HSI, Slaved compasses, and Flux Gates, oh my!

[labbadabba]

I'm a PPL who's reading up on my stuff getting ready to take the IR leap. One thing that baffled me when studying for my PPL (but didn't bother me since I never used one in my primary training) was the HSI.

Not the concept per se, I think it's an intuitive instrument on the front end. It's the guts of how it works and how it slaves to the flux gate makes my brain mushy.

So, with a HSI there is typically a remote compass controller on the panel. Is that controller supposed to indicate magnetic variation? Should it be set on '0' at all times? Do we use it to correct the HSI like we correct precession on a DG? Or should it reflect what we see indicated on the isogonic lines on our sectionals? Do slaved HSI's automatically correct for precession if they're slaved?

Is there ever a time (provided that the remote compass/flux gate is functioning) when you would set the HSI to 'Free Gyro' mode?

You see? Total mush up in here...

 

[Henning]

Normally when you disable a feature it is because it is broken and creating system error.

 

[Cpt_Kirk]

Is there ever a time (provided that the remote compass/flux gate is functioning) when you would set the HSI to 'Free Gyro' mode?

Yes.

Put it in 'Free Gyro' mode, adjust it left or right a set number of degrees (5 works), put it back in 'Slave' and see if it corrects itself.

 

[Gucci Pilot]

The controller basically lets you change from slaved to DG. In slaved mode, the HSI is getting its info from the flux gate, which is basically a magnetic compass usually mounted out of the way of everything(usually in the wing), that is sending a digital signal to the HSI. If you go super far north you may have to switch to DG to fly true north. I think the indicator is if you manually want to change the course, it tells you how far you have changed it from where the flux gate thinks it should be.

 

[poadeleted20]

So, with a HSI there is typically a remote compass controller on the panel. Is that controller supposed to indicate magnetic variation?

No -- the needle shows you how far apart the indicator is from the magnetometer.

Should it be set on '0' at all times?

You'd expect to see it near zero during unaccelerated level flight, and at a maximum deflection when turning through north or south.

Do we use it to correct the HSI like we correct precession on a DG?

If it's working properly, it takes care of precession all by itself.

Is there ever a time (provided that the remote compass/flux gate is functioning) when you would set the HSI to 'Free Gyro' mode?

Not when the magnetometer is working properly. Free Gyro is for when the magnetometer is malfunctioning and driving the gyro-stabilized indicator off heading instead of back to heading.

 

[flyingron]

It's rare (unless your flux gate is busted) that you ever touch the HSI controls. I've had a KCS55 (which is what those pictures on the exam are from I'm pretty sure) for over ten years now and I've only "unslaved" the thing to see if the controls worked in most normal operation.

There are a few times when you can get airborne very quickly after powering things up (typically when I land and shutdown for a fuel stop, and then fire up and head right to the runway), when you can get the thing reading goofy because the slaving and the gyro haven't gotten their act together. Usually, I just ignore the HSI for a bit until it catches up. You could use the controls to force it into alignment early.

This is one of the reasons why it's ill-advised to launch quickly into IMC. There was a bizjet crash a bunch of years back attributed to having to rely on the AI before it was stable.

There are some places (especially in the great white north) where the mag compass (be it the whisky one or the electronic equivalent) doesn't function well. In these cases, being able to switch off the slaving is essential.

 

[Henning]

You are really far N/S before you start getting those problems, like above 70° latitude. You will start seeing increased precession errors before you start seeing magnetic issues. If your gyro isn't latitude compensated by the time you have to uncouple it for magnetic issues, you are going to run into problems regardless.

For most GA pilots in the US, neither of these issues will ever come into play.

 

[flyingron]

I agree (other than the fact that Earth Rate errors while they result in drift of the gyro indication are NOT precession).

 

[Henning]

I agree (other than the fact that Earth Rate errors while they result in drift of the gyro indication are NOT precession).

Yeah, whatever they call the error, my old shipboard gyros with a latitude knob I set daily on N-S routes. Now it's all automatic/digital with solid state and GPS Gyro compensated. GPS Gyro has become an interesting low cost system, but my reviews on it in operation are mixed.

 

[labbadabba]

Cool, so I think I understand. Basically unless you're departing from a N/S heading or flying in extreme latitudes there should be very little deflection on the Remote Compass Indicator, yes?

In the event, let's say that the Remote Compass Indicator shows a deflection of -3 in straight and level flight, would I adjust the HSI to compensate?

 

[Henning]

Cool, so I think I understand. Basically unless you're departing from a N/S heading or flying in extreme latitudes there should be very little deflection on the Remote Compass Indicator, yes?

In the event, let's say that the Remote Compass Indicator shows a deflection of -3 in straight and level flight, would I adjust the HSI to compensate?

The slaved HSIs I'm used to had a little +/- toggle switch to make corrections, do you not have one? If not, just make corrections as the manual for what you have indicates.

 

[labbadabba]

The HSI I fly with is a retro-fit on a C175 so no flux gate just a free spinning gyro.

I'm asking simply because it came up in my reading and I didn't quite grasp how the whole thing works together from reading my Instrument Flying Handbook.

 

[Henning]

The HSI I fly with is a retro-fit on a C175 so no flux gate just a free spinning gyro.

I'm asking simply because it came up in my reading and I didn't quite grasp how the whole thing works together from reading my Instrument Flying Handbook.

Yeah, so you have an unslaved unit like I put in my Travelair, they just set same as you set a DG. The slaved systems typically have a couple ways to correct and need a way to disconnect to resolve problems.

 

[airdale]

... It's the guts of how it works and how it slaves to the flux gate makes my brain mushy. ...

Search on "king kcs55a HSI manual" and you'll find lots of stuff to read.

 

[petrolero]

My DPE asked me during my IR verbal exam to explain his diagram of an HSI setup.

The diagram he drew showed a box for the flux gate, a line to the PFM box, to the HSI indicator.

He asked me what the PFM box was.

I said "pure ****ing magic?"

ding ding ding ding.

Then he laughed and we discussed how they really work, which amounts to PFM! If mine ever breaks it is getting replaced with a something digital!