Every time an aircraft propeller turns, it places enormous cyclic loads on the crankshaft and the flange that connects them. If either component is bent, warped, or out of round even by a few thousandths of an inch, the resulting vibration can propagate through the airframe, accelerate bearing wear, crack engine mounts, and ultimately lead to structural failure in flight. The propeller flange and crankshaft runout inspection is one of the most critical checks an Aviation Maintenance Technician (AMT) performs during engine installation, after a propeller strike, and at scheduled overhaul intervals. Understanding the procedure, the tooling, and the manufacturer's tolerance limits is essential both for the FAA Powerplant knowledge test and for safe maintenance practice.
This article walks through the mechanics of runout, the correct inspection procedure, the reference standards that govern it, and the consequences of skipping or misinterpreting results — everything an AMT student needs to approach this topic with genuine confidence.
What Is Runout and Why Does It Occur?
Runout is the total deviation a rotating surface makes from a perfect geometric axis as it completes one full revolution. Think of it this way: if a crankshaft were machined perfectly and installed in perfect alignment, a dial indicator pressed against its flange face or its flange pilot diameter would read exactly zero movement through 360 degrees of rotation. In the real world, runout exists because of manufacturing tolerances, wear, impact damage, improper installation, or accumulated corrosion.
Two distinct types of runout are measured during a propeller-flange inspection:
- Lateral runout (face runout): Movement of the flange face in the direction parallel to the crankshaft centerline — essentially, how much the face wobbles forward and aft as the shaft turns. Excessive lateral runout causes the propeller to wobble in a plane perpendicular to its disk, creating an oscillating bending moment.
- Radial runout (pilot-bore or rim runout): Movement of the flange pilot diameter in a direction perpendicular to the crankshaft centerline — essentially, how much the center of the propeller mounting surface wanders off the true rotational axis. Excessive radial runout causes the propeller to orbit eccentrically, producing an imbalance force that rotates at propeller RPM.
Both types create vibration, but they differ in character. Lateral runout produces a rocking vibration; radial runout produces a rotating imbalance. Either one, if beyond limits, is a reject condition.
How the Inspection Is Performed
Required Tools and Setup
The primary instrument is a dial indicator (also called a dial test indicator or DTI) mounted on a magnetic base or a suitable fixture that clamps rigidly to the engine case or a stationary reference point. The indicator must have sufficient travel — typically at least 0.050 inch — and a resolution of at least 0.001 inch. The crankshaft must be rotated by hand or with a prop, so the engine must be safe: spark plug leads removed, ignition switches off, mixture at idle-cutoff, and the fuel selector off before any manual rotation.
Measuring Lateral (Face) Runout
The dial indicator stylus is positioned so it contacts the face of the propeller flange — the flat, machined surface perpendicular to the crankshaft axis — at a specified distance from the center. The technician zeros the indicator, then slowly rotates the crankshaft through at least 360 degrees while observing the total indicator reading (TIR). The TIR is the algebraic difference between the highest positive and the highest negative deflection observed. That total value is compared against the engine manufacturer's service limit, which is typically published in the engine's Type Certificate Data Sheet (TCDS) or the engine overhaul and service manual. Allowable lateral runout varies considerably by engine model and manufacturer — some Lycoming and Continental limits are as tight as 0.001 to 0.003 inch TIR depending on the specific engine and measurement point — so the technician must always consult the specific manufacturer's data rather than rely on any generic figure.
Measuring Radial (Pilot) Runout
For radial runout, the stylus contacts the cylindrical pilot surface of the flange — the raised diameter that centers the propeller hub. Again, the indicator is zeroed and the crankshaft rotated through 360 degrees. The TIR represents how far the pilot surface moves radially from the rotational centerline. Radial runout limits are likewise manufacturer- and model-specific and should not be generalized to a single range; some engines specify tolerances considerably tighter than others. Because even small eccentricities at the flange are magnified by the long moment arm of the propeller blades, manufacturers hold these tolerances extremely close.
