Whiskey compass vs. remote sensing

[B2N2]

Question:

Flew a fancy plane today with HSI.
Halfway through the flight noticed HSI in slaved mode did not match the good ol' whiskey compass.
Guess discrepancy between remote sensing compass in the wingtip and the wobbly thing on the glareshield.
If this occurs which one do you rely on?
Do you "free" the HSI and set it to your whiskey compass or do you leave it in slaved mode and use the remote one?
Which one is more likely to be correct?

DAZED AND CONFUSED
:O :O

[QAVION]

Did your high-tech aircraft only have one HSI?/Magnetic Heading Reference System???? :confused:

If your ADI differed from your Standby Attitude Indicator and there were no warning flags, I'd look for a third attitude reference for comparison. Similarly, I'd do the same with heading instruments.

Having said that, on the ground, under the influence of magnetic interference from terminal buildings, blast fences, etc, it is common to see large discrepancies between two remote compass systems (enough to generate comparator warning flags). In the air, however, there is much less chance of interference, so I'd be comparing the two HSI's with the Standby Compass. If you only had one Magnetic Heading Refence System, then the call might be a little harder...

Despite it's simplicity, the Whiskey Compass is not without it's faults. It is susceptible to magnetic fields generated by aircraft electrical systems. During maintenance compass swings, we carry out tests with most aircraft electrical systems switched on... and again with most electrical systems switched off. I've seen variations between the two of up to 5 degrees. The flux gates of remote compasses, on the other hand, can be found at the wingtips, further away from many of the disturbing influences (HF antennae, nav/strobe light power supplies, etc, aside).

Similarly, a slaved compass is not without its faults. It is an electronic device and, as such, is susceptible to failure. However, it should have fault detection circuits and will tell you if it has failed. As a MHR System may be integrated with your normal attitude gyros (for short term stabilization), I'd be checking to see that your respective attitude instruments were operating normally.

In your comparison between the two magnetic sensing devices, I'd be looking for any signs of stiction (stickyness) in the instruments. If possible, make a smooth turn in one direction, then return to your normal track, looking for any signs of jerkiness. The HSI compass card may not move smoothly, indicating a faulty mechanism within the instrument. Similarly, the ball in your whiskey compass may be binding (check that there are no bubbles in the liquid interfering with the movement). Not very scientific, but you could try, very gently, tapping the case, to see if that frees up the stickiness, allowing the compass to settle on the correct heading.

Without looking at the larger picture/knowing all the details it would be difficult to make a call either way, but hopefully this is of some help.

Regards.
Q.

[redsnail]

Did your aircraft have windshield heat on? Believe me, that can cause some interesting deviations to the magnetic compass.

[Tinstaafl]

What electrics were on?

[B2N2]

Single engine composite airplane, very little metal except a monster engine.
It's got one HSI and all the equipment was on except landing light.
Apart from checking on the ground on a compass rose what can you do in the air to figure out which one is correct?:O

[Keygrip]

Intercept the center line of an airway - fly fourty miles along it - twenty on the compass, twenty on the HSI - ask ATC which bit kept you in the middle.

Flew an Arrow recently - fresh air fan was labelled "Do not use in flight" - as it causes 10° error on magnetic compass (and it did.

Great idea - not to have a fan in a Florida based aircraft.

[B2N2]

Difference was 15 deg., so fairly significant .
The owner wasn't very worried about it since he flies GPS direct everywhere everytime.
There's a solution to everything it seems...:p :p :p

[Tinstaafl]

Try switching the electrics off. If you can see a swing on one, and not on the other then I'd suspect the moving one. Track a radial or ideally a LOC beam. With some idea of the wind (HW or TW ideally) check which compass reading compares favourably.

If in good VMC measure the bearing between a pinpoint you're over and a visible point as far away as possible. Point the a/c to the vis. point and read both compasses. Which one is more accurate?

Also make sure you're checking & comparing using the deviation card and not the raw compass heading.

[ATPMBA]

It looks like you are a US pilot. Don't take this wrong but US ATP's don't have any theory knowledge. The ATP written is 100 mutilple choice questions.

The JAA ATPL is about 14 exams that must be completed in an 18 month period. I'm working on it for a JAA license conversion. I am learning things I never knew and a lot of old material now makes more sense. I personnaly think the course is harder than a Masters of Business Administration which I recently completed.

