A polished propellor can be a thing of beauty, and is often coveted by the owners of classic aircraft. However, propellor polishing may affect the airworthiness of that component.

All parts for certified aircraft are designed and then produced to a set of specifications, many of which are in the type design data sheets, and some of which are only contained in the manufacturer’s "approved" data consisting of drawings, process specifications, and manuals. Technically, only the manufacturer’s specifications for coating process and materials may be utilized. Practically, enforcement actions against anyone maintaining the propellor in sound condition is a reasonable defense by the owner and mechanic.

If the owner and operator cannot provide the approved data authorizing a polished propellor, he can be judged by an inspector to be guilty of altering the FAA approved type design, and thereby grounded, or the aircraft denied re-certification. Such occurrences are rare, but possible.

Generally speaking, most aircraft components have some finish specification such a s prime, paint, anodize, plate, etc. The FAA with their aging aircraft fleet initiative have concentrated a great deal of effort into the area of corrosion in recent years, because that has proved to be one very serious Achilles heel to the aviation industry, and to proper maintenance (i.e. how much corrosion is too much). Corrosion is often a hidden defect.

Obviously zero corrosion is the fleet goal, but time and the environment, make that an unrealistic standard. Most manuals have some estimate for erosion and servicing of propellers, but the issue of corrosion has often been left to the opinion of the shop conducting work on a propellor. This can be good, or bad. To address the corrosion (and to some degree the surface abrasion issue), many propellor manufacturers have established an anodized finish, or a painted finish per some specification known to the manufacturer, and often detailed in its manuals. That manufacturer coating specification is controlling unless the owner obtains an approved deviation by field approval, STC, or can reference a particular manufacturer or FAA specification applicable to their propellor (make/model/application).

Often at conflict with this goal of no corrosion in the fleet is the approved general practice of "dressing" a propellor. As is described in AC43-13-1B at section 4, Repair of Metal Propellers (paragraphs 8-71 through 8-129), and also in AC20-37D, "Aircraft Metal Propeller Maintenance". It is immediately apparent that any attempt to dress out nicks and other deformities would remove the corrosion protection installed by the factory. What to do…

Similarly, it is generally accepted that the rear of the propeller blades are to be flat or satin black to prevent reflections and "flicker vertigo" situations. Most Mechanics or IAs require this inspection and paint at annual, along with the dressing of minor nicks. Few, if any, obtain paint from the manufacturer, and may be in technical FAA violation.

Also, the tips should be marked with indicator stripes of a contrasting color like red, yellow, white for better visibility on the ground by pedestrian traffic. Obviously all of this is approached with the goal of safety in mind, and the application of common sense and good maintenance practices. Common sense is not, unfortunately, an authority you can cite to the FAA.

Please note that Mechanics and IAs are responsible for returning to service those airplanes and modifications which met type design standards or other FAA approved data. Your IA cannot "Approve" something (like a polished prop), for which there is not some FAA approved basis to reference. He may ignore, or be unaware that the condition is unapproved, and you may get an annual signed off. Then again, you may not…

THE GOOD NEWS and the bad news

The good news is that many of the old propellers were delivered in a polished condition, or they had established standards for maintaining a polished condition.

Curtis Reed was known for its beautiful lines and bright luster. Polish seems to be the order on those units.

Hamilton Standard had a similar caveat with the polish maintenance standard established in its Overhaul manuals and service materials. The Hamilton Standard Aluminum Blade Overhaul Manual number 130B authorizes a procedure for blade polishing. That procedure describes the process by which an ‘Authorized’ Propeller Repair Station is permitted to polish blades during overhaul when the blades are removed from the hub. The procedure requires the removal of all of the polish material after polishing and that the blades be inspected and measured for thickness and diameter in several places while in the master jig before assembling the prop. According to this manual, only a propeller shop with the proper ratings can polish Hamilton Standard blades.

