Flight planning

Suppose a crew is constructing a railroad with pieces of steel tracks one half a nautical mile long so each is about 3000 feet. They managed to lay two pieces down, end to end, in the early morning and then left to take a long, deserved break. Upon their early afternoon return, the crew finds that the sections have expanded in the heat of the day by one foot and are now 3,001 feet long. The two segments remained straight, so the track buckled upward at the half-mile point and formed a triangle. How high is the midpoint of the track now? It turns out that the railway track sticks up more than 77 feet in the air!

Now, if we imagine tipping the tracks over so that they lie the ground, a flight planning lesson emerges: While a straight-line path is always shortest, we might not even notice the added distance, time, and fuel required for a more prudent path that helps us steer clear of obstacles or unfavorable terrain.

Recalling this astonishing fact helps with my own flight planning.

I once planned a VFR flight in a Cessna 172 from the Shelbyville Municipal Airport (SYI) in Tennessee to the beautiful Asheville Regional Airport (AVL) in North Carolina. I first connected the two airports with a straight line and saw that I would fly directly over the highest terrain in the Smoky Mountains, and projected the 191-nautical-mile trip to take one hour and 45 minutes. While the perfect weather would afford me a great view of the park, flying above such unforgiving terrain always gives me pause and sends me looking for alternatives.

While a straight-line path is always shortest, we might not even notice the added distance, time, and fuel required for a more prudent path that helps us steer clear of obstacles or unfavorable terrain.Electronic flight bags (EFBs) make quick work of evaluating various route scenarios, so I considered hopping on V67—T398—SUNET and then to Asheville and found it added just 25 nm and 13 minutes to the trip. For that small marginal cost of distance and time, the course still gave me a great view of the Smokies, but provided improved options should an in-flight anomaly arise.

Most practical exams include a flight planning component and, of course, it’s natural to start with a straight line between the departure and destination airports. Too often though, consideration of an optimal route stops there. It’s true that the shortest distance between two points in space is a line, so it may be tempting to think that hitting GPS direct will always economize both time and fuel, but that’s just not the case.

For example, on a recent exam for the instrument rating, I asked my applicant, Liam, to plan a flight from Portland, Oregon (PDX), to Yakima, Washington (YKM), for which he showed me a 104-nm straight course on his EFB. The route passed through a sector with an off-route obstruction clearance altitude of 16,800 feet, so he carefully studied that sector. Liam confirmed he would pass within four nautical miles of Mount Adams, whose peak reaches higher than 12,000 feet msl. He chose a cruise altitude of 15,000 feet msl and reported that the trip would take 47 minutes and use 13.4 gallons of fuel in his normally aspirated Beechcraft Bonanza.

Our discussion then turned to issues involved with high-altitude flying, and Liam explained that his trip required supplemental oxygen.

Although he had never used such a system before, he had done his homework and shared his plans for using the system on this hypothetical flight.

Interestingly, 15,000 feet msl is the highest eastbound altitude his EFB would allow him to choose, presumably because information on climb rates ends around there in the POH. I asked him to compute his climb rate during the cruise portion of the flight, and he squirmed a bit when he found it would be less than 200 feet per minute.

When I asked Liam what guided his decision to fly a straight course between the airports, he reported that he always flies GPS direct and that, frankly, airways seemed antiquated. I asked him to humor me by forming a plan along airways. His new route involved heading to the Battleground VOR (BTG) and then along V468 to the Yakima VOR, and then to the destination. Since it also follows lower terrain and he was GPS equipped, he could file for 9,000 feet and avoid the need to carry and use supplemental oxygen. Although the new route is 15 nm longer, the time en route is the same and he would even use a little less fuel. It was an “aha” moment for Liam that I know he won’t soon forget.

Winds can play an important factor in selecting an altitude because they generally change with varying altitudes. I left them out of this discussion to effectively compare factors such as time en route and fuel burn with a change in course.

Following airways might seem old-fashioned, as they are often defined by early- to mid-twentieth century navaids. But choosing a course other than “GPS direct” doesn’t necessarily mean adding much distance, and it can reap rewards in aircraft performance, time, fuel efficiency, and, most important, giving inhospitable terrain or high obstacles a wider berth.