The number of aircraft currently grounded and that have been in storage over the past year due to the pandemic is unprecedented. As vaccine distribution increases, travel restrictions are lifting, the world is inching its way toward “normal,” and people are ready to fly again. According to International Air Transport Association’s latest Airline Industry Financial Forecast, regions with large domestic markets – including the U.S., Latin America and Asia-Pacific – in particular have begun to see a resurgence in travel. As global demand for air travel continues to rise, airlines are now faced with a new challenge – getting parked aircraft ready to take to the sky.

When an aircraft is parked for an extended period, there is increased risk for deterioration of its components and structure if preservation procedures are not followed. Preservation procedures vary for traditional short-term storage vs. long-term storage. But regardless of the duration of parked status, it’s important to recognize that there are similar risks to the engines, hydraulic systems, fuel tanks and the airframe – namely water contamination and corrosion.

Engine lubricants in particular are susceptible to increased water contamination anytime an aircraft is parked for an extended period. Similarly, a major concern for fuel tanks is microbiological growth due to water contamination. As airline maintenance teams prepare their aircraft to return to service, there are some key steps to take.

It is essential that airlines follow the instructions included in aircraft OEM maintenance manuals and all relevant operational guidance. The considerations in this material are not intended to override OEM specific protocols.

Maintenance steps for the return to service

Returning an aircraft to service requires a multitude of steps, with lubrication and fuel quality being two key areas that could result in costly repairs if overlooked. There are several critical steps in returning the aircraft to service.

• Exercise the engines: Occasionally “exercising” engines while grounded by moving the aircraft can bring the oil to operating temperature, help evaporate any water, and renew the film of protective additives on the surfaces of engine components.

• Exercise the wheels: Moving the aircraft also rolls the tires, which renews the grease coating on the associated wheel bearing components and helps protect the bearings. This is important because wheel bearings are only re-greased when completely removed from the aircraft. Moving the airplane also flexes the landing gear which renews oil and grease films on landing gear struts and linkages.

• Test the oil: Technicians should periodically test the engine oil in parked aircraft for water (ppm) and monitor total acid number (TAN) and compare these to the oil condition limits set by the engine OEM. Be sure to work with a lab that specializes in aviation fluid analysis like Jet-Care and SGS, which use industry standard test methods, to get an accurate picture of your specific engine lubricant condition – and adjust maintenance practices accordingly. You can’t just assume you’re operating the engine enough and one airline’s approach may not work for another airline. In a limited number of cases, some engines with less than weekly engine runs in a high humidity, high heat environment did show excessive water and excessive TAN.

• Dewater the fuel tanks: All aircraft fuel tanks have drain points to drain water out of the tank. While drain frequency is based on OEM recommendations, more frequent drainage may be beneficial when an aircraft is parked. By draining regularly, maintenance teams can remove water that holds up in low points. When the aircraft is moved, water may also dislodge from spots that don’t drain as efficiently, and more draining may be necessary.

• Visually assess the fuel: Following any water draining, it is recommended that maintenance staff take a sample of the fuel in a clear container to visually assess the quality. Check for microbiological growth and particulate in the fuel, as well as the color. Depending on where an aircraft is stored, sand or dirt can find its way into the fuel tank. If particulate is present, it may be necessary to drain the fuel and clean the tank. In terms of color, jet fuel typically ranges from colorless to a light straw color. Anything darker than that may be cause for investigation.

• Clean the fuel tank – only if necessary: If there is particulate, or a buildup of microbiological growth due to inadequate drainage, it may be necessary to drain the fuel and physically clean the tanks. As there are significant cost and safety implications for this, it’s recommended that maintenance teams regularly drain the fuel tanks to avoid cleaning them.

As airlines prepare their planes to take flight again, it’s critical to test the engine oil and evaluate the water ppm and TAN, change the oil if needed, continue to exercise the engine, and drain and assess the fuel tanks.

Considerations for Long-Term Storage

While airlines worked to keep grounded aircraft parked only temporarily, some aircraft were put in long-term storage, or deep preservation. If an aircraft was put into long-term storage, exercising the aircraft is not an option.

Long-term storage requires preserving or “pickling” an engine, in which preservation additives are added to the lubricants to prevent corrosion. Once the long-term preservative additive is mixed in the lubricant, the engine operation is typically limited by the OEM as the preservative may interfere with other lubricant additives and create less load-carrying, or more deposits forming. For “pickled” engines, workload for return-to-service is very high. Lubricant should be drained, and engine flushed before return-to-service.

From a fuel perspective, many of the maintenance steps remain the same regardless of how long the aircraft has been parked. However, aviation fuel supply locations are required to re-test fuel that has aged for six months beyond its last certification. This same practice would also be beneficial for fuel stored in aircraft for that duration of time. Airlines should work with their supplier to understand the testing process and get clarity on any fuel quality concerns.

With these few additional steps, aircraft will be better prepared to take to the skies, and airlines can avoid costly repairs.