6th April 12:38
PP Written question drivin' me nuts
Excellent question. The answer is often not well explained.
Not necessarily. *If* two parcels of air have the same pressure but
different temperatures, the colder parcel is indeed denser than the warmer
parcel. But if their pressures are *not* the same, then either parcel might
be denser than the other.
That depends on what part of the airplane you're talking about. Both
temperature and density contribute to pressure, but the altimeter measures
only pressure. In contrast, the airspeed indicator and the wings respond to
density rather than pressure. That's why conversion between IAS and TAS
depends on density altitude rather than pressure altitude; more critically,
it's why aspects of airplane performance--such as distance required for
takeoff or landing--depend on density altitude rather than pressure altitude.
Think about how an altimeter works. It measures pressure. Under standard
atmospheric conditions, temperature, pressure, and density decrease with
altitude at a standard rate; pressure, in particular, decreases by about 1"
per 1000'. That's what the altimeter is calibrated for. If you set the
Kollsman window to 29.92", the altimeter will indicate all altitudes
correctly under standard conditions; it's just calibrated to read out the
altitude that corresponds to the standard pressure at that altitude.
Of course, pressure at ground level is often higher or lower than the
standard pressure for that altitude. So you compensate by adjusting the
Kollsman window to the current barometer reading; your altimeter then
indicates the correct altitude at ground level. And it does so *regardless*
of whether the temperature is abover or below standard; the altimeter
doesn't know or care how much of the pressure comes from temperature and how
much comes from density.
The trick is what happens when you take off and climb. Think of the layers
of different pressures in the air above you. If the air is colder than
standard, then those layers are vertically compressed, and the pressure
lapse rate is thus greater than standard. As a result, if you climb 1000',
the pressure decreases by a bit more than 1", so your altimeter thinks
you've climbed by a bit *more* than 1000', and thus indicates a slightly
higher altitude. *That's* the source of the instrument error.
Fortunately, unless you're very far north, the range of temperatures you'll
usually encounter will be such that the altimeter error due to nonstandard
temperature is small. But nonstandard temperature has a much larger effect
on density altitude, which is critical to take into account when calculating
your aircraft's performance.