Hypothetical question

[Kaye]

Once upon a time, the prop on an airplane had a dynamic balance. Not to long after, a vibration returned but wasn’t significant. So a few months later, the airplane is at a different shop to rebalance the prop. Shop No 2 finds no weights attached. Shop No 2 is also unable to balance the prop within recommended vibration levels using max permitted weights. Corrective action may require prop removal, static balancing, then dynamic balance. The owner is unavailable to make repair decisions for a couple of weeks, so he instructs the shop to put the plane back together and return it to the line.

Based on this info, is the plane airworthy???

 

[TMetzinger]

Did the prop shop sign off the propeller as airworthy? If so, then I'll defer to their judgement. The vibration "recommended levels" may not be a limitation in the airworthiness sense.

The part that makes no sense is "...he instructs the shop to put the plane back together..." Who is "he" in this case? The owner?

 

[AdamZ]

I don't know enough to comment. However given my lack of knowlege on the subject I'd probably deferr flying the plane until the situation gets replaced. Given that all big problems at somepoint start out as a small problems I wouldn't want to be the one flying it when the problem gets too big.

 

[Steve]

I had a similar issue with the IAR when I purchased it. I felt an RPM related vibration that just didn't seem right. Had a local mechanic check the prop balance and he got it within allowable limits using the maximum amount of weights. That didn't make me feel much better so I sent it off to a prop shop (it was due its 5 year o/h anyway) and they found the previous overhaul shop had mismatched the prop clamps (they're supposed to be matched by serial number per Hartzell). The shop I sent it to replaced the clamps and when the prop came back I had the balance checked again. It didn't need any weights following the overhaul as it was found to be in balance out of the box. So, depending on the shop you can get an improperly balanced prop even with the the paperwork filled out correctly.

That said, there are a lot of props out there that are not balanced and fly for years.

Hypothetically, I believe as a pilot you can make the judgement call as to whether a plane is airworthy. Just be prepared to explain the basis for your decision.

 

[Bob Noel]

>Based on this info, is the plane airworthy???

Wouldn't it depend on the airplane? For my cherokee 140, the answer would be
yes since (AKAIK) the service manual doesn't have limits for prop vibration.

 

[wsuffa]

>Based on this info, is the plane airworthy???

Wouldn't it depend on the airplane?

It would depend on both the plane and the prop - and possibly the engine - as both have type certificates. If it's a prop that has the maintenance/service instructions included in the type certificate by reference, it may well *not* be airworthy depending on the wording in the service instructions (Hartzell being notorious for this as it relates to TBO).

Aside from what Steve mentioned, it's entirely possible that the prop is mounted wrong on the engine (if there's a specified mounting alignment).

Personally, I wouldn't fly it if it didn't meet specs without substantially more information.

 

[Dave2]

I'd want some serious answers Kaye. There is a reason it is out of balance, especially since it was in balance privously. There are more things than a prop that can throw it out of balance.

Dave

 

[gismo]

Once upon a time, the prop on an airplane had a dynamic balance. Not to long after, a vibration returned but wasn’t significant. So a few months later, the airplane is at a different shop to rebalance the prop. Shop No 2 finds no weights attached. Shop No 2 is also unable to balance the prop within recommended vibration levels using max permitted weights. Corrective action may require prop removal, static balancing, then dynamic balance. The owner is unavailable to make repair decisions for a couple of weeks, so he instructs the shop to put the plane back together and return it to the line.

Based on this info, is the plane airworthy???

I think the airplane is unquestionably airworthy unless the prop ICA requires it to be dynamically balanced within the tolerance that wasn't met (unlikely) or there's some obvious other problem like different blades or a crack in the hub or blades. The "recommended vibration levels are normally just that, a recommendation not a requirement. There is likely a max limit on balance weights but I know of nothing that says your prop is unairworthy if you can't get the vibration down to minuscule levels with the max allowed weights.

That said, your symptoms suggest that there is some sort of undiscovered problem that I would want investigated if it were my airplane. If the engine has floating counterweights, one or more could be stuck, one of the prop blades could be at a different pitch than the other (happened to me), someone might have gotten too aggressive when filing out a nick etc. And any of those conditions would make the plane unairworthy.

-lance

 

[stingray]

I'd want some serious answers Kaye. There is a reason it is out of balance, especially since it was in balance privously. There are more things than a prop that can throw it out of balance.

Dave

Like wear and tear, when I had mine done he said you have to do it about every 5 years. just engine wear can bring it out of balance.

Dan

 

[Stache]

To answer your hypothetical question the answer is the propeller in NOT airworthy. Why because it has to be airworthy and airworthy has two conditions as follows:

AIRWORTHY--- Two conditions that must be met for and aircraft is considered airworthy.

1. The aircraft must conform to its type certificate. Conformity to type design when the aircraft configuration and the components installed are consistent with the drawings, specifications, and other data i.e. STC field approved alterations.

