Prepare the aircraft. At a minimum, chock all wheels. If chocks are not available, then consider using Vice Grip pliers to supplement the aircraft locks, or perform this service with the aircraft jacked. With experience, chocking is adequate. The gear selector is NOT moved during accumulator service unless the aircraft is jacked and SN 501 or below.
The objective of servicing the accumulator is to get 350psi of nitrogen gas in the accumulator with no hydraulic fluid in the accumulator at time of service. When servicing the accumulator there should be no indication of pressure on the hydraulic gage in the aircraft. If there is any indicated pressure, then use the flap selector (SNs 502* and on) or cycle an actuator (SNs 501* and prior) until there is NO pressure on the aircraft hydraulic gage. Note: It is not uncommon to bleed pressure to zero and then see pressure indicated when charging the accumulator. This “new” pressure must also be bled to zero. Likewise, consider removing the hydraulic dipstick, and remaining clear of hydraulic vents, reservoirs and fill ports as they may overflow when the accumulator is serviced. This is caused by the accumulator filling with fluid while discharged.
After bleeding hydraulic pressure to zero, install the service equipment to the gas end of the accumulator. On most aircraft, this will be under the instrument panel and located over the co-pilot’s left foot (early aircraft and 24 volt Renegades are different). Not all, but most have a fitting that is similar to the valve stem on a tube or wheel. This fitting, however, is equipped with a high pressure valve core that can handle about 2000psi. There are two common service fittings used to fill accumulators. One type has the ability to mechanically open the valve core and the other type uses nitrogen pressure to open the valve core. It typically takes 100psi or more to open a valve core with gas so this type of equipment would require input of at least an additional 100psi over the desired pre-charge of 350psi.
After adding nitrogen to the accumulator, close the valve core if you have that type of equipment. Using hand or electric pump, build pressure to about 600psi. Using flap selector, bleed pressure to zero. A properly charged accumulator while bleeding will show the characteristic of steadily losing pressure, then dropping suddenly from some point to zero. The pressure at which the sudden drop starts can be considered the accumulator pre-charge pressure.
If the drop point is below 350psi, then the gas pressure in the accumulator must be increased. If the drop point is above 350psi, then some gas pressure needs to be removed. In normal operation, a drop range of 250psi to 400psi gives acceptable performance.
Accumulator pre-charge can be routinely checked outside of scheduled maintenance checks. Pre-SN 501 aircraft are easy to check on the water as the gear can be cycled safely to reduce pressure and see the drop point. SNs 502 and post can use flap selector on ground as indicated above or on the water.
*SNs are approximate.
Disclaimer: Use this information at your own risk. This is provided as a guide and is meant to be used for informational proposes only.