Time to start thinking of Spring and Summer Adventures
In considering upcoming flights this Spring and Summer, we have considered trips not only coastal but also high altitude destinations. Considering high altitude and mountain airports, one must consider the aircraft to be used and its performance.
To satisfy our curiosity, Aaron and I flew the Colt to 12,000' today to see what our climb would be considering a high density altitude airport. As you can see, climbing at Vy (best rate) we are still producing over 500 feet per minute. This is more than sufficient performance for most mountain flying destinations.
Below is is a great article written by Amy L. Hoover considering mountain flying and performance. Read and enjoy.
Summer is a time to slip the surly bonds and go on that adventure to the coast or to the mountains, to load up the family and make your way to Disneyland, or airplane camping and fishing in the wilderness. It is a time to use your craft in the myriad ways for which it was intended. But, when the mercury rises, you know your airplane just isn't going to perform in the way to which you are accustomed, especially at higher altitudes.
At a given gross weight, increase in density altitude (DA) means a reduction in engine horsepower (thus a reduction in thrust), increased ground speed (thus longer takeoff and landing rolls), and decreased climb performance. The changes can be significant, especially at higher elevations. 
A turbocharger can solve some of the problems (except with your pocketbook). However, loss of propeller efficiency is a big factor, even on turbocharged aircraft. If your summer flying takes you to the mountains, you will learn to operate in the morning or late evening when your engine, propeller, and wings perform better. A major item you can control is how you load your airplane; you determine is how you load your airplane.
When you operate at
higher elevation airports in the heat of the summer, do you have a handle on
how much weight reduction is necessary for safe operations? How often do you
really know just what you can expect from your airplane at higher density
altitudes? Your aircraft performance charts are a good place to start. However,
many charts only give performance figures at gross weight, and some older
aircraft substantial charts or data. Remember that you are not flying a new
airplane and your performance will probably not meet the expectations of the
charts.
An excellent method of
determining effects of density altitude is to build your own performance
charts. Load your airplane to different weight and Center of Gravity
positions and record takeoff and landing distances at different density
altitudes for each weight/CG combination. This requires some time commitment,
but is worth it. Use a runway where you have references, such as spacing of
runway edge lights, to be as accurate as possible in recording takeoff and
landing distances. You may need to factor in other variables such as runway
gradient, runway surface conditions, and wind. If you get the chance, attend
a density altitude clinic, such as the ones sponsored by the FAA, to collect
data on your aircraft performance.

Determining Density Altitude (DA):
Following is a simple
way to determine density altitude (DA) at a given pressure altitude (PA):
Rule of thumb: To determine DA at a given
PA, add 600 feet to existing
PA for every 10° F above standard temperature for that altitude. 
To do this, you must know the
standard temperature at a given PA as shown in the table to the right. You
should know the standard temperature for your home field and otherelevations
you use often.

