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PROP STRIKE

 

August 2012

 

In this month's ShopTalk we will jump into the issue of a prop strike on a piston engine aircraft. You may not know this but todays typical prop strike with no airframe damage will set you back over 20 grand! First a little history.

 

Back in 1978 when I was a young and new A&P and my hair was blond instead of its current gray/silver, it was common to not disassemble the engine on a non counter-weighted engine after a prop strike. If the crankshaft flange run-out was still within limits then sometimes all that was necessary was to replace the prop with a serviceable one and you were off and flying. The thinking behind this was that the engine is mainly steel and the props are mainly aluminum or wood; they will adsorb the impact and take all the damage, not the engine.

Of course, back then obtaining an STC or field approval was just a minor hassle. Now I can attest to the fact that we in aviation are in a over-regulated industry to the point that it is almost impossible for the average person to obtain a field approval much less a STC! My current STC process I am in has kept me away from my beloved ShopTalk column for over a year! Boy, how the times have changed!

 

Also back in the 1970s, it was commonplace to just replace the damaged prop blade or in the case of a fixed pitch aluminum prop straighten it. Used serviceable parts were plentiful and within serviceable limits. You did not have to always purchase a new prop.

Boy, how the times have changed!

 

Today all the engine and prop manufactures have mandatory procedures in place that repair stations must follow after a prop strike has occurred. Hartzell and McCauley have both have a mandatory overhaul requirement on props and prop governors. Most of the time, the prop is a throwaway but the prop governor can usually be overhauled for $900 to $1300. A new prop alone will cost between 8,000 and 12,000 dollars. Even Part 91 operators will be caught by this as I will explain later in this article when we get to the FAA definition of a prop strike.

Another important issue is that magnetos have plastic timing gears in them. These plastic gears are subject to cracking, splitting or in some cases breaking a tooth loose or setting the internal timing of the magneto off. Magnetos must have a proper 500 hour service completed by a shop that specializes in aircraft magneto repair. This means a operational magneto run on a test stand.

 

On TSIO 360 TCM engines, Another item that is commonly missed is the #1 alternator shock drive on the TSIO 360 TCM engines. This shock drive is installed on the number one alternator and alternators are normally not inspected after a prop strike. This shock drive is very expensive to replace (up to $1600) so make sure it is torque tested for slippage before you re-install the alternator.

 

While we are talking about the accessories we need to mention that the engine driven vacuum pump has a plastic shear drive. Typically the vacuum pump is not inspected after a prop strike, so look at the shear drive for overall condition. Its good practice to replace the vacuum pump every 800 hours and never install a rebuilt pump. A new pump will normally run 800 hours safely, a rebuilt pump is anyone guess how long it may run. The reason I am not a fan of rebuilt vacuum pumps is a lot of auto pilots rely on a properly running vacuum pump to hold the wings level. This is especially important in IMC conditions. A new vacuum pump will set you back 500 bucks, but don't take any chances in this area, always buy a new one.

 

So at this point in our article we have talked about the engine accessories and now we will get into the engine itself, but first one must ask the question of what constitutes a prop strike? Prop strikes come in many forms.

 

In 1979, when I was working for a Cessna dealer, the nose gear collapsed on an unoccupied T210 parked on the ramp. The plane rested on one of the three prop blades which was stuck into the tarmac, holding the plane off the ground. Since this was a not a in-motion prop strike, the prop was removed to be repaired and the crankshaft flange was checked for run-out. The flange was OK and the repaired prop was reinstalled. There was no airframe damage. The nose gear down lock spring guide was found to be the cause of the whole affair so it was replaced and the plane was returned to service. The simplicity of that repair is a thing of the past because the FAA has put in writing an airworthiness directive that states what a prop strike is.

 

One only has to look at Lycoming AD 2004-10-14 to see why a prop strike is so expensive nowadays. Here is the letter of the law (the FAA definition) as ADs are mandatory for everyone, and I quote from the AD:

“any incident whether or not the engine is operating that requires repair to the propeller other than minor dressing of the blades”

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

”... sudden RPM drop while impacting water, tall grass, or similar yielding medium, where propeller damage is normally incurred.”

“The preceding definitions include situations where an aircraft is stationary and the landing gear collapses causing one or more blades to be bent, or where a hanger 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, front bearing, and seal”

 

Now AD 2004-10-14 may not apply to your aircraft, but because the FAA has now put their definition of a prop strike in writing in a regulatory document it becomes the letter of the law and applies to all certified aircraft. Most A&P mechanics will not take the chance of stating that AD did not apply if the FAA were ever instigate an investigation. Boy, how the times have changed!

