On the 1.2x safety factor for dry grass, note this is in the context of also applying the 1.33x take off safety factor, so nearly 1.6x cumulative.
In the case of soft ground and wet grass the safety factors add up: 1.33 × 1.3 × 1.25 (Plus!), so more than 2.16 x cumulative safety factor.
IMHO a guideline like 15-20% is reckless.
My club operates from a 630 m grass runway of moderate quality so I think I can confidently say that the 15-20% figure is quite reasonable. Of course it assumes that the grass is cut short and the runway is not waterlogged. Also you have to operate the aircraft correctly. Pilots used to 1000+ m tarmac runways have some problems — particularly landing — but that is an issue with the pilot, not the aircraft.
The regulations of my country says that if there is no correction figure in the POH, then +10% should be used as a guideline for grass runways where the grass is cut short.
IMHE the TBx wing does not want to fly until it is ready to fly, so trying to pull off early will increase the ground run considerably. In the TB20 I normally wait until at least 65kts, more like 70 if heavy and then positively rotate.
I’d be quite happy operating a TB20 off 600m of tarmac and have done, and it is no issue. But I would not use 600m of grass and even 800m is a bit too close to my comfort level and even then it’d have to be short, hard and dry.
Vs for a TB20 is 59kt so yes you would not try to pull it off below that.
A “max perf” takeoff would start at about 65kt. I normally rotate at 70.
450m tarmac is fine. Grass… variable, as grass always is 
If I had my own strip, and it was level (less than 0.5% gradient), and I was able to use the covert plastic mesh reinforcement, I would like to see 700m to be really safe at MTOW and ISA+20.
Very very few grass strips are in a really good condition. Generally, unless it is a small club with everybody being well solvent, the maintenance will be according to the lowest common denominator.
Very very few grass strips are in a really good condition.
I hope and trust that this is meant for UK only? I know several that are like billiard tables, such as EBAR and EBBN. EBML and EBLN are very nice, too. Even my own EBZH is quite good, only some unevenness in the taxi areas but the runway is as smooth as one could wish. And I do not know of a single grass runway in BE that is not at least “good”. Some tend to get waterlogged, though, in winter/spring.
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Panshanger was actually pretty good in terms of bumps, last time I was there which was some years ago. Justine used to live up there However, when it had rained, the plane would be covered in mud after taking off.
I can name two with tarmic runways in the UK (I wont), with more potholes and lumps and bumps than probably comparable to the entire surface area of the M25 on a bad day.
The worst one I have ever been to was a place called Spanhoe, near Peterborough. The concrete was covered in what I would call rocks. Before departing, I walked up ahead for a bit and kicked away the biggest ones. They had a club there of sorts, so I can only imagine that no agreement could be reached on the purchase of a broom.
What is taught in Germany for the PPL:
Add 20 % for dry, short, level grass runways
Plus 10% for wet grass
Plus 50% for soft ground
Plus 10% for damaged sod
Plus 20% for high grass
If the runway is sloped as well you might as well taxi to your destination I guess…
I was at Spanhoe today, a beautiful day visiting the folk at Windmill Aviation.
The Super Cub landed in the grass area south of the main hard surface runway, although they now have a very nice second grass runway. Only about 300 yards in the grass area, but in my years visiting plenty of room for a Super Cub.
It is one of my favorite spots with a well earned reputation for quality work for vintage classic aircraft. The amount of unique active aircraft that are serviced there is incredible.
Just today the following were either out on the grass or in the hangars: an Auster Agricola, Helio Courier of Air America vintage (soon to be placed into ‘company’ livery, so interesting to see how the CAA registration markings exemption process goes), Fairchild Argus (a neighbour of one of our forumites at Old Hay), a Globe Swift, at least a dozen Austers, half a dozen chippies, several military Super Cubs (including mine), Super Stearman with a 450HP supercharged behemoth of an engine, Stinson, Taylorcraft, Cessna 140, Focke-Wulf 149D, Pitts S2A and more than half a dozen rare air frames awaiting restoration.
Work comes in from all over the world with restored aircraft going as far afield as New Zealand.
About the most advanced piece of avionics kit I noticed was a military ADF in one of the Austers. I think it is fair to say I have never seen a GPS in one of the stationed aircraft, let alone a panel mounted GPS!
The only retractable based there is the Focke-Wulf, but I have flown twins into Spanhoe, and a SIAI-Marchetti 260 is a visitor. No prop nicks resulted, although nose draggers, and retractable ones, are in the minority at this field.
It is the sort of place that Richard Bach wrote about in A Gift of Wings. I hope next time Peter you have a happier visit.
I just brought my plane back from Spanhoe on Saturday (Windmill Aviation does our permit renewal and annual). It was a perfect picture of grass roots flying, three Chipmunks, an array of Austers (including ours), a Steaman, a Pitts, a really really sweet Cessna 140 in the final stages of a rebuild in one of the hangars. I used the hard surfaced runway since the wind was straight down it, but didn’t note any stones or potholes (but then again, I’m based at Andreas which is just an array of potholes and stones, so anything’s an improvement – I wouldn’t bring a nosewheel plane into Andreas unless it had massive prop clearance). The grass runway is in very good condition, I used it when I brought the aircraft down (I prefer to use grass runways if I can, the Auster is far better on them).
I should really go down there more. There’s a good group of people down there and the place is just about the purity of stick and rudder flying.
