Questions from the Cockpit: Decoding Cessna Performance Charts for Grass Runway Landings

Justin, a student pilot in Florida, writes: I’ve been studying the performance charts in my Cessna 172’s POH and was surprised to see a note on the short field landing table that said to add 45% to the figures for landing on grass. Why do you need more landing distance on a soft field? Shouldn’t the surface friction slow the plane down more than on pavement?

Good for you, reading your POH with critical thinking skills! And you are correct that if we landed two identical Skyhawks on parallel grass and pavement strips and both pilots used soft-field technique, the pilot on the grass would have a shorter ground roll from increased friction compared to the pilot on the pavement.

So what is up with this 45% more for grass business? Doesn’t this violate the laws of physics?

It would if we were comparing apples to apples, but Cessna has mixed apples and oranges on its charts for the newer versions of the venerable Skyhawk.

Rather than have one table for short field landings and another table for soft field landings, the company created a combined chart. Well, more correctly, it used one chart and provided a conversion formula that allows you to convert the apples to oranges all by yourself and, as you noticed, placed the formula below the chart in small print under the heading “NOTE.”

If you take any number on the short field table, and increase it by 45%, you now have soft field data. As to why those new numbers are illogically greater distances, it’s because the landing techniques for soft fields and short fields are not the same.

To understand what’s happening, take a look at the CONDITIONS of the chart in the upper left. Here you will see that the numbers on the chart were calculated using Maximum Braking after touchdown, the standard practice for short, paved runways — but not something you want to do on grass. So that extra 45% is the compensation factor between heavy brakes for short paved field landings and no brakes for soft field landings.

This is why it is important to read any fine print on any performance chart and to take the time to understand the conditions under which the data was collected.

I’m sure at least one person has wrecked a perfectly good airplane after casually looking at this chart and “logically figuring” they could land in less space on grass.