The Polar Keyhole??

A Global Express pilot friend of mine in Seattle, Val Trent – also an NBAA member – asked me a few weeks ago if I’d like to read something he’d written about the Polar Keyhole. At first, I thought maybe he’d started writing fiction and this was going to be the first installment. I was wrong.

This fascinating piece on the Polar Keyhole will expand the minds of pilots who have never flown way up north as it details some of the peculiarities of the basic magnetic compass we all fly with coupled up with the Flight Management Systems (FMS) found in most turbine-powered aircraft.  Val Trent is a former Army helicopter pilot with some 19,000 hours in his logbook and currently flies a Global Express. The only continent Val’s missed is Antarctica and he says that’s just fine with him.

Please welcome Val Trent to Jetwhine.

The first questions most people might ask are, “What and where is the “keyhole?” followed pretty quickly with so “What do we do about it.” I’ll answer the second question first. “We don’t do anything.” Now back to the first one …

True vs. Magnetic

In the Honeywell FMS‘ used in most large aircraft, the background navigation operations are always computed using “True Course” 100% of the time. If you’re one of those pilots who know that, count yourself as one of the relative few that enjoys mysteries. Transparent to the pilot is a background built-in conversion within the software that makes all of our operations appear to us in the cockpit seat to be magnetic so that we can operate in an environment with which we are familiar almost 100% of the time. The Magnetic environment is familiar, common to all aviation and marine navigation, and doesn’t change…right?

Well, … almost. Magnetic variation is constantly changing but the adjustments for accurate navigation are transparent to us up front in the cockpit.

As we navigate along, the Flight Management System takes the course you’ve programmed, looks at its database and checks the course with the magnetic variation along the route and converts back and forth so that we have an accurate route to follow. In the far north however, the lines of variation grow closer and closer together to the extent that in one particular area, they are so close that distance between the computed variation changes so rapidly with our groundspeed that the computer can’t keep up. This area is called the keyhole. I’ve flown through it several times and you can see it happen almost to the “degree” although it does vary minutely. A flight from Seattle (KBFI) to Stockholm (ESKN) for example, takes you right through the keyhole.

Upon entering this region, the FMS, along with the Horizontal Situation Indicator (HSI) in some aircraft such as the Global Express show a big red “HDG FAIL.” In fact, above 73° North and below 60° South, it won’t compute variation at all because the lines of variation are too close at those latitudes north (and south) and are not even stored in the variation database. That’s why all navigation above and below those regions is in True all the time.

Back to Square One

Ok, now we can more specifically answer the first question, “what is it” at the same time we answer the last half, “where is it?” Basically, the Polar Keyhole is a box from the ground up, the location of which is fairly specifically located between 90° and 120° west longitude and 70° to 72° north latitude. Because the FMS cannot keep up with the variation changes, your FMS will annunciate, “ACTIVE MODE IS MAG HDG” and your HSI’s annunciate the red “HDG FAIL.” This all reminds you to switch the FMS to indicate TRUE headings.

But what do you do about it? Actually nothing is the correct answer. In the Global Express, the HSI’s go into “HDG FAIL” mode but the airplane is still navigating as it always has. Nothing is wrong and nothing changes except what you see on the HSI and the FMS. If you do nothing, the aircraft will come out the other side of the keyhole on it’s own and operations will appear normal from there on. I believe that other types such as the GV series do change to TRUE mode on the HSI’s without pilot input.

Bottom line, not a big deal, unless you’re not expecting it and don’t know what it is. That can be a cause for concern until you figure it out. One more piece of useful pilot knowledge to tuck under your cap for that first trip up north. Be sure and send me some photos too.

Val Trent