# HSI (little more on-tpoic)

Another question I pondered today:

What exactly does the little line in the circular hole in the upper right hand corner of the Century HSI mean?

It has something to do with slaving, but what specifically does it indicate? The Century manual notes that it’s called a “Slaving Meter” but doesn’t really say what it shows.

I noticed my HSI “precessed” about 10 degrees during a flight today and I looked at that line, wondering if it could tell me anything about why it precessed.

Thanks as always, everybody…

Steve

Another question I pondered today:

What exactly does the little line in the circular hole in the upper right hand corner of the Century HSI mean?

It’s a measure of how much the slaving wants to crank the DG around. When it’s at a 45 degree angle, it’s synced up with the slaving system. You can see this by manually adjusting the DG–it will “null out” at the right heading. My HSI tends to require manual adjustment when I first start the plane and it is somewhat cold.

There is mention of it somewhere in the HSI manual.

You’re right, Dave…
More specifically, I read somewhere that if that needle stops moving altogether, it’s time to start worrying.

• Mike.

What exactly does the little line in the circular hole in the upper right hand corner of the Century HSI mean?

It’s a measure of how much the slaving wants to crank the DG around. When it’s at a 45 degree angle, it’s synced up with the slaving system. You can see this by manually adjusting the DG–it will “null out” at the right heading. My HSI tends to require manual adjustment when I first start the plane and it is somewhat cold.

There is mention of it somewhere in the HSI manual.

Mike & Dave,
Can either of you point to any documentation (on the web or hard copy) that describes this?
Unfortunately, you’ve sort of contradicted each other. (Dave says if it’s pointed at a 45 degree angle [presumably stationary at this position], all is good. Mike says if it’s stationary, watch out!)
The only “mention” I see of this in the Century manual is a little note pointing out that this line is the slaving meter, but doesn’t describe what it means. But I notice all the pictures of the HSIs in the manual have the line at a 45 degree angle.
Thanks again,
Steve

You’re right, Dave…

More specifically, I read somewhere that if that needle stops moving altogether, it’s time to start worrying.

• Mike.

What exactly does the little line in the circular hole in the upper right hand corner of the Century HSI mean?

It’s a measure of how much the slaving wants to crank the DG around. When it’s at a 45 degree angle, it’s synced up with the slaving system. You can see this by manually adjusting the DG–it will “null out” at the right heading. My HSI tends to require manual adjustment when I first start the plane and it is somewhat cold.

There is mention of it somewhere in the HSI manual.

Steve,
In my manual (Century NSD360A, 10-01-99, 68S85), the following appears in a paragraph on page 4:
… In slaved versions, the slaving meter should oscillate about a 45 degree point to show that the slaving circuits are accomplishing their function. Should the needle remain motionless, vertical, or horizontal for an extended period (two minutes) in level flight, the heading should be manually set so that the slaving meter is at the 45 degree point and then observe the performance of the heading card. If slaving difficulties are encountered, set the slaving mode switch to SL#2 or free gyro. In free gyro mode, the instrument must be periodically reset to manually counteract the effects of gyro precession.

The grammar may not be the best (they have a tendency to often split infinitives!), but hopefully this resolves the apparent contradiction between what Dave and I said.

• Mike.

Mike & Dave,

Can either of you point to any documentation (on the web or hard copy) that describes this?

Unfortunately, you’ve sort of contradicted each other. (Dave says if it’s pointed at a 45 degree angle [presumably stationary at this position], all is good. Mike says if it’s stationary, watch out!)

The only “mention” I see of this in the Century manual is a little note pointing out that this line is the slaving meter, but doesn’t describe what it means. But I notice all the pictures of the HSIs in the manual have the line at a 45 degree angle.

Thanks again,

Steve

Mike, Dave,
Thanks to both of you! I see it now. I had skipped by it… the paragraph starts with
The NSD360A has incorporated a heading warning flag to warn of loss of either air or electric power.
so I had just skipped the rest of the paragraph thinking it had to do with the warning flags.
That’ll teach me!
Thanks again,
Steve

Steve,

In my manual (Century NSD360A, 10-01-99, 68S85), the following appears in a paragraph on page 4:

… In slaved versions, the slaving meter should oscillate about a 45 degree point to show that the slaving circuits are accomplishing their function. Should the needle remain motionless, vertical, or horizontal for an extended period (two minutes) in level flight, the heading should be manually set so that the slaving meter is at the 45 degree point and then observe the performance of the heading card. If slaving difficulties are encountered, set the slaving mode switch to SL#2 or free gyro. In free gyro mode, the instrument must be periodically reset to manually counteract the effects of gyro precession.