Aircraft Corrosion

Clear Water Rinse Systems Mitigate Risk and Reduce Maintenance Costs

SITUATION: Corrosion – the natural deterioration of metal when it reacts with various environmental conditions – is a costly, hazardous problem that affects every sector of the economy, including consumer products, utilities, construction, and transportation. It is a particular challenge in commercial and military aviation, where corrosion compromises safety and performance, erodes productivity, and adds significantly to the cost of aircraft maintenance.

Corrosion can render an aircraft un-airworthy by weakening structural components, roughening the outer surface, loosening fasteners, hastening cracking, and facilitating the entry of water into electronic fixtures. Left untreated, corrosion can hasten other conditions that will eventually cause structural failure. Corrosion can quickly develop in key areas, where loss of even a small degree of material integrity

can allow moisture, salt, sand, and other contaminants to enter, sometimes with catastrophic results. The crash of an El-Al Boeing 747 in Amsterdam (1992), the crash of a China Airlines Boeing 747-200F (1991), and the incident in 1988 in which a hole was torn in the fuselage of an Aloha Boeing 737 as it flew over Hawaii were all traced to structural damage caused by corrosion.

Corrosion can render an aircraft un-airworthy by weakening structural components, roughening the outer surface, loosening fasteners, hastening cracking, and facilitating the entry of water into electronic fixtures. Left untreated, corrosion can hasten other conditions that will eventually cause structural failure. Corrosion can quickly develop in key areas, where loss of even a small degree of material integrity can allow moisture, salt, sand, and other contaminants to enter, sometimes with catastrophic results. The crash of an El-Al Boeing 747 in Amsterdam (1992), the crash of a China Airlines Boeing 747-200F (1991), and the incident in 1988 in which a hole was torn in the fuselage of an Aloha Boeing 737 as it flew over Hawaii were all traced to structural damage caused by corrosion.

Corrosion can affect entire fleets of aircrafts, causing delays and compromising military readiness:

  • The crash of a 28-year-old F-15C Eagle fighter in Virginia in 2014 and the break-up of another F15-C over Missouri in 2007 focused attention on the aging of the U.S. Air Force’s fleet of fighters, bombers, and tanker aircrafts. Concerns about metal fatigue and corrosion forced the grounding of the entire F-15 fleet during the investigation of the 2014 incident, and prompted placing F-15Es on ground alert while flying missions in Iraq and Afghanistan with other aircrafts.
  • Nearly one of every five of the Marine Corps’ aircraft – as many as 134 aircrafts, including F/A-18 Hornets, CH-53E Super Stallions, AV-8B Harriers, MV-22B Ospreys, and H-1 Hueys – were grounded in early 2015 due to high levels of corrosion.
  • Over many years, problems with the cabin pressurization system in the U.S. Air Force’s C-130 aircraft had severely sickened pilots and crews and grounded many of the aircraft, causing costly delays. The problem was eventually traced to corrosion of a critical part.

Aircrafts are particularly vulnerable because they are constructed from a variety of metals that are subject to different types of corrosion, and because they are constantly exposed to corrosive environmental conditions.

Other factors – including the age of the plane, where it is operated, how often it is cleaned, and whether it is hangared – will also affect how quickly and to what extent corrosion will develop.

JOINT BASE SAN ANTONIO-RANDOLPH, Texas
Up until recently, the 12th Flying Training Wing Maintenance Directorate corrosion control section washed the wing’s trainer aircraft outdoors, where temperature extremes, inclement weather and water-use restrictions often interfered with their work. The 14-member corrosion control team now has its own indoor wash rack in Hangar 42 with benefits that include washes unimpeded by weather conditions and an environmentally friendly Riveer water-recycling system that conserves water, saving the Air Force $174,000 per year said the aircraft maintenance supervisor.

ENVIRONMENTAL FACTORS

Corrosion occurs when a metal forms a chemical reaction with its environment, resulting in the oxidation, and eventual breakdown, of the metal. In aviation operations, both on the ground and in the air, specific environmental factors and the presence of certain substances make conditions right for corrosion to develop.

Atmospheric and environment conditions are the primary cause of corrosion, and they hasten the process once it has begun. Moist, oxygen-rich air, especially if it carries salts from ocean waters, is particularly damaging to metal components. Wind-borne sands and dusts are very corrosive, particularly in desert regions, where sand often carries salt from ancient oceans that once covered the arid lands. Industrial air pollution is highly corrosive. Volcanic ash is highly corrosive. The corrosion process is accelerated in hot environments.

Other substances that contribute to this corrosive mix include industrial fluids and cleaning solutions, oils and fuels, battery acid,

engine exhaust particulates, and even acidic residues from leaking galleys or lavatories.

