The Prop Strike/Sudden Stop

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"Dialing the crank" is not an adequate check for an FAA-defined prop strike, which includes sudden engine stoppage.

We get e-mail from readers who have had what they often prefer to call prop “incidents” as to possibly avoid the term “prop-strike,” and its frequent need for a recommended or mandatory engine tear down.

In our response to readers we quote the latest manufacturer directives and FAA guidance, and add our perspectives and experience to hopefully help the owner feel his or her ultimate decision is the right and legal one. At least that’s our goal.

We will not be discussing damage to the prop in this article since there is so much information on prop damage that does not involve engine tear downs.

It’s possible to do major engine/accessory damage without obviously damaging the prop by hitting water or heavy, tall grass that stops or slows the running engine. The FAA definition of a sudden stoppage, is the engine stopping suddenly in under one rpm. This can cause major internal engine damage without visible prop damage.

AC 43.13-1B has lots of general information in Chapter 8 on prop strikes. The general consensus is that at least some degree of tear-down is the only safe option under their definition, and largely follows manufacturer data in the general damage/repair evaluation techniques.

Moreover, 43.13-1B mandates the use of approved manufacturer data (providing it exists) rather than 43.13-1B data as the required source of both inspection and repair information (Manufacturer data will be more current with more detail and part specific is the thinking). In many instances prop strikes will fall into the major repair category, which necessitates repair station or manufacturer repair of the prop (if needed).

A tear down of most direct drive engines isn’t a major repair but we recommend using a certified repair station or the factory for that action as well as the level of NDT testing will be significant and required to be done by certified people for such critical NDT inspections. Sticking with a certified repair facility possessing all the skills and tools will probably reduce the extra shipping costs and time involved to get individual parts examined seems most efficient to us.

Considerations

One of the issues that is particularly hard to quantify if you elect not to tear an engine down, post strike and choose to disregard the fairly specific diagnostic procedures that the engine makers put out is do you have the FAA legal option of ignoring the service directives?

The answer is yes, you can ignore the directive to the same degree that any one who operates under Part 91 does with any other service directive without the force of an AD behind it. In this case however, there is an existing AD for quite a few different Lycoming engines that specifically mentions prop strikes and mandates some degree of tear down, but not necessarily a full tear down, and only if it is a prop strike as opposed to a sudden stoppage. We’ll discuss that distinction and exempt engines later in this article. The Lycoming information is more complicated and equivocating than the TCM directives in our opinion, as we’ll explain.

There is also a measure of anecdotal evidence in the engine accident/incident statistics that engines that have a prop strike/sudden stoppage and owners who did not follow the manufacturer recommendations such as only checking the crankshaft run-out, have a higher incidence of broken cranks or other engine failures a number of flight hours later.

It’s not a smoking gun, but perusing the accident reports makes one wonder about such coincidences. Some reports are quite specific about a recent prop strike in the engine’s history. Others are not necessarily so forthcoming, so reliable conclusions are difficult to draw.

We have personally seen an instance where a plane lost the entire prop in flight following a so called “minor” prior strike of hitting a runway light while taxing, but the crank on the IO-520 was only checked for run-out and prop repaired (and this was a Part 135 operated plane).

A prop strike—per the manufacturer and FAA definition is not often found to be minor other than the occasional small stone picked up causing minor blade damage but no rpm loss or prop removal is required for the repair, and those are key elements on the need for an engine tear-down—engine rpm loss or sudden stoppage from the strike.

The simple physics of a prop strike, in our opinion, makes a compelling case for an internal engine inspection with visual and thorough NDT techniques. The engine makers seem to agree quite strongly as does the FAA.

Do we think the engine makers are overly eager to have you tear down or dig into your engine? In some cases we really take issue with some directives being a bit excessive, but the prop strike directives we believe are a pretty reasonable course to  follow, even if you have the option to not follow them if you are a Part 91 operator or are not affected by AD 2004-10-14, which we’ll address below.

Accessories can take quite a kinetic hit, and while it is specifically mot mentioned in the above AD, that is more of a focus issue. Other Lycoming literature mentions the importance to check the accessories as does TCM.

Anatomy

A prop that hits so hard as to bend the crank flange is only one of several considerations. Bending loads alone are not the only destructive element, and nondestructive testing of the entire crank and associated structures is the only reasonable way to check it out.

