IFR No Gyro/HSI question

Center_Mid

Well-Known Member
This week in IR training I covered no gyro procedures with my instructor. I practiced in the sim under the assumption that the vacuum pump failure would take out the AI and DG. However, the 172SP that I fly in real life has an electrically powered HSI. I asked my instructor whether a vacuum pump failure would in any way affect HSI performance and he gave a confusing answer, suggesting that I would have to un-slave the HSI and fly by the compass. This did not make sense to me.

I actually like compass flying because it feels like a link to the past, but in real life would a vacuum pump failure affect the HSI at all?
 

Derg

Cap, Roci
Staff member
It shouldn't.

If you have an electrically-driven gyro in the HSI, it shouldn't affect it whatsoever.

Unless it's some weird set-up where your electric HSI is "slaved" off of a vacuum-powered DG, it shouldn't matter.
 

E_Dawg

Moderator
Look in the POH to find out how the HSI works... that should answer your question. Then show your instructor.
 

Center_Mid

Well-Known Member
[ QUOTE ]
Look in the POH to find out how the HSI works... that should answer your question. Then show your instructor.

[/ QUOTE ]

But I'm too lazy...I ask these questions on JC so that I don't have to check the POH!
 

sbav8r

New Member
then keep a cell phone and lap top in the plane at all times and hope that when it happens in IMC there are plenty of knowegeable JC's online that can respond to your emergency in a timely manner.
 

Derg

Cap, Roci
Staff member
Aww, I got off my butt and called the techincal program manager to get an MD-88 answer!

Crack that POM, baby!
 

Center_Mid

Well-Known Member
[ QUOTE ]
But I'm too lazy...I ask these questions on JC so that I don't have to check the POH!


[/ QUOTE ]

Joking here. I've flipped thru my POH so often that I plan on laminating the pages soon to keep the ink from fading. I don't have it with me in the office, so I thought I'd run the HSI question by you folks.
 

EatSleepFly

Well-Known Member
As Doug said, a vacuum failure will not affect an electric HSI.

The only times I can think of that you would un-slave an HSI would be:

a.) to re-adjust it to the proper heading (sometimes they get a little off),

b.) if there was a problem with the slaving system. On the [only] HSI that I am familiar with (KI 525A), if the slaving system fails, you can un-slave it, re-adjust it to the proper heading, and continue to use it as a regular heading indicator...it will just be subject to the same errors as your typical DG.

Of course, with an electrically powered HSI, if you have an electrical failure, you'll be using the trusty 'ol mag. compass.
 

pscraig

Well-Known Member
The POH does not always go into depth regarding the flight instruments. We had this same debate about some slaved HSI-equipped Seminoles, that were clearly marked "D.C." on the instrument case. One of our DE's insisted it had a vacuum driven gyro, and the POH didn't mention a thing about an HSI other that it is "optional equipment." We had to dig and find the King Instruments manual for it, and it was indeed a totally electric instrument. Of course, just pulling the breaker marked "HSI" would have been simpler...
 

Derg

Cap, Roci
Staff member
The POH is good only for things like the basic airframe and powerplant, but when you start installing optional or third-party avionics, you really need to research the specific unit.

Lets put it this way. There is pretty much one Cessna 152 POH that hasn't been revised since the 1960's. I do a revision on my MD-88/90 PRM (Pilots Reference Manual -- airline equivalent of a POH) about every 60 days.
 

n2o2diver

New Member
Slaved and Free

Slaved means that the Electric Compass Gyro is being corrected by the Flux Valve and is being compensated for drift/precession, the flux valve compensator is what the techs adjust when doing the annual compass swing.

Free its just out there pointing to north and if you sit there and watch it you will see the compass card move a few degrees every 5-15 mins, at least thats the way they worked on the A/C I used to work on

Also when it is in free you can usually use the knob to manually set the heading. Then try switching it back to slaved. Other wise as the system warms up it should automatically line itself up and generally agree with the wet compass.
 

E_Dawg

Moderator
[ QUOTE ]
The POH is good only for things like the basic airframe and powerplant, but when you start installing optional or third-party avionics, you really need to research the specific unit.

[/ QUOTE ]

Agreed, but the 172S POH is actually quite good. It describes pretty much any system you'd find in the airplane, unless of course you put some wierd third party aftermarket thing in it:

"ELECTRICAL POWER MUST BE SUPPLIED TO THIS INSTRUMENT FOR PROPER FUNCTIONING. ABSENCE OF WHICH WILL RESULT IN UNRELIABLE HEADING INFORMATION."

Yes, it's in all caps in the POH too
.

Section 9 - Supplements
Supplement 13 - FAA Approved
S13-7

If you want to look it up yourself (heavin forbid
), it's got lots more good info.
 

sixpack

New Member
Does anybody know how the electricity turns the gyro?
If there's a motor on the spindle, the wires would tug on it and cause precession.
Does the electric motor drive a blower, which blows on the spindle? Or is the motor mounted on the gyro?
 

EatSleepFly

Well-Known Member
Straight electrically operated gyros usually have a motor inside the rotor. Some instruments (probably electrically driven attitude indicators mostly) have both a motor inside the rotor, and the little "buckets" for the air from the vac lines to turn it. That way if one system fails, the gyro still spins.
 
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