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What is Density Altitude?

Summer is coming, and with it comes hotter weather: Good for the beach performance for you, Bad for aircraft performance. And this poor performance can get very scary.

This article is to review the danger of flying on hot days and tips for safer flight decisions on such days.

What Is Density Altitude?

In layman’s terms it is the altitude at which the airplane thinks it is at.

In the Standard Atmosphere, Pressure Altitude (ALTp) is the height measured above the 1013hPa pressure level, and Sea level temperature is 15oC.

Density Altitude (ALTd) is the ALTp corrected for non-standard Outside Air Temperature (OAT).  An increase in OAT and/or ALT causes air density to decrease.

How Will High Density Altitude Affect Flight?

Power & Thrust

a) Less Power and Thrust available (non turbocharged engines)

b) Slower take-off acceleration = longer take-off run

 

Aerodynamic and Performance

a) Need higher TAS to attain the same lift

b) Slower Rate of Climb.

c) Longer landing distance and faster touchdown speed (TAS vs IAS)

d) Reduced propeller efficiency.

 

Tips for Flying in High Density Altitude Areas            

  • Always assess H;H;H;H checks before flight (Hot;High;Heavy;Humid)
  • or W;H;A;T?“(Weight;Humid;Altitude;Temperature)
  • Plan flights when it’s cooler (mornings or evening)
  • Talk to local pilots for density altitude procedures at that airport.
  • Know your aircraft. Some aircraft climb more efficiently with first increment of flaps.
  • Apply a rule of loading to 90% of max take-off weight only.
  • Don’t load unnecessary fuel.
  • If you doubt your aircraft performance, fly a circuit by yourself to check.
  • Choose an abort point along the runway.
  • Attain 80 percent of your takeoff speed at the runway’s halfway point.

Calculating Density Altitude

ALTd in feet = ALTp in feet + (120 x (OAT – ISA temp))

© C.Gray 2016

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