Published: January 1999

 

They include water dripping onto passengers, electrical equipment failures, and wet insulation blankets. All airplanes will experience moisture-related problems, but the extent of these problems will vary tremendously among operators and among individual airplanes depending on how they are used. Boeing has studied moisture-related problems extensively and has developed recommendations for minimising moisture problems in its commercial airplanes.

All commercial airplanes that carry passengers will experience moisture-related problems in service. The chief source of moisture inside these airplanes is passenger respiration and the resulting condensation on the airplane skin. After working with operators to evaluate existing and proposed moisture-control methods, Boeing can now offer information to help mitigate the effects of moisture.

A Boeing team formed to address the moisture issue - known as "rain in the plane" - reviewed operator documentation on the subject and examined in-service airplanes with reported moisture problems. Operator reports identified where moisture problems were occurring and which operators were affected. The team then worked to develop cost-effective solutions for moisture control in all Boeing models, including out-of-production as well as current-production and future models.

The team developed these solutions after examining the following issues:

1. Root causes of moisture problems.
2. Service experience with moisture problems.
3. Available moisture-control systems.

Root Causes of Moisture Problems

When studying the origin of moisture problems, Boeing considered the following factors:

• Moisture sources and condensation.
• Drainage and dripping.
• Variables affecting condensation.
• Varying degrees of condensation and moisture problems across model fleets.

Moisture sources and condensation.
Most condensation on airplane structure occurs during flight when the temperature of both the outside air and the structure are very cold. Structure temperatures are usually below the dew point of the cabin air, causing some amount of condensation to form during most flights. In addition, because structure temperatures are normally below the freezing point of water, most condensation forms as frost (figure 1).

Condensation results when moist air moves to the cold structure (figure 2). The cabin air passes through small gaps in the insulation coverage and cools rapidly. Buoyancy forces induce a continuous flow of air and continuous movement of moisture to the cold structure.

The rate of condensation depends on the rate of buoyancy-driven air movement to the structure as well as the cabin humidity level. In-flight cabin humidity levels are low from a standpoint of human comfort (usually less than 20 percent relative humidity). However, the air is not completely dry, and any moisture it contains will condense as the air moves over the cold structure.

Drainage and dripping.
Frost melts rapidly during descent if conditions allow the airplane skin temperature to rise above freezing. This causes a sudden onset of drainage, which, if not managed completely, drips into the crown area (attic) of the airplane and possibly into the passenger cabin (figure 3).

The insulation blankets that cover the structure typically are fibreglass batting covered with waterproof non-metallic Mylar. This allows water to drain over the outboard Mylar surface similar to how rain drains over roof tiles or shingles. Ideally, all of the water flows to the bilge areas in the belly of the airplane, where it can drain overboard. However, some water may leak through gaps and drip into the crown and possibly into the passenger cabin. Some water may seep through unavoidable holes in the Mylar covering into the insulation blankets (figure 2).

Insulation blankets generally keep most of the water out of the airplane crown. However, a small amount of water may drip onto passengers or cause electrical equipment failures.

Variables affecting condensation.
The amount of condensation that forms depends on many factors, all of which belong to one of four categories (table 1):

• Airplane design/configuration.
• Airplane operations.
• Environment.
• Maintenance.

Airplane Design / Configuration	Configuration	Effect
	Seating Density	More people produce more moisture, causing higher cabin humidity levels and increased condensation rates.
	Insulation Design	An insulation design that minimises gaps will reduce condensation rates.
	Air-conditioning system design	The amount of outside air per occupant supplied to the airplane affects the in-flight humidity level. Increasing the outside air per occupant decreases the cabin humidity, which decreases the condensation rates.
Airplane Operations	Configuration	Effect
	Load factor (percent of available seats occupied)	More people produce more moisture, causing higher cabin humidity levels and increased condensation rates.
	Utilisation rate (hours per day the airplane is operating)	High airplane-utilisation rates result in more time during which the structure is below the dew point and subject to greater accumulations of frost on a daily basis.
	Mach number	High-speed flight results in aerodynamic heating of the structure. Higher Mach numbers will result in warmer structure temperatures and lower condensation rates.
	Cruising altitude	In general, the outside air temperature and the airplane structure temperatures will decrease with altitude. Higher cruise altitudes will generally result in higher condensation rates.
Environment	Configuration	Effect
	Air-conditioning system operation	For airplanes with overhead recirculation fans or crown ventilation systems, operating these fans or air-conditioning packs on the ground will help dry out the crown space.
	Outside temperature	Colder structure temperatures cause higher condensation rates. Colder structure temperatures on the ground inhibit the evaporation of moisture from wet insulation.
	Outside humidity level	Outside humidity level is not a major influence on condensation on structure. Most condensation on structure occurs during flight when the structure temperature is very cold and the outside air is very dry. In most cases, rate of condensation on structure will be much lower during ground operations than in flight, even if the outside humidity level is very high.
Maintenance	Configuration	Effect
	Insulation blanket installation	Gaps in insulation coverage created during maintenance can increase condensation rates. Damage to insulation cover material can increase moisture problems with wet insulation.
	Use of ground-based forced-air systems	Ground-based forced-air systems can be useful for drying airplanes parked for extended periods.

