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What Counts As A Prop Strike?

Derin Allard

Prop strikes can wreak havoc on your engine, not to mention your wallet. Here's what you should know...

What Is A Propeller Strike?

Prop strikes are one of the most costly incidents that can happen to propeller-driven aircraft. They usually require an engine overhaul, and they can easily take an airplane out of service for weeks, if not months. Lycoming Service Bulletin 533C discusses what's considered to be a prop strike (or sudden stoppage event) and subsequent required maintenance actions.

According to Lycoming, this is what's considered a prop strike:

  • Any incident during engine operation where the propeller impacts a solid object. This incident includes propeller strikes against the ground. Although the propeller can continue to turn, damage to the engine can occur, possibly with progression to engine failure.
  • Sudden RPM drop on impact to water, tall grass, or similar yielding medium where propeller damage does not usually occur.
  • Any incident, whether or not the engine is operating, where repair of the propeller is necessary.

Prop strikes can occur during anything from a hangar door hitting a stationary aircraft (without the engine running), to a gear-up landing. Sudden stoppage causes potentially severe side loading on the crankshaft of the propeller flange, front bearing, and seal. Poor aircraft control during gusty crosswinds, or a porpoised landing, are some of the most common causes of prop strikes.

Derin Allard

Wildlife Problems, Especially At Dusk

Wildlife on the runway is another major prop strike concern. Animals are typically more active during sunrise, sunset, and nighttime hours, making them difficult to see when you're taking off or landing.

Prior to landing, the pilots in the video below overflew the field to inspect the runway. But as they came back to land, deer started running across the runway as the pilots touched down in their Cessna 182. Unfortunately, it was too late for a go-around, and the deer strike caused damage to the propeller and the side of the aircraft.

We spoke with the pilots, and they said there was a 3" dent aft of the cowling, additional dents behind the door, and the deer impacted the prop, requiring an engine tear-down. Fortunately, everyone in the cockpit was OK.

Hidden Damage

The individual circumstances of a propeller strike can't always be used to predict the extent of interior engine damage. Future reliability of the engine, beyond how it currently operates, is a major concern. Initial damage may allow safe operation, but the progression of problems may become significantly severe with time and wear. Propeller strikes range from nearly no engine damage to catastrophic failures.

The problem is, you can't truly find out until you open up the engine. Generally speaking, the crankshaft doesn't get damaged easily in most engines. However, "some engine failures resulted from an overstressed crankshaft gear dowel which ultimately sheared. When this part breaks, all power is lost. Because of this, Lycoming Service Bulletin No. 475C and AD Note 91- 14-22 require a mandatory inspection and compliance with repair and reassembly procedures."

The dowel pointed to in the picture below is in the crankshaft on the opposite end from the propeller. That dowell has been called the "Jesus nut" of Lycoming engines. If that dowel fails, all of the accessory gearbox items stop turning (i.e. magnetos, camshaft, oil pump, fuel pump, etc.) Long story short...not good.

Here's a picture of a Lycoming engine with the accessory case removed. If these gears stop, you'll lose engine-driven accessories...

Required Inspections

Considering such high risks and no clear, quantifiable threshold for measuring engine damage, Lycoming has published multiple "Inspection Checklist After a Propeller Strike" documents. Before further flight, a mechanic must complete a sequential list of 30-40 inspections depending on whether the engine is geared or non-geared.

To complete these required inspections, you generally have to open up the engine. At that point, an overhaul is the next logical step.

Boldmethod

According to Lycoming, "any decision to operate an engine which was involved in sudden stoppage, propeller strike, or loss of prop blade tip without disassembly and inspection will violate the AD Note issued by the FAA and Lycoming Service Bulletin No. 475C, both of which are mandatory.

What You Should Look For On Preflight

When you're pre-flighting an airplane, especially if it's a shared aircraft, carefully check the propeller for damage. Look for dents, unusual scratches/scrapes, warping, grass stains, damage to the tip, etc.

Even the slightest damage to a propeller can signal engine damage, leading to an engine failure.

Boldmethod

Have You Seen A Propeller Strike?

Accidents happen, and propeller strikes are one of the most common. Have you either seen a propeller strike or the damage caused by one? Tell us in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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