August 6, 2000
Airline passengers often complain about the
debilitating effects of the dry atmosphere in aircraft cabins. Some also
express concern that reduced airflow and increased recirculation of cabin air
can increase the likelihood that infectious diseases are transmitted. And the
same is true for flight and cabin crews. In fact, maintaining a dry atmosphere
in the aircraft cabin is critical to the continued safe operation of the
airplane. Further, eliminating moisture helps to minimize the transmission of
Not A New Problem
40 years aircraft manufacturers have been trying to deal with problems
stemming from water condensation inside the cabin. Air conditioning is
provided by engine bleed air that is drawn from the engines, cooled,
dehumidified and passed into the pressurized envelope. At cruise altitude the
skin of the aircraft cools rapidly. Insulation blankets prevent the
transference of these cool temperatures into the cabin; however, small gaps in
the blankets inevitably develop and some cabin air always manages to come in
contact with the cold aircraft skin. Despite the fact that most of the water
is removed from the air before it is passed into the cabin for air
conditioning, passenger respiration adds moisture. The moist air that comes
into contact with the cold aircraft skin then condenses and forms a layer of
frost. When the aircraft descends, the skin temperature increases and the
frost layer melts rapidly. The hull is designed for this water to drain
overboard; however, if gaps in the insulation blankets are large enough, some
water may drip into the cabin interior or infiltrate into the blankets
Boeing has conducted extensive studies of various types of aircraft used by
different operators and found the rate of condensation that occurs depends not
only on cabin humidity levels, but on buoyancy rates. Buoyancy effect, or
stack rate pressure differential, is the difference in relative pressure
between areas behind and in front of insulation blankets. This pressure is
slightly negative near the ceiling of the cabin and slightly positive near the
floor. As a result, most frost forms near the crown of the cabin.
Other factors that generally increase condensation rates are load factor
(more people equals more respiratory moisture), high airplane utilization
rates (resulting in more time the aircraft structure is subjected to cold
ambient temperatures), low cruise mach numbers (less ram rise), and relatively
low cabin airflow rates and altitude (higher altitudes usually mean lower
ambient temperatures.) Interestingly, outside atmospheric humidity levels seem
to have little influence on this phenomenon. Therefore, some of the most
severe moisture problems occur on aircraft with high-density seating, high
load factors and high utilization rates.
Adverse Effects Of Cabin Moisture
blankets that cover the aircraft structure are typically fiberglass batting
covered with waterproof, nonmetallic Mylar. Generally, water drains over the
Mylar surface in a fashion similar to how rain drains over roof shingles.
Ideally, this moisture drains into bilge areas of the fuselage and then is
vented overboard. However, if there are gaps between blankets, or if some
blanket surface damage has occurred, water may drip into the cabin or
accumulate in the insulation blanket itself. If some of the accumulated water
drips on passengers, this is often a source of complaint; however, if
sensitive electrical equipment is exposed to dripping water, more serious
repercussions can and will occur. Wet-arcing has been cited as the cause of
various operators' electrical problems. On one 737-300 that Boeing studied,
the difference in weight between its existing insulation blankets and a set of
new, drier ones was found to be approximately 80 pounds. These older,
waterlogged blankets can increase operating costs due to added weight, reduce
the blankets' insulating properties and create a breeding ground for bacteria
and fungi. Accumulated water can also exacerbate corrosion and decrease the
service life of the airframe.
In the past, some maintenance technicians have wrung out insulation
blankets in order to drain accumulated water; however, this remedy causes
irreparable damage to the fiberglass material and further decreases the
blankets' effectiveness. Damaged blankets with accumulated water need to be
replaced with serviceable units.
virtually impossible to eliminate moisture from aircraft cabins. However,
based on extensive studies and testing, Boeing has determined that a number of
maintenance and procedural steps can be taken to reduce potential damaging
effects from cabin moisture. Boeing suggests that the best way to minimize
condensation is to eliminate holes in insulation blankets, overlap them during
installation and reduce gaps between blankets and aircraft structure.
Unrestricted flow channels for water to drain should be assured and plastic
bilge trays added in the lower section of the fuselage to prevent insulation
blankets from coming into contact with draining water. Addition of a
ventilation system that directs a small portion of cabin supply air to the
crown space of the fuselage is beneficial in reducing condensation and drying
wet blankets. However, this is not the case in aircraft equipped with overhead
re-circulation fans as part of the air conditioning system. Nomex-based felt
can be added to the upper surfaces of stowage bins and other ceiling areas to
absorb water before it enters the cabin. Water evaporation from Nomex felt
panels occurs whenever there is active airflow in the cabin.
