If I'm wrong, and all pilots scrupulously avoid embellishing the truth about airspeed, then I'm slower than everyone else flying the same make and model airplane.
The fabrications begin with the POH for my airplane. In the list of performance specifications, maximum speed is stated as 205 mph/178 knots true airspeed (KTAS). In fact, I routinely achieve that 178 KTAS - in a long descent from high altitude. Since the only time I fly at full power is on takeoff and initial climb, a more realistic specification is 75-percent-power true airspeed. The POH says that at 75-percent power, I should see speeds of 194 mph/169 kt.
Based on my experience in the airplane, those specifications are rubbish. In the 36 years since my airplane was built in a now-abandoned Piper plant in Pennsylvania, I don't think it has ever been capable of going as fast in level-flight cruise as the POH says it should.
My partner and I recently installed several airframe modifications, including sleek new engine cowls, fairings behind the retracted main landing gear tires, and wing-root fillets, that are supposed to increase top speed, among other benefits. Indeed they have, to the tune of about 3 to 5 kt depending on how generous we're feeling when asked. (There I go, embellishing.) Even so, the best that our modified speedster can manage in level flight at 75 percent power is about 163 KTAS.
The POH isn't the only source that says I'm flying slower than I should be. I've heard similar airspeed claims from owners of the same make and model airplane. The hyperbole usually is combined with fuel consumption claims that defy the physical laws of the internal combustion engine, especially the relatively crude, fuel-inefficient versions that we fly behind in light airplanes.
Airspeed enhancement isn't limited to the type of airplane I now fly, either. I owned a Cessna 172 for about five years, and after studying the POH and listening to other Skyhawk owners, I came to believe that either I was flying it incorrectly, or it was built on a slow day. It took time and perspective to understand that manufacturers' performance specifications - especially those for older airplanes - overstate real-world performance by perhaps 3 to 5 percent. Plenty of pilots do the same.
Along with overstating airspeed, pilots are guilty of understating flight time. The fiction begins when you base elapsed time on cruise true airspeed, then wanders even further away from reality if that airspeed incorporates the standard 3-to-5-percent fish-tale factor. For example, if I'm planning a 500-nm flight with my family, I might tell them it should take just under three hours. That's based on the POH claim of a 169 KTAS cruise at 75 percent power at 8,000 feet. Of course, my airplane doesn't fly that fast, so right away I'm guilty of stretching the truth in order to minimize the objections that I might hear about spending that much time in a noisy, drafty light airplane.
Even if the airplane were that swift at 8,000 feet, I'd probably fly higher. It's smoother at higher altitudes, especially here in Florida during the summer when thermals and towering cumulus grow thick and rise high. The airplane also is more efficient - uses less fuel for the same airspeed - at 10,000 to 12,000 feet mean sea level, and there's less traffic up high. The slight sacrifice I pay for flying higher is a lower maximum true airspeed, because the normally aspirated engines are only able to generate 60 or 65 percent power in the thinner air at higher altitudes. The airspeed loss that comes with flying higher only amounts to a few knots, but it adds to overall flight time, and that means the estimate of fewer than three hours for the trip that I've given my family is even more understated.
The biggest injustice I could perpetrate on my family is basing flight time on cruise speed rather than average speed block to block, takeoff to touchdown. If I faithfully accounted for the extra time it takes for taking off, climbing to cruise altitude, maneuvering for the approach, landing, and taxiing to the ramp, then I would have to add perhaps 30 minutes more to the elapsed time estimate.
Instead of averaging 169 kt over the 500 miles, the block-to-block average speed most likely will be under 150 kt, and even slower if I have to battle a strong headwind. In several hundred hours of flying the airplane on long cross-country trips, 150 kt is what I generally average from takeoff to touchdown.
True airspeed in cruise is an important specification for estimating flight time and fuel consumption between en-route fixes. It's also handy to know when you're swapping fish tales with fellow pilots. However, it is only the starting point when estimating total flight time.
You must factor in the wind component - headwind or tailwind - to arrive at a groundspeed and time en route, at least for the cruise portion. From there, start deducting speed and adding minutes for the takeoff, climb, approach, and landing phases of flight. In the end you'll arrive at a realistic estimate of total trip time - and an honest answer to your family's question of "How long will it take to get there?"
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