Calculating Takeoff Distance - Cessna 150

This document is an example for how to calculate takeoff distance for a Cessna 150. Please note, this is an example only. Consult the Pilot Operating Handbook (POH) for your particular airplane for doing real-life calculations. This example assumes you are a Canadian pilot - the logic is all the same as everywhere else, the lookup locations are different.

Summary

To calculate landing distance, the following general steps need to be done.

1. Calculate pressure altitude
2. Calculate temperature at altitude
3. Calculate headwind component
4. Look up and calculate takeoff distance

Things You'll Need

• Pilot Operating Handbook (POH)
• Canadian Flight Supplement (CFS)
• Current Weather (in this case via AWWS)
• VFR Navigation Chart (VNC)

Calculate Pressure Altitude

The first step is to calculate your pressure altitude at the particular aerodrome.

1. First, find the Aerodrome Elevation. You can look this up in the Canadian Flight Supplement (CFS).

We're going to use London International (CYXU) as our example airport. It's elevation is 912 feet above sea level (ASL).

1. Next, we're going to look up the METAR for London to get the current altimeter setting. You can access get that information from the AWWS.

Let's assume the METAR for CYXU (London) looks like this:

METAR CYXU 182000Z 29011KT 260V330 15SM FEW034 SCT250 11/03 A3002 RMK SC2CI1 SLP175=

We can see the current altimeter setting is identified by A3002, meaning our setting is 30.02.

1. Now, we subtract the current altimeter setting from our standard pressure of 29.92.

29.92 - 30.02 = -0.10

With a value of -0.10, the first digit before the decimal represents thousands of feet, the second hundreds, and the third tens. We then add the value of this to our airport elevation - in this case -0.10 represents -100 feet.

912 feet + (-100) feet = 812 feet

Therefore, the airplane, despite actually being at 912 feet, feels like its sitting at 812 feet, so we can expect slighly better performance. This is our pressure altitude.

Calculating Temperature At Altitude

Normally we can expect a 2 degree celcius drop for every 1000 feet of altitude climbed. Since however we have the METAR for London, and the temperature was measured at the elevation of the airport - we can simply use the value in the METAR.

METAR CYXU 182000Z 29011KT 260V330 15SM FEW034 SCT250 11/03 A3002 RMK SC2CI1 SLP175=

Our temperature and dewpoint is represented as 11/03 and thus 11 degrees at 912 feet.

Calculating Headwind Component

Next, we'll need to calculate our headwind component, as it will affect our takeoff performance. A stronger headwind will decrease our distance needed to takeoff and clear an obstacle, while a tailwind will increase the distance needed.

First, we need to make some runway decisions based on the weather. At a controlled airport like London, these decisions will be made for you.

METAR CYXU 182000Z 29011KT 260V330 15SM FEW034 SCT250 11/03 A3002 RMK SC2CI1 SLP175=

Converting to Magnetic Winds

Our winds are from 290 degrees at 11 knots. This value however is the true direction of the wind, not the magnetic. An easy way to remember is "if you read it, it's true, if you hear it, it's magnetic".

Since London is located in Southern Domestic Airspace, all runways are in magnetic degrees, not true, so we must convert our winds to magnetic degrees. In order to do this, we can look up our closest line of magnetic variation in our VFR Navigation Chart (VNC). For london, we have 10 degrees West of variation.

West is best, so to convert from True to Magnetic, we add the 10 degrees to the true. Therefore, our winds are 300 magnetic degrees at 11 knots.

Choosing A Runway

The CYXU tower is most likely using the runway that is most into-wind. In this case, they are probably using runway 27. Let's assume that moving forward.

Taking off from runway 27, would have our compass show 270 degrees. Our wind is coming from 300 degrees on the compass, so 30 degrees to the right of our airplane. Using this, we can now calculate our headwind component.

Calculating Headwind

In order to calculate headwind, we need a conversion chart - there's a great one in the early pages of your CFS that looks something like this.

To read this chart, we find the intersection of the 30 degree line, and the 10 knot arc (we're interpolating to find 11 knots), and then draw a line to the left to get our headwind component, which in this case is 9 knots.

Looking Up & Calculating Takeoff Distance

Now that we've got our pressure altitude, our temperature, and our headwind component, we can calculate our takeoff distance to clear a 50 foot obstacle. For this, we will need the Pilot Operating Handbook (POH).

Checking Conditions & Notes

We should first check the conditions and notes associated with this chart, and make sure we're meeting all of them.

Flaps Up

We can execute a flap-up takeoff, so this works for us.

Full Throttle Prior to Brake Release

We can do this, and will remember to do so.

Paved, Level, Dry Runway

Runway 27 at CYXU is paved, level, and dry today.

Zero Wind

We do not have zero wind, so we do not meet this condition - we need to look in the notes to see how we must alter our interpretations. The notes that correspond to this are:

1. Decrease distances 10% for each 9 knots headwind. For operation with tailwinds up to 10 knots, increase distances by 10% for each 2 knots.

Since we do have a headwind, so we'll keep this in mind for later after we've looked up our values.

Interpreting The Chart

Next, we find the values most appropriate for our current situation on the left (temperature and pressure altitude). We can then find the corresponding distances for the ground roll, and the distance required to clear a 50 foot obstacle.

We'll assume a fully loaded weight of 1600 lbs (which is pretty easy to get to in a 150), and find the closest value of our pressure altitude at 812 feet. In this case, 1000 feet. Our temperature is 11 degrees, so we look in the closest column (10 degrees). We can find that our ground roll is 775 feet, and our total distance to clear a 50 foot obstacle is 1465 feet.

Compensating For Wind

Now, since this chart assumes zero wind, we need to modify these values for our headwind component. Since we have a headwind of 9 knots, we can decrease our distances by 10%.

775 feet * (1.0 - 0.1) = 698 feet ground roll.
1465 feet * (1.0 - 0.1) = 1319 feet total distance required.

Things To Keep In Mind

Winds change between your calculations on the ground and you lining up for departure, so take calculations with a grain of salt and give yourself a buffer for clearing an an obstacle. Just because the math says it'll work, doesn't mean it will. Take a look at this video for an example.