Whisky Compass on a Jet

J Cole

J Cole

Well-Known Member
I just finished up IOE in the ERJ-145. On my last leg, I was looking at the whisky compass and noticed it was off a little. The HSI showed a 180 heading, and the compass showed a 215 heading. The compass correction card was for the radios and avionics on, but the windshield heat off. It also said for 180 heading, steer 183. Switching the windshield heat off caused the compass to swing back to a 193 heading.

I'm not going to include a tail number, but this was one of the older beasts in the fleet - some 40,000 hours or so on the airframe.

Has anyone else seen compasses this far off on a jet? I would hate to have to rely on one in an emergency. I'm going to start paying attention to the compass to see if they are all this way or if this was just a weird anomaly.
 
jtrain609 said:
If you get to the point of having to rely on the whiskey compass in that aircraft, you've likely got much more important things to worry about, like how you're going to shoot an approach with no other avionics working. Heck, at that point I'd just be happy that the engines are still running.
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It is also the only required instrument showing heading in most aircraft under VFR (most that I fly are only so equipped). You would be surprised how many instrument rated pilots are not capable of holding heading using a compass.
 
drunkenbeagle said:
It is also the only required instrument showing heading in most aircraft under VFR (most that I fly are only so equipped). You would be surprised how many instrument rated pilots are not capable of holding heading using a compass.
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You could probably count me into that group. I think I might know where it is located in my aircraft, but I wouldn't bet money on knowing which side of the canopy bow it's on :)

Seriously though, last time I tried to fly off one was doing a simulated no gyro approach in a T-45. I thought I was being pretty cool and resourceful by using those old mag compass rules of thumb that you learn in Cessnas, but that was all shattered when my crusty instructor in the back said something like "hey Ensign, what the hell are you doing? Just ask for no gyro vectors idiot" (or something to that effect). In my defense, I was holding the headings that ATC was assigning pretty well, but the point was made.....you don't really do that stuff in jets, at least if there is any other way.
 
///AMG said:
but that was all shattered when my crusty instructor in the back said something like "hey Ensign, what the hell are you doing? Just ask for no gyro vectors idiot" (or something to that effect). In my defense, I was holding the headings that ATC was assigning pretty well, but the point was made.....you don't really do that stuff in jets, at least if there is any other way.
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Hey, I love a gyro heading indicator as much as anyone. You certainly learn to appreciate it when it isn't there :)
 
drunkenbeagle said:
Hey, I love a gyro heading indicator as much as anyone. You certainly learn to appreciate it when it isn't there :)
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Agreed........certainly no gyro vectors are not an option in all places, or at least not to the point where you could use them as an approach.
 
J Cole said:
I just finished up IOE in the ERJ-145. On my last leg, I was looking at the whisky compass and noticed it was off a little. The HSI showed a 180 heading, and the compass showed a 215 heading. The compass correction card was for the radios and avionics on, but the windshield heat off. It also said for 180 heading, steer 183. Switching the windshield heat off caused the compass to swing back to a 193 heading.

I'm not going to include a tail number, but this was one of the older beasts in the fleet - some 40,000 hours or so on the airframe.

Has anyone else seen compasses this far off on a jet? I would hate to have to rely on one in an emergency. I'm going to start paying attention to the compass to see if they are all this way or if this was just a weird anomaly.
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There is a reason the compass correction card is not good with the windshield heat on. A whisky compass is unusable with electric windshield heat on. Think about it you have a bunch of electricity running around right next to the compass. From what I have noticed, if the heat is on and the temp controllers for the windshield are calling for heat, the compass will change to a certain heading and stay there no matter what heading the airplane is on. If you watch the compass as the temp controllers turn on and off the compass will change allot of degrees.
 
I would need a complete electrical and pneumatic failure to go to the compass. At that point we can't shoot an approach anyway so we're flying vfr now. I'll just look outside.
 
