By Joe Escobar
Much time is spent combating the effects of moisture on an aircraft. Every manufacturer has some type of preservation schedule in place for their aircraft and engines when not in service. Most require some type of desiccant and barrier to prevent moisture-related problems during long-term storage. There are even de-humidifiers that are manufactured that can treat hangars or even be directed at aircraft cabins or engines to help combat corrosion problems. But what about moisture problems associated with everyday utilization?
Moisture in the cabin overhead and sidewalls is a problem on most larger aircraft. Even without severe moisture introduction such as with a leaking window, the aircraft can accumulate significant amounts of moisture.
Aircraft manufacturers are aware of the problems that moisture can cause. Boeing published an article in its Aero Magazine No. 5, January 1999, titled "Controlling Nuisance Moisture in Commercial Airplanes." They formed a team to address the moisture issue in aircraft. The team reviewed operator documentation and looked at in-service aircraft that had reported moisture problems.
Boeing considered the following factors in relation to moisture problems:
• Moisture sources and condensation
• Drainage and dripping
• Variables affecting condensation
• Varying degrees of condensation and moisture problems across model fleets
Moisture sources and condensation
Condensation formation is caused by an aircraft’s operating environment. As an aircraft ascends to higher altitudes, the temperature of both the outside air and the structure becomes very cold. Because the temperature of the skin is usually lower than the dewpoint of the interior air next to it, condensation forms as water, frost, or ice. Since the cabin air passes through gaps in the insulation coverage and is continuously moving over the cold structure, more and more condensation forms. The amount of condensation that forms is directly related to the amount of air circulation and the amount of humidity in the air. As circulation and humidity increase, higher amounts of condensation form.
The frost that forms while the aircraft is at altitude melts as the aircraft descends (if the temperature of the skin gets above freezing). Theoretically, the moisture should drain out of the aircraft (through belly drains for example). But in the real world, the sudden onset of moisture drainage can drip into the crown area of the aircraft, possibly even dripping into the passenger cabin. Hence, the phenomenon known as "rain in the plane."
Moisture can also become trapped in areas of the overhead and sidewall. The fiberglass insulation blankets that are typically used in these areas are covered with a waterproof barrier. However, tears in the barrier material can allow moisture to soak in to the insulation blankets. This can add a significant amount of weight to the aircraft. In addition, trapped moisture can lead to the onset of corrosion. It can also cause electrical malfunctions either as a direct result of the moisture, or as a result of the corrosion on the electrical components.
Variables that affect condensation
The Boeing team addressed four categories that affect the amount of condensation that forms. These are:
• Aircraft design/configuration
• Airplane operations
Aircraft operations have a significant part in moisture formation. Condensation and associated moisture problems are affected greatly by seating density and aircraft operations. Load factors and utilization rates have an especially high influence on condensation formation. The higher the utilization rate, the more time the structure is at the lower temperatures that cause the condensation formation, allowing for more moisture formation. In fact, aircraft with a combination of high seating density, high load factors, and high utilization rates tend to have the most extreme moisture problems.