I was unable to reproduce the odd servos behaviour in some minutes on the ground with motor on (blades removed), so if I understand correctly your guess, this is in favour of electrostatic discharges. How can I try and reduce/avoid electrostatic discharges?
What can I do for the other problem (negative altitude constatly reported GEoLink flight logs)?

Hello,

please verify belt tension. Belt should not touch tail boom inside. Unfortunately when it is loose it is creating powder inside and all the parts of the model.
This powder is then multiplying static issues. It might hold there very well, so only some kind of washing could get it out.

Then please make sure no cable is going near the grounding wire. Preferably at least 3 cm far.
When humidity is low then problems are more frequent. With high humidity the problems might not appear.

There are also anti-static sprays that might at least help partially. Also please make sure that there is connection between tail boom and motor itself - measure at bolts as they are conductive.

_________________
Spirit System developer

Hi
Your ground cable have to go on the engine and not on the chassis.
Potential equalization
And then measure that you have a connection from the tail to the motor plate

_________________
official hacker distributor USA
www.hackermotors.us

Hi,

can you describe at least some information about your negative altitude measurement?

It is normal to see a negative value if your model is doing big pressure changes with roror blades.

_________________
Spirit System developer

Hello, I feel and understand your TREX 500 crashing Pain first hand .....

I built a TREX 500 as my "learn to fly" heli --- this was many years ago.
I have an incredible large bag of parts that is huge from the many crashes.

I also had Spirit Pro from those early days on my TREX 500. I still fly Spirit Pro and love it, especially with 3.1 now available

My TREX 500 would fly perfectly some days and then other days, it became "possessed" with the true evil described here in this thread. At any given time without any apparent reason, the heli would "lock up" and crash for what seemed to be no reason at all. EXTREMELY FRUSTRATING.

Finally there was only one solution that truly fixed this problem for good, and unfortunately this high quality precision company no longer makes this part. The company that made these parts I2RC went out of business years ago, and they made a metal tail box that is all metal which grounded the metal tail boom to the very substantial metal tailbox as shown in this thread in pictures:
https://www.helifreak.com/showthread.php?t=679091

This was the holy grail solution, and still is for me today.
I am still flying my now wonderful TREX 500 with that metal tail boom block and never ever have a static problem again.
I spent tons of $$$ on this heli wondering why it would out of the clear blue just lock up and crash.

So what will help you with this problem?
I highly recommend finding a "common ground wire" to as many metallic surfaces as possible to have a common ground across the metal components of the frame, motor, boom, etc anything metal is a must to resolve this issue. The good news is since adding that tail block many years ago from I2RC and/or doing the common ground wire to all metal parts I have had zero crashes (unfortunately, the I2RC metal tail block is no longer available, as the company went out of business).

I tried silicone sprays on the belts helped, but it has to be reapplied often, and running the ground wire mentioned to have a common ground across all metal parts as best as possible is the only solution.

Luckily with the I2RC tail block I purchased years ago, I have not had one static related crash since then. In fact I fly this heli a lot and maintain it, but the grounding is paramount to prevent out of the blue crashes due to static buildup issues that are pure evil.
Hope this helps, also if you have a plastic boom, consider changing it out to a metal boom, will help as well.
Trust me, I understand and lived this scenario, it was a night mare... hope you can use some of these tips to prevent static buildup crashes.
Dave

Thank you tomsl and HeliDave,
as I went forward experimenting, I give you a quick update. The tail block of my T-Rex was already a metal one. Following your advice I connected first tail boom to motor (evil still there) then I connected the boom to the negative pole of the battery, once again no good news. I also noticed again bumps of the servos even when the heli is still on the ground.
I'm almost hopeless. Only tentative left is substituting the plastic tail boom with the original metallic one (once I receive it from the shop).
Thank you for supporting me in my fight against

Are you sure your servos are Ok? You mentioned a 'bump' even when heli static on the ground. A crash can do nasty things to the internals,
plenty of people have had servos cause problems with FBL units. If you have a spare then go round all 3 cyclic ports to see if it settles down