Magnetic Compass Q&A

Some of you may remember the classic scene from the 1990s television show, Seinfeld, where disgruntled postal worker Newman proudly proclaims that zip codes are meaningless (1:20 in the clip below):

 

We might ask ourselves, is the magnetic compass relatively meaningless to pilots in today’s world of GPS navigation? Before we attempt to answer that question, let’s take a deep dive into the science, function, application and regulation behind the magnetic compass in your aircraft.

Q: Is the magnetic compass required for VFR or IFR flight? 

A: FAR 91.205 requires that a “magnetic direction indicator” be installed in any aircraft that is used for VFR or IFR flight. This indicator can be in the form of a legacy magnetic compass, or the magnetic heading displayed on the Garmin G5. A magnetic compass is not specifically required by FAR 91.205, but it is, however, listed as “R” or required equipment in most POH’s (see example below from our Cessna 152, N152DM). 

FAR 61.93(e), Maneuvers and Procedures for Cross Country Flight in a Single Engine Airplane, requires student pilots pursuing a private pilot certificate to receive and log flight training on the “use of aeronautical charts for VFR navigation using pilotage and dead reckoning with the aid of a magnetic compass.” Remember that this regulation applies to all student pilots, including those who are flying basic airplanes equipped with just a magnetic compass to those flying “Technically Advanced Aircraft” with “glass cockpit” displays and redundant navigation equipment. So to satisfy this training requirement, a magnetic compass must be installed. However, the Private Pilot Airman Certification Standards (ACS) requires applicants to “use the magnetic direction indicator in navigation, to include turns to headings.” There is no direct reference to the magnetic compass in this ACS. 

Q: Does the Garmin G5 replace the magnetic compass in our IFR equipped aircraft?

A: Legally, no, but functionally, yes. The Garmin G5’s that are installed in our IFR-equipped aircraft provide magnetic direction information provided by an installed Garmin GMU11 magnetometer. Each aircraft has two G5 displays, and therefore two magnetic direction indicators. If one unit fails, magnetic direction (heading) can be indicated on the other unit. 

In the event of a failure of the magnetometer (either on its own or as a result of an alternator failure and resultant loss of electrical power to the magnetometer), and the loss of magnetic direction/heading information from the magnetometer, the G5 will automatically display magnetic “ground track” information from the GPS. If both the magnetometer and the GPS lose power due to an alternator failure — which would be considered an emergency under IFR — the G5 will simply show an X and the pilot would be left with the magnetic compass for heading information.

Are there any continued airworthiness requirements for the GMU11? How can a pilot be sure it is providing accurate information to the G5? According to the Garmin G5 Maintenance Manual, a “compass rose swing” should be completed by the installer after the system is calibrated, but there is nothing in the Garmin G5 Airplane Flight Manual Supplement and Instructions for Continued Airworthiness – the documentation for the pilot – that says anything about checking the accuracy of the heading information on the G5.

 

Q: What is the compass deviation card, and is it required?

A: According to the FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 16: Navigation, “Because each aircraft has its own internal effect upon the onboard compass systems from its own localized magnetic influencers, the pilot must add or subtract these influencers based upon the direction he or she is flying. The application of deviation (taken from a compass deviation card) compensates the magnetic course unique to that aircraft’s compass system (as affected by localized magnetic influencers) and it now becomes the compass course.” Some examples of these “localized magnetic influencers” include electrical circuits, radios, lights, the engine, and magnetized metal parts of the airplane or installed equipment.

Fair enough. However, FAR Part 23, Airworthiness Standards: Normal Category Airplanes, no longer requires a compass deviation card (also sometimes referred to as a correction card) to exist at all.  As of this writing, FAR 23.1327, Magnetic direction indicator, no longer exists. 

AC 43.13-2B, Acceptable Methods, Techniques, and Practices – Aircraft Alterations, dated March 3, 2008, references FAR 23.1327. However, AC 43-215, Standard Procedures for Performing Magnetic Compass Calibration, dated August 7, 2017, states: “This AC is not mandatory and does not constitute a regulation. The content of this AC may be of interest to aircraft technicians performing compass swings or to users of aircraft compass systems. The contents of this document do not have the force and effect of law and are not meant to bind the public in any way. This document is intended only to provide clarity to the public regarding existing requirements under the law or agency policies.” AC 43-215 also states: “There should not be more than a plus or minus 10 degree difference between any of the aircraft compass readings and the sight compass readings.”

Q: Ok, so the compass deviation card isn’t required, but there’s one in my airplane. Do I still need to use it to factor in magnetic compass deviation on my navigation log when planning a VFR cross country flight?

A: There is currently no regulatory requirement for pilots to maintain, or to use, compass deviation information. The Private Pilot ACS makes no mention of this in Task D, Cross-Country Flight Planning or in Task G, Operation of Systems. However, it’s “fair game” for the examiner to query the applicant on any equipment installed in or located in the aircraft. You need to be able to intelligently articulate your reasons for including or not including magnetic compass deviation. 

From a practical standpoint, consider how the compass deviation information is actually intended to be used. For example, if you wanted to use the compass in N99725 to fly due north, you are supposed to fly (“steer”) a heading of 358 using the compass. Notice that most of the deviations are within 1-2 degrees of the intended heading – practically insignificant, even if the uncorrected heading were held for many miles. 

The most substantial published magnetic compass deviation in our fleet of aircraft is in N60264. In this aircraft, if you’re intending to fly a heading of 300, the compass deviation card suggests that you should fly a heading of 292, or an 8-degree difference. That’s fairly significant, no doubt, but remember that you would only do this if you are actually flying that heading using the magnetic compass itself. Most of the time we are using the gyroscopic heading indicator, which of course must be set periodically to correlate with the magnetic compass, due to gyroscopic precession. Notice that the most accurate compass heading, according to the compass deviation card, is north. So in theory, in flight we could reset our heading indicator on a north heading, and then continue to use the heading indicator with confidence. We could also correlate both the heading indicator and the compass indications with our GPS track, factoring in known wind correction.  