Corporate Pilot said:
There is a reason the compass correction card is not good with the windshield heat on. A whisky compass is unusable with electric windshield heat on. Think about it you have a bunch of electricity running around right next to the compass. From what I have noticed, if the heat is on and the temp controllers for the windshield are calling for heat, the compass will change to a certain heading and stay there no matter what heading the airplane is on. If you watch the compass as the temp controllers turn on and off the compass will change allot of degrees.
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Yes, I agree. It absolutely makes sense that the windshield heat would create two big magnetic fields either side of the compass (esp since the windshield heat uses inverters to create AC current). I was just surprised that in normal operations that the compass would be so inaccurate. However, my main point was that even with the windshield heat off, the correction card was still off by 10 degrees. I have lots of time in G1000 aircraft, and the compass was always fairly accurate in flight.

To be fair, not many failures would actually require use of the compass in this airplane. The two that come to mind are: a dual AHRS failure and an electrical emergency which led to a complete loss of battery power. In the first case, you would likely be screaming for radar vectors, and in the second, you'd better be VMC because you have no attitude information whatsoever.
 
J Cole said:
Yes, I agree. It absolutely makes sense that the windshield heat would create two big magnetic fields either side of the compass (esp since the windshield heat uses inverters to create AC current). I was just surprised that in normal operations that the compass would be so inaccurate. However, my main point was that even with the windshield heat off, the correction card was still off by 10 degrees. I have lots of time in G1000 aircraft, and the compass was always fairly accurate in flight.

To be fair, not many failures would actually require use of the compass in this airplane. The two that come to mind are: a dual AHRS failure and an electrical emergency which led to a complete loss of battery power. In the first case, you would likely be screaming for radar vectors, and in the second, you'd better be VMC because you have no attitude information whatsoever.
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Sorry I misunderstood but you got me wondering about it. This is from our Citation 560 maintenance manual, "Standard to strive for in compensating standby compass is to achieve best possible swing with all power OFF and to have all readings not exceed +30, -30 degrees when normal power is applied to airplane.".

Now I am going to have to look at the correction card in our airplane and see how big the correction is.
 
J Cole said:
Yes, I agree. It absolutely makes sense that the windshield heat would create two big magnetic fields either side of the compass (esp since the windshield heat uses inverters to create AC current). I was just surprised that in normal operations that the compass would be so inaccurate. However, my main point was that even with the windshield heat off, the correction card was still off by 10 degrees. I have lots of time in G1000 aircraft, and the compass was always fairly accurate in flight.

To be fair, not many failures would actually require use of the compass in this airplane. The two that come to mind are: a dual AHRS failure and an electrical emergency which led to a complete loss of battery power. In the first case, you would likely be screaming for radar vectors, and in the second, you'd better be VMC because you have no attitude information whatsoever.
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I would hold wings level with reference to the third attitude indicator (ISIS...is that AHRS driven?) while trying to recover an AHRS myself.


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Autothrust Blue said:
I would hold wings level with reference to the third attitude indicator (ISIS...is that AHRS driven?) while trying to recover an AHRS myself.
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The ISIS is independent of the AHRS and will run approx 45 min after an electrical emergency. It even has its own heading, so you wouldn't use the compass if you had one. Some of our older aircraft (including the one mentioned in this post) lack the ISIS system and have only mechanical standbys.
 
J Cole said:
The ISIS is independent of the AHRS and will run approx 45 min after an electrical emergency. It even has its own heading, so you wouldn't use the compass if you had one. Some of our older aircraft (including the one mentioned in this post) lack the ISIS system and have only mechanical standbys.
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ISIS is cool stuff and I wish I had one instead of the third gyro in the Bro.

If we get down to where we don't have any attitude-heading information we typically won't have any NAV radios either, so it's either (1) find the big VFR or (2) get some electrics back.
 
Autothrust Blue said:
ISIS is cool stuff and I wish I had one instead of the third gyro in the Bro.

If we get down to where we don't have any attitude-heading information we typically won't have any NAV radios either, so it's either (1) find the big VFR or (2) get some electrics back.
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Do your autopilot systems run off the AHRS or do they have their own source of attitude information?
 
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