2. The aircraft must be in condition for safe operation. Condition of the aircraft relative to wear and deterioration, i.e. skin corrosion, window delamination/crazing, fluid leaks, tire wear, vibration, etc.

NOTE: If one or both of these conditions are not met, the aircraft would be considered unairworthy.

First it is the pilot’s job to squawk things that don’t seem right. Now the mechanic is charged with determining if it is airworthy or not. It either is airworthy or its not.

So take the meaning above does the propeller met condition number 1. In this case let’s say yes it meets its time design of T/C.

Next condition for safe operation so lets take a look at the FAR rules. Under Part 35 propeller certification it states:

§ 35.4 Instructions for Continued Airworthiness (ICA).
The applicant must prepare Instructions for Continued Airworthiness in accordance with appendix A to this part that are acceptable to the Administrator.

Since you did not mention what hypothetical propeller we are talking about we can’t look up the ICA. However the manufacture has to perform vibration testing and set limits.

§ 35.37 Fatigue limit tests.
A fatigue evalution must be made and the fatigue limits determined for each metallic hub and blade, and each primary load carrying metal component of nonmetallic blades. The fatigue evaluation must include consideration of all reasonably foreseeable vibration load patterns. The fatigue limits must account for the permissible service deteriortion (such as nicks, grooves, galling, bearing wear, and variations in material properties).

The mechanic is charged with performing maintenance. Maintenance means inspection, overhaul, repair, preservation, and the replacement of parts, but excludes preventive maintenance. So the A&P or propeller repair station is required to follow FAR 43.13:

§ 43.13 Performance rules (general).
(b) Each person maintaining or altering, or performing preventive maintenance, shall do that work in such a manner and use materials of such a quality, that the condition of the aircraft, airframe, aircraft engine, propeller, or appliance worked on will be at least equal to its original or properly altered condition (with regard to aerodynamic function, structural strength, resistance to vibration and deterioration, and other qualities affecting airworthiness).

I happen to pull two examples of T/C’s for a Hartzell propeller the notes say:“(1) Airworthiness limitations, if any, are stated in Hartzell Manuals 108( ), 114( ) or Service Letter 61( ).”

McCauley T/C states; “Approved Vibration wise for Use on Normal Category Single Engine Tractor Aircraft. The maximum and minimum propeller diameters that can be used from a vibration standpoint are shown below. No reduction below the minimum diameter listed is permissible, since this figure includes diameter reduction allowable for repair purposes. “

So the question is what are the vibration limits in the manual or T/C did the propeller exceed them and if so the propeller is unairworthy and must not be returned to service. However the mechanic is required to make a maintenance record entry indication how they inspected it and to what data.

This is not a case where the owner can decide airworthiness. It’s the mechanic responsibility. The mechanic can reinstall it with a record entry indicating it is unairworthy in the maintenance record, because the owner operator is responsible for having the propeller repaired.

Propellers have inherent vibration characteristics, which are not usually harmful but can induce fatigue and in time cause failure of parts essential to the airworthiness of the aircraft. This is one reason why periodic inspection of the aircraft is essential.

A special word about propellers. Quite often a propeller blade becomes nicked, especially at the leading edges. These nicks become points of stress concentration. IT IS IMPORTANT THAT NICKS BE REMOVED AS SOON AS POSSIBLE AND IN A PROPER MANNER. Since the removal of nicks requires special skills and tools and a thorough knowledge of the procedure, such work may be accomplished by certificated personnel only. The importance of correct removal of even small nicks AS SOON AS POSSIBLE after incurring them, cannot be overstressed.

Below is some reference data that may help.
AC 20-66A Vibration and fatigue evaluation of airplane propellers
AC 23-16A Powerplant Guide For Certification Of Part 23 Airplanes And Airships
AC 20-106 Aircraft inspection for the general aviation aircraft owner
AC 33-2B Aircraft engine type certification handbook

AC 43.14-1B
8-109. VIBRATION. Although vibration can be caused by the propeller, there are numerous other possible sources of vibration which can make troubleshooting difficult.

a. If a propeller vibrates, whether due to balance, angle, or track problems, it typically vibrates, throughout the entire RPM range, although the intensity of the vibration may vary with the RPM. If a vibration occurs only at one particular RPM or within a limited RPM range (e.g. 2200-2350 RPM), the vibration is not normally a propeller problem but a problem with a poor engine/propeller match.

b. If a propeller vibration is suspected but cannot be positively determined, if possible, the ideal troubleshooting method is to temporarily replace the propeller with one which is known to be airworthy and test fly the aircraft.

c. There are numerous allowable tolerances in blade angles, balance, track, and blade width and thickness dimensions. These tolerances have been established through many years of experience. The degree to whìch these factors affect vibration is sometimes disputed and can involve significant repair bills, which may or may not cure a vibration problem. Reliance upon experienced, reputable propeller repair stations is the owner's best method of dealing with these problems.

d. Blade shake is not the source of vibration problems. Once the engine is running, centrifugal force holds the blades firmly (approximately 30-40,000 lbs.) against blade bearings.

e. Cabin vibration can sometimes be improved by reindexing the propeller to the crankshaft. The propeller can be removed, rotated 180º, and reinstalled.

f. The propeller spinner can be a contributing factor to an out-of-balance condition. An indication of this would be a noticeable spinner "wobble" while the engine is running. This condition is normally caused by inadequate shimming of the spinner front support or a cracked or deformed spinner

Just one man’s opinion.

