Has anyone come across this?
It’s been there for some years, at least. Every so often, when taxiing, the plane oscillates up and down a little, then settles.
The suspension units contain gas and AFAIK some damping liquid. The gas does escape eventually but mine are fine in that respect. I would think the liquid would be the last thing to get lost.
Not witnessed that symptom in any of the TB20s I’ve flown.
It is vital for the oil level in the dampers to be checked at regular intervals I have seen aircraft as diverse as Robin DR400 and Boeing 737 suffer damage that vastly more expensive than regular shock strut maintenance.
Yup, first thing to do is check proper servicing of the oleopneumatic shock struts. I believe you need to jack a TB20 up to service them, and you need high-pressure N2: your portable compressor will not do.
There is always some loss of fluid in the thin film that forms between the seals and the chromed areas. If the seals are good, yearly servicing should be enough, although our aircraft’s service manual recommends every 50h.
Mine have not been touched in 18 years 
I think the pressure was topped off once, a few years ago.
I have looked after the plane very well, cleaning the chromed portion on each preflight, to preserve the seals, so there is no visible fluid leak. But maybe some fluid has leaked out.
Yes it will need jacks because the gas struts support most of the weight of the plane.
The relationship between the oil quantity and the gas pressure provides the correct rising spring rate, get it wrong or let it go out of adjustment and you will get a DR400 that won’t steer correctly or a Boeing 737 prone to tail scrapes.
To many people don’t understand the science or just want to get the aircraft out of the hanger door and ignore this important issue.
Peter wrote:
But maybe some fluid has leaked out.
In oil damped motorcycle forks with rubber gaiters over the chrome portion and seals, you can pull back the gaiters after years and generally find no trace of oil after God knows how many full stroke cycles, unless it’s really wet due to a failed seal. However these typically use metal springs versus air springs and that may have some effect.
The damping oil may have reduced in viscosity after 18 years of service and an oil change may help.
The static sag is a function of the gas pressure. Assuming these are conventional oleos, the increase in effective spring force versus travel is a function of the remaining air volume with a given oil volume present. However on my plane the nose gear uses a fixed force car-hatchback-style sealed gas strut which has an effectively zero spring rate and is topped out, no static sag, when the plane is taxiing. This acts as an impact absorber for landing versus a suspension system for taxiing.
Even if there is no visible loss of fluid, over the years some fluid is lost. Perhaps some as you let some gas out after topping up with gas.
Either way, correct fluid level has two purposes: correct (if variable) spring rate and correct damping rate.
On a lot of light aircraft the procedure is simple if the aircraft is jacked up (but plse verify with your aircraft’s maintenace manual):
You can also top-off gas with the aircraft on ground using strut extension as a reference, but this is not so precise Some aircraft give different extensions for different weights, but especially for the nose, this is very dependant on cg . On my aircraft it is mostly a gamble. Last, in this case beware: you will need gas servicing equipment capable of much higher pressure, 500psi or more.
Hmmm very interesting and thank you for this great post!
So it is not necessary to dismantle the unit?
I was going to order two overhaul kits. I believe they are of the order of a few hundred $. But I am not seeing significant fluid leakage.
If you are not seeing evident leakage I would not bother. You can buy the OH kits if they are not costly, just to be ready, but if a servicing works, why dismantle the strut? I am all against unwarranted invasive maintenance. If however you go down that route make sure you follow strict procedures and that there is no strut pressure before doing any work.
If never serviced fluid, make sure you make a few fill-empty cycles to flush out the old fluid and then use clean fluid for the final refill.
Some people have reported success with Granville’s strut seal fluid mixed with hydraulic fluid as a fix for leaking strut seals since it rejuvenates them. You do not seem to need it, although it won’t hurt and it is much easier than replacing the seals, so you have little to lose.
