EAA Sport Aviation - January 2012

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110 knots). Similarly, a 10 percent increase in weight results in a 5 percent increase in stall speed.

TEMPERATURES, ALTITUDES, PRESSURE, AND PERFORMANCE

Many aircraft performance parameters, such as true airspeed and available power, are affected by altitude. As altitude increases, true airspeed increases and the available power decreases. We can estimate true airspeed (TAS) by adding 2 percent to the indicated airspeed (IAS) for every

1,000 feet of altitude. Example: If indicated airspeed at 5,000 feet is 100 knots, true airspeed is roughly 10 percent higher ( 2 percent x 5), or 110 knots.

Density altitude (DA) increases with higher altitudes and temperatures. To estimate DA, add 600 feet to field elevation for each 10 degrees over the standard temperature. At sea level, the standard temperature is 59°F ( 15° C), and it decreases by 3. 5 degrees for each 1,000 feet above sea level (or roughly 10 degrees for each 3,000 feet). Example: Field elevation is 6,000 feet, temperature is 80°F ( 27° C), and we want to estimate DA. Standard temperature at 6,000 feet is about 20 degrees lower than at sea level (decrease 10 degrees for each 3,000 feet), or about 39°F ( 4° C). Since the actual temperature is about 40 degrees higher than standard (80 - 39), we add 2,400 feet to the field elevation (600 feet for each 10 degrees). The estimated DA is 8,400 feet!

A non-turbocharged engine loses about 3 percent of its power for each 1,000 feet of DA above sea level. If the DA is 7,000 feet, we have less than 80 percent power available for takeoff. That’s if we lean properly for takeoff!

If the altimeter fails in flight (for exam-
ple, due to a blocked static port), we can
estimate altitude using the manifold pres-
sure gauge (if so equipped). With full
throttle applied, the indicated manifold
pressure is quite close to atmospheric pres-
sure. Atmospheric pressure at sea level is
about 30 inches ( 29. 92) of mercury, and
decreases roughly 1 inch for each 1,000 feet
of altitude. To estimate altitude in thou-
sands of feet, push the throttle full forward,
read the manifold pressure in inches, and
subtract from 30. Example: If full throttle
yields a manifold pressure of 24 inches, the
estimated altitude is roughly 6,000 feet
(( 30 - 24) x 1,000).

FUEL CONSUMPTION

Next to checking the weather, nothing is as important as knowing our fuel consumption and ensuring we have enough fuel to get to our destination with an adequate reserve. Here again, we should always consult the POH when calculating fuel

Some simple mental math can help determine the fuel needed to get there safely.

requirements, but sometimes a sanity check is worthwhile. When an unexpected diversion is needed, some simple mental math can help determine the fuel needed to get there safely.

In general, light general-aviation aircraft engines have similar efficiencies, so the fuel burn is dependent primarily on the horsepower. For normally aspirated engines, expect to burn about a gallon per hour (gph) for every 20 hp. A 100-hp engine will consume around 5 gph, a 180- hp engine around 9 gph, and a 260-hp engine around 13 gph. Of course, that’s with the engine properly leaned at normal cruise power.

Nothing is better than doing our homework and getting the right answer, but in a pinch, it helps to know if “that looks about right.” Having a good rule of thumb can help keep us out of trouble, and out of the danger zone!

Robert N. Rossier, EAA 472091, has been flying for more than 30 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.