How To Save 7 Of 8 Hours In Your Next Landing Gear Strut Overhaul.

 

 (Copyright 2004 - North American DARCO Inc All Rights reserved)

 

One of the very useful skills to have when flying to remote destinations is detailed knowledge of aircraft systems and some mechanical competence. Recently one of my main landing gear struts began leaking hydraulic fluid. It had not gone flat yet but what began, as an occasional small dribble would soon turn into a torrent!

 

The Bonanza and Baron landing gear is a sophisticated piece of machinery, which relies on Hydro-Pneumatic struts for supporting the airplane weight and absorbing shock. It is comprised of a forged assembly that pivots attached to the main and rear wing spar and is mechanically retracted by a worm and ring gearbox located under the front seats. Inside the barrel (#5) resides the chromed landing gear tube/piston (#13) to which the brake disk & caliper and wheel attaches at the bottom. Further inside the chromed piston strut and a separate part of the barrel is an Orifice tube (#4), which allows the hydraulic fluid to pass in a metered fashion cushioning bumps and compressing the inert gas (or air) to provide a smoother ride than what most light planes can deliver.

 

The equivalent of this assembly on automobiles is a shock absorber and they are usually a throw away item when they require fixing. In our Beech airplanes however we can overhaul these an infinite number of times. Most often the problem facing an Owner/Pilot is a “flat strut”. In this instance the Pneumo-Oleo mixture inside the assembly leaked out allowing the unit to fully compress. The aircraft should NOT be flown in this condition as serious impact damage can occur by just taxiing alone. In flight it is likely that the gear will jam in the retraction cycle and not extend again. Have it serviced before flying!

 

This problem typically happens when debris gets on the shiny chrome strut piston (#13) and works itself inside the strut cavity damaging the seals. Holding the oil-pneumatic mixture inside are four seals, as follows:

*      A wiper gasket with metal scraper at the very bottom that SHOULD keep contaminants out of the assembly (#10).

*      An O-ring located midway inside the strut located between the barrel forging and the chromed part (#6).

*      A small and large O-rings (#2b & 3) located at the top holding the air valve assembly (#2) for inflating the strut and to which the Orifice Tube attaches.

 

Your first order of concern in preventing a leaky/flat strut is to avoid operations in contaminated grounds including dirt fields, wet runways, etc. If unavoidable then take the time to wipe at the first opportunity with a clean rag and MIL-5606 hydraulic fluid, the exposed chromed strut piston (#13) and specially, at its very top, the exposed part of the bottom of the strut forging where the scraper wiper gasket is located (#10). This will prevent particles from entering the strut assembly and damaging the INNER middle O-ring (packing #6) which is responsible for holding pressure. Personally I have made it a habit to perform this duty after every flight whenever I return to home base. Unfortunately the most likely source of a leak is the inner O-ring located halfway inside the strut. Changing the O-rings on a strut is an 8-hour job per Beechcraft.

 

I had learned a long time ago from Norm Colvin and an old Bonanza mechanic a trick which will save you (or your shop) 80+percent of the work in changing these without having to remove the entire assembly from the plane and replacing the seals on a bench. I thought it would be a good idea to revisit this technique since it appears that this expertise has become forgotten as new repair talent appears in the aviation scene.

 

With the airplane jacked up and the strut’s valve core (#2a) removed to ascertain there is NO pressure inside, (lest ye loose a limb or your head and/or blow a big hole on top of the wing as the inner strut assembly departs violently under 1000+ PSI of pressure) disconnect the bolt which holds the two torque knees (#14 & 16) and the brake line enabling the chromed strut-wheel assembly to be removed from the bottom. Thence with your head inside the landing gear well and using a mirror and trouble light to see, remove the retainer ring (#1) at the top and lift the orifice tube with Air Valve Assembly (#4 & #2) a couple of inches. This enables you to change the small and large O-rings (#2b & #3) at the top and most importantly allows lifting the orifice tube enough to expose the middle packing (#6) inside from the bottom. Then with a heavy wire L-shaped tool (see picture of mine) the middle O-ring can be removed and replaced with a new one from the bottom. At that time the scraper ring (#10) on the bottom should also be replaced. Replacement of the orifice tube with its securing retainer ring should be carefully accomplished now and finally the chromed strut/wheel assembly can be reinserted from the bottom. Lastly attaching a small hose to the top Schrader valve and sucking oil by “pumping” the strut up and down a few times can add new Mil-5606 hydraulic fluid. Finally the valve core is replaced and the assembly is pumped up using 100-PSI shop air, or nitrogen, while the plane is still suspended. Alternatively, using a multiplier strut pump or high-pressure nitrogen if the gear has already weight on it can pressurize it. Of course inert gas (nitrogen) is the preferred pneumatic medium but air can be an acceptable alternative.

 

A proficient mechanic can accomplish this task in a little over one hour instead of 8.

 

(Copyright 2004 - North American DARCO Inc All Rights reserved)

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