Flying in the Rain - A Human Factors Perspective

By Gene Benson

Rain is, of course, a very necessary part of our existence on this planet. It provides moisture for our vegetation, drinking water for ourselves and our animal food sources, and cleanses our atmosphere. The list of essential benefits derived from rain is endless. That is, until we begin to contemplate flying through rain.

Nonsense some say. "The airplane doesn't care whether or not it is flying through rain," say many seasoned aviators. While that statement isn't entirely true, we'll let it stand for now. Even if the airplane was not affected by flight in rain, the pilot is significantly influenced both psychologically and physically.

Let's examine the psychological factors first. Most light airplanes have windshields made of Plexiglas or Lexan. While reasonable strong and light weight, the sound deadening ability of these materials is virtually non-existent. Flight in heavy rain is roughly the acoustical equivalent of spending time beneath Niagara Falls protected by an umbrella. Snug fitting headsets help some and noise canceling headsets make a significant difference, but there is still substantial rain noise. Any prolonged exposure to noise takes a toll in fatigue. A half-hour of flight in heavy rain can produce the same fatigue normally experienced in a two hour flight in clear air.

Even beyond the fatigue factor, a claustrophobic sense may be experienced. Most pilots have learned instrument flight under the hood or Foggles. While the technical skills can be mastered in this way, the knowledge that the view limiting device can be removed at any time is quite comforting. The incessant beating of the rain on the windshield serves as a constant reminder to the pilot that this is the real thing. He or she must persist in the task of flying the airplane on instruments. There is no quick relief from this very demanding task. Sometimes the pilot, particularly if newly rated without benefit of previous experience in rain, finds himself or herself battling back waves of panic. The onset of the encroaching panic is not brought about by any real emergency or even by the particularly heavy workload of single pilot instrument flight. It is simply induced by the knowledge that there is no quick escape from the present flight conditions.

Another psychological factor which may be very minor, but nevertheless a factor is a minor windshield leak. Flight through rain causes the water to strike the windshield from a different direction than when the airplane is parked on the ramp. Sometimes small leaks that weren't previously noticed manifest themselves in drips or fine sprays in the cockpit when water strikes the windshield at over a hundred knots. It isn't likely that any damage will be caused by these pesky leaks, but a distraction is most certainly introduced and, perhaps even more importantly psychologically, there may be a feeling that this seemingly hostile environment is invading the supposedly safe space of the cockpit.

Physical factors can also influence the pilot flying in rain. Since rain at altitude is almost always cold relative to the temperature of human breath, it is almost always necessary to run the airplane's defroster when flying in rain. Most of today's automobiles are equipped with air conditioning which allows the driver to dry the air in the passenger compartment. This luxury is not found on most light airplanes. The defroster on most airplanes works by simply ducting engine heat to the windshield and evaporating moisture through air which has been made warm enough to be relatively dry. The result, particularly in flight through summer rain, is an uncomfortably warm cockpit. If the pilot is already in a somewhat elevated state of anxiety, the addition of this temperature factor will make for a quite uncomfortable situation and will also serve to accelerate fatigue. Uncomfortably warm temperatures can also serve to induce motion sickness in passengers. When a sick passenger is added to the concerns of an already anxious pilot, the limit of the pilot's ability to function may be approached.

At least one physical factor may be added before the flight even begins. This one is proportional to how conscientious the pilot. A thorough preflight inspection of the airplane conducted in moderate rain will yield a drenched pilot. Initially, the pilot may not be aware of how wet clothing has become. The water might not have soaked through all the garments yet, or the excitement of beginning the flight might prevent the pilot from noticing the degree of dampness. The seemingly hostile environment of flight in rain, coupled with the extreme discomfort of sitting still in clothing which has now completely soaked through to the skin, can accelerate fatigue and possibly even influence decision making in an adverse way.

Since this article is primarily concerned with human factors, we will not delve into the physics of flight in rain. However, we would be remiss if we did not challenge the statement that the airplane doesn't know it is flying in rain.

Aircraft and engine performance is reduced when flying in rain. The engine produces less power because the air taken for combustion is less dense than dry air at the same temperature. The mixture will therefore be too rich, or the engine will have to be leaned to compensate for the reduced air density. Flight in rain is an open invitation for the formation of carburetor ice. If carburetor heat must be applied continuously, the mixture will again need to leaned and the result will be less power.

There will be more drag when flying in rain. Since water is more dense than air, more energy must be expended to move a raindrop out of the way than to move the corresponding volume of air. This is a relatively minor factor, but can result in the loss of a couple of knots of airspeed.

While some pilots are not comfortable in flying in moderate or heavy rain, no pilot is comfortable flying in freezing rain. It is essential to know the big weather picture before flying in any kind of precipitation. For example, a pilot flying in rain beneath a temperature inversion must closely monitor the outside air temperature (OAT). Air along the inversion having a temperature in the low single digits (degrees Celsius), along with the OAT decreasing toward the 0°C mark, might mean that freezing rain is just ahead. The simple solution is to turn back.

The intent of this article is certainly not to discourage flight in rain. Quite the contrary, being able to conquer the elements adds versatility to the pilots skills. By understanding the factors involved in this realm of flight, the pilot will be better equipped and prepared for the situations, both real and perceived as they are encountered.