Magnetic compass lag

[jasc15]

I hate mnemonics, and my instructor gave me one by which to remember magnetic compass behavior: UNOS (undershoot north, overshoot south). But it's still not enough to help me. Does it mean that I undershoot when turning to a north-bound heading, or from a northbound heading (and vice versa)? How do you folks remember this?

 

[dmccormack]

I hate mnemonics, and my instructor gave me one by which to remember magnetic compass behavior: UNOS (undershoot north, overshoot south). But it's still not enough to help me. Does it mean that I undershoot when turning to a north-bound heading, or from a northbound heading (and vice versa)? How do you folks remember this?

UNOS might be helpful.

But from the comfort of your computer desk, think through what happens when you turn away from a north heading -- what happens to the magnet in the compass?

 

[Areeda]

Joe,

To a heading. Best way for me is to go out and try it.

When turning to a north heading start about 25° before you get there (undershoot), when turning to a south heading start about 25° after you get there (overshoot).

East and west start your normal 7° before, anywhere in between interpolate.

It is so much easier in a sim or an airplane than it is on paper.

Joe

 

[dmccormack]

Joe,

To a heading. Best way for me is to go out and try it.

When turning to a north heading start about 25° before you get there (undershoot), when turning to a south heading start about 25° after you get there (overshoot).

East and west start your normal 7° before, anywhere in between interpolate.

It is so much easier in a sim or an airplane than it is on paper.

Joe

Yes it is -- and the amount of lead or lag in degrees correlates to your current latitude.

 

[poadeleted20]

Where I live (south of the Mason-Dixon), pilots know the South leads, and the North lags. Thus, when turning North, the compass is lagging, so I have to roll out early, and when turning South, the compass is leading, so I have to wait to roll out. That's worked for me for a very long time.

 

[dmccormack]

Where I live (south of the Mason-Dixon), pilots know the South leads, and the North lags. Thus, when turning North, the compass is lagging, so I have to roll out early, and when turning South, the compass is leading, so I have to wait to roll out. That's worked for me for a very long time.

See, boys and girls? Even mnemonics based on flawed premises are useful!

 

[mantakos]

I've found that most attempts to explain this principle are, characteristically, peculiarly terrible.

In particular, you'll always see talk about how "turning" causes compass errors. Well turning doesn't cause compass errors, it's _banking_ that causes compass errors, it just so happens that in an airplane, when we turn, we bank.

Imagine the Jolly Green Giant holding the plane in the air, wings level, at a normal cruise pitch attitude, nose pointed to magnetic North. The bit of the compass that has an "S" on it is attracted to the North Pole, so it's attracted straight ahead (it's the "back" of the compass, facing away from us, right now). However, the magnetic North Pole is actually "below" the plane's nose, so that "S" on the compass is also being pulled downwards slightly. With the plane's wings level, the compass' motion is constrained along that axis, and so our "S" can't dip downwards in response to that downward pull (and if it did, it still wouldn't change the compass' heading indication). But when our giant tips the plane's left wing low, our "S" now has a place to go in response to the downward pull, it slides "downhill" a little bit. It's just a little bit, because as it slides downhill, it ends up shifting a little away from North, so the shift is small, as it balances the pull towards both North and down.

So what happens when we dip our left wing, the "S" moves a little "downhill" (i.e. to the left)? Well, if the back of the compass moves left, then the front of the compass has to move right. Previously, we were staring at a big N, but as the big N moves to the right, the heading we'll be looking at is, say, 005 or 010. So dipping our left wing low causes the compass to read as if we had turned to the right. Similarly, dipping our right wing low would cause the compass to read as if we had turned to the left.

It's not the turning that caused this error, it's the "banking".
-harry

 

[bobmrg]

IMHO, time devoted to teaching compass turns could be better spent on something useful. There is no requirement to demonstrate them on any checkride. If, God forbid, I should ever be in a situation that required turning to a heading using the magnetic compass I would do it with wings level, feet on the floor, about ten degrees at a time. Sure, it would be a skidding turn, but this is an emergency situation, right?

Bob Gardner

 

[poadeleted20]

Bob's point is well taken. I doubt many folks are flying IFR these days without at least a handheld GPS, and in event of a vacuum failure, I'd be using that before I'd reduce myself to compass turns or even timed turns. In fact, I know examiners who'll fail you on an IR ride if you don't use any GPS in the plane to help make turns without a gyro heading indicator. Only for the CFI-IA ride is there a requirement to be able to actually do compass turns.

However, it remains a knowledge test requirement and a valid oral question on IR practical tests, so folks still have to know the answer.

 

[nddons]

I've found that most attempts to explain this principle are, characteristically, peculiarly terrible.

In particular, you'll always see talk about how "turning" causes compass errors. Well turning doesn't cause compass errors, it's _banking_ that causes compass errors, it just so happens that in an airplane, when we turn, we bank.