In its Technical Manual: Cleaning and Corrosion Control, the Naval Air Systems Command explains how corrosion can damage aircraft: “High strength steels used in landing gear and launch/ recovery systems are sensitive to pitting and stress corrosion cracking, which can lead to catastrophic failure. Aluminum alloys susceptible to exfoliation and intergranular corrosion are commonly found on wing skin and other load carrying structures. Even magnesium, one of the most corrosion sensitive metals known, is still used in canopy frames and gear boxes. Added to this is the ever increasing age of military aircrafts and the need to comply with stricter environmental regulations. All of these factors combine to make corrosion prevention and control a significant factor in the safe and economic operation of military aircrafts.”

ENVIRONMENTAL FACTORS

Corrosion occurs when a metal forms a chemical reaction with its environment, resulting in the oxidation, and eventual breakdown, of the metal. In aviation operations, both on the ground and in the air, specific environmental factors and the presence of certain substances make conditions right for corrosion to develop.

Atmospheric and environment conditions are the primary cause of corrosion, and they hasten the process once it has begun. Moist, oxygen-rich air, especially if it carries salts from ocean waters, is particularly damaging to metal components. Wind-borne sands and dusts are very corrosive, particularly in desert regions, where sand often carries salt from ancient oceans that once covered the arid lands. Industrial air pollution is highly corrosive. Volcanic ash is highly corrosive. The corrosion process is accelerated in hot environments.

Other substances that contribute to this corrosive mix include industrial fluids and cleaning solutions, oils and fuels, battery acid, engine exhaust particulates, and even acidic residues from leaking galleys or lavatories.

In its Technical Manual: Cleaning and Corrosion Control, the Naval Air Systems Command explains how corrosion can damage aircraft: “High strength steels used in landing gear and launch/ recovery systems are sensitive to pitting and stress corrosion cracking, which can lead to catastrophic failure. Aluminum alloys susceptible to exfoliation and intergranular corrosion are commonly found on wing skin and other load carrying structures. Even magnesium, one of the most corrosion sensitive metals known, is still used in canopy frames and gear boxes. Added to this is the ever increasing age of military aircrafts and the need to comply with stricter environmental regulations. All of these factors combine to make corrosion prevention and control a significant factor in the safe and economic operation of military aircrafts.”

AGING AIRCRAFT ARE AT SPECIAL RISK

The age of the aircraft is also a significant factor. In recent years, progress has been made in the fight against corrosion, with the development of better corrosion-resistant base materials, protective surface treatments, and coatings and the introduction of corrosion prevention measures into aerospace engineering and manufacturing processes.

But older aircrafts – particularly those beyond their 20-year design life – are particularly vulnerable to corrosion, not only because they lack the newer anti-corrosive protections, but because of their total exposure over years and

decades to the harsh environments and conditions that hasten the advance of corrosion. Even under ideal conditions, all aircrafts will experience some corrosion, but as an aircraft ages, corrosion is more likely to develop, and to be more extensive.

About one quarter of all the commercial aircraft currently in operation are more than 20 years old, and the average age of planes in the United States Air Force is 24 years.

AGING AIRCRAFT ARE AT SPECIAL RISK

The age of the aircraft is also a significant factor. In recent years, progress has been made in the fight against corrosion, with the development of better corrosion-resistant base materials, protective surface treatments, and coatings and the introduction of corrosion prevention measures into aerospace engineering and manufacturing processes.

But older aircrafts – particularly those beyond their 20-year design life – are particularly vulnerable to corrosion, not only because they lack the newer anti-corrosive protections, but because of their total exposure over years and decades to the harsh environments and conditions that hasten the advance of corrosion. Even under ideal conditions, all aircrafts will experience some corrosion, but as an aircraft ages, corrosion is more likely to develop, and to be more extensive.

About one quarter of all the commercial aircraft currently in operation are more than 20 years old, and the average age of planes in the United States Air Force is 24 years.

THE COST OF CORROSION

Corrosion imposes a tremendous burden on aviation operations, in both direct and indirect costs.
Direct costs include:

  • Special cleaning and corrosion control activities
  • Inspection and maintenance
  • Repair or replacement of corroded components

Indirect costs include:

  • Loss of productivity due to delays, failures, etc.
  • Taxes and related overhead on corrosion-related maintenance and other activities
  • Litigation, fines, and loss of public good will after accidents and crashes

The U.S. military, which is deeply affected by corrosion in aviation operations, has made careful study of every aspect of the issue, including costs. Among the findings of various studies conducted by or for the U.S. military are the following:

  • The Department of Defense spends more than $23 billion each year to control corrosion on aircraft and other equipment in its operations around the world. One source estimates this to be 20.5% of total maintenance costs for infrastructure, facilities, and weaponry.
  • The U.S. Army’s Aviation and Missile Command spends $1.6 billion each year to address corrosion on helicopters.
  • Naval Air Systems Command (NAVAIR) reports that corrosion accounts for half of all aircraft depot maintenance costs.
  • Over FY2009-FY2013, corrosion-related costs for the U.S. Coast Guard were $344 million, or more than 22% of the Guard’s total maintenance budget.
  • The House Armed Services Committee reports that about $7 billion of corrosion cost is preventable.