Condensation on structure and the resulting moisture problems are influenced heavily by seating density and airplane operations, especially load factors and utilisation rates. High passenger loads result in higher cabin humidities and higher condensation rates. High airplane-utilisation rates result in more time during which the structure is below the dew point or frost point and greater accumulations of frost on a daily basis. Some of the most severe moisture problems occur on airplanes with combinations of high seating density, high load factors, and high utilisation rates.

Varying degrees of condensation and moisture problems across model fleets.
The amount of condensation and the severity of resulting moisture problems vary dramatically across airplane model fleets. The variation in daily crown area condensation for the 757 fleet is illustrated in figure 4.

Service Experience With Moisture Problems

As part of its study, Boeing reviewed operator reports to learn where moisture problems were occurring and which operators were affected. Many operators have reported water dripping into the passenger cabin and problems with extremely wet insulation blankets.

Inspection of the upper surface of ceiling panels and stowage bins for water stains indicated that water was dripping through penetrations and gaps in the insulation blankets. Inspection also showed that water pooling on the upper surface of the ceiling panels and stowage bins (figure 5) migrated through joints into the passenger cabin.

Boeing conducted numerous in-service reviews to determine the scope of the moisture problem. As an example, while inspecting airplanes with the most severe moisture problems, Boeing weighed each existing insulation blanket on three 737-300 airplanes (figure 6). Comparing these weights with a new shipset of insulation blankets revealed that the removed blankets contained up to 80 lb (36 kg) of water per airplane.

Other service experience results showed that water dripping into electrical equipment has caused some failures.

Available Moisture-Control Systems

Because moist air will inevitably come in contact with cold structure, condensation cannot be eliminated. As a result, Boeing chose to evaluate potential moisture-control systems that can help operators accomplish the following:

• Minimise condensation.
• Minimise dripping onto equipment and into the passenger cabin.
• Maximise liquid drainage.
• Optimise evaporative drying from wet surfaces and insulation blankets.

Boeing used a test section of a 757 airplane in an environmental test chamber to simulate flight cycles. Over an extended period of time, the test section was used to evaluate frost levels, the amount of water retained in insulation blankets, and new moisture-control methods. Video cameras recorded frost formation, melting, drainage, pooling, and drip paths into the passenger cabin. Cameras were also used to evaluate the performance of some potential moisture-control methods: insulation types, water diverters and collectors, and evaporative materials.

In-service airplanes, including those equipped with alternative materials for water collection and evaporation, were also tested. Results of these in-service evaluations determined that proper placement of moisture-control devices is crucial for their performance.

An analytical model was created to simulate the buoyancy-driven airflow from the crown volume to the skin. The model also estimated the amount of condensation (frost) that forms on the structure. The model was validated using in-service data and lab testing and showed how gaps in insulation, structural temperature variations, and cabin humidity levels affect condensation.

The testing produced the following information to help Boeing and operators reduce moisture-related problems:

• Test results.
• Moisture-control system design recommendations.
• Maintenance recommendations.

