Knowing the differences in performance
Recently we departed Kendall-Tamiami Executive Airport south of Miami headed northeast. The initial clearance was to 2,000 feet, but before we got there the controller cleared us to climb to 4,000. I set the power for climb and pitched the nose up until the airspeed indicator settled on 120 mph/104 knots-best rate of climb speed in the airplane I fly. The needle on the vertical speed indicator wavered in the vicinity of the 1,000-feet-per-minute mark, give or take a hundred feet. Not bad, I thought.
Later that evening I spent a few minutes leafing through the airplane flight manual and was surprised to see that the specifications call for a climb rate of 1,490 fpm at best-rate-of-climb speed. That's nearly 50 percent better than I saw in our airplane. But it gets worse. The book specifications are supposed to reflect performance at maximum gross weight in standard conditions. It was a good 30 degrees above standard temperature (59 degrees Fahrenheit) when we flew, so that would account for some loss of performance, but with just two people aboard and less-than-full fuel tanks, I figured we were about 16 percent under maximum gross weight. Our lighter weight should have more than made up for the higher ambient temperature. In fact, according to the "Climb Rate vs. Density Altitude " chart in the flight manual, we should have been climbing at about 1,600 fpm.
What's going on here? I was doing a good job of maintaining VY and we were way below max gross weight, yet the climb rate was nowhere near what the book promised. That's not only perplexing, it's discouraging. If climb rate doesn't measure up, what other performance numbers in the flight manual are out of sync with reality?
The fact is, performance specifications represent an ideal case-a new, factory test airplane flown by a trained and experienced test pilot under controlled conditions. Our airplane is flown by a standard-issue commercial pilot, the airframe is 33 years old, and although the engines (it's a piston twin) have been overhauled, they have a combined 1,500 hours of use since the overhauls. Compression is less than in a new engine, the magnetos and plugs aren't delivering the same ferocious spark as when new, and the induction and fuel-injection system isn't as efficient as it once was. All of that means the engines produce less power than that factory test machine. The propellers have fewer than 100 hours on them since overhaul, so at least they should be up to the book-performance challenge.
No doubt many other factors contribute to the poor showing compared to book specifications-errors in the pitot-static system that lead to errors in airspeed and vertical speed indications; variations in the rigging of ailerons, flaps, and trim tabs that add to drag; and dents and dings in the wing and fuselage that disrupt the smooth flow of air over the surface, affecting both lift and drag. Finally, given our weight and the density altitude, the climb-rate chart says I should have maintained an indicated airspeed of about 110 mph/95 knots in the climb to achieve best rate. The lower speed/higher angle of attack would have helped close the climb-performance gap somewhat, but not entirely.
The climb-rate chart in the flight manual falls under the heading of book smarts. The exercise of comparing actual performance when we departed Tamiami to book performance has given me some useful insight into the airplane I fly. The result is that I now have a street-smarts filter through which to view the performance-planning charts in the flight manual.
Street smarts are especially useful when you're looking at operating near the edges of the performance envelope. That's the territory where the margin separating a safe operation from a potential problem can get uncomfortably narrow.
We were looking forward to a trip to Pittstown Point, a small fishing resort in the Crooked Islands in the southern Bahamas. The resort has its own paved airstrip, but it's narrow and just 2,000 feet long. On the positive side there are no obstacles-the approach end of the runway is just a few feet from the water-and there's a 400-foot-long overrun on the departure end. It would be challenging, but in my opinion well within the capabilities of the airplane and pilot.
To confirm my confidence, I consulted the "Accelerate-Stop Distance " chart in the flight manual. The chart assumes a worst-case scenario-you're about to take off when a serious problem occurs that requires you to abort the takeoff run. From the chart I can calculate the total distance required to accelerate to rotation speed and then come to a complete stop by chopping the throttles and applying maximum braking without locking up the wheels and skidding the tires. The chart tells me the minimum length of runway I'll need to accomplish that.
Given the expected weight of the airplane, wind, temperature, and density altitude, I concluded the accelerate-stop distance when departing Pittstown Point would be about 1,800 feet. That gave me a 200-foot cushion, plus the overrun. However, my street smarts caused me to add a 10-percent fudge factor, which took the accelerate-stop distance right to 2,000 feet, or the full length of the runway. I was comfortable with that, and in fact we lifted off and began climbing in less than half the length of the runway.
Book smarts-the takeoff planning chart in the flight manual-said we could have been as much as 400 pounds heavier when we started the takeoff run and still have met the 2,000-foot runway-length limit. Street smarts said no way. The airplane and pilot simply do not perform up to the standards that the planning charts are based on, so to be safe back off a comfortable distance from the limits.
Another flight-planning exercise where street-smarts are needed to convert book-smarts to useful and safe reality is when computing range. The "Range vs. Density Altitude " chart for my airplane says that if I depart with the tanks full, I can blissfully fly along at 10,000 feet and cover a bit more than 1,050 statute miles (912 nautical miles) and land with 45 minutes of fuel. Great! Think I'll start planning a flight from Tampa, Florida, to Corpus Christi, Texas, a great circle distance of 915 nautical miles, all of which is over the Gulf of Mexico. And why not-book smarts says go.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.
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