# What is a short field?

No airport is officially declared to be a short field because a field that might be short for a 747 could be very long for a Piper Cub. Every airplane is different.

However, a good rule of thumb is to consider an airport short if you will need more than 50% of the runway length to takeoff or land.

# Performance Charts

To determine how much runway is needed we use performance charts provided by the manufacturer of the plane. Let’s try an example.

We are flying a 152 out of Morgantown airport (O03 -oscar zero three). The wind is 320@6 and the temperature is 15 degrees C. We expect to takeoff and then fly around the pattern. The aircraft is at max weight.

## To the charts

IMPORTANT: Before digging into the numbers on ANY performance chart read ALL of the notes.

Click on the chart above to open it up in its own window. First, read the condition. We will need to depart with flaps set to 10 at full throttle on a paved level dry runway with zero wind. Unfortunately for us, Morgantown is NOT paved, and there IS wind.

This is where the notes come in. First, we must read section 4 to make sure we are carrying out the short field takeoff procedure correctly.

The second note doesn’t apply to us since Morgantown is at 600 feet elevation.

The wind is handled in note 3. Since we are departing runway 28 with a wind of 320@6 we can expect a headwind of about 5 knots, roughly half of the 9 knot headwind in the note, so we will decrease the distance by 5%.

Finally, because we are operating on grass with will increase the distance by 15% for the ground roll.

Now we can look at the table itself. Starting from left to right notice that this chart is only if the aircraft weighs 1670 lbs, which is the plane’s max weight. If the weight is lower performance will be a bit better. Continuing, the instructions say that we should lift off at 50 and try to climb at 54, which is Vx.

## Interpolation

Now we don’t have a column for 15C or a row for 600 feet so we need to interpolate.

First, let’s get the figure for ground roll at sea level by interpolating 695 ft at 10C and 755 ft at 20C. Interpolating to 15C means we need to find the middle between these two numbers, which is 725 ft. The total to clear the obstacle is done the same way, and it is 1340 ft.

Next, let’s get the same numbers for 1000 ft elevation. Interpolating between 10C and 20C comes out to a ground roll of 795 ft and a total to clear the obstacle of 1475 ft.

Next, we need to interpolate between sea level and 1000 ft. The airport is at 600 feet so we need to be 60% between these numbers. First, find the difference between the 1000 ft ground roll and the sea level ground roll: 795-725 = 70 ft. Then multiply by .6 to get 42 ft. Then add to the lower number: 42 ft + 725 ft = 767 ft. The total to clear the obstacle is done similarly to get 1421 ft.

Now we can apply the percentages based on the notes. First, the wind which will decrease the distance. 5% of our ground roll is 38 (rounded) and 5% of the total to clear the obstacle is 71 (rounded). So we can subtract these to get a ground roll of 729 ft and a total to clear obstacles of 1350 ft.

This next part is tricky so read carefully. The note says to increase the ground roll figure, but doesn’t say anything about the total distance to clear the obstacle. This is because once you are airborne the runway type doesn’t matter anymore. To begin let’s calculate the distance in the air to clear the obstacle by just subtracting the ground roll from the total distance to clear an obstacle. This is 621 ft of forward distance once airborne to reach an altitude of 50 ft. Let’s hang onto this number.

Now let’s increase the ground roll distance for the grass by multiplying our ground roll of 729ft * 1.15 = 838 ft (rounded). Next, we can add the 621 ft from the last paragraph back in to get 838 ft (ground roll) + 629 ft (distance in the air to reach 50 ft) = 1467 ft (total distance to takeoff and climb to 50 ft.

For this exercise, we are assuming Morgantown has a 50 ft obstacle, even though it really doesn’t.