I've been planning a cross country flight in the DC-3 and have been reading up on dead reckoning and navigating in VFR conditions with a magnetic compass. This has raised a question.When you are taking off in a modern airplane that has a gyro compass, why is the heading on the gyro compass the same as the magnetic compass - which, if I understand correctly, is also the runway's approach course. Should not the gyro compass show the "true" heading and not the magnetic one. For example. Taking off from SEATAC, the magnetic variation according to the sectional is 19 degrees. So, if you are lined up with the centerline, your mag compass should read the approach course published for the runway - but why does the gyro compass show that course as well. When you get up in the air, they start to diverge.I'm confused.Many thanks!Colin Ware

Colin:I need to first correct your use of terms. Heading, in it's aviation usage has wind correction applied. In true dead reckoning, you start with an aviation chart, a sectional being most common. Using basic geometry and a protractor, your true course is determined. Much like shooting an azimuth with a compass. This true course is an angular measurement in relation to the true north pole as the lat/long are measured in relation to that pole. From that determination, the following formula applies:TC +/- Variation = magnetic course. I won't go into a lesson on geology so you'll have to accept that the magnetic north pole is situated in a different location than true north (Because the earth is tilted in space). At any location on the earth, there is an angular difference between TN and MN. That difference is called variation. Westerly Variation (or where you are clockwise from MN) is added. Easterly variation (where you are counterclockwise from MN) is subtracted thus the euphemism east is least. I teach my students east is least...it's also odd. That helps keep the altitude rules straight.Magnetic Course +/- wind correction angle = magnetic heading. Now for the part that unless you've actually had aviation ground school you haven't heard of and that FS doesn't model. The whiskey compass in aircraft are magnets. They align themselves to the magnetic north pole. Care to guess what airplanes are made of? Metal. And in some instances...iron. This introduces a 'confusion' into the whiskey compass. It's attempt to seek magnetic north is affected by local magnetic disturbances. This is called deviation. Deviation is recorded, on a 'Compass Correction Card' and is part of an aircraft's annual inspection. The mx personnel tow the aircraft to a compass rose, aligned to magnetic north and they check to see how far off it is at the cardinal directions. If you have Dreamfleet's Cardinal or Archer, look closely immediately below the compass. That is the correction card. Deviation is usually less than 3 degrees but I have seen it as much as eight in older aircraft...like the DC3. That era didn't know a lot about magnetic shielding. All that being said, comes the final calculationMH +/- Deviation (from the card and yes...each aircraft is different) = Compass Heading. The main thing to remember here ..whether it's true, magnetic, or compass...heading has wind correction angle in it.Now to answer your question: It depends. Less complex aircraft that have a gyroscope operated directional indicator and a whiskey compass (like the C172 and the DC3) are subject to both variation and deviation. But bear in mind these corrections are to permit an adjustment to true course. Except for deviation, the whiskey compass reads correctly. Variation is applied to adjust True Course to Magnetic Course. Not to correct the compass. What you read is correct because the magnets are trying to align themselves to the lines of flux/magnetic field that are present around the earth. And it therefore, in that sense, is 'self correcting'. When setting the directional gyro, you read the whiskey compass (it's correct); apply deviation (from the card) and set that course into the directional gyro. Since FS doesn't model deviation, the two therefore are the same. Once adjusted for deviation, the directional gyro should be within 5 degrees of Runway heading. It can be as much as five degrees out because RW are numbered to the nearest ten degrees. For example, a RW that is surveyed as 184/004 is numbered 18/36. To the untrained then, there is a four degree margin between the two.Directional Gyros have an inherent limitation: gyroscopic precession. That's another topic. Suffice it to say that they drift. And in FS, it's w-a-a-a-y over exaggerated. I turn it off regardless of whether I have a slaved gyro or not. In more complex aircraft, the gyro is slaved meaning you don't match it to the compass. There is a master gyro usually in a wing tip or the Vertical Stabilizer that the directional gyro is slaved to. It self adjusts. During taxi-out you have to check it's orientation and that's it. And in even more complex aircraft, the heavies for example, the whiskey compass is for emergencies only. Their primary compass is inertial.I hope this answers your questions. First, a gyro compass will never show heading. It shows course. Second, variation doesn't affect the compass reading. It's used to adjust TC to MC. Consequently, your RW hdg and MC will be within five degrees of each other. They diverge not because of variation but because FS is doing a really bad job with gyroscopic precession.If you actually read this far...congratulations. And yes,...I'm a flight instructor...BobL

An easier way to define heading is the direction the nose of the plane is pointing. And course is easier defined as the intended direction of travel. Then you have track, which is your current direction of travel.John S. MorganReal World: KGEG, UND Aerospace Spokane Satillite, Student 60+ hours.Virtual: Fly!II, KPHX, MidCon, PMDG Boeing 757-200

BobL - As an interested lurker, may I thank you for this.I've been through the Thoms series of training manuals, and those by Jeremy Pratt, and not made much sense of the explanations they offer on this subject. I now understand a little more.And to think your students pay to learn about this! (:-)Brian

Getting back to the original question, there are gyrocompass systems which are north seaking, but I guess these aren't installed in aircraft. The north seaking ones, as long as you are not too far north or south, will provide true heading info.scott s..