Performance: Hopelessly optimistic

Given the airplane’s 3,100-pound takeoff weight, time en route, landing runway elevation and slope, surface condition, current wind, and density altitude, ForeFlight crunched the numbers and calculated a ground roll of 911 feet. It also anticipated a takeoff ground roll of 912 feet. But how accurate was this information, and could it be trusted?

ForeFlight points out that its performance metrics are taken directly from manufacturer-provided data. The information comes straight from the pilot’s operating handbooks of more than 200 aircraft models, and ForeFlight lets pilots customize their own aircraft information by adjusting for weight changes and modifications. The Texas firm doesn’t independently test the data or verify it, so we decided to put the manufacturer data to the test.

Richard McSpadden, executive director of the AOPA Air Safety Institute, was a few minutes behind me flying a Cessna 182 Skylane, and ForeFlight expected his airplane to stop in 587 feet. ASI’s video crew was standing by the runway with cameras and measuring devices to document the results.

Student pilots learn in ground school to accurately read the tables and charts aircraft manufacturers devise. Then, once we become pilots, we develop a deep distrust of those calculations as hopelessly optimistic and we round up. Way up.

ASI has long recommended adding 50 percent to takeoff and landing distances as a fudge factor. The manufacturer numbers, after all, were derived from professional test pilots flying factory-new airplanes under carefully controlled conditions, and regular general aviation pilots shouldn’t expect to match them. It’s wise to give ourselves a healthy margin to compensate for aged aircraft and our own less-than-perfect technique.

I configured the Bonanza for a normal landing with full flaps and a 75-knot approach speed, and aimed for the fixed-distance marker at the 1,000-foot mark. The Bonanza touched down on the main gear, and I lowered the nosewheel to the pavement and braked moderately. It came to a stop after a ground roll of less than 1,000 feet, just about what the app had predicted.

Next, I lined up for takeoff using the manufacturer recommended short-field technique: Flaps up, maximum rpm, full power, and brake release. I expected to get off the ground in about 900 feet. Fifteen seconds after brake release, I reached 70 knots, rotated, and the main wheels broke free of the runway just shy of the 1,200-foot mark. My maximum-effort takeoff was about 30 percent longer than the ForeFlight figure.

Using textbook technique, McSpadden touched down on the main gear and braked aggressively. Yet his actual stopping distance was 760 feet—about 30 percent more than ForeFlight had anticipated.

We both repeated the exercise multiple times, and our results were consistent.

In the Bonanza (a 1988 model), landing distance was accurate but takeoffs were well in excess of the predicted value. In the Skylane (a 2001 model), ForeFlight underestimated both takeoff and landing distances by roughly 30 percent. Both aircraft are extremely well maintained, regularly flown, and were operated for this test by current, experienced GA pilots.

The information ForeFlight provides is meant to increase pilot awareness of aircraft performance and terrain, critical factors for flight safety. The app also warns pilots if runways are unsuitable or, for twins, whether single-engine performance is unsatisfactory for current conditions or terrain. (Takeoff and landing distance calculations are only available to ForeFlight customers who pay for $300-a-year premium subscriptions.)

My first impression of ForeFlight’s performance calculator is that ASI’s original recommendation of adding 50 percent to estimated takeoff and landing distances was about right.

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