Hey I'm finishing up on my instrument but I'm having a little bit of a hard time grasping the concept of how temperature affects pressure levels. I understand the whole "true altitude is lower than indicated altitude when the temperature is colder than standard" deal and how it's because the pressure levels are spaced differently apart, but the FAA books don't really do a good job of explaining the how and why of it (my biggest gripe with all their material). Can anybody shed some light on this?
Temperature and pressure levels.
- Thread starter JordanD
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lhornaday
Well-Known Member
I confused myself while writing this. Hopefully there is some truth here...
The altimeter would work without error if temperature and pressure never changed. But they do. We compensate for pressure difference by the altimeter setting of course, but there isn't a way for us to make changes for nonstandard temperature.
When the air temperature begins to increase, air density will decrease. This is because hot air molecules move very fast, pushing each other away. Colder air molecules don't. Colder air becomes more dense. This is only true if the other air molecules have somewhere to be pushed to. Air near the ground doesn't get pushed as far; therefore the altimeter error is lesser near the surface of the Earth.
Air entering into an altimeter of an airplane flying along at an altitude of 5,000 feet indicated will begin to slow down if it become cooler. The air won't push as hard on the aneroid wafers. In order to keep the plane flying along at 5,000 feet indicated, the pilot will have to descent to keep the the altimeter indicating the same value. This is done very slowly and with an autopilot of course.
The altimeter would work without error if temperature and pressure never changed. But they do. We compensate for pressure difference by the altimeter setting of course, but there isn't a way for us to make changes for nonstandard temperature.
When the air temperature begins to increase, air density will decrease. This is because hot air molecules move very fast, pushing each other away. Colder air molecules don't. Colder air becomes more dense. This is only true if the other air molecules have somewhere to be pushed to. Air near the ground doesn't get pushed as far; therefore the altimeter error is lesser near the surface of the Earth.
Air entering into an altimeter of an airplane flying along at an altitude of 5,000 feet indicated will begin to slow down if it become cooler. The air won't push as hard on the aneroid wafers. In order to keep the plane flying along at 5,000 feet indicated, the pilot will have to descent to keep the the altimeter indicating the same value. This is done very slowly and with an autopilot of course.
You've pretty much got it there. Consider a column of air: if the temperature is high, that air will want to expand. Because of all the air around it, however, it cannot expand laterally, so it much expand vertically. As it does, the pressure levels will become farther apart (kind of like a spring being stretched). If the temperature drops, then the air will want to contract, and the presence of the air around it will make that contraction happen vertically as well - the pressure levels become more closely spaced. Hope that makes sense.
Doesn't necessarily have to be done with an autopilot - all it takes is the pilot noticing the altitude deviation and "correcting" for it. This can create some real problems on instrument approaches at high altitudes when the weather is cold - you can get some serious errors on MDAs and the like. Fortunately, there are correction tables out there - there's one in the Instrument Flying Handbook (Figure 3-7 according to my brief search).
lhornaday
Well-Known Member
It would be the opposite since decreasing temperature slows everything down. Think of a hot air balloon, the warm air makes it expand and climb (more pressure in the balloon. If it were cold air, it wouldn't do so well. Much like cold water does to... well, you get the point.
Sorry, that was an attempt at a joke.
I was in the pool! I was in the pool!
gestrich19
New Member
Non-standard temperatures essentially change the lapse rate of pressure change. Imagine a stack of air getting shorter when the temperature cools. The density of the air of any given altitude takes the property of the air that was above it - it becomes less dense, so your altimeter reads a higher altitude than actual. (hot to cold, look out below). This is the only way I could make sense of it.
The error increases with the deviation from standard temperature and the height above the reporting station. Even with the correct altimeter setting from the reporting station, the error will still be experienced.
The error increases with the deviation from standard temperature and the height above the reporting station. Even with the correct altimeter setting from the reporting station, the error will still be experienced.
That kinda makes sense now.. Thanks for the input everyone.
:yeahthat:
Anyone know the video that explains this? They do a wonderful job, but I cannot locate it.
PS Nice to see you posting here gestrich.