A permitted repair with unforgiving limits

Twenty-four elements, six station elements, and only two Risk Management elements — which badly understates the danger.

1. Two risk elements for the section's defining hazard

R1 ground operation. R2 propeller maintenance and inspections.

Two is the lowest risk count of any substantial subject in Powerplant, and the propeller is what makes a piston engine lethal to maintain.

PP-A §1 states the rule and PP-H §1 explains it: a magneto is a generator, the switch only grounds it, and a broken P-lead leaves it live with no outward sign.

In this unit your hands are on the blades. S1 removes and installs a propeller. S6 lubricates one. S3 and S10 inspect one closely. Every one of those puts you in the arc.

So the control here is physical, not procedural: disconnect and ground the magnetos at the harness before touching a blade, and treat it as live anyway.

S8, adjusting a governor, is done with the engine running — which adds the whole of GEN-F Procedures Procedure 7 to the list.

2. A constant-speed propeller is an oil-pressure problem

K3, K4, S8. Candidates try to understand the linkage. The linkage is not where the control is.

The governor is a speed-sensing oil valve. Flyweights driven by the engine sense RPM against a spring set by the pitch control. When RPM rises above the setting, the flyweights move a pilot valve, and oil is directed to or released from the propeller hub to change blade angle.

Higher blade angle means more load means lower RPM, and the reverse. The system continuously hunts toward the set speed.

Two consequences:

• Most constant-speed faults are oil faults — supply pressure, a blocked passage, a leaking transfer bearing, contaminated oil, or air in the system after maintenance.
• The direction of failure depends on the design. Some propellers go to fine pitch on loss of oil pressure, some to coarse or feather. Knowing which your propeller does tells you what a loss of oil pressure will look like — and it is in the manual, not guessable.

A candidate who traces the oil path can troubleshoot. One who memorized the linkage cannot.

3. S5 — dressing a nick is permitted, and doing it slightly wrong is catastrophic

Perform a minor repair to a metal propeller blade.

This is one of the few tasks where a mechanic may repair a component that will fail catastrophically if the repair is wrong, and it is permitted precisely because there is a prescribed method.

Why a nick matters at all: a propeller blade is a rotating beam under enormous centrifugal and bending load, cycling every revolution. A nick is a stress concentration, and stress concentrations start fatigue cracks. A blade failure in flight is usually unrecoverable.

Why the repair works: dressing removes the sharp notch and replaces it with a smooth, generously radiused hollow. You are not removing the damage — you are removing the geometry that concentrates stress.

Why it is unforgiving:

• Depth limits — how deep a nick may be and still be dressable.
• Blend radius and length — the transition must be gradual, and a short steep blend is itself a stress raiser.
• Location limits — leading edge, face, camber side and tip all have different allowances, because stress differs by location.
• Cumulative removal — there is a limit to total material removed over the blade's life, and it is tracked.
• Both blades — removing material from one blade unbalances the propeller, so the manual's requirements about matching apply.

S11 is the documentary half. K8: a propeller has its own type certificate data sheet, with its own limits and its own list of acceptable minor alterations, independent of the engine and airframe. Candidates do not expect this.

An older propeller may have no TCDS at all. That is why the element names three documents: propeller specifications, TCDS, and listings. FAA-H-8083-30B Ch. 2 — Aircraft Specifications explains that specifications are FAA recordkeeping documents for both type-certificated and non-type-certificated products: "Although they are no longer issued, specifications remain in effect and will be further amended." Group II and Group III propeller approvals, from before August 1938, "have met minimum airworthiness requirements" without formal type certification. The handbook's index files these under Aircraft Listings. So find which document governs this propeller before looking for its limits — a missing TCDS is not a missing approval.

And GEN-I Concepts §2 adds the limitation that catches people: an A&P may not perform major repairs or major alterations to propellers. Dressing a nick within limits is minor. Beyond that, it goes to a propeller shop.

4. Track, balance and vibration

S2, S7, K1.

Track is the path each blade follows — whether the tips pass through the same plane. Balance is mass distribution. They are different faults corrected differently, and both produce vibration.

Vibration is not a comfort issue on a propeller-driven aircraft. It fatigues engine mounts — PP-C §4's cracked mount is frequently a vibration story — and it loosens everything around it.

So a change in propeller vibration is a finding, and the diagnosis starts with track and balance rather than with the engine.

5. What consumes this unit

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