuum system?

 

[GRG55]

Do you really believe that two electrical systems offer the same redundancy as a totally different and separate vacuum system?

Sure do. I can split the electrical bus on my Aztec and drive either side from either alternator. A standby alternator on the 182 can be set up the same way. Add in the battery backup for the solid state gyros, completely independent of the airplane electrical system, and why would one want to keep the vacuum pump?

If you are in IMC and the engine quits the vacuum system fails immediately and completely. Oh joy! Both ship power and the individual instrument batteries don't do that.

Admittedly I also have the advantage of redundant vacuum pumps. But the only reason I am keeping the vacuum system is because I need it to run the de-ice boots. Not applicable to the OP's 182.

 

[MBDiagMan]

So, you have two batteries?

 

[GRG55]

So, you have two batteries?

Only one battery for the airplane electrical system. All the other batteries in the airplane are directly powering or backing up specific devices.

Why would anybody need more than one battery for the aircraft electrical system? Is there anybody here that has ever had an aircraft battery actually fail in flight? I mean an actual failure of the battery itself in flight, not a discharge of the battery after a charging system failure.

 

[MauleSkinner]

Is there anybody here that has ever had an aircraft battery actually fail in flight? I mean an actual failure of the battery itself in flight, not a discharge of the battery after a charging system failure.

Yes. But I didn’t know about it until the next starting attempt, as the generator still worked.

 

[Salty]

Only one battery for the airplane electrical system. All the other batteries in the airplane are directly powering or backing up specific devices.

Why would anybody need more than one battery for the aircraft electrical system? Is there anybody here that has ever had an aircraft battery actually fail in flight? I mean an actual failure of the battery itself in flight, not a discharge of the battery after a charging system failure.

Why would you separate the two scenarios, in either case you’re in the dark.

 

[mondtster]

Standby alternator and instrument battery back up should be plenty of redundancy for a 182.

Im not sure I’d even bother with a standby alternator in this case. It’s more to maintain and lug around.

When Piper put the G500 in the Archer they did away with the vacuum system and installed an electric backup attitude indicator. The electric backup included a backup battery built onto the unit. There was only one alternator on that airplane and it didn’t bother me to fly into any IMC I’d be willing to fly into in a non-deiced single engine piston airplane.

If you are in IMC and the engine quits the vacuum system fails immediately and completely.

It’s a minor nit to pick but that may not be true. Is the prop still turning and the accessory drive still functional? The vacuum pump may still work well enough to provide some air flow...

 

[GRG55]

Yes. But I didn’t know about it until the next starting attempt, as the generator still worked.

Then how do you know it failed "suddenly" in flight, and not until after you landed.

Batteries are chemical devices. They don't hold a charge one minute and suddenly fail to hold a charge the next. They fail gradually and give more than ample warning that's happening. Mine is subjected to a load test at least every annual.
But if you really feel the vacuum system is the better back up alternative so be it. We will just agree to disagree.

Im not sure I’d even bother with a standby alternator in this case. It’s more to maintain and lug around..

On a 182 I wouldn't either, but I think it would depend on how much the owner is using it in IMC and under what conditions. He may feel that level of redundancy is appropriate if his panel is all electric.

IWhen Piper put the G500 in the Archer they did away with the vacuum system and installed an electric backup attitude indicator. The electric backup included a backup battery built onto the unit. There was only one alternator on that airplane and it didn’t bother me to fly into any IMC I’d be willing to fly into in a non-deiced single engine piston airplane.

It’s a minor nit to pick but that may not be true. Is the prop still turning and the accessory drive still functional? The vacuum pump may still work well enough to provide some air flow...

The prop and vacuum pump may still be turning, but if the output is below the specification for the gyros there's no assurance they are giving an accurate indication. Certainly you won't find me wanting to depend on that possibility to safely resolve getting out of IMC.

 

[MauleSkinner]

Then how do you know it failed "suddenly" in flight, and not until after you landed.

Batteries are chemical devices. They don't hold a charge one minute and suddenly fail to hold a charge the next. They fail gradually and give more than ample warning that's happening. Mine is subjected to a load test at least every annual.
But if you really feel the vacuum system is the better back up alternative so be it. We will just agree to disagree.