Corrosion also affects productivity, as demonstrated in studies done by or for the U.S. military:

  • In 2010-2011, the average number of days per aircraft that were unavailable due to corrosion was 15.9 days for Air Force aviation, 17.4 days for Army aviation, and 26.5 days for Navy and Marine Corps aviation.
  • In a similar study from FY2012, corrosion was found to be a contributing factor in 18.1% of total non-availability time for Army aviation assets, an average of 472 hours, OR 19.7 days per year per unit.
  • Naval Air Systems Command (NAVAIR) estimates for each aircraft, corrosion-related activities require 350-400 person hours of labor (labor hours) and result in 9-10 weeks of unavailability per year.

Post-flight rinsing helps mitigate corrosion, keeping aircraft out of rework and in the air.

FIRST LINE OF DEFENSE: CLEANLINESS

Aircraft designers and manufacturers have made significant improvements in the corrosion-resistance of modern aircraft. On the ground, airport and military managers are paying closer attention to corrosion, and have implemented more frequent inspections and better cleaning and maintenance.

Despite these improvements, corrosion remains a significant challenge for commercial and military aviation, requiring constant vigilance and continuous preventive maintenance.

The most effective means of preventing and mitigating corrosion is to keep aircraft clean – in particular, by removing corrosive contaminants that accumulate on the exterior of the aircraft during flight. And cleaning offers other benefits, including:

  • Reducing drag and overall weight, thus improving fuel efficiency
  • Facilitating inspections, done more easily on a clean aircraft
  • Maintaining the plane’s appearance

The frequency and intensity of cleaning operations are largely determined by the operating environment. Aircraft operated in hot, humid areas, within ten miles of sea coasts, or in deserts, or in areas where industrial air pollution is present, or those that are not hangared, will require more frequent cleanings than aircraft operated in dry, pollution-free environments that are protected from the elements between flights.

In its Technical Manual: Cleaning and Corrosion Control, the Naval Air Systems Command (NAVAIR) specifies these cleaning schedules for its aircraft:

  • In the absence of aircraft specific requirements, Navy aircraft shall be cleaned at least every 7 days when aboard ship and at least every 14 days when ashore.
  • Under certain conditions, depending on the type of aircraft and usage, the normal wash cycle may not be sufficient. More frequent cleaning may be required for certain types of aircraft when exposure to salt spray, salt water, or other corrosive materials occurs.
  • When deployed within three miles of salt water or when flown below 3000 feet over salt water, daily cleaning or wipe down is required on all exposed, unpainted surfaces, such as landing gear struts and actuating rods of hydraulic cylinders.

And for Army aircraft:

  • The frequency of cleaning of army aircraft shall be 30 days … unless aircraft are stationed within two miles of salt water [when more frequent cleaning is required].
  • Extended or low level operations over salt water require daily fresh water rinsing.

The Air Force mandates the following intervals for thorough aircraft washing, dependent on the degree of harshness of the operating environment:

  • Mild: every 180 days
  • Moderate: every 90 days
  • Severe: every 30 days

The Federal Aviation Administration suggests the following intervals:

  • Mild zones: every 90 days;
  • Moderate zones: every 45 days
  • Severe zones: every 15 days

The military also has requirements for aircraft cleaning for air bases in specific geographic locations or used for certain operations:

  • All aircraft stationed within 1.25 miles (2 km) of salt water require a clear water rinse (CWR) at least once every 15 days unless washed first. All aircraft deployed to stations within 1.25 miles (2 km) of salt water for 10 days or more must follow the CWR requirements of the deployment location.
  • Aircraft making two or more take-offs and or/landings, including touch-and-go landings, when the runway approach is under 3,000 feet and over salt water require a CWR after the aircraft completes the last flight of the day.
  • Any aircraft (primarily transient aircraft) performing only a single takeoff and/or landing requiring low-level flight (below 3,000 feet) over salt water in a single day are excluded from CWR unless there are ten or more occurrences within a 30 day period.
  • Search, rescue, and recovery missions or any other low-level flight operations that require aircraft to operate over salt water at altitudes under 3,000 feet require a CWR after the aircraft completes the last flight of the day.