Moreover, the crank is not the only critical element in a prop-strike incident.

A prop-strike also imposes terrific jamming forces backward on the rear of the case—where the gearing is located and easily damaged. Lycoming is particularly concerned with this area and outlines specific inspection criteria with the force of AD 2004-10-14 to support their bulletin SB 475C depending on engine model.

Also, some engines with a gear-driven alternator have a large, relatively thin and very vulnerable gear bolted to the forward end of the crankshaft. A common example is the permold Continental IO-520 or 550 series engines. Say the engine is turning only 1000 rpm, barely above an idle. The alternator is turning 3000 rpm because of gearing. Studies have shown that a 100-amp alternator can impose a

600-ft lb load on the crank gear on a sudden stop from 1000 engine rpm.

Some geared alternators shafts use gears that are designed to slip on overloads, but older versions of this alternator gear do not have this design, and the gear shear design is for an alternator failure not a crank stoppage. In short, all accessories become suspect to damage under either a prop strike or sudden stoppage incident.

Definitions and Bulletins

As mentioned earlier, Advisory Circular 43.13-1B, Chapter 8 defines a sudden engine stoppage as stopping an operating engine in one revolution or less for any reason, period, and includes prop-strike damage as a subset of the of sudden stoppage definition (at least the way we interpret the way it is written).

AC 43.13-1B has a great deal to say about what should be done in the manner of inspections and actions, and all the statements are preceded by the word “must” such as following the manufacturer directives as the priority action, with emphasis on the importance of a teardown when the event meets the criteria.

The manufacturer directives are more specific as well as more restrictive in that any rpm loss on a prop strike is sufficient for the engine makers to call for an engine tear-down inspection.

There is another FAA document with additional specifics for many Lycoming engines, and this is found in AD 2004-10-14, which defines and differentiates a prop strike and sudden prop stoppage and references Lycoming SB 475C, Jan 2003 as the supporting manufacturer document for the inspection and applicable engines.

This AD, defines a propeller strike as follows: “(i) For the purposes of this AD, a propeller strike is defined as follows:

(1) Any incident, whether or not the engine is operating, that requires repair to the propeller other than minor blade dressing.

(2) Any incident during engine operation in which the propeller impacts a solid object that causes a drop in revolutions per minute (RPM) and also requires structural repair of the propeller (incidents requiring only paint touch-up are not included). This is not restricted to propeller strikes against the ground.

(3) A sudden RPM drop while impacting water, tall grass, or similar yielding medium, where propeller damage is not normally incurred.

(j) The preceding definitions include situations where an aircraft is stationary and the landing gear collapses causing one or more blades to be substantially bent, or where a hangar door (or other object) strikes the propeller blade. These cases should be handled as sudden stoppages because of potentially severe side loading on the crankshaft flange, front bearing, and seal.”

It’s entirely possible after a sudden engine stoppage for there to be severe internal damage and yet the prop looks perfectly OK even if the prop was either totally stopped or the rpm dropped by the incident. In fact it is a very insidious situation since there is often no obvious damage—no harm, no foul so to speak.

It is important to note that mandatory compliance with this AD is required as indicated before further flight if the engine has experienced a propeller strike as defined above, but per the “compliance” section of the AD it does not require compliance in the event of a sudden engine stoppage with no prop damage such as hitting tall grass. This may seem to contradict itself since sudden stoppage is mentioned earlier in the AD in the prop strike-definitions section, but that’s the way it is written with an orientation to certain types of direct drive Lycoming engines, with only Lycoming O-145, O-320H, O-360E, LO-360E, LTO-360E, O-435, and TIO-541 series engines being exempt. It’s also important to point out that this is not the only Lycoming bulletinthat discusses prop strikes.

Lycoming also has a short, two-page separate, specific prop-strike bulletin, SB 533A, which is dated in 1999, five years earlier than AD 2004-10-14, and four years earlier than SB 475C. Service Bulletin 533A has the pertinent Lycoming prop-strike definitions for all Lycoming direct drive engines. Per the Lycoming website it is still an active bulletin, which we quote in part 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).”

A propeller strike (according to this Lycoming bulletin) is defined as follows: (It has all the same definitions found in AD 2004-10-14 that we printed in italics but has additional information):

“Circumstances 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 engine 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.”

