instrument student (mag dip errors)?

eric_ocean

eric_ocean

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osprey1
i have just started my instrument training, and really could use some help with the mag dip errors....i understand the ANDS accelerate north deccelerate south and OSUN overshoot south undershoot north...its just the northerly and southly turning errors, that i could use some help on....rt turn north subtract half the bank subtract the lat.
rt turn south add the lat subtract the bank....etc
is this something i just have to memorize?...any help sure would be appreciated....thanks eric
 
I've never bothered worring about half the bank angle -- the mag compass is so sloppy and the methodology is so gross that it falls in the noise level on any turn you want to be making on nothing but the TC/needle. I just use the "South leads, North lags" (effectively the same as OSUN), add/subtract the latitude from the desired heading to get target heading, and roll out when the mag compass hits the target heading. Further, due to the grossness of this whole system, I use 40 degrees for latitude anywhere in the Continental USA.

Remember, though, that the error is equal to latitude only at N/S -- it's zero at E/W, so you have to alter the correction factor accordingly (i.e., 20 degrees at NE/SE/SW/NW). And again, since the methodology is pretty gross, I don't bother trying to fine tune much beyond that, using 20 degrees for anything between 030 and 060, 40 degrees north of that, and zero east of that. As soon as I roll out, I check the MC, and correct any remaining error with a timed turn. Trust me -- it's good enough for the real world, as well as an IR practical test.
 
You're not going to be asked to perform a compass turn on your checkride, according to the PTS, so why sweat it? Learn to do timed turns, which are on the checkride, and forget about compass turns. Ask your instructor why s/he is requiring them.

Bob Gardner
 
And in the real world take advantage of all sources of information available to you, e.g. a GPS if you fly with one, either IFR or VFR. If you've got a vacuum failure for real, ANYTHING is legal to use to help you out, because that's an emergency situation.
 
From www.pilotsweb.com:
[SIZE=-1]The center of gravity of the float assembly is located lower than the pivotal point. As the airplane turns, the force that results from the magnetic dip causes the float assembly to swing in the same direction that the float turns.
nor_turn.jpg
The result is a false northerly turn indication. Because of this lead of the compass card, or float assembly, a northerly turn should be stopped prior to arrival at the desired heading. [/SIZE]
 
eric_ocean said:
its just the northerly and southly turning errors, that i could use some help on....rt turn north subtract half the bank subtract the lat.
rt turn south add the lat subtract the bank....etc
is this something i just have to memorize?...any help sure would be appreciated....thanks eric
Click to expand...

yikes now you've got my mind spinning, overshooting, undershooting, and dipping. seriously ive never heard of that, or if i did i immediately recognized it as worthless and forgot it. thankfully its not the 30's anymore so you dont need to be able to do turns solely on the compass.
 
bobmrg said:
You're not going to be asked to perform a compass turn on your checkride, according to the PTS, so why sweat it? Learn to do timed turns, which are on the checkride, and forget about compass turns. Ask your instructor why s/he is requiring them.
Click to expand...
There is no longer a specific requirement for compass turns, but there is a requirement to perform a partial panel (which in most cases means AI/HI covered and using the MC for heading information) nonprecision approach. During turns of more than about 90 degrees (especially if you have to make a 210 degree turn on a teardrop or parallel entry for a holding pattern in lieu of procedure turn), a compass turn rather than timed turn may be a more accurate way to get there. In addition, Area II Task B1g requires that the applicant "Exhibits adequate knowledge of the elements related to" the MC "and [its] operating characteristics," which would include all that northerly turning error, dip, ANDS and OSUN stuff (and I routinely hear that asked during the oral portion). Finally, Area VIII Task A "Checking Instruments and Equipment" requires the applicant to determine during the flight whether any instruments (including the MC) are malfunctioning -- and if you don't understand the dip/northerly turning errors, you can't make that determination for the MC.

If nothing else, an understanding of those errors is important for understanding why you should generally ignore the MC when you're turning, accelerating, or not level.
 
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I was most definitely tested on making turns with the mag compass on my Instrument practical. In/not in the PTS? I dunno, but my instructor taught it to me, and I was glad he did.

'Course, I was hell on wheels at NDB apps, too, and may never get to use that skill again.
 
you probably did timed turns spike? or did you do turns solely with the compass. if the latter you are much better than me.

come to Iowa, we will fly across the state using nothing but NDBs. It'll be fun!
 
tonycondon said:
you probably did timed turns spike? or did you do turns solely with the compass. if the latter you are much better than me.

come to Iowa, we will fly across the state using nothing but NDBs. It'll be fun!
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Truth to tell, I do not recall, guess I probably did timed turns...

