I recently purchased a 1978 Sierra. I noticed a small streak of oil from the propeller hub running along side one blade prop after my last flight. I checked the oil level and it didn’t appear as though much oil had been lost.
Any ideas on the cause and recommendations on possible remedies? Would such a condition be cause for grounding the plane for repair?
You need to clarify whether what you are seeing is really oil, or whether it is prop grease. If it is oil, it will most likely be coming from the flange at the rear of the prop, or the dome on the front. If it is grease, it will be coming from around the root of the blades, where they disappear into the hub.
If it really is oil, and is coming from the rear flange where it bolts to the crank, it could be as simple as having a new hub o-ring installed. That means some labor time, but almost nothing for parts (just the o-ring cost).
If it is oil coming from the front dome, the prop needs repair. If it is grease coming from between the mating hub halves, you can have the hub bolts re-torqued and check it again. If it is grease coming from around the root of the blade, the seals there are leaking. This is often the result of moving the plane by the blades on the constant-speed prop. In flight the blades are locked solidly in place by the centrifugal force, so the thrust forces can’t flatten an aged seal. On the ramp, and especially if the seals are cold, the tiniest blade shift from an eager arm can start a leak.
If it truly is a blade seal leak, the good news is that it isn’t a crisis. The bad news is that it won’t stop until the prop is repaired. The prop will slowly go out of balance, as it is nearly impossible to restore exactly the same grease load in both sides, once it has changed.
Note that there are special instructions for greasing this prop. The rear fittings are removed, and the proper grease (as specified on the last overhaul label on the prop), is pumped in until it is full. Then the rear fittings are reinstalled. The actual instructions say to put the same number of pumps in each side, but I consider that to be unreliable. I have never seen a hand grease gun that always pumped the same volume every time (especially with the typical aviation greases). “Full” may vary, depending on air pockets, etc.
Two other key aspects:
1. How long since the prop was last overhauled? Leaking blade seals are often a sign of an overdue overhaul (the seals harden and no longer seal properly).
2. Prop leaks can be the first warning sign of a very dangerous crack in the hub. There is an inspection protocol spelled out by Hartzell to examine for cracks. There is also an AD out by Hartzell that mandates a new hub for applications of 300 HP and above. That AD was driven by cracking hubs. While we don’t have more than 200 HP, this still points toward a need for careful inspections any time a leak shows up.