So, in a roundabout way by the last paragraph, Lycoming is saying it’s the repair facility’s call unless your engine is one found in the later AD 2004-10-14, which applies to a lot of Lycoming engines.

And as we previously mentioned, the Lycoming 475C bulletin is broader than a prop strike; it is oriented to general engine overhaul crank inspections or crankshaft gear removal for inspection as well as prop strikes.

We know of no ADs on TCM engines in this area to muddy the waters, so it’s the owner’s decision unless they are operating other than under Part 91, such as Part 135 where compliance with service directives is typically mandatory.

More on Damage Locations

Both Lycoming and Continental express concerns about prop-strikes causing damage to the rear of the crank in the vicinity of the rear radius of the oil slinger, which can be shoved back into the case rear. Dialing in the flange is worthless for this.

Nitride hardened cranks have a greater tendency to crack, since the nitride hardened materials resist bending. Think of it as the equivalent of a high-speed twist drill, with great resistance to bending—it tends to snap or develop cracks from damage and fails suddenly.

Another problem area is in engines with crank counterweights, a very common design. These counterweights move on relatively soft bushings. A slam stop will “detune” the weights and damage the bushings so they can no longer do their job of helping to balance the crank while operating.

Highly stressed connecting rod bolts also take a beating from a prop-strike, which can cause a slight stretch. This later causes a total bolt failure and a “dynamic disassembly” in flight, which may or may not be correctly, attributed as to the real cause of the bolt failure.

Consider the accessories. They are often ignored in the prop-strike evaluation, yet they have prescribed inspection criteria even if involved in only a sudden stoppage.

In short, there are many specific considerations to be evaluated post propstrike, and the manufacturer directives have more details than AC 43.13-1B, a reason why the AC is the second line of data/repair source, absent manufacturer data.

To be undiplomatic, dialing in a crank after a prop strike or sudden stoppage as has been defined by both the manufacturers and the FAA is essentially a fool’s errand—at least as possible decision making criteria on the serviceability of the crank. Unfortunately, disassembly and nondestructive testing procedures are the only way to provide a reasonable level of assurance that the engine is truly fit for continued service.

Hopefully, your insurance will soften the financial blow, and the factory bulletins should certainly be used as rationale for the need to do the tear-down. The argument goes “would you rather be paying for an in flight engine failure 100 hours from now.” Dealing with them on this issue is fodder for another article, however.

Lycoming Service Literature Details

To comply with AD 2004-10-14, if it applies to your engine, the engine does not need to be completely disassembled and access to the accessory gear train can be accomplished, in most cases, with the engine still installed in the aircraft. The degree of difficulty of this action will bevaried, depending on the installation and location of accessories, turbos, intercoolers, etc.

Lycoming, in their prop-strike Service Bulletin 533A, takes the approach that the safest procedure is to take 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 or she feels that it is the prudent and responsible thing to do. In light of the preceding information, would you be comfortable doing that? How much liability insurance do you have?

What this all boils down to is that in the case of any accidental damage to a propeller installed on a aircraft operating under Part 91 of the FARs, it is up to the inspecting mechanic to determine if the engine should continue in service without total disassembly and inspection, or only the SB 475C level of inspection if a Lycoming engine falls into that category.

How TCM Views Prop Strikes

TCM is less equivocal. TCM-powered aircraft operating under Part 135 of the FARs that have to comply with all manufacturers service bulletins would have to comply with Service Bulletin 96-11B, dealing with prop strikes, dated July 2008, requiring total disassembly and inspection after any incident that requires removal of the propeller for repairs or if the engine physically lost any rpms during the incident. Note the recurring theme of the loss of rpm or sudden stoppage of the engine runs as a key criteria for an internal inspection among the makers as well as the FAA.

TCM SB 96-11B defines a propeller strike as follows, and we paraphrase: First, any incident, whether or not the engine is operating, that requires repair to the propeller other than minor dressing of the blades, for instance where the propeller is damaged by a small foreign object during operation, such as a small stone. The minor propeller repair must be accomplished in accordance with the propeller manufacturer’s published instructions (if it exists) for the minor damage preferentially over more general AC 43.13-1B with the exceptions we noted earlier).

Second, any time foreign object damage requires propeller removal for repairs other than the minor dressing of the blades as mentioned above, the incident is considered a propeller strike and must be inspected as outlined in SB 96-11B.

