Preservation of GA Aircraft - a short guide

Flight Training Europe SL • 29 April 2020
in community General Aviation
1 comment

An aircraft on the ground for an extended period of time is never a good thing, not for the pilot nor for the airframe itself. It is generally recognized that flying an aircraft every 30 days (at the very least) is the best preventive maintenance that can be done, as it operates all the different systems, from flight controls to engine(s) and avionics.

Unfortunately, with the operational restriction currently standing in many countries, it is almost impossible to have every (if any) airframe flying that regularly, and preservation measures should be implemented. Preservation is any maintenance action that will prevent (or slow) any aircraft system to degrade over time due to inactivity. This is sometimes confused with "storage", although storage is more related to the conditions the airframe/components are stored, and preservation to the maintenance actions required (for storage or during storage).

GA aircraft manufacturers are not too kin on developing extensive maintenance technical documentation, unlike what can be found on larger aircraft. As such, most lack proper procedures to store and preserve aircraft for long periods of time, and bits and pieces of information can be found in the Service Information (SBs, SLs, SIs) from the specific products or components manufacturers. The focus for GA aircraft preservation is typically on engines and both Lycoming and Continental have good documentation on the subject (not exhaustive):

- Lycoming SL L180B

- Continental MM M-0, SB 20-03, SB 99-8B

The message is that engines should definitely be preserved if operation is not forecast within the next 30 days as internal corrosion could start "attacking" very soon. The procedure is relatively simple: run the engine on a mixture of preservative and mineral oil, coat the inside of the cylinders with that mixture, and then seal off any air intakes with silica gel bags. We have found that 50g bags work best for the exhaust and 100g bags for the intakes. Remember that both carburated and injection engines have alternate air supplies that also need to be sealed off.

Although not mandatory, the usage of dehydrator plugs as an indication for the moisture content inside each cylinder is definitely recommended, and a great indicator of when to re-preserve the engine.

As an approved service center we have been asked if ground running the engines every now and again would suffice. We most definitely recommend AGAINST this practice: water and acids are a by-product of combustion and accumulate in the oil - if the oil is not kept at a high temperature (typically above 165ºF) for a large period of time (roughly 1H) this water will not vaporize and will in time corrode the inside of the engine and turn into acid. Furthermore, inadequate cooling on ground-runs will result in hotspots in the cylinders and baked ignition harness and oil seals (causing leaks).

Moving on from engines, other maintenance procedures are important and generally lacking from appropriate documentation. This is a general list of additional procedures based on our own experience and knowledge, we follow to keep over 30 GA aircraft in good condition:

- Fill the fuel tanks - the fuel will act as a preservative to the tank walls;

- If possible hangar your aircraft - this will prevent direct exposure to the environment (rain and direct sunlight), minimizing both water ingress and thermal shocks and cycles. If it is not possible to hangar the aircraft, cover it as much as possible (especially if it has large transparent canopy);

- Cover pitot and static ports, ventilation inlets and outlets, and any other probes or "holes" in the airframe;

At least every 7 to 30 days, depending on the area you are in:

- Move the aircraft a bit (about 1/3 of a full rotation) to avoid flat spots on tires and apply brakes to prevent brake binding/seizing;

- Turn on electrical and avionics systems to warn them up and hopefully vaporize some of the humidity that may have settled in;

- If equipped, turn on electrical hydraulic and/or fuel pumps - the fluids will lubricate the gears and the valves;

- Ventilate the aircraft (open doors/windows) to allow air renewal - try to perform this during a nice sunny day with as low relative humidity as possible - we keep humidity sensors in the cabin and have observed the tremendous positive impact this can have on the cabin stale (and possibly humid) air;

- Drain any water from the fuel tanks and at the lowest part of the fuel system (typically a drain in the engine vicinity);

- Exercise all flight surfaces, including flaps, using the controls;

- Load/unload the landing gears by lifting the tips of the wings;

If you are planning for a (very) long period of inactivity, you should also consider the following:

- Remove batteries and avionic systems and store them in a controlled environment (dry);

- Cover seats and carpest to prevent moisture absorption and microfungal contamination;

- Jack the aircraft up and cycle the retractable landing gear (at least once a year).

To determine if you are at a  high risk of corrosion area, you could use Cessna SID D971-3-13 Corrosion Severity Map as a reference as it is generally applicable to any aircraft. The more severe the corrosion area, the more frequent the maintenance should be performed.

Hopefully, this will give you a general idea of how to maintain your aircraft during these troublesome times. If you have any questions, drop us a line here, we would be more than happy to help!

Stay safe and keep your aircraft equally safe!

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