Accident Report: A shaky indicator
How much fuel do you really have left?
Given a reasonable selection of airports along the route, the choice can boil down to what’s most comfortable for you and what facilities are available at the airports you will overfly. As for that comfort factor, at what point along the route will you start evaluating airports as possible fuel stops? Is there more to making the decision than the price of fuel and the menu in the airport café?
Perhaps you have decided that you will skip any airport that you reach before, let’s say, a minimum of 60 percent of your fuel load has been consumed. Then what’s the maximum fuel burn you are willing to accept before you want to be taxiing up to a fuel pump? Seventy-five percent?
What about pushing on until between 80 to 90 percent of the fuel load is gone—does that sound like a reasonable plan?
Remember before you answer that the decisions you make will be based not on hard facts about the fuel burn but estimates of the rate of your aircraft’s fuel consumption, contingent on such variables as the aircraft’s past performance (which, of course, you have been tracking meticulously over time), the current condition of the hardware, and your ability to extract the published performance.
Then there’s the impact of any deviation from your planning. Climbing to a higher altitude than that which you planned, or making a series of altitude changes to avoid turbulence or find improved visibility for spotting checkpoints, would eat into fuel remaining.
Even if your flight is proceeding as your trip log states, snares await those who cut their margins too close. So make careful and early calculations of your groundspeed, comparing it to estimates you based on a winds-aloft forecast you acquired as close as possible to departure time.
If the groundspeed is revealed to be lower than you expected, it may mean more than the aggravation of a late arrival at the destination. If not recognized promptly, it could elevate the problem to the level of an emergency.
On August 2, 2015, a Piper PA 28-181 with a pilot and two passengers aboard lost power and landed off-airport near American Falls, Idaho, on a flight from Lincoln, California, to Idaho Falls. There were no injuries, but the aircraft was substantially damaged.
The National Transportation Safety Board said the accident was probably caused by the 92-hour pilot’s “inadequate in-flight fuel planning, which resulted in fuel exhaustion, a loss of engine power, and an off-airport landing in rough terrain.”
The NTSB noted that the leg from Lincoln was the second leg of the day, and the aircraft had been fueled there. “The pilot stated that with full tanks the airplane could fly 5 to 5.5 hours, and he intended the next fuel stop to be 4.5 hours later at Idaho Falls,” it said.
The report noted that the flight’s groundspeed disparity did not make itself evident until late in the game. Then, it said, the pilot relied on fuel-gauge readings—not a trustworthy source of information when exactitude is required—when planning what to do next, including whether to stop at an airport the flight would overfly in short order.
Four hours into the flight, the NTSB said, “the pilot determined that the groundspeed was less than planned. The left fuel tank indicated five gallons, and the right indicated more than five gallons, so the pilot estimated that there was enough fuel for one more hour of flight.”
The Piper overflew the Burley, Idaho, airport. But the pilot, unfamiliar with Burley, decided to continue to Pocatello, Idaho, estimating that he would arrive there with 30 to 45 minutes of flight time still possible.
He almost made it.
“About 10 miles and three to five minutes from Pocatello, the right fuel tank indicated empty and the left tank two to five gallons with the indicator shaking. This was four hours, 40 minutes into the flight, and then the engine stopped,” the report said. The aircraft landed in a field. The main landing gear departed the aircraft, the nose gear was crushed upward, and the left wing was damaged. “After the landing, the pilot noted that the left fuel gauge still indicated five gallons,” the NTSB noted.
What percentage of your fuel is the maximum amount you would consume before looking for a place to land? Does a paved 4,092-foot runway seem like a reasonable haven if you are starting to feel squeezed? Could a fuel gauge be off by five gallons?
A thoughtful answer to each of those questions should raise your safety margins to a safe level.
Fuel quantity indicators must be calibrated to read zero during level flight when there is no usable fuel in the tank, but accuracy may vary at other fuel quantities.
Fuel tools
Do you know which airports along your route have fuel? The AOPA Flight Planner can display fuel prices at airports along your proposed route and will mark legs where your reserves are expected to be running low, considering forecast winds aloft and your aircraft's fuel usage. But don't forget to reevaluate in the air as conditions change.
