Prop Strike Inspections
Better safe than sorry
By Greg Napert
The following was overheard recently at a maintenance facility after a very irritable looking pilot walked in with a look of embarrassment on his face. As he approached the director of maintenance he asks,
Pilot: "When does a prop strike require engine tear-down and inspection?"
Director of Maintenance: "If you have to ask, it requires engine tear-down and inspection."
Kiss the old days goodbye. No more filing blades a bit shorter after a tip strike, or performing a run-up and signing off on an engine after the runway lights have been mowed a bit shorter. Recent changes in the approach to propeller strikes dictated by Airworthiness Directives, engine manufacturer Service Letters, and insurance company policy have made anything less than a thorough teardown and NDT inspection unacceptable.
Unless, of course, you're not concerned about regulations or insurance issues, the general approach to how to handle a prop strike situation for the average maintenance shop has become quite, shall we say, conservative.
According to John DeJoris, president of Aircraft Propeller Service Inc., in Wheeling, IL, "The FAA has embraced the policy that any type of propeller strike is a cause for teardown — and engine manufacturers are quickly jumping on board."
DeJoris adds, "As a result, there is no real relationship today between the extent of damage of the prop and the type of inspection that has to be done to the engine. There used to be, but it was kind of informal and there was a lot open to interpretation."
Case in point
DeJoris, who deals with propeller strike issues quite frequently says, "We recently had a customer with a prop strike who struck a steel stake that cut 1/4-inch of the tip off one blade of a three bladed prop. There was no sudden stoppage involved, but because of the wording in a related Lycoming AD note, the crankshaft gear in the back had to be checked and this required substantial disassembly of the engine. Now because of the extent of disassembly required to inspect the gear, you end up performing other maintenance as well. Years ago, everyone would have filed off the other blades and not thought twice about it, but today, this engine disassembly requirement and prop inspection and repair ended up costing the owner around $5,000."
In addition to regulatory changes and manufacturer recommendations, insurance companies are also playing a major role in determining what you do to your engine and prop. DeJoris says he is dealing with claims adjusters on a regular basis, and many adjusters today choose to have you go the extra mile and tear down the entire engine and propeller and do a thorough inspection." He adds, "This limits their liability exposure. Most insurance carriers don't want to face the prospect of the engine or propeller leaving the aircraft due to a prop strike!"
Propeller inspections are fairly clear cut — damage on the blades is typically quite apparent, and the existence of damage beyond field repair limits dictates that the unit is disassembled and given a searching internal inspection, and in some cases, complete overhaul.
According to DeJoris, "Hartzell requires hub replacement if any blade in the assembly is bent beyond repair limits due to impact damage. McCauley hubs must be retired if any of the blades within the assembly are bent beyond repair limits inboard of the 85 percent blade radius. The rationale for this hub retirement criteria revolves around the fact that forces great enough to bend blades beyond their respective repair limits represent loads that are in excess of those that the hubs were designed to withstand, while providing sufficient structural safety margins."
Engine inspection requirements after prop strikes can become complicated, however, and the regulations and recommendations seem to overlap and intertwine.
FAA: Special Airworthiness Information Bulletin on All Lycoming Engines (Lycoming) Direct-drive Reciprocating Engines,
Lycoming engines must be in compliance with Airworthiness Directive (AD) 2004-10-14 if they have had a propeller strike.
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