...from time to time, I encounter airplanes with RMIs, which I think are really cool instruments. Maybe I came along after my time?
 
perhaps, me too i guess, I love RMIs, no math!
 
tonycondon said:
thankfully its not the 30's anymore so you dont need to be able to do turns solely on the compass.
Click to expand...
True, but do you know how many "disorientation in the clouds" accidents and events include "compass failure"? I've read several ATC transcriptions, and know at least one survivor who claimed compass failure in the clouds.

In all likelihood, it was the normal reaction of the compass to the gyrations they were making with the airplane, but there was definitely a lack of knowledge about how the instrument worked that added to the stress and confusion of the situation.

The knowlege probably wouldn't have changed the outcome, but then again, it might have. Who knows.

The point being that maximum knowledge of ALL systems on an airplane improves the safety of flight dramatically. If nothing else, it allows you to identify a failure or malfunction before it combines with something else to give you REAL problems.

Oh...and I got to do turns solely by reference to the compass as recently as the 90's. And if I ever get to visit him, I've got a bet with a buddy of mine that I can still do it. :)

(I know...a little thread drift. We now return to our regularly scheduled broadcast. ;))

Fly safe!

David
 
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Ron Levy said:
Nothing but compass? No turn needle/coordinator? I know for sure I wouldn't want to bet my life on my ability to do that.
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I wouldn't want to bet my life on it either, but my CFII did have me down to nothing but mag compass and airspeed indicator in actual. :hairraise:
 
flyingcheesehead said:
I wouldn't want to bet my life on it either, but my CFII did have me down to nothing but mag compass and airspeed indicator in actual. :hairraise:
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and with a hand hovering over the isntrument cover(s) undoubtedly :)
 
flyingcheesehead said:
I wouldn't want to bet my life on it either, but my CFII did have me down to nothing but mag compass and airspeed indicator in actual. :hairraise:
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In actual instrument conditions?:hairraise::eek::hairraise::eek: I'm pretty good at partial panel from the right seat, and am happy to cover the AI and HI in actual with an instrument trainee flying, but to take it down to "nothing but mag compass and airspeed indicator" is a bit beyond even my fairly high risk tolerance level.
 
Ron Levy said:
Nothing but compass? No turn needle/coordinator? I know for sure I wouldn't want to bet my life on my ability to do that.
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To be honest, Ron, it was in "legal" VFR conditions, but it was done in your neck of the woods in the summertime in an airplane that didn't have enough forward/downward visibility in the climb to see the ground anywhere but behind me once I got above a couple thousand feet. (I could still count 3 miles vis, though)

In a constant airspeed condition, in relatively smooth air, any movement of the compass is due to a turn...if you know that error set well enough, it is possible.

Not, of course, that I'd bet my life on it either... :no:

Fly safe!

David
 
tonycondon said:
and with a hand hovering over the isntrument cover(s) undoubtedly :)
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Mostly the one for the altimeter, which he checked every 10 seconds or so. :yes:

Ron Levy said:
In actual instrument conditions?:hairraise::eek::hairraise::eek: I'm pretty good at partial panel from the right seat, and am happy to cover the AI and HI in actual with an instrument trainee flying, but to take it down to "nothing but mag compass and airspeed indicator" is a bit beyond even my fairly high risk tolerance level.
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It's not like he went at it all at once - It was maybe a 3-hour cross-country flight. After takeoff, it was byebye AI. Cruise, seeyalater DG. Sit on regular partial panel for a while, then no more turn coordinator.

Now, I still had all of the pitch instruments and just had to get used to using the compass to keep us pointed in a straight line. It was November and the air was very smooth, so it was relatively easy compared to attempting the same thing in any sort of turbulence or chop.

Do that for a while, then I got back the TC and it was goodbye altimeter.

That was a "whattheheckdoIdonow" moment. Well, keep an eye on the VSI. If it shows +100 for 5 seconds, pitch down to -100 for 5 seconds and then back to 0 and you should be at the beginning altitude. Once I got the hang of that (and well within ±100), there goes the VSI and I had to control pitch using the same averaging technique only using airspeed. Again, in a short time I was able to keep it well within ±100 ("get the hang of" in both cases here meant I was within ±20 for the most part, and never outside ±40).

Finally, the TC went away again, and I was down to ASI/MC only. Keep in mind that this sequence took at least an hour and a half. After a couple of minutes with ASI/MC, it was back to full panel - "This ought to look like day VFR to you now." And it did. I'd never really seen the little nuances of the individual instruments before I had to rely on them each on their own. I was especially lazy with the airspeed indicator - I'm not gonna stall so who cares, right? - but seeing the 1 or 2 knot excursions not only gave me the ability to control altitude to an extremely tight tolerance, it also proved quite valuable later on in the flight when I noticed that I'd lost a knot or two but was still maintaining the same altitude. I had carb ice and I'd have never noticed without this lesson.

Honestly, it was the most valuable instrument lesson I've ever had. :yes: CFII was watching like a hawk and never let me get out of PTS tolerances (especially since it was HIS name on the flight plan :D) so IMHO there was an adequate level of safety.
 
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