Magnetic Misalignment In Aviation

Screenshot_2018-06-15-13-57-18I’m a Flight Instructor (CFI, CFII, MEI).  I love teaching my students.  But there is topic that is a growing concern in navigation; magnetic field changes.

The magnetic compass was perhaps the earliest navigation instrument to be installed in an aircraft; And why not, it had been successfully employed in nautical navigation for centuries.  It is self contained and self powered.  However, it has several drawbacks in an airplane.  First, it is only points to the magnetic heading when one is in straight and level flight.  Start a turn and it leads or lags one’s actual heading.  (This is compensated by using a gyro stabilized directional instrument that is periodically aligned with the compass when in straight and level flight.)  The compass also has errors because of electronics and magnetic metals in the aircraft which the pilot has to compensate For another, it doesn’t actually point to “north”, but to “magnetic north”.  And even that isn’t actually pointing to the north magnetic pole as the Earth’s magnetic field isn’t a simple bar magnet.  Global and local distortions exist.  So, the pilot depends upon geomagnetic surveys that are updated periodically.

And therein lies the rub.  That magnetic field is changing, shifting, faster than ever.  Aviation maps are updated several times a year, but there are other references that are updated much less infrequently.  This leads to different “magnetic north” references that can be very confusing to the pilot.

First up, land based radio navigation aids, particularly the Very high frequency Omnibearing Range or “VOR”.  These radio stations transmit “light house” like rotating radio beams whose relative timing (phase) compared to reference signal tells instruments in the cockpit of an aircraft what the bearing is from the station.  But, since the system was put into place with the assumption that airplanes all had compasses as their one and only reliable directional instrument, those stations were all aligned and calibrated to the local magnetic field.  This is a problem.

The rapidly shifting magnetic field has left many VOR stations no longer aligned.  As instructor, I have had to supplement my students navigation instruction on how to use the VOR ‘magnetic bearings’ as idiosyncratic, independent references that only roughly correspond to the magnetic bearing indicated by their compass.  Fixing this problem would be simple, provide a magnetic flux compass on every VOR and it would ALWAYS remain aligned.  However, this would cost money and the FAA has other plans.

The FAA is shutting down most of the VOR stations, leaving only a minimum network of stations as a back-up to GPS navigation.  But here’s the fun part; those GPS units also report direction of flight and bearing to waypoints and airports using magnetic reference!  The GPS system suffers from the same shifts in magnetic fields as the compass vs. the calibration date of the on-board GPS data base.  That magnetic data base is only updated very infrequently, unlike the data base for the location of waypoints and airports which is defined by latitude and longitude.

The mismatch between the VOR and GPS system can be very noticable in flight, often by several degrees.  Yet another thing I have to teach my students is how to keep these in mind and not get confused.  This difference can lead to very different locations in airspace for what is nominally the same point.  For example, not far from my home base of Santa Rosa / Sonoma County / Charles M. Shultz Airport (Shultz was the creator of the Peanuts comic strip) is SNUPY, an intersection (a point in airspace) that is defined by the intersection of two VOR radials (magnetic bearings) from two stations and is also in the GPS database.  But those two points, which by definition should be identical, are actually a bit over a mile apart.  I’ve never been able to get a straight answer from the FAA which is right.

Research suggests that the rapid shifts and reduced strength of the Earth’s magnetic will continue.  Thus, problems with navigation will continue.  However, their is a very simple solution provided by the increasing sophistication and decreasing cost of avionics.  The “glass cockpit” (named from the use of glass substrates found in Liquid Crystal Displays, LCD) can be driven by differential GPS receivers that detect heading & track, displayed as true, with no need for reference to the Earth’s magnetic field.  All our maps, instrument procedure charts, everything would be with respect to true north, no matter the location, vastly simplifying navigation, reducing pilot workload, and reducing training requirements, increasing aviation safety, and lower costs by eliminating the need to update charts as the magnetic field shifts.

This is a plea to the FAA and ICAO, please update our navigation philosophy.  Its time to switch from magnetic to true.  Its time to have a TRUE 21st Century navigation doctrine.

Recommending A Great Space Technology Blog

It’s rare that I find a really great blog writer covering technology with deep analysis and insight, so when I do, I feel compelled to share it.  Casey Handmer is such a writer:

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