A prop strike is not limited to visible propeller damage. When a blade contacts the ground, an obstacle, water or tall grass, or experiences a sudden stop or severe RPM reduction, the resulting load can be transmitted directly into the engine. The most important damage is not always visible from the outside.
A propeller that still looks straight, an engine that turns by hand, or even an engine that restarts normally therefore does not, by itself, prove that the engine is undamaged. After certain prop strike or sudden stoppage events, the applicable inspection goes well beyond an external visual check.
What exactly counts as a prop strike?
The definition is broader than a dramatic blade impact. Under Lycoming’s current Service Bulletin 533D, a propeller strike can include several types of events involving contact, sudden RPM reduction or damage requiring more than minor propeller correction. The exact definition and required action must always be checked against the current manufacturer data for the engine and configuration involved.
Even a relatively low-speed contact may fall within the applicable definition. The key issue is not only how the propeller looks afterward, but how the event may have transmitted loads into the engine.
What can happen inside the engine
Think of the propeller and crankshaft as one rotating mechanical line. When a blade is stopped or slowed abruptly, other rotating masses in the engine still carry inertia. For a fraction of a second, that load can travel through the engine rather than remaining isolated at the propeller.
Depending on the event, the crankshaft may be loaded or deformed, connecting rods and gears may experience abnormal forces, and the crankcase or valve-train components may be stressed in ways that are not obvious externally. This is not a list of damage that occurs after every prop strike; it illustrates why the effect of an impact cannot be assessed from the propeller alone.
A prop strike does not automatically mean severe internal damage. The outcome depends on RPM, contact angle, the object involved, propeller type, engine configuration and how the event occurred. Two events that look similar from the outside can produce very different internal results.
That is why both shortcuts should be avoided: assuming everything is damaged, or assuming everything is fine because the engine appears normal. The purpose of the required inspection is to establish the actual condition of the engine.
1. The crankshaft is one of the primary areas of concern
Because the propeller is attached directly to the crankshaft, an impact can transmit a significant load into this part of the engine. Depending on the circumstances, deformation may be local or may affect other areas of the crankshaft assembly, and an obvious external sign may not be present.
The inspection therefore does not stop at a simple flange run-out measurement. Other areas of the crankshaft and related components must be evaluated as required by the applicable instructions. Any disposition of a bent or otherwise damaged crankshaft must follow current manufacturer limits and approved repair data.
2. The crankshaft gear and gear train can also be involved
The crankshaft also drives the engine gear train. A sudden stop can therefore transmit an abrupt load into the crankshaft gear, retaining system and mating surfaces. Fretting, marks, deformation or other conditions may not become apparent until the engine is opened and the parts are cleaned for inspection.
Again, these components are not automatically damaged in every event. Inspection distinguishes serviceable parts from those that no longer meet the applicable limits.
3. Other internal parts can tell the story of the impact
Once the engine is opened, some conditions may be visually obvious while others require cleaning, dimensional inspection or NDT. Connecting rods, valve-train parts, gears and crankcase areas can all provide evidence about how loads travelled through the engine.
Push rods are another useful example. They are long, relatively slender components that must remain straight to transmit valve-train motion correctly. Manufacturer troubleshooting data recognize that bent push rods can occur under certain mechanical conditions. Their inclusion in an inspection or diagnostic procedure does not mean they will necessarily be damaged after every prop strike; it means hidden damage should not be dismissed without the required checks.
The fact that a component appears on an inspection checklist does not imply that damage is expected. It means the condition must be established rather than assumed.
4. An important point for Lycoming roller-tappet engines
Lycoming engines equipped with roller tappets, commonly called roller lifters, have a specific requirement when the engine is disassembled for a propeller strike or sudden engine stoppage: Lycoming Service Instruction 1514D states that the applicable roller tappet body assemblies must be replaced. For those components, visual inspection alone is not the return-to-service criterion.
This requirement applies to the relevant roller-tappet configuration and must be implemented using the current documentation for the engine involved. It is a good example of why two externally similar Lycoming engines may require different actions after the same kind of event.
5. Engine-mounted accessories may also require attention
A prop strike does not affect only the basic engine assembly. Depending on the configuration and applicable instructions, magnetos, the propeller governor and other engine-driven accessories may also require inspection or other action under their manufacturers’ data.
Why a flange check alone is not enough
Measuring flange run-out can be part of an evaluation, but it does not establish the condition of every internal component. Internal damage cannot be reliably excluded from one external measurement alone.
For Lycoming reciprocating engines, current SB 533D specifies recommended corrective action before further flight after events within its applicability. Depending on the engine and event, the work can require engine removal and disassembly so that internal components can be inspected before return to service.
Can a small impact really cause damage?
It can, but damage is not automatic. The external appearance of the propeller does not directly reveal the load transmitted into the engine. A brief contact can generate substantial forces, while another visually dramatic event may produce a different result.
The objective is not to dramatize every contact, but to recognize that serious damage cannot always be ruled out simply because an event looked minor. Evaluation must be based on the event, engine configuration, applicable instructions and inspection results.
Lycoming and Continental: there is no universal procedure
The physical principles may be similar, but inspection procedures must not be generalized from one manufacturer to another. A Lycoming engine must be evaluated under the applicable Lycoming documentation; a Continental engine under the Continental data for the specific model and configuration. Applicable Airworthiness Directives must also be reviewed.
What a shop actually does after a prop strike
Depending on the engine and applicable data, the work may include removal and teardown, cleaning, dimensional inspection, NDT, crankcase and crankshaft inspection, and evaluation of affected components and accessories.
At that point, the engine stops being a black box. Each component provides evidence: a part that remains within limits, or a mark, deformation or crack showing where loads travelled. Inspection results determine which parts remain serviceable, which must be replaced and what repair scope is actually required.
At Aéro Atelier, we perform inspection and necessary repairs after prop strike or sudden stoppage on Lycoming and Continental engines according to the applicable technical data.
After a prop strike: the next step
If your aircraft has experienced a prop strike, propeller contact with the ground or another obstacle, or a sudden stoppage, do not assume the engine is safe simply because it still turns or runs. Identify the engine, document exactly what happened and determine which instructions apply to that configuration.
You can send Aéro Atelier the engine and event information through our engine quote request process so that the applicable documentation and expected inspection scope can be identified.
Technical sources
- Lycoming Service Bulletin No. 533D: Recommended Action for Sudden Engine Stoppage, Propeller/Rotor Strike or Loss of Propeller/Rotor Blade or Tip, revised February 10, 2026.
- Lycoming Mandatory Service Bulletin No. 240AA: Mandatory Parts Replacement at Overhaul and During Repair or Maintenance.
- Lycoming Service Instruction No. 1514D: Roller Tappets Part Information Update.
- Lycoming Service Bulletin No. 475C: Crankshaft Gear Modification and Assembly Procedures.
- Lycoming HIO-390-A1A Engine Maintenance Manual.
- Continental Aerospace Technologies: Service Bulletins.
Note: this article explains general principles. The current Instructions for Continued Airworthiness, Service Bulletins, Airworthiness Directives and manuals applicable to the engine model, configuration and serial number remain the controlling references for determining required maintenance action.