“Suddenly” is a whole other discussion, and no, I don’t know that it failed in flight, but based on the fact that it had started to melt the case, it’s a pretty good assumption that it happened while the generator was trying to charge it.

I made no statement about which is better. I merely answered your question.

 

[GRG55]

Why would you separate the two scenarios, in either case you’re in the dark.

Because the probability of occurance of one is infinitesimally small compared to the probability of the other. And when one is considering the level of desired redundancy and the methods & costs (and in light airplane the additional weight) by which that can be achieved, the number of potential failure modes, and the probabilities of occurance factor into those decisions.

 

[GRG55]

“Suddenly” is a whole other discussion, and no, I don’t know that it failed in flight, but based on the fact that it had started to melt the case, it’s a pretty good assumption that it happened while the generator was trying to charge it.

I made no statement about which is better. I merely answered your question.

Sounds to me like something else in your charging system failed if the battery was melting.

 

[MauleSkinner]

Sounds to me like something else in your charging system failed if the battery was melting.

No...battery replacement is all that was required.

 

[mondtster]

The prop and vacuum pump may still be turning, but if the output is below the specification for the gyros there's no assurance they are giving an accurate indication. Certainly you won't find me wanting to depend on that possibility to safely resolve getting out of IMC.

Neither would I. I just wanted to clarify for others that the air flow may not immediately go to zero, which is the way I read what you wrote.

 

[MBDiagMan]

Only one battery for the airplane electrical system. All the other batteries in the airplane are directly powering or backing up specific devices.

Why would anybody need more than one battery for the aircraft electrical system? Is there anybody here that has ever had an aircraft battery actually fail in flight? I mean an actual failure of the battery itself in flight, not a discharge of the battery after a charging system failure.

Easy Cowboy! It was only a simple question.

You have all this alleged redundancy so I just wondered if the alternators are in parallel or on completely separate circuits. Since they are on the same battery, then it would seem that a dead short across either circuit would take out both, eliminating the redundancy. Just curious how far it all went.

 

[tspear]

Do you really believe that two electrical systems offer the same redundancy as a totally different and separate vacuum system?

Vacuum systems have a much higher failure rate than alternators, vacuum instruments fail more than solid state electrical systems....
The actual redundancy level is likely much lower.

Tim

 

[manac]

Thanks for all the replies.
Remember the original question was do I keep the vacuum driven mechanical AI or do I keep the electrical driven mechanical AI.

The question wasn’t how to build the million dollar 182. We already have that thread.

I think some people are confusing redundancy and reliability.
A twin has the redundancy of a second engine at the expense of half the reliability.

Some said the electronic AI is going to be much more reliable than any mechanical one. Ding, ding, ding we have a winner!

Correct answer: get rid of both mechanical AI’s.
Increase overall reliability.
Reduce electrical load.
Reduce weight.
K.I.S.S.

 

[Tarheelpilot]

My first thought before opening the thread was keep the one that does not want to destroy humans.

Dump the vacuum, install second alternator and get an electronic instrument with solid state gyros.

 

[manac]

My end result was a bit of a surprise.
My T&B had an Elec flag so I assumed it was electric. Turns out it drove the ap and was another vacuum gyro. Ended pulling that too.
End result no vacuum system, no gyros.
Still playing with the cover getting it centered.

 

[WDD]

You have two G275's, each with their own independent battery back up and either can work as an Attitude Indicator. Looks like you picked what I would have voted for if there was that choice in your poll - get rid of both of your old vacuum and old electric AI.

Different topic - you have your G275 AI that also shows altitude, along with the traditional altimeter next to it. When you adjust your Kollsman on your mechanical altimeter does it change the G275 as well, or do you have to adjust two altimeters each time you get a new altimeter setting?

 

[manac]

The 275 STC only requires 1 battery. I could add a second for $400 but don’t think it’s worth it/needed.
Two adjustments for the altitude setting, fortunately not 3. The two 275’s sync.