com·plain/k m‘plan/verb Express dissatisfaction or annoyance about something. In pilot parlance: a squawk.
Sometimes people confuse our technical, studious, perfectionist, Type A pilot-personality with that of a complainer. My mom and dad would say, “If you can’t say something nice, don’t say anything at all.” Failure to abide by this maxim or using “bad” words (even euphemisms) could result in mouths being washed out with soap. If the infraction so deserved, Grandma liked a “switch” from a nearby tree. Dad preferred his belt, and mom’s favorite instrument was a paddle. The complaining for which I may be due a scolding this time is aircraft maintenance. The paddle in this story, however, provided relief and not pain from a difficult-to-diagnose propeller squawk.
Patience is a Virtue
I haven’t flown the Duke in many months. And since I stopped flying for a living in July, the last few non-flying months have been painfully unique. I am non-current for the first time in five decades; my Santa-like beard a prickly reminder. Thankfully, a week at Oshkosh and a month of hunting in New Mexico helped placate my withdrawal. Perhaps my angst can be blamed on a moderate lack of patience. Well, perhaps more than moderate. The Duke’s annual inspection scheduled to begin on April 1 didn’t start until late August. And therein lies grievance number one (hide dad’s belt).
Unfortunately, the long delay was not unique to me, my shop or the Duke. A handful of fellow GA pilot/owners experienced similar maintenance delays and shortages, apparently due to the “COVID Effect,” which disrupted providers, manufactures, suppliers, distributors and shippers. In addition to my shop having a backlog longer than the line at Chick-fil-A, I’d been fighting a squawk that surfaced after last year’s engine overhaul: The right propeller developed a propensity for uncommanded, self-feathering. Thus complaint number two (hide grandma’s switch).
Paddle Me
Fortunately, the self-feathering only occurred with hot oil, low RPM and on the ground – like on landing roll or while taxiing to the hangar. Two or three times I made it all the way to the hangar before the right prop feathered, but a few others, it feathered during the landing roll or as we cleared the runway. This resulted in an inconvenient and embarrassing call to ground control and the FBO to coordinate a tow. The Duke will not taxi on one engine. After the oil cooled, it was sometimes possible to unfeather the prop by attempting an engine start, but the technique and finesse required were often beyond my ability. And for those who have never attempted to unfeather a stationary prop by manually twisting the blades using one or two sets of hands, the spring tension holding the prop in the feathered position is significant – brute hand strength is insufficient.
I always believed that a device known as a “prop-paddle” was an acceptable un-feather tool. Hartzell told me that it is not and internal damage is possible if attempted. And this new prop-paddle knowledge is why I stopped using a homemade version of the tool, stopped flying the Duke and awaited the annual inspection for diagnosis and repair. It took calls to the Tiffin Aire prop shop, Duke guru Bob Hoffman and Hartzell Propeller’s technical help-line to explain to me the intricacies of propeller mechanics so as to develop a plan of action. Eventually, Lycoming Service Instruction 1462A provided the answer.
Full Feathering Props 101
Piston engine, controllable-pitch propellers come in several flavors, but the majority share a common design. Prop blade angle is controlled by the motion of a piston inside the propeller dome. This piston is moved by oil pressure on one side, with counterweights and a strong spring (usually augmented by a charge of compressed nitrogen in the prop dome) on the other. The counterweights do most of the work to drive the blades into feather (high pitch). The springs help to feather the prop when the centrifugal loads acting on the counterweights do not have enough centrifugal force to drive the blades completely to feather. As oil pressure changes, the piston moves, and the blade angle changes through gearing between the piston and the blades themselves. If oil pressure drops below a set minimum, the propeller blades drive to the feather position. In this situation with most twins, the propeller goes to so high a pitch that the blades flatten out relative to their direction of rotation, being twisted to the low-drag, feather position (82 degrees in the Duke, for example).
To drive the blade angles from feather (high pitch low RPM) back to low pitch high RPM, or to keep them there, you must have oil pressure. The oil psi of which we speak is not only from the engine’s oil pump (the psi you read on the gauge), but the prop governor gear pump boosts oil pressure before it heads out of the governor and into the propeller hub. With the boosted pressure, you get a better, quicker response from the propeller when you move the lever in the cockpit. Low oil pressure can occur intentionally through the use of this blue propeller control lever by pulling the handle to the feather position; this opens a valve that dumps all oil from the prop dome and drives the blades into feather. Or, the low pressure can occur due to a massive oil leak or a “mechanical failure.” The mechanical failure mode is what I was experiencing with the right prop in the Duke – more on that and Lycoming SI 1462A in a bit.
Feather Me Not
Under normal operation, a component called an “anti-feather lock” is supposed to catch the prop as it attempts to feather when RPM and oil pressure decrease. The anti-feathering lock pins are held out of contact by flyweights when the engine is running and engage when propeller speed drops to 600 to 800 RPM. But for the anti-feather lock to move into place, there still must be a certain amount of oil pressure as the engine slows to a stop. Conversely, while sitting on the ground with the engine shut down, the prop sits on that latch until you start the motor. The locks help keep some oil in the prop dome and avoid friction and vibration when we shut down the engines.
In an in-flight emergency or training scenario with air load driving the propeller blades, moving the prop control to feather causes the prop blades to twist to feather pitch before the lock pins engage, and the propeller feathers. During a normal on-ground shutdown, the air load is absent, and when the engine shuts down, the prop reduces RPM slowly enough that the anti-lock pins drop into place as the rpm drops through that 600 to 800 RPM range. The prop blades will twist no further, so they don’t go into feather. No vibration, no oil-starved prop dome gears, and no cranking against dry metal on the next start-up. Enter Lycoming Service Instruction 1462A.
My Kingdom for Oil Pressure
According to Lycoming, SI 1462A should be accomplished “Whenever sluggish propeller action is reported, when the engine does not hold RPM during cruise, climb or descent, or whenever the engine (I’m certain they meant propeller) is going into feather during landing roll out with reduced throttle setting.” The purpose of the air pressure check to the propeller governor system is to determine if the governor oil passages have excessive clearance, leaks or blockages (tight clearance). The air pressure check shows the condition of governor oil passages, front bearing clearance, and positioning of the governor circuit oil plug. The test procedure directs you to attach a test plate and pressure gauge to where the prop governor mounts to the engine. First, the test plate is used to verify oil psi with the engine running – this verifies you have good baseline oil psi. Next, you shut down the motor, attach a differential air pressure tester to the same test plate and input 40 PSI.
The output (prop governor circuit) should be between 6 and 35 PSI. The Duke’s was around five. The reason for the low number was that during the engine overhaul, while within tolerance, the crankshaft was undersize. The overhauler installed new “normal” sized bearings on the crankshaft when, since the crankshaft is undersized, he should have installed oversized bearings. The undersized crank and normal size bearings created too large of a clearance at the front bearing, resulting in too low of oil pressure to the prop governor. With the excessively low oil pressure, the system acted just as it should – it allowed the prop to feather.
Grumpy Old Man
Since I’m officially old and on Medicare, and being a grumpy old man was moved to the Normal Procedure section from the Abnormal and Emergency sections, perhaps I can get away with some venting – as long as I comply with the Limitations section. But after grounding my poor Duke for a few months and paying $26K to fix the main-bearing misstep, complaining is difficult to hold back. I can taste the soap.