Temperature/pressure/altitude

[88MPH]

This is from ASA CFI Prep.

"True airspeed and true altitude are based on the existing outside air temp, which affects the density of the air. While flying at a constant power setting and a constant indicated altitude, an increase in OAT will cause the air to become less dense. Both the true airspeed and the true altitude will increase."

I understand that higher temp makes the air less dense, so you're altimeter/wings will interpret that as being at a higher true altitude, when it's just density altitude.

What I'm not getting is "while flying at a constant ... indicated altitude ..."

Theoretically, if you can hold TRUE altitude, altimeter should indicate climb in warmer, less dense air. But if holding a constant INDICATED altitude, you would have to decend the plane in response to the fooled altimeter.

I don't think it's a typo, as its repeated in the book. What the H-E-double-hockey-sticks am I missing here???? :banghead: :banghead: :banghead:

 

[Minuteman]

Quoth the Jeppesen

When the air temperature is warmer than standard, the altimeter will indicate a lower altitude than that actually flown. When the temperature is colder than standard, the altimeter will indicate too high, that is, true altitude will be lower than indicated altitude.

So they're saying the same thing as your ASA CFI prep.

This happens because the altimeter is "hard-coded" to expect a specific change from sea-level pressure will correspond to a specific height above sea-level. The altimeter displays a barometric altitude, as opposed to a density altitude.

When the atmosphere is cooler than standard, it shrinks in height (= total height required to go from sea-level pressure to ~0 inHg), and the true (tapeline) altitude required to achieve certain change in pressure is less than normal. Vice versa when things are warmer: the "height" of the atmosphere is taller and you have to go further to achieve that same pressure change.

Higher temps result in higher density altitudes for a given barometric altitude. So if you're stooging along at a constant indicated airspeed (not sure why they chose to say a constant power setting in the ASA), your true airspeed will be higher because the "rho" (density) in "one-half-rho-vee-squared" is less. The "vee" (true airspeed) will have to be higher to achieve the same ram-air pressure differential to get the same indicated airspeed.

 

[88MPH]

So, I get the airspeed/ram air density thing no problem. What I was confused with was with less dense warm air (interpreted as the same as less dense upper air) indicated altitude would be LOWER while in less dense upper air, indicated altitude would be HIGHER. Both (warmth and height) make air less dense, but one lowers true alt while the other raises it.

So is there any downfall of always picturing the "egg" depiction of the atmosphere around the earth? If I imagine the atmosphere shallow at the poles (cold air) and thick at the equator (warm air), that theory works. Just wanna make sure there are no exceptions to figuring out density/true alt in regards to temp.