Recording and Marking High Spots
A best practice during the inspection is to use a grease pencil or chalk to mark the crankshaft or a reference disk at the point of maximum deflection on each pass. This lets the technician verify repeatability — the high point should appear at the same angular position on repeated rotations. If the high point wanders, the indicator fixture may be moving, or there is a bearing issue causing crank deflection under different support conditions. Both scenarios require investigation before the reading is considered valid.
When This Inspection Is Required
FAA regulations and manufacturer guidance mandate runout inspection under several conditions:
- After any propeller strike: A propeller strike — defined as any incident where a propeller contacts an object other than air — requires immediate engine inspection, including crankshaft runout, per manufacturer service instructions. Even a ground strike at low RPM can bend the crankshaft without obvious external evidence.
- At engine overhaul: Runout is checked as part of the standard overhaul teardown and reassembly sequence to ensure the crankshaft meets new- or service-limit dimensions before the engine is returned to service.
- During troubleshooting of unexplained vibration: If a pilot reports persistent vibration and dynamic prop balancing has not resolved it, runout inspection is an early diagnostic step.
- After any event involving sudden engine stoppage: A sudden stoppage — water ingestion, foreign object damage causing a blade failure, or a severe overspeed — generates bending forces that can distort the crankshaft.
Why It Matters: Safety and Regulatory Implications
A crankshaft with runout beyond limits is not merely a comfort issue — it is a structural hazard. Cyclic bending stress is proportional to deflection and RPM, and crankshafts operate in a particularly dangerous fatigue environment: the stress reverses direction with every revolution and never fully recovers. A crankshaft that is bent even slightly beyond limits will accumulate fatigue cycles far faster than a straight one, potentially cracking at a journal, fillet, or the flange itself. A crankshaft fracture in flight means an immediate loss of the propeller, sudden engine failure, massive imbalance forces, and possible structural damage to the firewall and engine mounts.
From a regulatory standpoint, returning an aircraft to service with crankshaft runout beyond the manufacturer's published limits constitutes approving the aircraft for return to service in an unairworthy condition — a violation of 14 CFR Part 43 and, in operation, 14 CFR Part 91.7, which prohibits operating an aircraft that is not in airworthy condition. The AMT who signs off such a discrepancy bears direct legal and ethical responsibility for the outcome.
Key Numbers and Rules
- Always use manufacturer data: The only legally authoritative runout limits are those published by the engine manufacturer in the applicable service or overhaul manual, not generic industry values.
- Lateral (face) runout limit: varies by engine model and manufacturer, with some limits as tight as 0.001–0.003 inch TIR — always verify in the specific manual.
- Radial (pilot) runout limit: also manufacturer- and model-specific — never assume a generic figure applies; verify in the specific manual.
- Propeller strike = mandatory runout check: No exceptions; the engine may not be returned to service without it per the manufacturer's service instructions.
- Resolution of dial indicator: Use an instrument reading to at least 0.001 inch; coarser instruments cannot detect borderline conditions.
- Ignition off and fuel off before any manual crankshaft rotation — non-negotiable safety requirement.
- TIR = Total Indicator Reading: The full swing from the lowest to highest reading in one complete revolution, not just the deflection from zero.
Common Test Traps
- Confusing TIR with one-sided deviation: If the indicator swings from -0.002 to +0.002, the TIR is 0.004, not 0.002. The FAA test often phrases questions to catch students who record only the peak deflection from zero rather than the total swing.
- Applying generic limits instead of manufacturer limits: Test questions sometimes offer a plausible-sounding number; the correct answer almost always references consulting the manufacturer's maintenance or overhaul manual, not a generic standard.
- Forgetting that a propeller strike mandates an engine inspection: A question may describe a minor ground contact at low power and ask whether further inspection is required — the answer is yes, regardless of how minor the strike appears.
- Mixing up lateral and radial runout: Know that lateral runout is measured on the face (axial direction) and radial runout is measured on the pilot diameter (radial direction). Getting these backwards is a common error.
- Assuming a passed runout check clears the engine after a strike: Many manufacturers require a full teardown inspection after a propeller strike regardless of runout results, because internal crankcase and bearing damage may not be detectable externally. Runout is one check, not the only check.