Pressure altitude......... Standard temperature
Sea level....................59.0° F.............15° C 1000 feet ....................55.5° F.............13° C 2000 feet ....................52.0° F.............11° C 3000 feet ....................48.5° F...............9° C 4000 feet ....................45.0° F...............7° C 5000 feet ....................41.5° F...............5° C 6000 feet ....................38.0° F...............3° C 7000 feet ....................34.5° F...............1° C 8000 feet ....................31.0° F..............1° C 9000 feet ....................27.5° F..............3° C 
Standard Temperature at Different
pressure altitudes
Here is an example using this simple
rule of thumb. Suppose you want to depart from an airport at elevation 5000
feet on a summer day and the temperature is 85°F.
Example:
Pressure altitude (PA)...............= 5000 feet Temperature.............................= 85° F Standard temp at 5000 ft...........= 41.5° F Temp = 85  41.5......................= 43.5° above standard Using the Rule of thumb, round off the numbers to make it easier: Density altitude (DA).................= 5000 + (600 x 4.4) ...............................................= 5000 + 2640 ...............................................=7600 feet 
Thus, A typical summer density
altitude in much of the mountain west is over a mile and a half high when
you are sitting on the ground!
Another example:
PA..........................................= 9000 feet Temp......................................= 70° F Standard temp at 9000 feet.......= 27.5° F Temp .....................................= 70  27.5 ..............................................= 42.5 degrees above standard Density Altitude (DA)...............= 9000 + (600 X 4.3) ..............................................= 9000 + 2580 ..............................................=11,600 feet 
In the mountains, where you will be flying at actual elevations ranging from
over a mile to as much as 17,500 feet, increased density altitudes create a
problem! Do you fly an airplane whose service ceiling is only a few hundred
feet above the density altitude? What will your climb rate be? These are
questions you should be posing when the temperatures start to rise. Having a
quick idea of the density altitude is the best starting point. The next step is
to determine…
Reduction in engine horsepower due to DA increase:
As density altitude increases, engine horsepower decreases. If you know what the decrease is and how to apply that knowledge, you can determine roughly what your performance degradation will be and how much weight you may have to take out of the airplane to operate safely. The following is based on standard atmospheric pressure lapse rate and reciprocating engine efficiency.
Rule of thumb: A
normally aspirated aircraft engine loses approximately
3.5% hp per 1000 feet increase in DA 
Let's use a common airplane as an example, a Cessna 182.
Example: 230 hp airplane at the 5000 ft airport where
the DA = 7600 feet
HP reduction.................= 3.5% x 7.6 ....................................= 27% reduction (approximately 73% available) 230hp x (73%)...............= 168 hp available at 7600 ft DA 
Another way to determine the amount of power reduction is to know what your engine will produce at full power on the MP gauge at sea level on a standard day. Then, determine the amount of power per inch it is producing, and calculate power reduction based on the fact the engine will lose one inch of manifold pressure per 1000ft DA.
Example: given a 230hp engine that produces 28" MP at full
power, sea level standard day
Horsepower per inch..................= 230hp divided by 28" ...............................................= 8.2 hp per inch At 7600 ft DA the engine will produce approximately 20.4" (28"  7.6") at full power and the horsepower will be: 8.2 hp per inch X 20.4"..............= 168 hp available at 7600 ft DA 
Either of the above methods work in estimating your power reduction and give similar results. The amount of reduction may seem a shock at first, but it should alert you to the realities of density altitude related performance problems.
The next question is, how much
weight must you remove to compensate for the reduced power if you want to
operate at a roughly equivalent sea level performance? One way to do this is
do determine the aircraft power loading at which you want to operate, and
load the aircraft accordingly.

Power loading and weight reduction
Sea level standard day power loading
is defined as gross weight (GW) divided by horsepower available. In many of the
newer Owners manuals or Pilots Operating Handbooks sea level power loading is
given with other performance figures, but it is easy to calculate. Using the
Cessna 182 again:
Example: The Cessna 182 is A 2950lb, 230hp airplane at sea level:
Power loading ......................= 2950lb......= 12.8 lb/HP
...............................................230HP
Power loading ......................= 2950lb......= 12.8 lb/HP
...............................................230HP
To get sea level power loading out
of the airplane at higher density altitudes, you would have to remove enough
weight such that the new weight to hp ratio equals 12.8 lb/HP. To do this,
first compute net HP for density altitude:
From the previous example:.....5000 ft PA and 85° F
Density altitude......................= 7600 ft
Horsepower available..............= 168 hp
Original (certified) GW............= 2950 lbs
Density altitude......................= 7600 ft
Horsepower available..............= 168 hp
Original (certified) GW............= 2950 lbs
To calculate the
"effective" gross weight that will give equivalent power loading, multiply
the net HP available by the desired sea level power loading:
net HP available x SL power loading........= new GW
..............168hp x 12.8...........................= 2150 lbs
..............168hp x 12.8...........................= 2150 lbs
Thus to obtain sea level equivalent
power loading for the Cessna 182 on a summer day at 5000ft elevation and 85° F
you must remove roughly 800 lbs!! That is equivalent to two
180lb passengers, two 75lb bags, and 48 gallons of gas!
The calculation may be done using
any power loading you choose to get an approximation of the expected
performance. For example, if you usually operate from a 3000ft elevation
airport and the performance there is adequate, simply calculate the power
loading using 3000ft pressure altitude and standard temperature, and use that
figure as the standard of comparison.
Aircraft loading and weight are
items over which pilots have direct and immediate control. We have looked at
how to reduce aircraft weight to get better performance. However, you may
choose to fly with a given weight and accept the decrease in performance. If
so, you should have a good idea of just what that decrease in performance is
going to be. If an aircraft has inadequate performance charts, you may choose
to make your own, as suggested earlier. Or, you may have to make some choices,
such as flying only in the morning or evening, or making multiple trips
carrying lighter loads.
The bottom line is, be aware of the
effects of density altitude on the performance of any aircraft, and learn to lighten
up!!
Author: Amy L. Hoover
Author: Amy L. Hoover
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