Performance in the Real World – Part 2

4 min read

More Than Just the Calculations

In my last post, I discussed Pressure and Density Altitude calculations and their contribution to the total performance equation. In this post, I’ll cover some of the other values in the takeoff equation that we don’t always keep in mind.

When calculating performance for our airplane, we use the charts found in the Pilot’s Operating Handbook (POH) for our aircraft. These charts generally account for density altitude and include conditions and notes to address other factors. When was the last time that you took a close look at the conditions and notes associated with the chart? They are important and are there for a reason.

As an example, the “Short Field Takeoff Distance at 2550 Pounds” table for the Cessna 172S lists the following conditions:

  • Flaps 10°
  • Full Throttle Prior to Brake Release
  • Paved, level, dry runway
  • Zero Wind
  • Lift Off: 51 KIAS
  • Speed at 50 Ft: 56 KIAS

It also includes the following notes:

  1. Short field technique as specified in Section 4.
  2. Prior to takeoff from fields above 3000 feet elevation, the mixture should be leaned to give maximum RPM in a full throttle, static runup.
  3. Decrease distances 10% for each 9 knots headwind. For operation with tail winds up to 10 knots, increase distances by 10% for each 2 knots.
  4. For operation on dry, grass runway, increase distances by 15% of the “ground roll” figure.

Many of these items are related to technique and aircraft configuration and are easy to comply with. Some can be tougher.

Take the “Paved, level, dry runway” condition. This seems simple enough but have you checked the grade on the runway at your home airport? What about the runway where you are planning to land and will have to later takeoff? Do you know where to find this information? Do you know how it will affect your takeoff distances when you find this information?

Runway slope information is published by the FAA in the Airport/Facility Directory. The information can also be found on the Airport Diagram or Airport Sketch for the airport in the Terminal Procedures Publication (approach plates). Many commercial sources, GPS databases, and pilot apps also include the information.

Looking at the Clermont County / Sporty’s Airport, we find a slope of 0.9% uphill on runway 22 and the same slope downhill on runway 4. This equates to about 32′ of elevation change from one end to the other of the 3566′ runway. The grade is not a consistent slope of 0.9% but there isn’t information readily available that tells us what the slope is at different points along the runway so we’ll stick with the 0.9%.

The chart in the POH doesn’t provide any indication as to what we should do when the runway is not “level.”

For help in addressing this adjustment, I would look to an expert in flying on “non-level” runways. The late Sparky Imeson, author of the Mountain Flying Bible Revised suggests the following rule of thumb with regard to gradient. For a 1% upslope, which is approximately runway 22’s slope, increase the takeoff distance by 7.5%. While not specified on the Imeson family’s website, this should be 7.5% of the ground roll. If a 2% upslope, use a 14% increase; 4% upslope, 25.5% increase; and 6% upslope, 39.5% increase.

Another aspect of the notes which has no resolution in the POH is departing from a wet or otherwise contaminated runway.

A wet runway will change the friction between the tires and the surface. While this may decrease friction, I wouldn’t expect it to shorten the takeoff roll.

Standing water will increase the takeoff roll. This is due to “displacement and impingement drag” as the spray from the tires is displaced and strikes the aircraft. The FAA’s AC 91-6A regarding Water, Snow, and Slush on the Runway hasn’t been updated in over 30 years. At that time there was no clear engineering data on how much water or slush on the runway affected the takeoff roll.

You should not attempt a takeoff when standing water or slush on the runway is more than one half an inch deep.

Imeson’s website has some rules of thumb regarding surface contamination as well.

Cessna recommends increasing the ground roll figure by 15% when taking off from a dry, grass runway. This guidance may be incomplete as it does not account for the length of the grass or the roughness or softness of the surface. 15% will be fine when the grass is short and the ground is firm and smooth but may be insufficient if the grass is longer or the ground is softer or rougher. Using a soft-field technique rather than the chart’s indicated short-field technique (see Note 1 above) may also influence this distance.

All this said, the chart in the POH is our best place to start on any takeoff calculations. You just have to keep the conditions and notes in mind and know when they won’t account for your current situation. Have fun and stay safe!