A possible answer to your question is remote compasses that use flux valves, such as your HSI can have an error introduced when flying at high speed and making a slow turn while heading in a northerly direction.

:cool:

[ft]

ATPMBA,
what would be the mechanism behind that error in the flux gate remote?

Cheers,
Fred

[ATPMBA]

To answer FT’s question, several errors with fluxvalves:


Fluxvalve tilt error – turning error caused by slow prolong turns at high speeds.


Northern Instability – heading oscillation that can be induced when flying on a northerly heading close to magnetic north, banking can induce an error from the tilt to the fluxvalve. Errors as much as +- 6 degrees.

[B2N2]

OK I was nowhere near magnetic north and my guess is 180 kts does not qualify as "high speed".
So please take it easy on the ATPL stuff, remember I'm only a US pilot with no theoretical knowledge.:} :} :}
Now if the remote sensing compass is off how do you correct it?
:p

[Tinstaafl]

Visit whoever does your maintenance.

[STC]

The owner wasn't very worried about it since he flies GPS direct everywhere everytime.

And what heading does he steer when the GPS gives him a new DTK? GPS can't resolve heading.

Your error may have been induced by a turn. HSIs in smaller aircraft are almost always connected to a magentic stabilized gyro. The gyro gives your heading reading, "short term" stability. The remote flux sensor gives your heading reading "long term" correction. Following a turn, it is not uncommon for errors to be induced into directional gyros. It sometimes takes a few minutes for the flux detector to correct the error. Typically, corrections are about 1 degree per minute.

If I saw an difference between the HSI and standby compass, I would immediatly look at my slaving meter and check if the slaving circuits have completed their correction. If they have, the next thing to do is see if the aircraft is configured properly in accordance with POH/AFM or compass placards. Sometimes there is a placard that limits operation of the standby compass with certain equipment running. Windscreen heat is one that has been mentioned, but some less obvious ones may be applicable such as one or two radios in the stack.

After all this, check the correction cards for the HSI and standby compass. It's not impossible to have 10 or 15 degree differences between the two. For instance, on one particular heading, the HSI correction may be +7 whereas the standby compass correction may be -7.

[Burger Thing]

It looks like you are a US pilot. Don't take this wrong but US ATP's don't have any theory knowledge. The ATP written is 100 mutilple choice questions.

Looks like the USA is quite a dangerous place to fly as a Passenger. Imagine to sit in an aircraft and your flight crew does know only shxt...

But then, looking at the statistics someone could draw the conclusion that the system in USA isn't that bad after all?

Yes, there are only 80 multiple choice question, but that doens't mean you don't have to study your material. Especially if you go into your 2 1/2 hours oral... Does JAA have an oral exam?

But then, I know probably also only shxt, because I don't have a JAR ATPL. On the aircraft I fly (737), I often wondered, if we could take the little standby compass out of its housing, and replace it with a tiny fish. So, in times of high workload, you would just pull down the standby 'fishpass' and have a relaxing look at your little companion. :}

[Blacksheep]

The apparent inability of some 'pilots' posting here to determine which instrument is correct, when there's a split as large as 15 degrees is a bit alarming. I'll be charitable and assume you're all non instrument rated and never fly airways.

Generally speaking this avionics engineer would put more faith in the HSI than in the Standby Compass. While STC is correct to say that you might have a +ve error on one and a -ve error on the other, there should never be enough to give you a 15 degree difference unless you're reading the standby with something that influences it switched on. Redsnail points the way...

To work out what you should switch off before reading the standby compass, try reading the AFM, there's often a clue or two in there; not just about compasses either... :rolleyes:

**************************
Through difficulties to the cinema

[error_401]

Happended to me on a Seneca III.

If it is a King HSI with flux valve and the HSI heading reference comes from remote gyros there is a deviation indicatior together with the "SLAVE" "FREE" Switch. Look at this.

I once was than 40° off correct HDG. For some weird reason that slaving mechanism was unable to slave the HSI. No warning Flag no nothing. Just noticed the deviation to the standby compass and that flag indicating a deviation on the slave.

Possible reasons:

Low voltage, low battery, low alternator output.

A quick swing during line-up. One of these that happen on good days when everything works fine and you get an immediate departure - up to the moment when ATC calls you "Confirm HDG 270"...