 

[Let'sgoflying!]

[ Shop No 2 is also unable to balance the prop within recommended vibration levels using max permitted weights.

I know this is asked as one of those challenging legal questions that can have multiple interpretations, but to me the bottom line on these things is, "Is that safe?".
I agree the legal definitions are interesting and important, but the part of the post quoted above should make us pilots say "Whoa, there - that doesn't seem right" Ie; this really needs looking into before I'm flyin' it.

 

[Kaye]

The mechanic is charged with performing maintenance. Maintenance means inspection, overhaul, repair, preservation, and the replacement of parts, but excludes preventive maintenance. So the A&P or propeller repair station is required to follow FAR 43.13:

So if the shop/mechanic signed off after putting it back together, does that make the assumption that it's legally airworthy?

I know this is asked as one of those challenging legal questions that can have multiple interpretations, but to me the bottom line on these things is, "Is that safe?".
I agree the legal definitions are interesting and important, but the part of the post quoted above should make us pilots say "Whoa, there - that doesn't seem right" Ie; this really needs looking into before I'm flyin' it.

And that right there is the million dollar question.

 

[Stache]

Just because a mechanic signs something off does not make it airworthy, it just mean he did something. If the mechanic signs it off in accordance with part 43.9 referencing the data used and what was accomplished we would know if it was within limits or not.

Just because something is safe for flight does not make it airworthy. Safe for flight requires a Special Flight Permit. Keep in mind on the airworthiness certificate it calls out three FAR's 21, 43, and 91 all three have to be meet to be considered airworthy.

Even us mechanics make mistakes and many do trying to help a customer out by just signing off a maintennace record.

 

[gismo]

By what standard have you determined that the propeller isn't airworthy? The only specific I've seen so far is that the technician wasn't able to achieve the "recommended" low levels of vibration (acceleration actually) when applying the maximum amount of balancing weight per the dynamic balance procedure. Nothing in that says the balance or vibration of the propeller is beyond the type certificate limits. I do agree that if there was such a limit in the TC and the prop didn't meet this limit it would be unairworthy but that's not necessarily the case here.

To answer your hypothetical question the answer is the propeller in NOT airworthy. Why because it has to be airworthy and airworthy has two conditions as follows:

AIRWORTHY--- Two conditions that must be met for and aircraft is considered airworthy.

1. The aircraft must conform to its type certificate. Conformity to type design when the aircraft configuration and the components installed are consistent with the drawings, specifications, and other data i.e. STC field approved alterations.

2. The aircraft must be in condition for safe operation. Condition of the aircraft relative to wear and deterioration, i.e. skin corrosion, window delamination/crazing, fluid leaks, tire wear, vibration, etc.

 

[Stache]

When propellers are manufactured they are static balanced. And when a propeller goes to a propeller repair station for overhaul it is again static balanced to the current overhaul manual.

Without knowing the whole story and what type of propeller the situation is speaking of it is difficult to come up with clear answers. But it sounds like the propeller will not meet static balance. Not meeting the static balance would make the propeller unairworthy.

So the next question is who performed the maintenance/balance? What manuals (data) did they use? I assume the propeller was only a dynamic balance on the aircraft so we don't know if the propeller was static balanced.

I would say an A&P can perform dynamic balance if they have some sort of service instructions to follow. Most dynamic balancing equipment has service instructions I am very familiar with them since I used to teach how to perform dynamic balance. Its not rocket science.

I would highly recommend all GA aircraft have the propeller(s) dynamic balanced to the engine. The repair station or manufacture can only static balance the prop. But when you install it on the engine with the spinner, washers, and screws it changes the balance and a dynamic balance should bring it very close to zero ips. In most cases, the vibration level due to mass imbalance can be brought down to under .100 IN/S very easily.

Dynamic Propeller Balancing is the process whereby an electronic balancer is used to measure the vibration produced by the aircraft power plant. Small trim balance weights are added to the propeller/crankshaft assembly to correct for errors in mass distribution and to reduce power plant vibration due to mass imbalance to the lowest level practical. These should be noted in the maintenance record entry.

Knowing what I do this problem may have nothing to do with the propeller and everything to do with the crankshaft or crankshaft damper(s).

Bottom line is if the vibration level is over the overhaul manual, type certificate or propeller manual limits the propeller is unairworthy. The aircraft should not be flown until the vibration can be resolved. Vibrations have a tendency getting worse or cause other problems.

Just one man's opinion.