These are guidelines to help you make your final decision about
taking off from Lake Tahoe in the afternoon.
Part I) Is the runway long enough for you to take off safely? You
need to base your decision on these conditions - the
temperature at Lake Tahoe airport in the afternoon is
now 32 oF and the real altitude of the airport is 6253 ft.
Determine the total weight of the plane including passengers,
fuel and cargo.
For the total weight of the plane, determine your plane's take
off speed in KIAS (Knots Indicated Air Speed) from a Takeoff
Performance chart to clear 50' barrier.
From a map, determine the length of the runway at Lake Tahoe in
feet.
Given the temperature at Lake Tahoe at the end of the day,
determine the calculated density altitude.
Using your calculated density altitude as the pressure altitude
on the Takeoff Performance Chart, and including the air
temperature of the day, your plane's total weight and
headwind, determine the takeoff distance needed to
clear a 50' barrier at the end of the runway. Why do
you think it's important to have a clearance of 50'
at the end of the runway?
Compare the real length of the runway and your calculated take
off distance required to clear a 50' barrier. Do you go
ahead with your plans to take off?
Part II) There are mountains around Lake Tahoe and your plan
is get over Martis Peak to get home. For
simplicity, after taking off, your flying speed
relative to the ground is 90 mph (this already takes
into account the headwind) and your starting height
is now 6300 ft. You need to climb to 11,743 ft to clear Martis Peak
by at least a safe 3000 ft.
Determine the horizontal distance from Lake Tahoe
Airport to Martis Peak.
At your flying speed, determine how many minutes your
plane will take to reach the mountain.
With the height of the mountain peak you are trying to
clear and the calculated time to reach the peak, how many feet per
minute will your plane need to climb? This would be your calculated
climb performance.
But, your plane has a maximum climb performance for a
given weather temperature and resulting density altitude. From the
Climb Performance Chart of your plane, determine the maximum climb
performance for your plane under these conditions.
Compare your calculated climb performance with the
plane's actual climb performance from the Climb Performance Chart.
From your comparison, do you still make the decision to take off and
safely make it over the mountain?
Part III) Your Climb Performance Chart gives you information
for a given temperature and resulting density
altitude. In Part II, you used the
temperature and density altitude at the airport. But guess what? Is
this a wise decision? Should you instead use
the temperature and density altitude at the altitude over
Martis Peak? Let's go through the charts
again to see if you should stick with your decision to take off.
As you climb, the air temperature and pressure will
decrease and thus affect density altitude. Given that the temperature
drops 1.8 oC for every 1000 ft, determine the temperature
at 11,743 ft.
Knowing the temperature at the peak of the mountain,
determine the density altitude from the Density Altitude Chart.
Calculate the climb performance of your plane at this
density altitude.
From the Climb Performance Chart of your plane,
determine the maximum climb performance for your plane under these
new conditions.
Compare your new calculated climb performance with the
plane's actual climb performance from the Climb Performance Chart.
From your comparison, do you still make the decision to take off and
safely make it over the mountain?
Part IV) Challenge Extension - What would the temperature at
Lake Tahoe airport have to be in order to make if over Martis Peak
safely?
Now that you've made your decision, go back to the You Decide: Decision-Making Process and complete the
matrix provided.
