This is why people tend to stall during takeoff and landing, because they feel they are going faster than they normally do. Keep in mind your visual reference to the ground speed for takeoff and landing is going to go up 13%. Second, if you rotate at 60 knots indicated airspeed at sea level, then at 8,000 foot density altitude your true airspeed (ground speed in calm air), rotation speed would be 68 knots. So at 8,000 feet density the 100 horse engine is only producing 76% of the thrust, a 24% loss of thrust. This is derived from various performance parameters, and it could be 7,000 foot density altitude but 8,000 feet is a good round number for this rule of thumb.įirst, for every 1,000 feet increase in density altitude, the engine loses about 3% power or thrust. Take this with you.Īt 8,000 feet density altitude, it takes about twice as long to lift off as it does from sea level. Now, how does density altitude affect performance? Let’s start with the rule of thumb I teach and use as a basic for density altitude performance. You can also listen to the AWOS you have various ways to determine density altitude. You can use any app, the classic E6B computer, or an electronic E6B to find density altitude. Temperature makes a much bigger difference in density altitude. Note that variations in temperature can cause the density altitude to go up 3,500 feet altitude and pressure altitude only 500 feet variation. If we have a hot day say 80 degrees F, the pressure altitude is 6,500 feet, but density altitude goes up to 9,500 feet. Standard temperature for 6,500 altitude is about 37 degrees F. Typically, it would be less but for calculation expect atmospheric pressure could raise or lower your standard temperature altitude by 500 feet. Falling pressure raises your density altitude by up to 2,000 feet 6,500 to 8,500. From the well-known FAA diagram (pressure altitude) we can see that with higher barometric pressure you could lower your pressure altitude by by up to 1,000 feet or so, from 6,500 to 5,500.
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