Long Trips On Short Legs
Even simple, short-legged airplanes can be used to travel long distances. All you need is patience, time and a realistic approach to flight planning.
For most of us tooling around the airstrip and to the occasional pancake breakfast, the size of our fuel tanks doesn’t matter. But when you’re planning a longer flight, your aircraft’s range becomes a consideration. Put another way, if you want to travel more than 500 nm, tank-size matters, and not all of us are endowed with long-range tanks. It may be your tanks are too small, or your passengers are too big. You may want to bring along too much gear for use at your destination, or you’re carrying too many gadgets to use while airborne. Of course, your fuel burn rate may be too high, or perhaps your airspeed and headwinds make your groundspeed too slow. Any combination of these will give your aircraft short-leg syndrome.
The syndrome manifests itself through an inability to make nonstop flights to seemingly far-flung destinations. The fuel stop itself imposes burdens such as additional wear and tear on tires, brakes and starter motors, consumes the time we’ve set aside for the trip and potentially exposes us to poor weather we’d rather avoid by flying past. The syndrome can involve all aircraft types and destinations, doesn’t respect our personal schedules and consumes much of the mental bandwidth we have available for our pre-flight planning.
We refer to planes like Skyhawks and Cherokees as four-place aircraft, but that is a rather generous description. Though the FAA still uses 170 pounds for the average person’s weight in its advisory circulars, weight and balance handbooks, and seat and restraint design criteria, we are not as svelte as we were when those aircraft were designed. The limitations imposed by seating, fuel capacity and useful load typically require trading one for another, results which ultimately will be liberated from the fuel tanks. For aviation, the upshot of the obesity crises is reduced endurance of our aircraft.
Today, 170 pounds is not an average weight; it is an aspirational weight that would require most American men to skip the pancake breakfast altogether. According to a 2008 report by the U.S. Centers for Disease Control, the average U.S. male weighs 194.7 pounds. The average female? She’s 164.7. Bigger people means less fuel. If you need to preserve short field or climb performance to deal with hot days, high density altitude, short run- ways or airstrips with obstructions, you may need to reduce your fuel load even further. The end result is what I’ll call short-leg syndrome.
The Need For Speed
If you study almost any airplane’s Pilot’s Operating Handbook or Airplane Flight manual (POH/AFM), you’ll notice that the best economy cruise with the greatest range corresponds to a speed you’ll never put on a flight plan. You can extend your stride and burn less gas by flying slow, but if you are trying to make time, beat weather or arrive before dark on a long trip, you may not be willing to mush along at the airplane’s economy cruise setting. On the other hand, if you plan to use an economy-cruise power setting, you’d better stick with your plan. Planning a max-range trip on an economy-based fuel burn and then flying the trip at full-tilt boogie is begging to come up short.
Anecdotally, I have found that pilots with fast planes tend not to achieve best ranges because they have fast aircraft for a reason—they like flying fast. On the opposite end of the spectrum are low-and-slow pilots. While endowed with patience, pilots facing a long cross- country flight in a slow plane will typically strive to get every ounce of speed they are entitled to or risk losing a day or two on the cumulative trip.
My point here is that the aircraft we fly typically have more range capabilities than their pilots have patience. When planning a trip based on POH/AFM numbers, be realistic and understand that speeding up will shorten your legs and slowing down will extend them. Don’t just plan your flight, also fly your plan.
Speed makes a huge difference on leg lengths. If you convert your fuel capacity into range measured in hours, a faster plane has longer legs. That’s because the slower your aircraft is, the more susceptible you are to headwinds, and the more dramatically they will affect your short-leg syndrome.
Take three hypothetical aircraft with three hours of traveling fuel and one hour of reserve. A Mooney traveling 150 knots will travel 450 nm in three hours. With a 30-knot headwind, that range drops to 360 nm—a 20-percent decrease. A Skyhawk or a Husky flying at 115 knots will cover 345 nm. With a 30-knot headwind, you’ll only see 255 nm, a 26-percent hit. A Cub cruising at 80 knots will cover 240 nm in a no- wind scenario but only 150 nm with that same 30-knot headwind. That’s a 37.5 percent reduction in range, more than one-third. The slower you go, the more headwinds affect your stride.
My Cub has 15 gallons of usable fuel and typically burns five gallons per hour, giving me 2.5 hours of endurance with a half-hour reserve. At 80 knots groundspeed, I can travel 200 nm on primary fuel, with a maximum leg of 240 nm by burning some of my reserves. A 10-knot headwind takes me down to 175 nm, or 210 nm on my reserves.
