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I understand that for any specific aircraft, at a constant power setting, you can use the aircraft's attitude as a proxy for airspeed.

For e.g. In aircraft X, when the dashboard is 1 inch below horizon, at power setting X, means the aircraft will be flying at around speed X.

My question is, if you are much higher in terms of altitude, won't your attitude based on visual reference e.g. How many inches away is your dashboard to the horizon, be different when you are much closer to ground?

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  • $\begingroup$ I would say not significantly, at least not in a small plane. The horizon is 60+ miles away, you are 1 to 2 miles above the ground (5000 to 10000 feet). Draw a right triangle and do the math, you will find the angle is pretty small. If you go higher, you may find you need more back pressure to hold altitude, but then you are compensating for loss of horsepower with altitude (unless the engine has a turbo to maintain sea level power, most small planes don't). The other thing you will find is that due to lower power at altitude, airspeed drops, but the thinner air lets ground speed go up. $\endgroup$ – CrossRoads Feb 19 '20 at 17:17
  • $\begingroup$ "... the thinner air lets ground speed go up." Unless you are facing a stronger headwind! ;) $\endgroup$ – Michael Hall Feb 19 '20 at 20:37
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Yes, due to the curvature of the earth. But you'd have just as much variation with a small change in pilot eye position within the aircraft. The exact "sight picture" of the top of the nose (or the top of the instrument panel) relative to the horizon is not really something that you memorize and use as an absolute reference point with the expectation that it will be exactly the same every time you fly a given airplane. It's more just something that you keep an eye on to watch for changes. By the time you've climbed to altitude, you've had plenty of time to (perhaps unconsciously) get used to the resulting slight change in the sight picture between the nose and the horizon line. All the while, other instruments such as the altimeter or airspeed indicator or vertical speed indicator help tell you whether the place you are putting the nose relative to the horizon, is really the correct place to make the airplane do what you want it to do. Of course, there many instances where the horizon is not clearly discernible anyway.

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  • $\begingroup$ Wow this answer is perfect. Thanks so much! $\endgroup$ – Flightsimrightnow Feb 19 '20 at 17:18
  • $\begingroup$ Yes, good answer, but curvature of the earth isn't much of a factor until you are WAY up high... $\endgroup$ – Michael Hall Feb 19 '20 at 18:26

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