What is the blade angle of a feathered propeller?

Feathering drives the blades to roughly 80 to 90 degrees, close to parallel with the oncoming air. At that angle the blade produces almost no lift, the propeller stops windmilling, and the drag of the dead engine drops sharply. That drag reduction, not engine protection, is the reason the FAA gives for feathering a propeller after an engine failure on a multiengine airplane.

What is the difference between blade angle and pitch?

Blade angle is a physical measurement: the angle between the blade chord line and the plane of rotation at a given blade station. Pitch describes travel per revolution. Geometric pitch is the distance the propeller would advance in one turn if it did not slip, and effective pitch is the distance it really advances. Blade angle sets pitch, but the two are not the same number.

How does a constant-speed propeller governor hold rpm?

Engine-driven flyweights are balanced against a speeder spring that the propeller control loads. On-speed, flyweights and spring balance, the pilot valve sits neutral and blade angle holds. If rpm drops, the flyweights tilt inward and the valve ports oil to reduce blade angle; if rpm rises, they tilt outward and blade angle increases until rpm returns to the selected value.

Are propellers on the Airframe or Powerplant written test?

Propellers sit in the Powerplant section of the Mechanic ACS as subject area AM.III.M, so propeller questions appear on the Powerplant written test rather than the Airframe test. Blade geometry, governor operation, feathering, reverse and propeller inspection all come from that area, and the supporting public-domain reading is the powerplant handbook, FAA-H-8083-32.

Can an A&P mechanic repair a nicked propeller blade?

Small nicks and scratches can often be dressed out, but only within the limits the propeller manufacturer publishes: depth, location and permitted material removal are all specified, and blending must leave no sharp edges. Major repairs are different. 14 CFR Part 65 bars a certificated mechanic from performing major repairs or major alterations to propellers, so retipping or straightening a bent blade goes to a propeller repair station.

Propellers — FAA A&P Test Questions (ACS AM.III.M)

Propellers is the Powerplant subject area where blade geometry meets governor hydraulics. The FAA expects you to define blade angle and geometric pitch precisely, explain why a blade is twisted from shank to tip, and trace what happens inside a constant-speed system when the engine drifts off the selected rpm. Questions run from single-engine constant-speed installations through feathering and reverse thrust, plus the inspection and ground-safety rules that go with a propeller that can turn. This page shows what the area covers, where candidates lose points, and lets you drill the published questions.

What ACS AM.III.M covers

Propeller questions start with definitions, and the FAA words them tightly. Blade angle is the angle between the blade chord line and the plane of rotation, measured at a stated blade station. Geometric pitch is the theoretical distance the propeller would advance in one revolution with no slip; effective pitch is the distance it actually advances, and slip is the difference. Blades are twisted so the blade angle is greatest near the shank and smallest at the tip, which holds the angle of attack roughly even along a blade whose tip travels far faster than its root. From there the area moves into control. On a typical single-engine constant-speed system, governor-boosted engine oil pressure works against a spring, an air charge and counterweights to set blade angle. The propeller is on-speed when engine rpm matches the rpm the governor is set for and the flyweights balance the speeder spring, so no oil moves to or from the piston; underspeed and overspeed conditions and the pilot valve response are tested directly. Multiengine work adds feathering, driving the blade to roughly 80 to 90 degrees so it stops windmilling and sheds drag after an engine failure, and turboprop work adds reverse, where blades rotate below flat pitch into a negative angle. The rest is maintenance: blade tracking, static balance, torque and safetying, dressing damage within published limits, ice control by electric blade boots or slinger-ring alcohol, and safe handling around a propeller. FAA-H-8083-32 is the reference behind most of these items, and 14 CFR Part 65 sets what a certificated mechanic may and may not do to a propeller.

Where this sits on the test

ACS AM.III.M is tested on the FAA Powerplant written test, one of 1,355 ACS-tagged questions in the Powerplant bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-32

Three traps candidates fall into

Geometric pitch gets read as how far the propeller actually moves forward in one revolution. That is effective pitch. Geometric pitch is the theoretical no-slip distance, effective pitch is the real distance, and slip is the gap between them. Reverse the two and every slip question falls apart.
Because the tip moves fastest, candidates assume the tip carries the largest blade angle. It is the opposite. Blade angle is greatest at the shank and decreases toward the tip, and the twist exists precisely because the fast-moving tip needs a smaller angle to hold a workable angle of attack.
Centrifugal force is assumed to throw the blades toward high pitch. Centrifugal twisting moment actually rotates a blade toward low pitch, and it is the stronger of the two twisting forces. Aerodynamic twisting force is the one that tries to increase blade angle. Swapping the pair costs points repeatedly.

7 free sample questions from ACS AM.III.M

AM.III.MTap an answer
On a propeller, the blade angle is best defined as the angle between the
AM.III.MTap an answer
Propeller blades are usually given a twist so that the blade angle is
AM.III.MTap an answer
Geometric pitch of a propeller is defined as the
AM.III.MTap an answer
In a typical single-engine constant-speed propeller system, engine oil pressure acting on the propeller piston is used to
AM.III.MTap an answer
A propeller is in the on-speed condition when the
AM.III.MTap an answer
Feathering a propeller on a multiengine airplane following an engine failure is done primarily to
AM.III.MTap an answer
When a propeller blade is in the feathered position, the blade angle is approximately

On a propeller, the blade angle is best defined as the angle between the

ACS code: AM.III.M

Correct answer: blade chord line and the plane of rotation

Rationale: Blade angle is the angle formed between the chord line of a propeller blade section and the plane of rotation of the propeller. It is a geometric, built-in or set value. This is distinct from the angle of attack, which is measured between the chord line and the relative wind and changes with airspeed and rpm. Defining blade angle relative to the plane of rotation is fundamental to understanding pitch and pitch distribution.

