Controlling Nuisance Moisture in Commercial Airplanes

Moisture-related problems have been reported on commercial airplanes for the past 40 years. 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 minimizing 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.
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 (fig. 1).

Condensation results when moist air moves to the cold structure (fig. 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 (fig. 3).

The insulation blankets that cover the structure typically are fiberglass batting covered with waterproof nonmetallic 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 (fig. 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):

Condensation on structure and the resulting moisture problems are influenced heavily by seating density and airplane operations, especially load factors and utilization rates. High passenger loads result in higher cabin humidities and higher condensation rates. High airplane-utilization 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 utilization 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 (fig. 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 (fig. 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:

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.
Testing and inspections revealed the following findings:

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

Key to controlling moisture, overlapped blankets (fig. 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.

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

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 recir-culation fans as part of the air-conditioning system.

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

Bilge trays are sheets of molded plastic (fig. 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 traveled toward the drain valves.

Equipment that is sensitive to wet environments should be protected or moved from these environments. Sealed electrical connectors should be used to minimize 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

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.

A service letter (see sidebar) 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 provide better protection than strings and nets currently used in the cargo compartments of most airplanes.

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.

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.

Boeing Service Letter
Boeing issued the following multipart service letter regarding the installation of felt to prevent water from dripping into the passenger cabin of 707, 727, 737, 747, 757, 767, and 777 models. The letter was issued on Jan. 16, 1998.

Table 1

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 minimizes 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.
Utilization rate (hours per day the airplane is operating) High airplane-utilization 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.

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.

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.











Paul Huber
Specialist Engineer
Payload Systems
Boeing Commercial

Karl Schuster
Specialist Engineer
Control Systems

Boeing Commercial

Rob Townsend
Moisture Control
Team Program Manager
Control Systems

Boeing Commercial

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