What It Looks Like
Proper Nose Strut Inflation
Nosewheel landing gear come in a variety of shock-absorbing designs, from a simple one-piece spring-steel strut, to one that incorporates rubber "doughnuts" to soak up the bumps, to the most common type - an air-oil (or oleo) compression strut.
The shock-absorbing component in an air-oil strut is a sealed cylinder filled with a combination of nitrogen and oil. When the nosewheel touches down or encounters a bump on the runway or taxiway, the strut compresses, forcing the air/oil mixture through a metering orifice inside the strut cylinder.
The orifice is larger at the beginning of strut compression. The more the strut compresses, the smaller the orifice becomes. Like a spring being squeezed, compression becomes progressively more difficult as the metering orifice decreases in size. This prevents the strut from bottoming out and transferring damaging shock loads to the landing gear and airframe.
The seals in air-oil nosewheel struts eventually leak, allowing air, oil, or both to seep out of the cylinder. The first indication of a leaky strut might be a bit of a mess where some oil has run down the shiny strut and onto the nosewheel. In that case, a shot of nitrogen in the strut can restore it to full function. A more serious condition exists if the strut is obviously compressed, with little or no shank visible. That calls for an immediate trip to the maintenance shop to have the strut rebuilt.
How much should the strut be extended when the airplane is parked and empty of passengers and bags? About three inches is normal, but it differs according to make and model. Check the pilot's operating handbook for precise guidance.