Propeller blades are usually given a twist so that the blade angle is

ACS code: AM.III.M

Correct answer: greater near the hub than at the tip

Rationale: Because a propeller is a rotating airfoil, sections near the tip travel through a much greater distance per revolution and therefore move faster than sections near the hub. To produce a more uniform angle of attack and thrust along the blade, the blade is twisted so the blade angle is highest near the hub (slower-moving root) and decreases toward the tip (faster-moving). This blade twist, called pitch distribution, compensates for the difference in section speeds.

Geometric pitch of a propeller is defined as the

ACS code: AM.III.M

Correct answer: distance the propeller would advance in one revolution with no slippage

Rationale: Geometric pitch is the theoretical distance a propeller would move forward in one revolution if it were advancing through a solid medium with no slippage, based on the blade angle. Effective pitch is the actual distance it advances through the air, which is less because air is not solid. The difference between geometric and effective pitch is propeller slip. Distinguishing these three terms is core propeller theory.

In a typical single-engine constant-speed propeller system, engine oil pressure acting on the propeller piston is used to

ACS code: AM.III.M

Correct answer: move the blades toward low pitch (high rpm)

Rationale: In most single-engine constant-speed propellers, governor-boosted engine oil pressure is directed to the propeller piston to drive the blades toward the low blade-angle, high-rpm position. Opposing forces such as counterweights, springs, or nitrogen pressure move the blades toward high pitch when oil pressure is reduced. The governor meters oil to and from the piston to hold the selected rpm. Knowing which direction oil drives the blades is essential for troubleshooting.

A propeller is in the on-speed condition when the

ACS code: AM.III.M

Correct answer: speeder spring force and governor flyweight force are in balance

Rationale: In a governor, centrifugal flyweights tend to fly outward with rpm while the speeder spring pushes them inward according to the pilot's rpm setting. When these two forces are balanced, the pilot valve is centered, no oil flows to or from the propeller, and the system is on-speed at the selected rpm. When flyweight force exceeds spring force the system is overspeed, and when spring force exceeds flyweight force it is underspeed. Recognizing these three conditions explains governor response.

Feathering a propeller on a multiengine airplane following an engine failure is done primarily to

ACS code: AM.III.M

Correct answer: reduce drag from the windmilling propeller

Rationale: Feathering rotates the blades to an angle of approximately 90 degrees, edge-on to the airflow, so the propeller stops rotating and presents minimal frontal area. This greatly reduces the drag that a stopped or windmilling propeller would otherwise create on an inoperative engine, improving controllability and performance on the remaining engine. Feathering also protects a damaged engine from further wear, but reducing drag is the primary aerodynamic purpose.

When a propeller blade is in the feathered position, the blade angle is approximately

ACS code: AM.III.M

Correct answer: 90 degrees

Rationale: In the feathered position the propeller blades are turned to approximately 90 degrees so the blade chord is roughly aligned with the line of flight, presenting the edge of the blade to the airstream. This stops the propeller from windmilling and minimizes drag on a failed engine. A near-zero or flat blade angle would instead create maximum drag, the opposite of the feathering goal. Knowing the feather angle is basic to feathering-system work.

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Frequently asked questions

What is the blade angle of a feathered propeller?: Feathering drives the blades to roughly 80 to 90 degrees, close to parallel with the oncoming air. At that angle the blade produces almost no lift, the propeller stops windmilling, and the drag of the dead engine drops sharply. That drag reduction, not engine protection, is the reason the FAA gives for feathering a propeller after an engine failure on a multiengine airplane.
What is the difference between blade angle and pitch?: Blade angle is a physical measurement: the angle between the blade chord line and the plane of rotation at a given blade station. Pitch describes travel per revolution. Geometric pitch is the distance the propeller would advance in one turn if it did not slip, and effective pitch is the distance it really advances. Blade angle sets pitch, but the two are not the same number.
How does a constant-speed propeller governor hold rpm?: Engine-driven flyweights are balanced against a speeder spring that the propeller control loads. On-speed, flyweights and spring balance, the pilot valve sits neutral and blade angle holds. If rpm drops, the flyweights tilt inward and the valve ports oil to reduce blade angle; if rpm rises, they tilt outward and blade angle increases until rpm returns to the selected value.
Are propellers on the Airframe or Powerplant written test?: Propellers sit in the Powerplant section of the Mechanic ACS as subject area AM.III.M, so propeller questions appear on the Powerplant written test rather than the Airframe test. Blade geometry, governor operation, feathering, reverse and propeller inspection all come from that area, and the supporting public-domain reading is the powerplant handbook, FAA-H-8083-32.
Can an A&P mechanic repair a nicked propeller blade?: Small nicks and scratches can often be dressed out, but only within the limits the propeller manufacturer publishes: depth, location and permitted material removal are all specified, and blending must leave no sharp edges. Major repairs are different. 14 CFR Part 65 bars a certificated mechanic from performing major repairs or major alterations to propellers, so retipping or straightening a bent blade goes to a propeller repair station.

All 40 ACS areas

General

Airframe

Powerplant

Primary sources

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