Propeller Storage and Preservation

General Information

• Propeller storage and conservation varies based on propeller type for different airplanes.
• Preservation differs based on manufacturing material and whether the propeller remains installed.
• Propeller preservation and de-preservation is located in the component maintenance manual (CMM) and service bulletins (SB).
• If installed on a stored airplane, instructions can be found in Chapter 10 of the aircraft maintenance manual (AMM).
• The main goal of preservation procedures for metal propellers and components (PCU, governor) prevents corrosion.
• Calendar time and operation hours determine the time between overhauls for propellers, governors, and accumulators.
• The calendar limit starts from the first installation date on an engine, not from the manufacture or overhaul date.
• Manufacture or overhaul date matters for long-term storage inspections.
• Time Between Overhaul (TBO) calendar limit still applies if the propeller is removed from service and not long-term storage.
• A calendar month spans from the first to the last day of the month, allowing compliance anytime within that period.
• A propeller with a 60-month inspection interval, that is inspected on any day of the month, will be due for inspection on the last day of the same month, 60 months later.
• Propellers on engines out of use for up to three months should be cleaned and inspected for corrosion regularly.
• Variable-pitch propeller internal parts are protected by running the engine weekly; if not possible, feather and unfeather the propeller using the feathering pump.
• If an engine is out of use for more than three months, the propeller mechanism should be flushed with inhibiting oil.
• Lanolin or rust preventative should be applied to all external propeller parts.
• Propeller operating mechanisms should be covered with waxed paper, visible parts should be inspected for corrosion regularly.

Propeller Preservation (In-Situ)

• Airplane parking is categorized by time out of service.
• For Saab S2000, short-term parking is 0-45 days without special propeller conservation.
• After 45 days, maintenance should run the engines for 15 minutes, then park for another 45 days.
• For 46-180 days of parking, Saab requires fresh oil and new oil filters to prevent damage from acids and oxides.
• Restraining bridles or blade protective covers (blade socks) should be installed.
• If an aircraft remains serviceable after 180 days, it should be de-preserved, engine run for 15 minutes, and re-preserved to maintain serviceability.
• Outdoor propellers should have restraining bridles to prevent windmilling.
• Propellers stored outdoors for extended periods should be protected from the environment (rain, snow, moisture, birds, insects) with full protective covers or blade socks.
• Hangar storage poses the greatest threat to propellers, especially composite ones, because of damage during maintenance, referred to as 'hangar rash'.
• Blades are recommended to be protected using purpose made blade socks, often made from neoprene rubber.

Propeller Preservation (Uninstalled)

• Uninstalled propellers should be stored in clean, dry, warm conditions free of corrosive fumes.
• Two-bladed propellers are typically stored in racks for air circulation; propellers with more blades are stored vertically on stands to save floor space.
• Retain propellers in manufacturer's packaging or wrap in moldable wrap and waxed paper, otherwise, use a moisture-proof bag ('cocoon').
• Silica gel desiccant bags can be added inside moisture-proof bags.
• External parts of metal propellers should be coated with lanolin or an alternative.
• Hydraulically operated propeller pitch change mechanisms should be inhibited with approved oil.
• Loose parts like oil tubes and mounting cones should be coated with lanolin and wrapped in waxed paper.
• When disassembling a variable pitch propeller for storage, immerse individual mechanical parts in inhibiting oil, then drain off.
• Bearings should be coated with mineral jelly and electrical connections smeared with petroleum jelly.
• All electrical equipment (motors, slip rings) should be cleaned, connections smeared with petroleum jelly, external surfaces treated with rust preventative, and each part sealed in a moisture-proof bag.
• Wrap all propeller parts in waxed paper and pack in a suitable carton or crate.
• Bearings in assembled propellers or pre-loaded blade assemblies must be exercised after six and nine months of storage.
• Bearings must be removed and examined for false brinelling and corrosion after twelve months in storage, if satisfactory, clean, grease, and reassemble on the blade for six more months.
• The maximum storage period differs by propeller type, but if retained in the manufacturer's packaging, it's three years if checked after that period as described above.
• If not in the original packing, propellers/components require re-inhibiting every twelve months and overhauling after three years.
• Rubber components have a specific life from the cure date and must be discarded at the overhaul nearest to their life expiry as sourced in the relevant Maintenance Manual.
• Loose rubber components must be stored unstressed, in the dark, and in manufacturer's packing until needed.
• Label all propellers/components in storage with their part number, modification status, original storage date, and details of actions taken post storage date.

Return to Service of Variable Pitch Propellers

• The following instructions apply to new and overhauled propellers before installation or after removal from service.
• Storage time is from the date of manufacture, overhaul, or removal, and should be in in a clean, dry, and above ground place.
• If storage is less than two years:
  • Perform a general visual inspection and correct questionable conditions.
  • Check current service bulletins and manufacturer technical information.
• If storage exceeds two years, do the above and:
  • Check for internal/external damage/corrosion without removing paint/plating unless damage is found.
  • Replace parts, seals, and gaskets as necessary.
  • Lubricate as laid out in the overhaul manual.
  • If applicable, test the de-ice system, ensuring boots are well bonded without blistering.
  • Repaint/replace components.
• With procedures completed, the propeller may be released for full TBO and calendar life.
• The procedure must be done by an approved propeller repair station following the overhaul manual.
• If storage exceeds five years, follow special inspection and parts replacement processes before returning the propeller to service, including:
  • Disassemble to replace rubber seals and lubricants; total disassembly is not needed unless extensive damage is found.
  • Work must be accomplished by an approved propeller repair station.
  • Inspect everything for damage and corrosion, repair or replace any defective parts.
  • The steps must be undertaken by an approved propeller repair station.

Return to Service of Governors and Accumulators

• These instructions are for new and overhauled governors/accumulators before engine installation or removal.
• Storage time is determined from the date of manufacture, overhaul, or removal from aircraft.
• Storage should be in a clean, dry place, stored above the ground in the original shipping carton.
• If stored for over two years, inspect the unit with the following steps:
  • Inspect externally for damage and corrosion.
  • Test run the governor on a test bench to verify correct operation and check for leaks, this must be done only by an approved repair station.
  • Pressure check accumulator for correct operation and leaks.
• If stored for over five years perform the following inspection and parts replacement:
  • Completely release air or nitrogen pressure before any disassembly, as pressurized retaining rings are dangerous.
  • Disassemble to replace all rubber seals and gaskets; total disassembly is not needed unless corrosion is found.
  • Inspect parts for damage and corrosion, repair/replace parts as necessary.
    • Critical inspection areas for governors are the drive gear, gear of the pilot spool, and ‘toes' of flyweight, this work must be done by an approved governor repair station.
    • The accumulators should be inspected inside the cylinder for corrosion.
  • Test run the governor on a test bench to ensure the unit is operating correctly and check for leaks.
  • Pressure check the accumulator to ensure the levels are correct and check for leaks.

Storage of Wood Propellers

• Wood propellers should be stored with their blades in a horizontal position, as verticle blades may collect moisture in the lower blade and cause the unit to be unbalanced.
• Leave one blade pointing down while the aircraft is stored outside.
• Do not store propellers on their tips.