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Density altitude - the secret
killer 
Density altitude is perhaps just as hazardous as
structural icing. In a recent NTSB study, density altitude
contributed to just as many accidents as structural icing as shown on
the right. Click here to view a larger image.
To be fair, some of the density altitude accidents
in the NTSB study were caused by pilots departing in an over-gross
weight aircraft or using improper procedures (e.g., improper flap
usage). However, pilots need to be aware that gross mistakes
such as this are not forgiving when the density altitude is
high.
What is density altitude? In
simple terms, density altitude (DA) is pressure altitude corrected for
non-standard temperature. Therefore, if the pressure and
temperature throughout the atmosphere matches the standard, then
pressure altitude and density altitude are the same. Of
course, during the warm season, the temperature is generally above
standard in most locations around the U.S.
While a pilot can determine pressure
altitude in the cockpit (by setting the altimeter to 29.92"), there's
no instrument that you can use to directly measure the density
altitude. It must be calculated based on the pressure,
temperature and dewpoint temperature.
The best way to think about density
altitude is to first calculate the density of air without moisture
based on a standard atmosphere. The table below is that
calculation up to 4,000 feet for brevity. The column on the
right is the resulting density of air based on the standard temperature
and pressure while disregarding moisture (for now).
Let's say you are planning to depart an
airport that is located at sea level. Assuming the pressure is standard
(29.92"), but the temperature is 86°F (non-standard) and the dewpoint
is 64°F, what is the density altitude? If we want to utilize
the table above, we first need to calculate the density of air using
the temperature and dewpoint. After plugging the data into
the proper equations you calculate the density of air to be 0.002242
slugs per cubic foot. Using the table above, this density
equates to the same density as air at 2,000 feet. Therefore,
at sea level when the temperature is 86°F
and the dewpoint is 64°F, 2,000
feet represents the density altitude.
Of course, no pilot is going to
calculate the density of air. This was just an exercise to
understand how the actual density of air at the surface relates to a
given altitude. In this particular case, the aircraft will
perform as if it is at 2,000 feet not sea level.
Using this online calculator, the
density altitude can be determined by simply obtaining the
observed station pressure, observed station temperature and observed
station dewpoint temperature.
You can easily get the surface temperature and dewpoint from
the ASOS or AWOS broadcast, but the surface pressure (altimeter
setting) is adjusted to mean sea level. But for now, let's
assume we are at sea level with standard pressure.
What about that moisture? The
table above considers the atmosphere to be completely dry. Of
course, that is not the case in a real atmosphere. On most
warm weather days the dewpoint temperature may range from 60°F to 80°F
providing plenty of moisture. The moisture in the atmosphere
makes the air less dense which leads to a slightly higher density
altitude.
So is there a need to correct the table
above? Actually no. The correction is taken when
the actual density altitude is calculated. In order to
properly calculate the density altitude, virtual temperature must be
used, not the actual temperature. Virtual temperature is the
temperature that air will achieve if all of the moisture was removed.
Many thermodynamic equations become much easier with virtual
temperature since it greatly simplifies the thermodynamic equations.
The best way to visualize virtual
temperature is using two parcels of air - assuming the same pressure
for both parcels, of course.
Parcel One: This air parcel has some measured temperature T1.
It also has some moisture in the parcel. As a
result, the parcel has a density D.
Parcel Two: This air parcel also has density D, but has no
moisture. Obviously if it has the same density with no moisture, it
must have a different temperature. To achieve density D, the
temperature of the second parcel must be greater than that of the first
parcel. This new temperature T2 is called the virtual
temperature. The virtual temperature is higher and
essentially accounts for the moisture that is in the air.
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