Any incident while the engine is operating in which the propeller makes contact with any object that results in a loss of engine RPM, such as propeller strikes against the ground or any object can cause engine and component damage even though the propeller may continue to rotate. This damage can result in catastrophic engine failure.

“Following any propeller strike, complete disassembly and inspection of all rotating engine components is mandatory and must be accomplished prior to further flight. Inspect all engine driven accessories in accordance with the manufacturer’s maintenance instructions.”

“Prior to any ground operation of the engine assure that the propeller flange and the crankshaft flange area are free of any crack indications in addition to the engine component inspection.”

You will note TCM does not get into the concept of sudden stoppages possibly being handled differently as Lycoming does. It also does not equivocate to any degree on an engine tear down being the mandatory procedure, which is described in the bulletin.

That Great Deal

A special note to experimental aircraft builders or someone looking for an engine upgrade: Beware of that great deal on an engine with a less than stellar or unknown history. Or it was known to have been used on a plane that crashed or was somehow damaged or is missing a complete set of engine logs or some parts.

Be especially wary if this is some form of private cash sale that has to be done right away, unless you are planning to have it torn down and rebuilt by a shop with the right inspection equipment to inspect for crank and gear damage, and be prepared for costs potentially exceeding a factory reman engine i.e. don’t do it.

This dream price just may be the cast off of a prop-strike incident that has been covered up or minimized in some way—or a prop with blades that are out of spec from metal removed from previous overhauls. Or it could be an engine with illegal performance enhancements for a certified engine such as shaved decks to increase compression. That engine may be a time bomb, and an illegal one if the data plate is still on the engine, as after some mods it no longer qualifies as a certified engine, and may not be economically able to be recertified, depending on what was done to it.

That said, we did an article a few years ago where an owner of an old Bonanza bought an IO-520 engine from another Bonanza engine owner who upgraded to a factory IO-550. Everyone was happy and the prices were right. We don’t have such trust unless we know a whole lot about who we are dealing with and have all the proper logs and background checks—especially in the Internet age there is no excuse to not do a lot of digging on who and what you are dealing with.

Summing Up

These are the requirements as we see them. Actually they are fairly consistent. We can even see situations where a prop sustains a fairly nasty nick in a gravel strip, but no rpm loss occurs or serious blade damage occurs. This is a minor or major prop repair issue, but not a potential engine tear down issue. It still could end up at a prop repair station if inspection deems it a prop major repair requirement depending on manufacturer inspection criteria of the prop.

Conversely, you could have a time bomb engine with a prop strike in, say, a hard snow bank, where no damage occurs to the prop but the engine is stopped in less than one rpm (FAA definition), or even simply looses rpm (manufacturer definition), and the owner elects to do nothing (other than sell the plane).

There may be other additional requirements mandated by insurance policies or engine manufacturer and/or overhauler warranty. Either may require additional inspection but neither may negate the inspections required by the FARs.

Never allow an insurance adjuster to dictate the inspection requirements after an incident. Always rely on the FAA certified inspecting technician, applicable service data and the FARs to dictate how thorough an inspection is necessary to continue the engine in service.

After the extent of the inspection has been determined, it is important, as with any major repairs that are accomplished on your aircraft, to find out exactly what is included in the estimate to repair your engine following a prop strike. Are the minimum legal requirements being met?

Is the engine being completely disassembled and inspected? What other services or inspections are being performed at the same time as the inspection? If the engine is being disassembled does the estimate include testing after reassembly?

Are any of the engine’s accessories inspected as they should be, and if so to what extent? Are there any hidden costs? After finding out the answer to these questions, it’s time to discuss with your insurance company what it will pay for and what it won’t, before it’s a big surprise after the inspection is done.

Many insurance companies will not pay for any inspection requirements unless damage from the incident is found during that inspection. Others will pay for all costs for the inspection and for any parts needed due to the incident. Still others will only pay for the labor to do the job and will not pay for any parts.

Finding out what needs to be done and whether that agrees with what you feel should be done, who’s going to do it and who is going to pay for it, should help make the experience of a prop strike/sudden stoppage as painless as possible.

And again, Chapter 8 of AC 43.13-1B is a good place to start gathering unbiased information. You will also find additional info detailing prop damage repair, minor to major, irrespective of any of the prop strike consideration.

This article originally appeared in the December 2012 issue of Light Plane Maintenance.