If the forecast is for no winds, I could optimistically stretch each leg to the full 200 nm and even feel safe because I have a half-hour reserve. But if the wind is beyond 13 knots on the nose, that same 200 nm leg is now outside of my reserve capacity. It is easy to see how a little headwind can make you an NTSB statistic.
The most critical part of cross-country planning with short legs is the reliability of refueling stops. The shorter your legs, the more stops you need to make and the more critical each one is to reaching your destination according to plan. The path you pick to optimally connect the dots, also defines the number of options you have when and if a planned fuel stop proves to be a no-go.
Traveling in the West and Midwest, this becomes a geography exercise in mapping population density. There are definitely some vacant parts of the sectional you have to work around. While you can skip across some of those vacant areas via straight lines, straying away from populations and highways can be perilous. Also, just because an airport is on a sectional doesn’t mean it has fuel. Just because the Inter- net shows it has fuel services is no guarantee it be available when you arrive. Calling ahead is a good idea because you might not have cell phone coverage at a remote rural fuel stop, and pay phones are quickly vanishing from public places like airports.
Some Points To Ponder.
When flying with short legs, there are three decision points to ponder en route. The most familiar of these is the point of no return. As you travel along your planned route, your legs get increasingly short as you near your target fuel stop. When you have enough fuel to make it to your destination but not enough to make it back to your point of departure, you have gone beyond the point of no return.
The next point is the point of no departure. Once you have started burning reserve fuel, the next place you land will be a point of no departure because you can’t depart without refueling. You can’t depart because legally you need fuel enough to arrive at a planned destination plus at least a half hour reserve for standard VFR. If the gas pumps are locked, there is no cell service, or the FBO tells you their tank is empty, too bad; this is your home until something gives and fuel arrives. If you take off to find fuel elsewhere, it is not so much a violation of the FARs as it is mailing your self-nomination for a Darwin award.
The next thing to ponder is the dynamic shape of your range ellipse. At the point of departure, with no wind, your range is a nice circle centered over your departure point extending equally in all directions. You can draw an inner ring showing your range on primary fuel and an outer ring showing your extended range using reserves. Beyond these two rings is your range as a glider. I don’t recommend using the last ring.
If the wind is blowing, the ellipse’s radius will increase downwind and decrease upwind. The “bow shock” of wind, effectively compresses the range of your reserves in the direction of the headwind.
Since wind direction and speed will vary along your route, the real shape of your range would actually be a cone-like amoeba shape. It comes to stubby point because your range decreases over time as fuel is burned. It will have an amoeba like shape because wind directions and speeds may vary along your route. The important thing to understand is how this shape overlays with your plan for connecting fuel stops and available off-route options and alternates. One obvious point of the shape is that it has less area as you proceed. The further you travel, the narrower is your lateral wiggle room for options.
When the Venn diagram of your range ellipse or range amoeba no longer includes your departure, you have passed the point of no return. When your intended destination is in the Venn diagram of your reserve fuel, it has become a point of no departure. When the Venn diagram of your range includes no airports, it represents the limits of your planning skills. If you allow your range-shape to come to a point, your engine will stop.
When the Wave Collapses
When looking over your flight plan and connecting the dots of your short-legged trip, try to find the critical path. If any stop is critical to mission success, call ahead to make sure they will have fuel when you arrive.
The thing to remember about short-leg syndrome is there are many variables creating feedback loops that change your options in novel ways. As you fly along, your options will shrink with the shape of your range projection. This isn’t bad; it just creates decision points and opportunities.
On my long trip to Kansas, when I arrived at Westport Airport (71K, a.k.a, “Dead Cow International”) in Wichita, Kan., I had just hit my reserve fuel as I passed into Wichita’s Class C airspace. No big deal; there are lots of airports in Wichita, but by definition, the next landing would be a no-departure landing until I could refuel.
As I pulled my plane up to the pump at Dead Cow, a guy walked out of the hangar and said, “I hope you don’t want gas; we just ran out.” Fortunately, a delivery truck was en route to refill the tanks. While the airport was getting restocked, the proprietors bought me lunch.
The fun part of long trips on short legs is the airports you get to see and the people you will meet. If you are lucky, somebody might even buy your lunch.
Mike Hart is an Idaho-based commercial/IFR pilot with 1000 hours, and proud owner of a Cessna 180.
This article originally appeared in the May 2013 issue of Aviation Safety magazine.