The classic rule of thumb is a handy and relatively easy-to-remember formula for arriving at an approximate answer to an otherwise involved or obscure calculation. A good example is estimating true airspeed based on indicated airspeed. The rule of thumb is to multiply your altitude in thousands by 2 percent of indicated airspeed, and add that to indicated airspeed. For example, you show 115 knots indicated airspeed at 6,000 feet. Make the following calculations: 115 x .02 = 2; 2 x 6 = 12; 12 + 115 = 127 KTAS. If you use precise calculations instead of the rule of thumb shortcut, it gives an answer of 128.8 kt-a minor difference. To ease the mental arithmetic, rules of thumb generally are based on rounding off.
Since no laws govern rules of thumb, they don't necessarily have to involve any mathematical calculations. For example, a rule of thumb for determining the most advantageous climb speed says that to minimize the effects of a headwind in the climb, use a cruise-climb airspeed; to maximize a tailwind in the climb, use best-rate airspeed.
Rules of thumb exist for every phase of flight. To balance that last one on climb speed, here's one to use for descents: Multiply the altitude you need to lose in thousands by four, and that's how many minutes it will take to descend at a passenger-pleasing 250 fpm. For example, if you are cruising at 9,500 feet msl and need to descend to 1,500 feet msl, you should begin your descent when 32 minutes from your target altitude waypoint (9.5 - 1.5 = 8 x 4 = 32). It works for any ground speed. Halving the multiplier to 2 halves the time to 16 minutes and doubles the descent rate to 500 fpm; a multiplier of 1 advances the descent rate to 1,000 fpm, regardless of ground speed.
One of the delights in aviation is flying with a veteran pilot from whose wise mind rules of thumb tumble like silver dollars from a slot machine. Art Ronan is such a pilot. Way back when, Ronan worked as a pilot for a company that manufactured several models of twin turboprop aircraft. He got to know those airplanes as only a pilot who flies them all day every day can. Today he is an independent instructor, specializing in initial and recurrent pilot training in those same model aircraft. Several years ago, I took a training class from Ronan, and the hours spent in the cockpit with him were a joy. Ronan had more flying tricks up his sleeve than I had memory slots to store them. I've always regretted not turning the controls over to Ronan while we were training so that I could write down each of his many useful and entertaining rules of thumb. But then, I wouldn't have done any flying.
A few weeks ago I was browsing in Banyan Air Service's fine pilot shop at Fort Lauderdale Executive Airport and came across the guide I should have published after flying with Art Ronan. It's a palm-size booklet titled Pilot's Rules of Thumb, written by Art Parma (Flight Time Publishing; 877/926-6570 or www.flight-time.com ). Parma, who lists convincing CFII, ATP, and A credentials after his name, has collected an interesting and useful array of aviation rules of thumb and quick tips.
He's arranged them into several topics: temperature/altitude/airspeed; time/ distance/fuel; takeoff and climb; flight maneuvers; flight planning/navigation; and approach and landing. The final section has the quick tips. There's good stuff in each section. For example, on page two Parma provides an easy rule of thumb for calculating density altitude: It increases or decreases 120 feet for each one degree Celsius variant from standard temperature. If, for example, it's 10 degrees warmer than standard at 5,000 feet pressure altitude, the approximate density altitude is 6,200 feet. That's a good rule of thumb to know if you fly where it's hot and high.
You say you can't use that shortcut because you don't know what the standard temperature is at 5,000 feet? Turn to page three to learn that standard temperature decreases 2 degrees C per 1,000-foot increase in altitude. Standard temperature at sea level is 15 degrees C. At 5,000 feet it is 5 degrees C (2 x 5 = 10; 15 - 10 = 5).
OK, but the rule of thumb for determining density altitude specifies pressure altitude. How do I find pressure altitude? Simple. Check page one to find two rules of thumb on the subject. The easiest one to remember is to set the altimeter to 29.92 inches. The indicated altitude is pressure altitude. The formula is a bit more complicated: Add or subtract the difference between 29.92 inches and the current altimeter setting to the field elevation at the rate of 1 inch per 1,000 feet. For a high altimeter setting (above 29.92 inches), subtract the correction. For a low altimeter setting, add the correction. If, for example, field elevation is 5,250 feet and the altimeter setting is 30.17 inches, the pressure altitude is 5,000 feet (30.17 - 29.92 = .25 x 1,000 = 250; 5,000 - 250 = 5,000 feet).
Some rules of thumb are too simple to work really well, such as the one that states: "The fuel consumption of an aircraft engine is equal to one-half the horsepower divided by 10." My airplane has two 160-hp engines, so according to this rule of thumb I should see total fuel consumption of 16 gallons per hour. In fact, my partner and I figure on block-to-block fuel consumption of 17 gallons per hour. That figure, however, is highly dependent on altitude and power setting. At low altitude and high power, fuel consumption may be 9.5 gallons per hour per engine. It would be foolish to use a one-size-fits-all fuel-consumption rule of thumb for actual flight planning - a fact that Parma acknowledges.
On the other hand, some rules of thumb are decidedly on the mark. Need to make a quick distance calculation but don't have a plotter handy? The rule of thumb is to use your thumb. The length of the average person's thumb, from tip to knuckle, is 10 nautical miles on the sectional chart scale. I tried it, and I am exactly average.
Rules of thumb are clever, fun to play with, and in many instances highly useful tools. Having missed my opportunity to catalog Art Ronan's collected wisdom, I'm happy to have discovered Parma's 49 pages of neat-to-know stuff.
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