Imagine the Jolly Green Giant holding the plane in the air, wings level, at a normal cruise pitch attitude, nose pointed to magnetic North. The bit of the compass that has an "S" on it is attracted to the North Pole, so it's attracted straight ahead (it's the "back" of the compass, facing away from us, right now). However, the magnetic North Pole is actually "below" the plane's nose, so that "S" on the compass is also being pulled downwards slightly. With the plane's wings level, the compass' motion is constrained along that axis, and so our "S" can't dip downwards in response to that downward pull (and if it did, it still wouldn't change the compass' heading indication). But when our giant tips the plane's left wing low, our "S" now has a place to go in response to the downward pull, it slides "downhill" a little bit. It's just a little bit, because as it slides downhill, it ends up shifting a little away from North, so the shift is small, as it balances the pull towards both North and down.

So what happens when we dip our left wing, the "S" moves a little "downhill" (i.e. to the left)? Well, if the back of the compass moves left, then the front of the compass has to move right. Previously, we were staring at a big N, but as the big N moves to the right, the heading we'll be looking at is, say, 005 or 010. So dipping our left wing low causes the compass to read as if we had turned to the right. Similarly, dipping our right wing low would cause the compass to read as if we had turned to the left.

It's not the turning that caused this error, it's the "banking".
-harry

Harry, are you related to John, or Martha (King)? Nice explanation.

I hear what Bob and Ron say about compass turns, but my CFII had me doing partial-panel approaches last night, and without my HSI or a DG, it's still comforting, if not necessary, to know how the old compass acts. Even if you're getting vectors, it's helpful to know when to roll-out of a turn based on lead or lag. (My CFII was nice enough to hold the flashlight on the compass because the rarely-working light inside the compass was inop, but that's another story.) We had a GPS, but he only allowed me to use that for DME.

 

[poadeleted20]

We had a GPS, but he only allowed me to use that for DME.

I make my trainees do some timed turns early in training just so they know what it is and can do it in someone else's airplane with no GPS handy, but after that, I let them use the GPS. After all, what are the odds of losing both the gyro heading indicator and the GPS -- totally independent systems?

 

[dmccormack]

IMHO, time devoted to teaching compass turns could be better spent on something useful. There is no requirement to demonstrate them on any checkride. If, God forbid, I should ever be in a situation that required turning to a heading using the magnetic compass I would do it with wings level, feet on the floor, about ten degrees at a time. Sure, it would be a skidding turn, but this is an emergency situation, right?

Bob Gardner

+1 quadjillion

Actually, the compass exercise is one of those "let's see how much you know..." data points in aviation.

BTW -- my "nav instruments" consists of a Whiskey compass and plexiglas.

If I ever fly into inadvertent IMC in the Chief, I will fly needle, ball, airspeed and do a very careful, deliberate 180. Period.

If I lose everything while flying IFR, I have a handheld GPS and a handheld radio.

 

[CT Arrow]

However, it remains a knowledge test requirement and a valid oral question on IR practical tests, so folks still have to know the answer.

Yup, been there answered that!

 

[gismo]

IMHO, time devoted to teaching compass turns could be better spent on something useful. There is no requirement to demonstrate them on any checkride. If, God forbid, I should ever be in a situation that required turning to a heading using the magnetic compass I would do it with wings level, feet on the floor, about ten degrees at a time. Sure, it would be a skidding turn, but this is an emergency situation, right?

Bob Gardner

I agree that trying too learn more acronyms and which way leads vs lags is pretty much a waste of time. OTOH, I wouldn't be making skidding turns to achieve a particular heading, I'd just turn enough to be close (based on a standard rate turn and timing) then go wings level long enough to detect the residual error and repeat until I was on course. I do think it's important for pilots to know that the compass does "lie" and maybe even worthwhile for them to know why, but I'll bet a stack of Madoff IOUs that more than 95% of pilots who don't regularly turn to headings using nothing but a compass won't remember what acronym to apply or how to apply it if such a turn became necessary in a high stress situation.

 

[crash7]

I hate mnemonics, and my instructor gave me one by which to remember magnetic compass behavior: UNOS (undershoot north, overshoot south). But it's still not enough to help me. Does it mean that I undershoot when turning to a north-bound heading, or from a northbound heading (and vice versa)? How do you folks remember this?

It's cold up NORTH. Everyone will be sluggish & lagging to head North. It's nice & warm down SOUTH. Everyone is in a hurry & rushing to head South.

Northern Hemisphere, of course.

 

[Geico266]

How do you folks remember this?

They don't, they buy a GPS.

