This seems like the place to ask! 
Peter and Howard will know what is doable on an experienced level.
New to the TB20, I had no problem getting out of LOWZ with full fuel and 3 pob (avg 80kg).
And landed there before with a part snowy rwy.
Sure Zell am See LOWZ is no problem, but that’s tarmac. The takeoff performance chart is here
Grass is highly variable. You can see a factor of two difference in the ground roll, according to the quality of the ground and the length of the grass.
I would not do any grass under 700m at MTOW, ISA. I have seen a 350m ground roll once but that was quite hard ground and almost no grass on it.
One of the problems with grass is the difficulty of getting a reliable report. It’s the same thing if you have a Ford Ka and ask a farmer with a Land Rover whether a particular track is OK to drive on Some of the grass strip flyers are obviously very experienced but some others I know are pretty casual about it (prop strikes are not an encouraged discussion topic; one tends to hear about it years later). I once flew to a “really smooth 1200m” strip and the ground roll was about 1000m! The ground was horrible and the grass was up to 20cm, so that is an extreme example, but what other due diligence could I have done?
If I had my own grass strip I would spend some money on it and 700m would be fine, even for hot days. But usually the whole point of a grass strip is that nobody wants to spend money on it. Only occassionally people want to tarmac it and cannot get the permission, or don’t want to draw attention.
The other thing about grass is that the aircraft – any aircraft – inevitably gets more dirty and if this is the normal day to day operation, this will translate into the landing gear needing more care if it is to last a long time. And this is an area in which practically the whole maintenance business is dreadful. The “standard £2500 Annual” won’t really do anything on the landing gear, beyond a quick squirt of oil, which is no good at all. You are looking at spending perhaps another £500 on the Annual for a proper job.
I was based at grass strips for more than 20 years and can confirm that there is huge seasonal variability. In the summer months, the ground is hard, the grass almost stops growing and it resembles a hard runway. In the winter months I sometimes could not take off at all. I remember one day in February taxiing to the runway when I heard a call on the radio from a female pilot who said she was stuck on the runway and could not move with full power. It turned out to be a TB20. She was the instructor with a couple of student pilots.
We removed the wheel pants and pushed it out of the soft ground. They had to get the chief pilot to come over by car and fly it off from a less boggy side later in the day.
TB20s are used for flight instruction in France and I often saw them at our 800 m grass strip.
Simon
750m is minimum for me with TB20 and the length is not the only condition for me as well as for the other TB-20 flyers on the forum. Typically I am OK with 750m, two people onboard and reasonably hard surface with trimmed grass. When the grass is tall, it is hot, there is an upslope or more than two people on board I would rather decide to use other airport or have 1000m. This comes from my own experience as I learned to fly on a grass airfield (1000m) and visited quite many other grass airports as we have plenty of those here in CZ. Today I really try to avoid grass whenever it is possible and take a helicopter when there is no reasonable airport. TB-20 is built for tarmac with occasional use on good quality grass. Prolonged use on grass can induce mechanical problems with landing gear, wetwing leakages and other nasty staff. You want to avoid grass with aircrafts with retractable landing gear and with pressurized aircraft like Cessna P210. And aircraft with wet wings :-).
Btw, there was one famous TB-10 accident at LKVR where a German owner tried to depart from a muddy (after prolonged rain) grass strip. He tried several times without success. He even had to remove the wheel pants as those quickly filled with mud. Then he tried again, managed to lift off very close to stall speed and never managed to accelerate before he landed in an adjacent hotel. He survived the accident to die a month later in a hospital. LKVR is 840m with slope, 1600ft altitude.
Final report (English) on the D-EANH muddy grass TB-10 accident is here: Final report D-EANH
Simon – it probably wasn’t a TB20 since it has no wheel pants. Maybe a TB9 or TB10 (the TB200 is very rare).
I have found the TB20 will “always move” under its own 250HP, presumably due to the trailing link main gear, but a pilot with more than half a brain is going to think twice before using full power for any length of time / any distance because the Q has to be asked “what is going on that needs so much power?”. And since this power requirement is is invariably due to heavily undulating ground, there is a high risk of a prop strike because such power implies you are trying to climb over a big bump and you know what will happen to the prop clearance once you are the other side of it 
Once I was forced onto grass at Friedrichshafen (the first and absolutely the last time I was going to obey that) and luckily there were 3 of us inside so I stopped the engine and we pushed/pulled it onto the (very rough) grass surface manually. But upon departure (usually the safer scenario because one has an opportunity to walk and eyeball the surface ahead) it briefly needed full power to get out of there. The only time I judged it unsafe to continue to move under own power was on heavily waterlogged and undulating grass at Southend (doing a bit of the FAA CPL there via a now-long-defunct outfit; normally one would not have to park on grass at Southend) and they brought a van, proceeded to try to pull by the nosewheel, I managed to stop them just in time (!!) so they came back with a rope to put around the main wheels, the wheels on the van just spun around, so they came back with a fire truck and that did it. A pilot I used to know who had a 421C had a similar experience doing a Project Propeller run to some grass place and got totally stuck.
The above comments apply equally to any other half similar type, not just a TB20 (20cm prop clearance with the 3B prop, 18cm with the 2B prop) and definitely not just a retractable. The SR22 has/had widely reported issues with collecting grass inside the wheel cowlings which then catches fire from braking. I reckon the TB20 is more likely to get out of really undulating ground than a non trailing link gear type. But ultimately the Q is how badly do you want to abuse your plane?
IMHO, watching people for the last 17 years, the biggest factors in “grass capability” are
and this is NOT tongue in cheek! I wish I had a camera running at EGHE last week…
Having had a 20k prop strike in 2002 I tend to be rather forthright on this topic
Pytlak – why the wing tank leakage? I would not think the bouncing around is any worse than during flight in turbulence.
Peter – you bet it is much worse on the grass than in the air. Mooneys are extremely sensitive to this. The problem is in the force source, which, in case of taxiing on uneven surface, comes from single point, which is the gear attachement point whereas the force or loading on the wing when flying is much better distributed over its entire area. The same applies to pressurized hulls.
Peter wrote:
But usually the whole point of a grass strip is that nobody wants to spend money on it.
Or that they don’t have money to spend! We’d be happy to spend €1M on our runway to get a tarmac surface if we had that kind of money, but that’s five times the annual turnover of our club. What’s more realistic is reinforcing the grass which would cost about €100k.
