This is really a 2 person job, easy way is to have a hose hook up to the struct valve, let out the pressure, the hose is hooked into a bottle of fluid, extend the strut ( Your helper ) which sucks in the new fluid, then  put the valve back on and add the air pressure. It should be the same fluid that you use in your brake system.

You have to have the right amount of fluid and air, and might need new seals. The maintenance manual and an A&P are a good reference.

In adding air to the strut, my 172 service manual prescribes an air pressure of 45 psig when fully extended.  This could be different when just setting the extension by just filling it with air from fully compressed position to some arbitrary height. 

I see two problems with doing the latter and would like to get some of you A&Ps to respond to how it is "normally" done. 

1) Due to frictional forces, setting it from fully compressed to a nominal height results in a higher pressure than it would be if you do it per the service manual.

2) If the air pressure is too great to get it to move, can you blowout a seal or do some other damage?

Another thought, if it is partially extended and you need to add air, it is easy to collapse the strut if the little bit of air is released too quickly.

Do exactly what howard has said. The only things i would change and this is what we do:

1. push the tail down with sandbags or anything (cases of oil) to hold the tail on the ground. make sure to position them on the spars and equally on each side.

2. Instead of fully compressing strut after servicing leave approx 1/2 inch of piston showing. This helps if the strut blows out, so that it is sitting on fluid and not metal to metal.

3. Inflate strut with air, we just push it on until no more air will enter. 25 gal compressor, 125 psi. allow small amount of air out, push strut up and down and check extension. repeat until right extension is correct.

I prefer to set aircraft back down on nose then inflate with nitrogen to correct height.  If I have to inflate with strut off ground I fully extend strut prior to inflation to prevent strut from suddenly slaming to full extended position.  I use the four finger method also, works well for the cessna singles.

Thanks for the input.

Here are Some thoughts on the input all have provided, from a different perspective.

The nitrogen purge prevents corrosion in the air space (Corrosion requires oxygen). Corrosion from any moisture that might be in the strut would be limited to the content of the dissolved oxygen in the moisture (negligible).  Condensed moisture in the hydraulic fluid should not be allowed to accumulate but unless you get a lot it won't make much of a difference. Where's a good place to get bottled nitrogen?

As I see it, the air in the shock is an "air spring."  It works in conjunction with the hydraulic damper to reduce shock to the superstructure on a hard landing and affects the response of the strut to "bumps" in the road.  The quantity of air makes a difference in the damping coeficient.  Too much will tend to be overdamped and too little could be underdamped. Overdamped transmits the largest forces (shock) to the superstructure (Not good). Underdamped would make the nose bounce up and down excessively (Not good).  One hopes that the specified quantify of air in the service manual optimizes this for us.

My question, really goes to the latter point. However, you all have provided me with a wealth of practical experience for accomplishing this.  Thank You!

Barry,

One thing to check is service the strut with air then use a soapy solution of water and spray it on the strut around the schrader valve and air seal. If there is a leak it will bubble. Check anywhere you feel the strut could leak. It most likely is the schrader valve, you could try tightening it or taking it out and inspecting it for a bad seal.

My strut went flat last week (about -5 F) and once two winters ago, again well below O.  Twice in seven years.  Maybe the seals shrink just that much more, but other times it has been as cold with no change. go figure.  I am planning to replace seals soon only because they are 1968 original.  I find that 45 psi is too low..just fill to the right extension height and seems to work well.