 

[DutchessFlier]

Deja Vu all over again...amazing but this was one of the points of my IR ground lesson just this afternoon, (correction for lag or lead I was told is my latitude plus half the standard turn rate so for me so its about a 50-deg lag or lead) complete with UNOS and partial panel discussions. I must say that the thoughts expressed in POA for training discussions about using the GPS, covering up the GPS, mag compass only, etc etc etc, partial panel, full panel are all over the place. Some say NEVER to using the GPS in training, some say intro it early on. I guess its up to the individual CFII and how he/she learned. Sheesh

 

[Clark1961]

They don't, they buy a GPS.

Naw, just time the turn. Much, much cheaper. I really like flying by the clock. The concept is just so simple and it's easy to do. Just start the clock and put the little airplane wing on the mark. Roll out at the proper time. Glance up at the compass and it's magically showing the right number. Ya don't even need to look out the window!

When deciding which instrument to pull for the Aspen installation, the avionics shop suggested dumping the turn coordinator. That didn't happen and it's not ever going to happen in my panel.

Ok, ok, I've got two GPS's but I don't use'em for turning...well maybe I'll use the panel mount for turning in the future if the Aspen software update actually works and enables GPSS...:smile:

 

[poadeleted20]

Some say NEVER to using the GPS in training, some say intro it early on. I guess its up to the individual CFII and how he/she learned. Sheesh

Any CFI-I who says "never" is setting their trainees up for failure, as there are DPE's who will fail you for not using it if it's available after your gyro heading indicator is covered. Further, if they teach folks "never" use it in training, that's just what they'll do if it happens for real (see laws of primacy and exercise in the AIH), and that could get them killed. That's why I teach them how, but once they show they can do it, they get to use the GPS for the rest of training. Kind of a carrot-and-stick approach -- if you learn mag compass, you can use the GPS and have it easier; if you don't, you get punished by more mag compass work.

 

[bobmrg]

I agree that trying too learn more acronyms and which way leads vs lags is pretty much a waste of time. OTOH, I wouldn't be making skidding turns to achieve a particular heading, I'd just turn enough to be close (based on a standard rate turn and timing) then go wings level long enough to detect the residual error and repeat until I was on course. I do think it's important for pilots to know that the compass does "lie" and maybe even worthwhile for them to know why, but I'll bet a stack of Madoff IOUs that more than 95% of pilots who don't regularly turn to headings using nothing but a compass won't remember what acronym to apply or how to apply it if such a turn became necessary in a high stress situation.

Another ploy is to roll until the TC indicates a standard rate turn and then roll back to level flight...compass heading will change about ten degrees. The Feds consider the loss of the gyro instruments to be an emergency, so I'm using my emergency authority to turn by whatever method I choose.

I remember reading in the dim and distant past about a pilot and her father who crashed a Mooney out of control after losing the gyro instruments. Sad and unnecessary.

Bob

 

[Unbeliever]

My instrument DPE expected me to do a compass turn during my checkride. I was flying a heading to join an airway. (Fly runway heading, join Vxxx). I was expected to fly partial panel, watch the needle, and make a nearly 90 degree turn to nearly due North with the compass and roll out within PTS standards to the right heading.

--Carlos V.

 

[poadeleted20]

My instrument DPE expected me to do a compass turn during my checkride. I was flying a heading to join an airway. (Fly runway heading, join Vxxx). I was expected to fly partial panel, watch the needle, and make a nearly 90 degree turn to nearly due North with the compass and roll out within PTS standards to the right heading.

That DPE is operating off the reservation. The 1998 Rev. C of the IR PTS and its predecessors did not require lead/lag compass turns, only timed turns, and even that went away when Rev. D came out in April 2004. Since then, you can do it any way you want with whatever equipment you have left when your "primary flight instrument" fails.

 

[MauleSkinner]

After all, what are the odds of losing both the gyro heading indicator and the GPS -- totally independent systems?

Depends on your power sources...Never seen a vacuum-powered GPS, but I've flown a lot of electric DGs & HSIs.

 

[poadeleted20]

Depends on your power sources...Never seen a vacuum-powered GPS, but I've flown a lot of electric DGs & HSIs.

In that case, you've had a two-point failure, but you have the AI and a radio for "no gyro" vectors, and even then, you may still have a handheld GPS. Or are we talking total electrical failure and no handhelds?

 

[gismo]

In that case, you've had a two-point failure, but you have the AI and a radio for "no gyro" vectors, and even then, you may still have a handheld GPS. Or are we talking total electrical failure and no handhelds?

Think EMP

 

[murphey]

Explanations are great. But what really impresses on your brain - take a pocket or hikers compass and walk (that's right, walk on the ground) and wander. Watch what
the compass needle is doing. Start by walking (briskly) to the north, then
turn briskly (keep walking) east, then west, then south, then....

You get the idea. Not exactly the same as with the liquid compass in the airplane, but no need to worry about flying the plane.