Let’s say that you are planning a cross country flight in N60254 from Craig to Flagler. Winds are calm, and you determine that your on-course heading should be 170 degrees. If we interpolate from the compass deviation card (which suggests flying 4 degrees right to effect a heading of 150 or 7 degrees right for a heading of 180) we should fly a heading of approximately 5 degrees to the right, or 175 to effect a heading of 170 degrees to Flagler. But, we ignore the card and fly a heading of 170 on the compass. The error card suggests that if we do this, we would end up 5 degrees left (or east) of course, on an actual heading of 165. If we plot this in ForeFlight, we can see that a 5 degree heading difference over 55 miles results in a lateral displacement of 4 nm. This is consistent with the so-called “1 in 60” rule of air navigation, which states that each degree off course over a distance of 60 nm will result in a lateral displacement of 1 nm. I would hope that a pilot would notice this discrepancy fairly early on in the flight by using visual ground references. After all, pilotage (using ground references) and dead reckoning (flying a heading for a given time) go hand in hand. 

It’s worth noting that the Private Pilot ACS requires an applicant to “verify position within 3 nm of the flight planned route.” Recall that AC 43-215 allows for a maximum 10 degree difference between the heading displayed on the compass and the verified magnetic heading. If you flew a  heading on which a compass deviation card suggested a 10 degree correction, and you ignored the compass deviation card entirely and flew a heading that was 10 degrees off course, you’d end up 9 miles off course after flying 50 nm, again consistent with the 1:60 rule. It should be very obvious to the pilot after only a few miles that he’s off course, using ground based visual references. 

Still, the compass deviation card in N60254 has been bugging me for a while, and stressing out students who are trying to do the right thing when planning cross country flights. So one morning recently, I decided to taxi N60254 onto the compass rose located near Runway 5 on Taxiway B here at Craig to see for myself whether this compass correction card is accurate.

First, I observed what the compass read while taxiing straight on Taxiway A parallel to Runway 14, and sure enough it showed 140 degrees (the deviation card suggests a 2-3 degree error on this heading). Then, I turned right onto Taxiway Bravo parallel to Runway 23, and the compass showed 230 degrees (the deviation card suggests a 1-2 degree error on this heading). Then I got to the compass rose at the end of Taxiway B. Even with deft foot and throttle work to taxi the airplane into position with the spinner pointed exactly south and the tail pointed exactly north, it was near impossible for me to feel confident that I had it dead on to the degree. Still, I did this several times and compared my compass heading to what I saw out the window, and the compass consistently showed 175 while facing south – closer to a 5 degree deviation, not 7 degrees as suggested by the correction card. Similarly, I tried several times to orient the airplane on a 300 heading according to the paint marks on the ground, and each time I observed that the compass read 305 – again, a 5 degree deviation, not 8 degrees as suggested by the correction card.

The runway numbers themselves are allowed by FAA regulation to vary plus or minus 5 degrees on either side to account for variations in the Earth’s magnetic field over time. So at any airport, on any given day, even if you do everything exactly by the book you still might be up to 5 degrees off course the moment you’re wheels up. For example, if you depart Runway 14 at Craig exactly aligned with the runway centerline, you’re actually flying a heading of 141.7 degrees, not 140. But who among us is good enough to hold a heading to within a degree right after takeoff while we’re paying attention to arguably more important things like wind, air traffic, birds, clouds and oil pressure?

One last point to consider: When ATC instructs a pilot to “fly heading 360” what the controller is really trying to accomplish is to keep that aircraft’s target on a specific ground track. So, much like how Newman allegedly didn’t care about what zip code was written on a piece of mail he was trying to deliver, the controller really doesn’t care (and honestly has no way of knowing) exactly what heading the pilot is actually flying (or thinks he is flying) at any given moment. All the controller really cares about is that the aircraft’s target on the screen keeps moving along the desired track. ATC assigned headings are educated guesses based on desired track and winds aloft.

The FAA Pilot Controller Glossary defines the phrase “fly runway heading” to mean “the magnetic direction that corresponds with the runway centerline extended, not the painted runway number.” So in our example above with Runway 14 at Craig, if ATC instructs you to “fly runway heading” you are expected to fly the published runway heading of 141.7 degrees, rounded up to 142 degrees. But most pilots will fly a heading of 140, because again, who among us can hold a heading to that level of accuracy? The FAA Instrument Rating – Airplane Airman Certification Standards require a pilot to fly a given heading plus or minus 10 degrees in cruise or while on an instrument approach. Only an applicant for an Airline Transport Pilot certificate is required to maintain a heading plus or minus 5 degrees during certain procedures.

Think about how we actually navigate our small airplanes from place to place. It’s hard to believe that a century ago, Charles Lindbergh managed to cross the Atlantic Ocean in an airplane without killing himself by using only a compass, some nautical maps and a bit of celestial navigation. Around that same time, airmail pilots were able to use the new  Transcontinental Airway System which included lighted beacons and huge arrows on the ground to point the way. Still, they were limited by weather conditions. Fast forward about 50 years, and pilots were able to navigate fairly accurately even in bad weather using more advanced ground based navigation aids including ADF, then VOR, ILS, and now, GPS.

Based on all this, my advice to pilots is to take compass headings and compass deviation information “with a grain of salt,” so to speak, and instead use “all available information” (per FAR 91.103) – compass, AHRS, GPS, VOR, ground references – to stay on course and get safely to the destination. 

Remember the “original” meaning of IFR — I FOLLOW ROADS!