Test results.
Testing and inspections revealed the following findings:

• All blankets have holes and penetrations. Attempts at sealing them during testing were not effective.
• The best way to reduce condensation is by eliminating holes and gaps around the insulation blankets next to the structure.
• Nomex felt is an excellent material for collecting and evaporating moisture.
• Dehumidification systems (ground-based or onboard) are effective at removing moisture but are not cost effective.
• Water will seep into the insulation blankets through holes, penetrations, and edge seams. A drainage path is necessary to allow the water to drain.
• Evaporation is required to dry wet insulation blankets.
• Spray-on insulation is too heavy to meet thermal and acoustic requirements and makes visual inspection of the structure difficult.
• Inspections of aging airplanes revealed that corrosion in the crown area is extremely unusual and should be eliminated by incorporating corrosion-inhibiting compounds in the crown.
• Fibreglass batting treated with an enhanced hydrophobic coating does not reduce water retention.
• Any type of system that increases cabin humidity will exacerbate the moisture problem.

Moisture-control system design recommendations.
Boeing determined that a system (figure 7), rather than an individual component, is required to effectively address a moisture problem. The system includes

• Insulation blankets.
• Moisture-control devices.
• Airflow systems.
• Structural drainage.
• Bilge trays.
• Electrical- equipment protection.

Insulation Blankets.
Key to controlling moisture, overlapped blankets (figure 2) and minimal gaps for structural supports can reduce air movement and condensation. Penetrations for wire runs, electrical brackets, and other equipment should be kept to a minimum. In addition, all blankets should have a drainage path.

Moisture-control Devices.
Nomex felt should be used to control water on ceiling panels (figure 8), stowage bins (figure 9), and structural penetrations. Active airflow will promote the evaporation of water collected in the felt.

Airflow Systems.
Onboard systems for ventilating the crown space will help control moisture problems. A crown ventilation system that provides a small portion of the cabin-supply air to the crown space will help reduce in-flight condensation and enhance drying of wet surfaces and wet insulation.

The addition of a crown ventilation system is not recommended for airplanes that have overhead recirculation fans as part of the air-conditioning system.

Structural Drainage.
Water drainage through holes and channels should be considered in structural designs such as stringers and intercostals.

Bilge Trays.
Bilge trays are sheets of moulded plastic (figure 10) intended to support the insulation blankets. Bilge trays should be used in the lower lobe of the airplane to keep insulation blankets away from any water that has travelled toward the drain valves.

Electrical-equipment Protection.
Equipment that is sensitive to wet environments should be protected or moved from these environments. Sealed electrical connectors should be used to minimise moisture entry and to reduce the number of system failures.

Maintenance Recommendations

Operators can take several steps to reduce moisture-related problems. These actions are related to

• Insulation blankets.
• Moisture-control methods.
• Bilge trays.
• Ground-based dehumidification systems.

Insulation Blankets.
Reducing exposed structure and excessive gaps between insulation blankets will decrease the amount of condensation that forms. Ensuring that blanket joint areas - whether butt joints or overlaps - are properly installed will also reduce the creation of condensation and subsequent dripping into the crown area. If the blankets are overlapped, drainage holes will remove most of the water and keep it away from the passenger cabin.

Maintenance personnel remove wet insulation blankets during maintenance checks and often wring them to expel water. This helps dry the blankets, but it also damages the insulative material, reducing the blanket's thermal and acoustic capabilities.

Moisture-control Methods.
A service letter has been distributed to all Boeing operators regarding the use of Nomex felt on ceiling panels and stowage bins. Applying Nomex felt to these areas will reduce the amount of water that could drip into the passenger cabin.

Bilge Trays.
Bilge trays provide better protection than strings and nets currently used in the cargo compartments of most airplanes.

Ground-based Dehumidification Systems.
Ground-based dehumidification systems can maintain very low humidity levels in an airplane. They can significantly enhance the drying of wet surfaces and wet insulation. However, a considerable amount of time is required to dry an airplane using these systems, and the airplane doors must be kept closed for the duration of the process. As a result, most operators are not likely to choose this method in their daily operations. However, the systems may be useful for drying airplanes parked for longer periods.

Summary Moisture in commercial airplanes is a complex issue, and its severity depends on many variables. Condensation on airplane structure is impossible to eliminate without prohibitive cost. However, Boeing has developed cost-effective methods for managing moisture once it has condensed that are both feasible and effective. The design improvements and other solutions recommended by Boeing were developed with assistance from operators and considered cost, weight, and ease of installation.

Paul Huber
Specialist Engineer
Payload Systems
Boeing Commercial Airplanes

Karl Schuster
Specialist Engineer
Environmental Control Systems
Boeing Commercial Airplanes

Rob Townsend
Moisture Control Team Program Manager
Environmental Control Systems
Boeing Commercial Airplanes

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