Most pilots can tell you how much power their aircraft engine is capable of developing according to the book. How many can say for sure what percentage of power is being produced right now, how much more is available if needed, and what kind of performance it might be capable of delivering?
Pilots of piston-engine aircraft tend to think of power in absolute terms: full power for takeoff, idle for landing, and a mid-range value for cruise. You can check performance charts to find out what percentage of brake horsepower (bhp) you will use for that cross-country leg given a throttle setting, cruising altitude, and other values. But the percentage of power is usually accepted as a product of the input variables, not a performance objective. Maybe that’s why some pilots forget that full power isn’t always 100-percent power. Consider that according to one rule of thumb for normally aspirated aircraft engines, 30 percent of its horsepower is lost at 10,000 feet.
That’s what a density altitude calculation tells you—indirectly—when you check expected performance before that operation at a high-altitude strip on a hot day. But the exercise provides its insights in terms of expected climb rates and landing roll distances. The emphasis isn’t on percentage of power.
Your pilot’s operating handbook probably advises you to check engine power early in the takeoff run for any signs of “sluggish engine acceleration,” as a Cessna 152 manual describes it. Any doubt means discontinuing the takeoff, taxiing back to the runup area, and performing a full-power static runup in accordance with the manufacturer’s instructions.
Inexact and gut-feeling-based as a takeoff-run power check may be, it’s the only measure available absent the ability to make a direct reading of percentage of power being developed in a piston engine. At least it promotes a pilot’s awareness of engine performance before the aircraft leaves the ground.
Compounding the already critical situation for the Cessna Skycatcher’s pilot on August 1, 2011, was the distraction of incoming bad weather, said the National Transportation Safety Board.
“The pilot reported that prior to takeoff, he noted an approaching storm but estimated that the wind was light and variable for the takeoff roll,” the report said. “The airplane was slow to accelerate and reached the 50-knot rotation speed halfway down the runway. The pilot stated that the airplane lifted off and maintained an altitude a few feet above the runway, but lost altitude at the end. The airplane struck the ground and subsequently nosed over, coming to rest inverted. A few minutes after the accident, the pilot reported that it began to rain.”
The NTSB report noted that the airport density altitude at the time of the accident “was calculated to be approximately 9,000 feet, which is 1,000 feet above the manufacturer’s recommended maximum takeoff altitude as outlined in the pilot’s operating handbook.” A five-knot quartering tailwind recorded minutes before the accident had become a quartering tailwind at 22 knots, gusting to 32 knots, shortly after. The NTSB determined the probable cause of the accident as “the pilot’s decision to attempt a takeoff at a density altitude that was outside the performance envelope of the airplane. Also causal was the flight’s likely encounter with a sudden tailwind gust.”
On August 29, 2011, a Cessna 172 attempted a landing in Atlanta, Idaho, where the airport’s 2,460-foot runway sits at an elevation of 5,500 feet msl, with higher mountains close by.
“According to the pilot, on short final approach in his low-performance airplane, he realized that his approach path was too high, so he attempted to go around,” the NTSB reported. “After full engine power was applied, he recognized that he had insufficient distance from the approaching trees and terrain to perform a go-around. Therefore, he aborted the maneuver and forced the airplane onto the ground in a clearing beyond the departure end of the runway. The airplane hit the ground hard, breaking the wing and fuselage structure.”
The pilot told investigators that he was unfamiliar with the airport, and “had not read theFAA's published remarks for the airport in its Airport/Facilities Directory. The remarks included a recommendation that only pilots proficient in mountain operations and flying high-performance aircraft use the strip. The remarks also informed pilots that a go-around would not be possible ‘due to rising terrain and trees.’”
The NTSB attributed the accident to the pilot’s misjudgment of his final approach path, and inadequate planning.
You can check how much power your aircraft engine can make, and you can estimate fairly accurately whether it will be enough for a given set of conditions. But don’t consider your precautions complete unless the sound, the feel, and the readings on your power gauges assure you that your takeoff is proceeding as planned.
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