 

Even though TCM has no regulatory requirement for a tear down of the engine after a prop strike has occurred it will only damage the resale value of the aircraft if the owner does not complete the job correctly. I would be hard pressed to find a reputable IA mechanic that would sign off on the annual with out a proper prop strike inspection being completed first.

From a mechanic's perspective, it is not our job to determine airworthiness. It is our job to advise the owner/operator of the regulatory requirements that he must comply with. We also do not want to caught up in an FAA investigation that may cost us our FAA privileges and endanger our livelihood and income. Its just that simple.

 

Once your engine is removed and sent in to your favorite engine shop, they will have mandatory service bulletins that must be followed. These bulletins will dictate instructions of how and what parts are to be inspected, including a list of parts that must be replaced regardless of condition. Some of the things they must do are:

 

Magniflux all steel parts for damage.

Zyglo all non-ferrous parts for damage.

Verify the crankshaft is straight and in some cases ultrasound it for cracks.

Replace all counter weight pins, clips and bushings in the crankshaft.

Replace the crankshaft gear attach hardware and possibly rework the attachment point.

Check crankcases for misalignment or line bore.

Typically the cylinder assemblies are removed and set aside, but on Lycoming engines the camshaft may be a problem; not because of the prop strike but because of corrosion and pitting. The camshaft and the lifter bodies that run against the cam lobes may not be airworthy and may have to be replaced.

At this point one can see that no stone has not been overturned in this process.

 

Unfortunately there is often something else that is found during this process that has nothing to do with the prop strike. Unless an engine is new or first run, components may have wear and tear that precludes reinstallation. This is especially true if components have been replaced after engine overhaul or overhauled components were installed.

Recently I completed a prop strike repair on a M20 K(231). After reinstalling the engine it developed an oil leak when it was hot. As it turns out the oil cooler was a overhauled cooler the owner had recently had installed and it had a crack in it that would open up when the engine was hot.

 

Now the TCM TSIO 360 series engine has been around since the 1960s and when you purchase a rebuilt oil cooler or any component you may get one that is almost fifty years old! Our airplanes and their parts are getting so old that some of the parts just need to be scrapped and not overhauled. The TSIO 360 oil cooler is a good example; replace with a new one only.

 

A prop strike will require your mechanic to remove and replace the engine and prop on the aircraft and this job can be a lengthy experience. All of these items I have mentioned are why a prop strike with no airframe damage can cost $20K or more. With airframe damage it can easily become $40K-$50K.

 

If you carry hull insurance and the pilot is properly qualified (FAA and insurance requirements) when the prop strike occurred then your insurance company will most likely cover the cost of repairs, less the deductible. Most insurance companies are reasonable and understand that you nor your mechanic can front this type of money and wait for them to repay the costs and they will pay up front a certain amount to get the job going.

Unfortunately this is the time you will find out if you have a reasonable or difficult insurance company. I say this because one insurance company told me that is is the responsibility of the aircraft owner to pay the mechanic and they will reimburse their client, the policy holder (aircraft owner). This situation can put you, the policy holder, in a bind for getting your airplane repaired in a timely manner. Fortunately, most insurance companies do not practice this policy.

 

If you do not carry hull insurance then you are on your own for the repairs. Its just that simple.

Back in the 70s and 80s there were plenty of used serviceable props and parts for them. Today that is not the case and some of the used parts are so old that you would not want them in your prop. In most cases one can only purchase a new prop and this defiantly runs up the price real fast. Remember, a new prop will set you back $8K-$12K.

When the cost of repairs get to 70 percent of the wholesale value of the airplane the insurance company will likely total the plane and pay off the hull value of the policy. When one considers the pitiful resale values of our airplanes in this never-ending economic aviation depression you can see that a 1964 M20C will most likely be headed for the aircraft junk yard instead of being repaired.

 

From a long-time Mooney mechanic's perspective it is so important that you the airplane owner protect your airplane as best you can from getting caught in the prop strike experience. One way is to be proactive with your mechanic and make sure that the landing gear down lock tensions are checked at each annual using the proper Mooney tools and a quality torque wrench while the airplane is on jack stands. I can tell you that I still see many airplanes that have the gear tension set improperly.

 

Your Mooney landing gear system is a robust and well designed system, and when kept in proper working condition with correct gear down lock tensions it will provide you with years of trouble-free operations, keeping you out of the prop strike experience.

 

As with this article and any other ShopTalk article, if you have questions feel free to contact me at my aircraft repair shop: 307-789-6866 or via e-mail.

Until the next ShopTalk, enjoy flying your Mooney!

KNR-e-mail