COMMANDER TIPS How to Check Hydraulic Fluid

When performing a Twin Commander (non-Skydrol) aircraft preflight check you must first verify two things to get an accurate indication of hydraulic fluid level:

1. The flaps are fully retracted
2. Hydraulic system pressure shows 0 PSI

If either of these requirements is not met, the aircraft will present a false low-level indication.  The aircraft gets serviced with what is thought to be the needed amount of the magic red stuff, goes on a trip, comes home, and is parked for a few days. The next time the pilot comes out to the aircraft he finds a puddle of red “blood” on the ground under the left nacelle. Then the questions start flying.

So, how to prevent this loss of the precious fluid? The answer is to perform the following simple process. It does not have to happen right after aircraft shutdown, but should be sometime soon after. First, run the flaps down, then back up to fully retracted, then down part way and finally back up again. This uses some of the residual energy within the accumulator, causing the accumulator piston to move towards the bottom of its bore, pushing hydraulic fluid back into the reservoir. The idea is to drop a lot of the energy out of the accumulator but end up with the flaps retracted before the energy is dissipated.

Now, sitting in the pilot’s seat and while watching the hydraulic pressure gage, work the brake pedals. The hydraulic system pressure will start to decrease, and at some point the pressure will take a sudden dive towards 0 PSI. Note that point. Work the pedals a few more times. If the gauge seems to bounce around 0 PSI whenever you press the pedals you are ready to get a good reading of the fluid level in the aircraft. The reservoir is vented to the atmosphere (not pressurized as was the case with aircraft using Skydrol) so you can open the nacelle cover, turn and lift the cap on the reservoir and note the height of the fluid between the two marker holes in the dipstick. 

The distance between the holes is equivalent to about one quart of fluid. If the reservoir needs to be serviced, use clean hydraulic fluid, put the cap back on, close the nacelle cover, and the system is ready to go.

I bet you think I forgot about the pressure gauge needle drop comment. When you are depressurizing the accumulator the point at which the hydraulic system pressure gauge suddenly drops is a very rough indicator of the accumulator precharge. You expect it to be around 600 PSI or so before it suddenly drops. However, if it suddenly takes a rapid dive towards 0 PSI when you start the process that is an indication of a low accumulator precharge. If that happens I would recommend contacting your favorite Twin Commander Authorized Service Center to have the system checked for an incorrect accumulator precharge that needs to be serviced, or an internal system leak that needs to be fixed.

Eagle Creek Aviation Service’s Mike Grabbe has been servicing Twin Commanders since 1974. He has been a factory service representative, and is currently the Twin Commander Factory Authorized Training Provider. He can be contacted at [email protected].