How do I use the takeoff and landing distance charts in the C172 POH?
There are many ways to calculate aircraft performance. Some ways are entirely manual through the use of formulas. Some ways use electronic means like apps and special calculators. Some ways use pre-calculated Tables. Then there are ways that use graphs and charts. Let’s tackle the charts way.



First, remember one thing. These charts are made from data obtained using highly experienced and skilled test pilots using brand new airplanes with ideal performance and under ideal conditions. Since you probably meet none of these parameters (most of us don’t), it is best to add as much as 50% fudge-factor to your results as a safety buffer. 

Think of the Takeoff and the Landing Performance Charts as each being a combination of 4 separate charts. 


The Pressure Altitude to Density Altitude conversion chart.
The effect of Density Altitude on weight chart.
The effect of wind on distance chart.
The additional distance needed to clear an obstacle chart.


To start your calculations, read any associated text on the chart. It will give you parameters that must be met in order to make the chart as accurate as possible. They may also give you factors to consider and/or adjust in your calculations.

The Pressure Altitude to Density Altitude conversion chart.

Next, find out the Pressure Altitude and Temperature at Field Elevation for your point of departure. You may have to obtain the Pressure Altitude by calculating it using your Field Elevation and Atmospheric Barometric Pressure reading. Another chart may be handy for this. 

Start your graph entering at the horizontal Temperature axis at your Field Elevation Temperature. Follow the graph line vertically up to where it intercepts the line representing your Field Elevation Pressure Altitude. Follow the graph line horizontally to the right until it intercepts the Reference Line for the next section of the chart.

The effect of Density Altitude on weight chart.

Enter the next section at the point where your drawn graph line from the previous section intercepts the Reference Line. Draw a line that roughly parallels the diagonal lines immediately above and below your drawn line. Find the point at which your drawn line intercepts the vertical graph line representing the actual total weight of your aircraft at the point the performance data would be used (takeoff weight or landing weight). Draw another line horizontally from the point of interception, to the next Reference Line.

The effect of wind on distance chart

Enter the next section at the point where your drawn graph line from the previous section intercepts the Reference Line. Draw a line that roughly parallels the diagonal lines immediately above and below your drawn line. Find the point at which your drawn line intercepts the vertical graph line representing the actual Wind Component at the point the performance data would be used (takeoff or landing). Draw another line horizontally from the point of interception, to the next Reference Line. If the winds are calm, use the 0 wind graph line that coincides with the Reference Line. Your drawn line would then be an individual horizontal line from the reference line to the next Reference Line.

The additional distance needed to clear an obstacle chart.

Enter the next section at the point where your drawn graph line from the previous section intercepts the Reference Line. Draw a line that roughly parallels the diagonal lines immediately above and below your drawn line. Find the point at which your drawn line intercepts the vertical graph line representing the actual obstacle height AGL you would have to navigate/avoid at the point the performance data would be used (takeoff or landing). Draw another line horizontally from the point of interception, to the vertical axis on the right. If there is not an obstacle, use the 0 height graph line that coincides with the Reference Line. Your drawn line would then be an individual horizontal line from the reference line to the vertical axis. 

Results.

The value you read on the y-axis of the chart is the minimum distance you need to accomplish the stated phase of flight under the given conditions. As a reminder, there will be different charts for different phases of flight. A phase of flight may have multiple charts. An example of this is a chart with flaps and a chart without flaps. Make sure you are using the correct charts and conditions for your aircraft and situation.