The aircraft in question was my FT337GP being flown by an airline Captain on a post annual test flight while I was away on business. This actually happened almost 5 or 6 years ago now.
So what happened you might ask? On this annual all the hoses were replaced and the suspicion was that the right main gear down lock hose position allowed the down lock to partially pressurise. The main gear actuator was begining to fail so did not have enough pressure to over come the down lock pressure and thus the gear would not come down & lock.
There were a number of attempts per the POH to rectify the problem, but in the circumstances, nothing was ever going to work with that combination.
The pilot did a very nice job of landing the airplane with minimal damage. There might have been even less if he had landed on the runway and not the grass.
The FAA was informed and they were helpful, but only partially interested and there was no follow-up action.
Learning out comes? There are quite a number and I list them in not particular order.
1) When doing the gear test as part of your annual, simulate high air loads to ensure the actuator has enough to push you out of the way when the gear is going down. After the accident when the gear was swung, it was only when I suggested we simulate air loads that we realised the actuator was failing.
2) Make sure the main gear down lock stays out of the way until the gear pushes it, which then allows it to pressurise. If it pressurises too early and is partially down, you may have trouble. In my case, it may have been like that for years which ultimately caused the actuator failure.
3) Don't land on the grass unless it is really smooth. The majority of damage to the belly skin panels was caused by the rough grass landing. The total damage was minimal with the gear doors, exhaust stack and two belly skins requiring replacement along with various antennas as well. My A&P figures some of the damage was caused when they put the plane on a plate and dragged the plate to a place where the plane could be lifted and gear could be extended.
4) Always feather the front and get the prop horizontal.
5) When landing assured pull the rear and stop it as well. We had a prop strike on landing on the rear engine. The POH suggests an abrupt flair which puts the rear prop in a vulnerable position.
6) Don't buy the belly skin from Cessna as it is not drilled and it is very expensive. Use sheet material of the correct thickness and you will save tonnes of money.
7) Search the scrap yards for gear doors. They are now getting quite scarce as an FYI.
8) There will be a number of rivets positions on the belly skins where you will have to used bolts in place of because there is no way a rivet could ever be set.
9) If you can find someone who does skin repairs as a specialty, get them to do the work as most A&P's just don't do enough to be expert enough.
10) Fully check the geometry and be prepared to replace the wing bolts and the boom to wing fasteners.
11) On pressurised airframes, be prepared for some extra work to re-seal the cabin. Mine is still not quite right, although I can get to a little over 18 k now. Use the down time to get that Garmin stack or similar you have always wanted installed.
12) Don't be in the plane yourself when this sort of thing happens. Be 4,000 miles away so when you call the insurance company you can truly say it was nothing to do with me.
Epilog: this was not a big deal. The plane was out of action for a while, but no one was hurt and the plane flies fine today with its Garmin stack.
If anyone has any questions, please let me know.
Regards - Dave