Boeing also concluded that one seemingly obvious solution of applying an
adhesive insulation material to the interior surface of the airframe would not
only be weight restrictive, but would also prevent adequate inspection of
metal surfaces for corrosion and fatigue failure.
used to have a cockpit humidifier on their 747 Classics that was turned on and
off by the flight engineer when passing through 10,000 feet on ascent and
descent. When operating with Qantas, I routinely witnessed condensation rates
in the cockpit that resulted in water dripping from metal frames around
cockpit windows during descent. Often it was necessary to place towels on the
glare-shield or around the front windows in order to prevent water dripping on
pilots' legs and/or instruments. Now I fly with "another airline"
and cockpit humidifiers are not included in its 747 Classics' air conditioning
systems. I haven't noticed any significant difference in how I feel after a
long flight without a cockpit humidifier. Therefore, I wonder whether
humidifying cockpit air is worth the potential trouble dripping water could
cause. It would be interesting to know if electrical failure rates and
corrosion problems have been greater in Qantas airplanes versus other
operators, such as this "other airline."
What's This Mean To Passengers And Us?
It is easy
to understand how passengers wonder why they sometimes get rained on during
descent while the atmosphere in the cabin generally has humidity levels below
those found in the desert. As a consequence, many passengers and crew members
complain about stagnant air, incessant thirst and physical discomforts, such
as red and itchy eyes.
"Will I catch the flu?" they sometimes ask after witnessing a
nearby passenger sneezing.
"What was that funny smell in the cabin during take-off?"
"It's too cold ... too hot ... too stuffy."
(They think they have it bad? This is our office!)
In reality, however, passenger questions such as these are reasonable and
deserve an honest answer.
One fact has remained constant over my career: After a long flight, I feel
badly no matter what I do. However, there a number of steps that can be taken
to ensure that both crew and passengers feel as well as one can in these
circumstances. I have found that the effects of jet lag are less debilitating
when I drink a sufficient amount of fluids during a flight, do not drink
excessive amounts of a diuretic -- such as coffee -- and regularly dampen my
nasal passages with water. For the latter, I have found that breathing through
a wet towel for a few minutes every hour helps. (Perhaps you have other
remedies you could suggest to passengers in order to make them feel more
The amount of fluid intake necessary on a long flight depends on each
individual; however, one doctor advised me that if you need to urinate every
couple of hours, then your body hydration level is probably adequate. Some
people also recommend that immersing your body in water, such as a bath or
swimming pool, can help in the re-hydrating process after a long flight .
contaminants in cabin air, such as ozone, burned and unburned combustion
hydrocarbons, other gases and toxic particulate aerosols, engine lubricating
oil seal leakage, hydraulic fluid leak ingestion and deicing fluid ingestion
have attracted much attention over recent years. However, in normal aircraft
operations, the air in the cabin is usually as safe, if not safer, than that
found in many large cities. One added benefit of in-flight cabin air over city
air is that cabin air is completely changed every two to six minutes. However,
if a malfunction occurs in an aircraft system and noxious fumes are ingested
into the air conditioning airflow, cabin air quality degradation can have
severe, adverse effects on passengers and crew -- thankfully, these occasions
are relatively rare. In these circumstances, quick reactions by pilots and
flight engineers to isolate the cause of the contamination are imperative.
Some passengers and travel industry affiliates have also expressed concern
about the possibility of spreading infectious diseases in flight, particularly
when air conditioning pack operations are reduced and subsequent lower air
exchange rates exist. After conducting a study in 1998, the American Medical
Association (AMA) concluded that under usual aircraft procedures, cabin air
quality does not represent a significant risk for transmission of infectious
diseases. Passengers should be reminded that risk of infection is an everyday
occurrence, but that this risk is most likely lower on an airplane due to the
constant, total exchange of cabin air. I have no empirical evidence to prove
my theory; however I think that the risk of infection is probably greater in
other public areas, such as elevators, stores and offices -- or even airport
terminals and waiting areas.
The bottom line is that passengers and crew have to endure an air
conditioned aircraft cabin in order to travel from one point to another. Being
armed with the facts may assist crew members in allaying passenger fears about
detrimental effects of dehydration and contaminants. However, it should also
be realized that some people may never be satisfied with rational
explanations. Conspiracy theorists are everywhere and paranoia about the water
we drink and the air we breathe is rampant. Therefore, for some people, no
amount of reassurance will convince them they will not catch the Bubonic
plague or shrivel into an unrecognizable mass during the course of their
For this category of person, the alternate actions for crew members might
be to use their sense of humor ... give them a bottle of water and tell them
to hold their breath for the rest of the flight.