Oleos are shock absorbing tubes constituting the main flexible connection between the wheels and the aircraft. When an oleo goes bad, the airplane can’t fly.

Well, it can fly, but it can’t land. Consider the construction of an oleo: high-pressure compressible nitrogen, a pool of hydraulic fluid, and a couple layers of nested movable cylinders and rubber O-rings keeping the two apart.

If the O-rings leak, both contained fluids can escape the enclosure. This robs it of its springiness, much like a flat tire. A completely flat oleo has several serious effects. First, of course it can no longer absorb the shocks of taxiing and landing. All those harsh forces would be transferred right to the airframe, possibly damaging the structure.

Second, with retractable gears like those in our Aztec, the oleo is required to extend fully as soon as we are airborne, so that the wheels swing up and into their slots in the nose and the engine nacelles. Flat oleos have no residual pressure with which to fully extend the wheels. This can cause a wheel retraction cycle to smash the wheels into an inappropriate region, possibly crushing gear doors or other structure. The wheels may not come down again. If for some reason flight with a flat oleo is unavoidable, one should leave the gears down throughout the flight.

GXRP’s nose wheel oleo has started seeping the red hydraulic fluid about two weeks ago. It was very slight, and it didn’t stop me from flying with it. The oleo would emit a little bit of fluid just after landing, then seal okay again. But after two or three flights, it was clear that the problem was not getting better, and the pattern of fluid loss could not continue much further. I cancelled several flights over the previous weekend, concerned about a sudden failure, and scheduled an urgent visit at Brantford for a repair. I didn’t realize how close I was.

The Brantford folks kindly allocated an early weekday for the work only two days away. By the time this day rolled around though, the nose oleo has decided to completely deflate. Only about 1/4” of the inner chrome showed. Flying to Brantford for its repair was not safe. Instead, John Cabaco’s maintenance crew right at the Island made themselves available to fix the oleo. It had to be removed and rebuilt, replacing all O-rings and refilling the fluids. 24 hours later, they fit us in and the airplane was good to go. A quick overnight trip to Ottawa Rockcliffe showed the rebuilt oleo working perfectly.

Lessons? Most airplane maintenance shops are willing to handle urgent maintanence like this, even putting some of their scheduled work aside briefly. Owners whose scheduled-work airplanes are sitting for a few more days should not begrudge such temporary reprioritization: they will need it one day.

Also, keep the oleo clean. John suggests using a rag, soaked with hydraulic fluid, to wipe down all oleos before/after each flight. Dirt can be caught by a moist oleo, and its movement can drive the gunk right into the mechanism, shredding the O-ring seals. This can happen surprisingly quickly. The failure modes, as described above, are nasty. So keep them clean!