Prop Strike Inspection: Better safe then sorry

Prop Strike Inspections

Better safe than sorry

By Greg Napert

September 1999

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.

More than the prop and crankshaft
The other thing to consider is all of the accessories on the engine or parts of the engine that are spinning at high speeds that could come to a sudden stop. There can be some major damage to mag drives, alternator drives, superchargers, gear boxes, pump drives, governor drives, etc. as a result of sudden stoppage.

DeJoris explains, "Take for example the prop governor drive — the rotating mass in the prop governor, mainly the flyweight assembly can really be problem if suddenly stopped. There's a tremendous amount of energy stored in the rotating flyweight, and when all of a sudden you stop the prop and engine, that energy has to be dissipated somewhere."

"The result is that those flyweights that are hanging on by 1/8-inch pins can easily bend, twist, or damage those pins. We see flyweights and connecting hardware, in some designs, brutalized quite frequently by sudden stoppage. The pins are more than adequate for running loads, but they are totally inadequate when you subject them to sudden stoppage. Not every prop strike results in sudden stoppage, but it doesn't take much to damage some of these accessories. Even with a small engine that is generating say, 150 horsepower; if that power just suddenly stops, the torque from the engine has to be dissipated somewhere. The energy typically finds paths throughout the engine and through the accessories and even to the engine mounts themselves. On severe prop strikes, the mounts can be easily twisted and loaded in a way that it was not designed to withstand," he says.

"You naturally think of the crankshaft and things that are directly connected to the engine, but you don't think of other accessories. The oil pump on the Continental IO-360 is driven by a very small shaft, for instance, and can be easily sheared as a result of a strike," says DeJoris.

Some elbow room?
Mahlon Russell, production manager for Mattituck Aviation Corporation, an engine overhauler in Mattituck, NY, says, "The only pertinent FAA definition that I have been able to find for Ôsudden stoppage' is in Advisory Circular 43.13-1A. It defines a sudden engine stoppage as; stopping an engine in one revolution or less for any reason, be it from propeller impact or from an engine failure of some sort. Both major engine manufacturer's have service literature that explains the desired course of action after accidental propeller damage and, in the case of Teledyne Continental, defines what their interpretation of a propeller strike is."

He continues, "T.C.M.'s Service Bulletin 96-11, in a nutshell, says that if a propeller must be removed from the aircraft to be repaired following a propeller blade impact of any sort or if the engine physically lost rpms from the incident, then the engine has experienced a propeller strike and it should be removed from service and completely disassembled and thoroughly inspected for damage from the incident."
But it's getting harder
Just a few short months ago, Textron Lycoming's Service Letter L-163C recommended taking the engine apart for inspection following any incident involving propeller blade damage. However, they have the caveat that the inspecting mechanic may override that position and return the engine to service without disassembly and inspection if he feels that it is the prudent and responsible thing to do.

Yet, the company recently published a new Service Bulletin, SB No. 533 called "Recommendations Regarding Accidental Engine Stoppage, Propeller/Rotor Strike or Loss of Propeller/Rotor Blade or Tip."

The bulletin reads as follows:
On numerous occasions, Textron Lycoming has been consulted about recommendations on whether to continue using an aircraft engine that has been involved in the separation of the propeller/rotor blade from the hub, the loss of a propeller/rotor blade tip or sudden stoppage following accidental propeller/rotor damage (such as propeller/rotor strike).

Conditions which surround accidents are many and varied; therefore the circumstances of the accident can not, in our opinion, be used to predict the extent of the damage to the engine or assure its future reliability.

Textron Lycoming must take the position that in the case of a sudden stoppage, propeller/rotor strike or loss of propeller/ rotor blade or tip, the safest procedure is to remove and disassemble the engine and completely inspect the reciprocating and rotating parts including crankshaft gear and dowel parts. Any decision to operate an engine which was involved in a sudden stoppage, propeller/rotor strike or loss of propeller/rotor blade or tip without such an inspection must be the responsibility of the agency returning the aircraft to service.

Even tip damage is considered cause for teardown
Lycoming's Service Bulletin 475B requires that in the event that the engine has experienced a propeller strike, inspection and possible rework of the accessory gear train as well as the rear of the engine's crankshaft is required. Compliance with this service bulletin is mandatory per AD 91-14-22. The AD specifies the inspection at each engine overhaul, after a propeller strike, sudden stoppage, or whenever gear train repair is required. Lycoming's new SB seems to generalize the term propeller strike and sudden stoppage to be quite inclusive.

Russell says, it should be noted that to comply with A.D. note 91-14-22, the engine does not need to be completely disassembled and that access to the accessory gear train can be accomplished, in most cases, with the engine still installed in the aircraft.

Type of operator/engine makes a difference
In the case of any accidental damage to a propeller installed on a aircraft operating under Part 91 of the FARs, it is ultimately up to the inspecting technician to determine if the engine should continue in service without total disassembly and inspection. A Textron Lycoming engine, that is being operated on a Part 91 aircraft, that had a sudden engine stoppage, not a propeller strike, must comply with AD 91-14-22 and Service Bulletin 475B at a minimum.

Teledyne Continental Motors has a slightly different approach, but not much different in practice.

Russell explains, "Aircraft operating under Part 135 of the FARs must comply with all manufacturer's service bulletins, and would have to comply with Service Bulletin 96-11 requiring total disassembly and inspection after any incident that required removal of the propeller for repairs or if the engine physically lost rpms during the incident. Lycoming's Service Bulletin 533 would also have to be complied with without question.

Putting the puzzle together
Although many in the industry still like to interpret rules to their advantage, the mechanic is being faced with fewer and fewer "outs" for the aircraft owner. The pressure from an owner to ignore the engine and fix the prop may be great, but it is certainly in the best interest of the mechanic's career to persuade the owner that any amount of damage to the propeller constitutes the need for a thorough inspection. So carefully consider all factors related to propeller strikes. Start with all relative ADs, collect all service bulletins and service letters offered by the particular engine manufacturer, and sit with the insurance adjuster. Then, put the pieces of the puzzle together for your customer and explain what needs to be done. A bit of homework will go a long way towards helping you persuade your customer to do the right thing.