Cessna 120-140 Association

What would be a percentage increase of take-off run when operating off grass? The POH (I have) lists for hard surface runway only. I suspect longer runs due to friction from the grass.

Will be doing some higher altitude runways in a few weeks, paying attention to density altitude. One stop is going to be Cavenaugh Bay (66S) with a 3100' grass runway at 2484' elevation.

I"m not too worried about performance of my bird (0-200 with climb prop pitch) but thought I would start getting ready for more mountain flying in the future here in the NW. I've been to Bridger WY (KFBR) 6404' at 7038' and early morining off the asphalt was a non-event.

I use +10% for solid field with good dry/short grass and +25% for long or especially wet grass or soft field.

TOP COMP calculator puts grass at +12-13% standard day for all planes.
The 120/140 has big tires for its weight and does really well on grass compared to most.

I just flew up to an island in Lake Michigan with a decent grass runway. Because we were concerned about weight with trees on each end, we did some fairly thorough calculations. Just one data point, but with as close to identical winds as we could manage, 6x6.00 tires, absolutely fully loaded: asphalt to dry, short, reasonably smooth turf. Wheels up was too close to even measure - less than 5%. Distance to 80’ (our tree line) was about 8% more due to the fact that I was a little slower at wheels up than on the asphalt and had to stay in ground effect for a second. I would be pretty confident using 10% as a worst case when on decent grass.

Wet or dry made no difference on TO but I found at gross weight the brakes were pretty useless when the grass was super wet - seems like natural friction and wind resistance did as much as the brakes.

Just my experience this weekend.

FYI: 3000 ft turf, 76 deg, density altitude 2650, 85hp, full gross weight: wheels up by 1000-1100 ft, 80’ asl by 25-2600 ft.

Well, just got back and I over-thought the whole thing. Turns out I was airborn in about 900' on a cool morning from wet grass, density altitude maybe 3000', no problems.

Taking off from Kalispell MT this morning on the other hand with a DA of 4000' on asphalt actually took a little longer, haven't figured that one out yet. 7 hours from Kalispell to Vancouver WA, not bad.

The DA on most of my adventure was 3-4000 ft and the plane did fine, but cruising at 7-8000' to stay out of any rollers from the mountains was a slow affair.

I don’t have easy access to the documentation which FAA provided on this subject, but only recently I read FAA’s recommendation to add 10% to dry 4-inch tall grass…and 30% to grass taller than 4”. They also suggested adding 30% to wet grass, but I do not recall them mentioning height of that wet grass (presuming 4”.) The reference given was a Canadian-study publication.

15% is, I believe, a good starting number. But, a lot depends on the grass length. If it is tall, say higher than 4” and/or wet, more wiggle room is needed. I would never plan on less than 15%. Pilot technique is also a critical factor as well as the condition of the airplane engine (as well as the usual stuff like density altitude, winds, runway slope, obstacles, etc).

So, to render it down, I would suggest adding at least 15% and if that is tight, skip the takeoff and go play golf. Wait for another day after the grass is cut, dry, and the winds are working in your favor.