The Hamilton Standard Propeller Service Manual No.110D dated March 19, 1945 sec 5 "Service, Inspection, Maintenance, and Lubrication" paragraph 3, "Maintenance of Aluminum Alloy Blades" specifies that: "In special cases where a high polish is desired, a responsible party may authorize the use of a good grade of metal polish, provided that upon completion of the polishing all traces of polish are immediately removed and the blades cleaned and coated with a thin film of clean engine oil."

Most IA’s and A&P’s would consider the owner qualified and responsible to do this so long as there is no trace of the polish compound left.

(note: most polishing compounds have a corrosive accelerator in them to speed the polishing process. Any failure to remove these compounds increases the rate of corrosion on most aluminum without the application of wax, clear-coat or other protection)

SENSENICH PROPELLER MFG CO (ALUMINUM) 717-569-0435 http://www.sensenich.com has several documents on Polishing out cracks and corrosion and warnings about corrosion and the application of protective finishes.

http://www.sensenich.com/bulletin/r-15a.htm - 4 K - 25 Sep 102

http://www.sensenich.com/bulletin/r-12a.htm - 3 K - 25 Sep 102

http://www.sensenich.com/bulletin/r-11.htm - 3 K - 25 Sep 102

http://www.sensenich.com/directiv/ad601108.htm - 1 K - 25 Sep 102

http://www.sensenich.com/bulletin/r-8-1.htm - 1 K - 25 Sep 102

McCauley Met-Al-Prop Repair and Maintenance Handbook, dated 2-15-55 says specifically:

14. Finish

a. If the repaired propeller is to be used in the vicinity of salt water,  it shall be anodized.

b. The face side of the blades shall be painted with a non-specular black lacquer for glare elimination.

c. The tips shall be painted with red, orange, or yellow lacquer.

Other Sources for good information and overhauls:

Maxwell Aircraft, Crystal Minnesnowta is an excellent propeller repair facility, which I have used. Maxwell Aircraft Service, Inc.; (800)964-4247 or (763) 533-8611 www.hcsweb.com/maxwell

Jim Fix of Nebraska. worked at Maxwell and is a similar expert, and very reasonable. JIM FIX 402-470-2346 METAL PROP REPITCHING, TEMPERFOAM AND SUNMATE FOAMS

Other useful links at:

http://www.aopa.org/whatsnew/newsitems/2003/03-3-140x.html

http://www.stearman.net/forum/messageview.cfm?catid=3&threadid=824

Hamilton Standard responded with, "The Hamilton Sundstrand position would be found in the applicable documentation that has been published for the propeller systems you are involved with, i.e. Maintenance and Overhaul Manuals. All requirements contained in these documents are to be adhered to as written."

Hartzell's response was that "Hartzell has no approved procedure for polishing blades to a high polished or mirror finish. All of our procedures regarding blade finishing require a corrosion preventative and paint".

McCauley's reply was that "McCauley has manufactured a very few propellers as polished versions at the request of Mooney, for Mooney aircraft. These propellers are identified as such and are the only McCauley propellers that are legally allowed to be polished. However, due to susceptibility to corrosion, McCauley encourages owners of these propellers to have them painted as well. It is not legal for a propeller, which was not originally manufactured as a polished propeller, to be released as a polished propeller, because it does not meet type design requirements."

Both Hartzell and McCauley tech supports mentioned that they are getting reports of FAA inspectors grounding aircraft with polished props.

The reason for the paint is that it protects the prop from corrosion. The prop has to be balanced. A small prop nick causes a lot of stress . Balance and corrosion is due to wear and upkeep. Most IAs will allow the prop to be painted and kept free of corrosion by the owner. The point is that the prop is another part of the plane that is in need of upkeep and an IA will work to maintain its airworthiness.

McCauley states that the propeller must be painted according to the way they manufactured and certified it with the FAA.

* PROPELLOR is an acceptable variant...per Merriam-Webster's Dictionary...

Main Entry: pro·pel·ler Variant(s): also pro.pel.lor /pr&-'pe-l&r/ Function: noun

Date: 1780 : one that propels; especially : a device that consists of a central hub with radiating blades placed and twisted so that each forms part of a helical surface and that is used to propel a vehicle (as a ship or airplane)

Copyright © 2004 Doug Combs (original work only)

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