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True Airspeed vs Aircraft Performance

Asked by: 980 views Aerodynamics

If I am off on any of these basic subjects let me know.

True airspeed increases as air density decreases, because the air is thinner which allows your aircraft to move more quickly through the air.

However, the same factors that affect density also affect aircraft performance.

On a hot day at my high elevation airport, my cruise TAS would be higher than on a winter day, but I wouldn't be able to reach more than 60% power. Conversely on a cold day I can have better performance but my TAS would technically be lower? 

How do I make sense of this? It would seem like there could be a point where there is a perfect match of density to allow for best possible TAS without sacrificing performance.

On the cruise performance chart for GA aircraft (I'm going to use the CE-172) as my %BHP decreases my TAS increases.  This is for cruise, obviously not climb. So am I right in saying that in the summer, it is more difficult to climb because of density altitude, but once we are in cruise then "performance" would overall increase because we are traveling faster through the air?

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2 Answers



  1. Mark Kolber on Aug 25, 2023

    I’m not completely sure what you are asking, but there are two related principles at work. (1) TAS increases *relative to IAS* as density altitude increases. (2) power and thrust decrease as density altitude increases due to both decreased engine performance and airfoil efficiency. Between the two it means you may not be able to get the IAS needed to get a substantially increased TAS.

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  2. CA787 on Mar 02, 2024

    Answer:
    Normally aspirated engines in aircraft generally perform (top TAS) best between 8000-10,000 MSL, see your C172 POH. You can get a little more power (% BHP) when its colder but that thicker air slows down the airplane (we are talking about <1% here – 1knot.)
    Generally, your C172 performs better with respect to TAS, and Fuel consumption when it is warmer out. 8000 Feet Pressure altitude looks like the sweet spot – 122 KTAS, and with above standard temperatures you can get that TAS at 7.9 gallons per hour.
    At 10,000 MSL you can expect 121 TAS but with the savings of 5% fuel – 7.5 vs 7.9 GPH. so you have a choice on what\'s more important to you -Time or Fuel.
    The aircraft will take a little longer to climb to Cruise Altitude when it\'s warmer than standard but the climb speed is the same. This is all theoretical really, since we are talking about such small numbers. Throw in a 50 Knot headwind at 10,000 MSL and you would definitely fly lower, assuming less headwind.
    As an airline captain, I calculate- speed, altitude and wind problems constantly on every leg, no pilot has ever complained about extra fuel and/or arriving early.
    Hope this helps.

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