 

[poadeleted20]

Think EMP

If that happens, I don't think the mag compass will work too well, either. Like my buddy John Eastabrooks said, "I'm not half as worried about surviving nuclear winter as I am about surviving the nuclear summer that precedes it."

 

[MauleSkinner]

In that case, you've had a two-point failure, but you have the AI and a radio for "no gyro" vectors, and even then, you may still have a handheld GPS. Or are we talking total electrical failure and no handhelds?

FWIW, I consider a "total electrical failure" to be a multi-point failure even with only one alternator or generator. And I've had enough multi-point failures that I don't use the phrase "what are the odds of that?" anymore I'm also bugging Hawker Beech to produce some rewiring documentation to make my life a little less miserable in the event of a really bad day (on the Hawker I fly, the left pitot heat is operable for the entire 60 minutes of ship's battery that I've got in the event of a dual generator failure. Unfortunately, the right pitot tube is the one driving the only airspeed indication I'll have at that time.)

But that aside, one of the reasons I don't carry a handheld is because I have a compass and a wristwatch.

 

[Areeda]

That DPE is operating off the reservation. The 1998 Rev. C of the IR PTS and its predecessors did not require lead/lag compass turns, only timed turns, and even that went away when Rev. D came out in April 2004. Since then, you can do it any way you want with whatever equipment you have left when your "primary flight instrument" fails.

That DPE is the one my students use most often. The Unbeliever was one of those.

For the record Carlos (Unbeliever) is a JPL Engineer who seems to remember/hear everything. Except when I'm posting here "Carlos said" is pretty much good enough for me. That man knows what he's talking about.

Ron would not use this DPE, he's criticized his position before. I have spent some time and a few checkrides dealing with his hot spots. He's an experienced pilot and a fair examiner.

Tough but fair, working with the PTS, is my description. I know what he wants and my candidates can fly in actual.

My next IR candidate goes on Wednesday with a FSDO inspector. What is the difference? I am much more concered with flying to minimum in actual the day after the checkride than I am with the checkride.

Joe

 

[rottydaddy]

There's always the "counting seconds based on standard-rate turn" thing, which I intuitively remember more reliably than the lag/lead rules:

In a steady, standard rate turn (as indicated by most turn coordinators and old-style turn-and-bank indicators), the airplane should change heading at a rate of 3 degrees per second. Therefore:

360 degrees: 2 minutes
180: 1 minute
90: 30 seconds
45: 15 seconds
Etc.

I've never used that method in earnest (can't remember the last time I did it even in practice, to be honest)... I'm curious as to how useful it might be, especially in IMC.

 

[Skip Miller]

I've never used that method in earnest (can't remember the last time I did it even in practice, to be honest)... I'm curious as to how useful it might be, especially in IMC.

That's how I was taught to do the inadvertant VFR into IMC 180 turn during my primary instruction.

-Skip

 

[dmccormack]

That's how I was taught to do the inadvertant VFR into IMC 180 turn during my primary instruction.

-Skip

If the a/c has an a/p. I make sure the student knows how to use it to do the 180.

 

[MauleSkinner]

I've never used that method in earnest (can't remember the last time I did it even in practice, to be honest)... I'm curious as to how useful it might be, especially in IMC.

I wouldn't have the foggiest clue...

 

[Unbeliever]

Ron would not use this DPE, he's criticized his position before.

I would agree with Ron on a few things. This DPE was the same DPE that gave me the "I don't hold charts" line during my PP ride.

My instrument checkride was chronicled here:
http://www.pilotsofamerica.com/forum/showpost.php?p=277460&postcount=30

Clearance was heading 260 intercept V363, POM, EMT. 430W doesn't do airways, so I pulled the chart, found the segment that I would intercept if I flew 260, and defined that leg. So my flight plan was CNO, PRADO, POM, EMT. Activated the leg from PRADO to POM and planned to turn to intercept when the needle came in. Did the RAIM check at EMT and selected the VOR/GPS-A. On the climbout my vaccuum pump "failed" with a pair of stickies over the DG and AI. Flew 260 until the needle came alive and did a compass turn to intercept V363.

Now I'm going back through my memory, I'm not entirely sure whether or not he said "Good job, you know you could have used this. <pointing to the 430>" after the turn, or if he said "don't worry, you can use this anyway <pointing to the 430>" when I tried to keep the sticky over the DG from falling off. I know he turned the DG knob some random amount to at least keep me honest. I know he did point to the NAV page on the 430W at some point.

--Carlos V.

 

[poadeleted20]

Tough but fair, working with the PTS, is my description.

Working with, perhaps, but not within the PTS, based on the checkride reports posted here. And I don't consider that "fair."

 

[jesse]

For me, understanding the why and the how of a compass makes it rather easy. I don't try to memorize any mnemonics to help me.

In the end though -- It's pretty unlikely I'd need to be THAT damn precise with the compass.