The Abstract Truth

Archive for August, 2006

Flying at High Density Altitude

Posted by rbpasker on August 9, 2006

[Before reading this article, it might help to review the definitions of indicated , ground, and true airspeed]

High density altitude (DA) affects both aerodynamic and engine performance.

In terms of aerodynamics, the difference you will see at high density altitude (DA) is your true airspeed (and, with no wind, ground speed) will be higher than at sea level, for the same indicated airspeed. For example, you when you rotate at 67knots indicated, at sea level, you will be at 67 knots TAS. At 8,000ft DA, however, 67KIAS is 76KTAS. With no wind, you’ll be rotating 9 knots faster ground speed. Try this TAS calculator.

Some things to watch out for:

  • You will have a longer take-off and landing roll than at sea level. Expect that the ground will be moving past you much quicker on take off and landing.
  • Your turning radius will be higher than at sea level (for the same indicated airspeed and bank angle) because your ground speed is higher. This means you will need to start your turns in the pattern EARLIER, since you don’t want to slow down or increase your bank angle in the pattern.

From an aerodynamic perspective, your plane is certificated upto its gross weight. there should not be any difference when landing at 0′DA versus 8000′DA.

From a POWER perspective, your non-turbocharged airplane will not produce as much power at 8000ft DA as it does at sea level. It is especially important not to be behind the power curve at high DA is because POWER equals ALTITUDE. Remember your slow flight? Pitch controls airspeed, power controls altitude. If you get behind the power curve, you may not have any reserve power to climb or arrest your descent, even if you firewall the throttle.

Carburetors and most fuel-injection systems mix air and fuel by VOLUME, not weight. But your engine cares about the weight of air, not the volume, because weight is a measure of the number of air molecules. At high DA, therefore, there are many fewer air molecules in the same volume of air, so using the same air-fuel mixture at high DA means that your engine will be running much richer. You need to learn how to lean your engine to produce best power. Best power is the term used to describe the air-fuel mixture ratio at which the engine produces the best amount of power.

In a nutshell:

  1. expect longer landing and take-off rolls
  2. make your pattern turns earlier, so you don’t have to increase bank angle
  3. stay ahead of the power curve
  4. lean to lean for best power

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