Cleaning, particularly for aircraft operated in harsh environments, is a lengthy, labor-intensive process. Both the military (through its branches) and commercial aviation (through the FAA) have implemented specific, detailed procedures for accomplishing this essential task, which is typically carried out by specially-trained cleaning crews working by hand on each aircraft. The process requires that the aircraft be taken out of service for many hours, sometimes for an entire day or night, depending on the type of aircraft and the harshness of the operating environment.

In addition to regular washing or cleaning, industry and military experts recommend (and often require) that aircraft operated in harsh environments be rinsed with clear water immediately after use. While rinsing cannot replace cleaning (and does not satisfy regulatory requirements for cleaning), it significantly reduces corrosion risk through the immediate removal of salts and light soils before they have a chance to interact with aircraft surfaces and materials.

Because corrosion control and prevention activities will always be necessary and will always be a top priority, military and commercial aviation managers constantly seek to streamline the process, reduce aircraft down time, and minimize costs.

A PRACTICAL APPROACH: AUTOMATED
FRESH-WATER RINSING

Though rinsing can be accomplished manually with hand-held hoses or other spray equipment, military regulations recommend the use of taxi-through fresh-water rinsing facilities, by which rinsing of the entire aircraft can be accomplished more effectively and far more quickly than by manual methods. The military further recommends that automatic, taxi-through rinse facilities, which use multiple jets to reach every part of the aircraft exterior, including parts difficult to reach with manual rinsing, “should be used as frequently as possible.”

Automated fresh-water rinsing facilities are a valuable supplement to cleaning operations, and offer many advantages over hand-rinsing operations, including:

  • Speed – While it can take several people many hours to (RINSE) wash an aircraft, an automated rinse facility can rinse an entire fleet at under five minutes per plane.
  • Effectiveness – While rinsing cannot replace washing, rinsing removes most of the salts and corrosive residues that build up between scheduled washes, thus reducing the time and effort needed for cleaning, an essential factor in large fleet operations.
  • Safety – During manual rinsing procedures, workers must climb on and around aircraft, carrying unwieldy equipment and hoses, and risking slips and falls on wet surfaces. Taxi-through rinsing systems can be operated remotely, thereby reducing the risk of worker injury.
  • Efficiency – Automated rinsing facilities capture, re-cycle, filter, and re-use rinse water. This is an efficient, cost-effective, environmentally-friendly approach for any facility, and an especially important consideration in desert environments.

RIVEER OFFERS PROVEN SOLUTIONS

The main proponent of aircraft washing in U.S. and global applications is Riveer. As a pioneer of clear water rinse systems and aircraft washing, Riveer has the performance data and experience gained from engineering, construction and operating permanent and semi-portable systems. This company’s Tactical Rinse System (TRS) technology, for example, offers inline automatic clear water rinsing for aircraft of virtually any configuration and complexity.

The pre-engineered rinse system, which operates entirely aboveground, can be quickly installed in a fixed location or as a temporary facility, and can be deployed rapidly to any location. Designed for easy entry and exit, the rinse deck area is (only slightly elevated above the ground allowing easy safe taxi through or tow through of all military aircraft) flush with the ground, custom-sized for the aircraft in use, and requires no significant site engineering.

Using Riveer’s fully-automated onboard Pilot Activated Rinse (PAR) feature, the captain can activate the system from the cockpit while the aircraft taxis to the rinse pad. Through multiple, automated, precision oscillating spray nozzles and (undercarriage spray nozzles arrayed in configurations to clean the undercarriage and wings) the rinse system drenches all sides of the aircraft with a flow of water calibrated to meet military and commercial requirements (of flow and pressure) for aircraft rinsing. The system can be adjusted to operate effectively with any type of aircraft and in varied weather conditions.

The Riveer rinse and filtration system efficiently and effectively rinses the aircraft, removing dust, salt, and other corrosive deposits. The TRS then collects the dirty rinse water; filters the water to remove salts, suspended solids, heavy metals, chemicals, microbes and other corrosive contaminants, recovering up to 80%; and stores the purified water for later use. The system can also be arranged to capture rain water, resulting in further savings.

Because the TRS is installed above ground, the time and expense required for infrastructure modification and associated site engineering or environmental considerations is eliminated. Importantly, the TRS is procured as kit not as a construction project, greatly simplifying the procurement process. The modular configuration consists of steel pad sections forming the wash rack, the recycling system and tanks are housed in special ISO containers. Since inlet water is automatically processed to standard by the recycling system all that is required of the user is electric power and a water source.

Riveer’s systems, which effectively and economically combat corrosion, provide superior results in military and commercial applications.

RESOURCES CONSULTED

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