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If there’s one thing we have in good measure out here in lower BC it’s rain. I can’t speak with authority for the rest of the country, but I suspect rain is to be found in other jurisdictions, as well. Even as I write these words, I glance outside and what do I see? Good guess: rain falling gently to the ground, soaking into the grass verges, pooling on the runway and taxiways. All this lovely green in British Columbia doesn’t come without a price. Aircraft operations are affected by rain in a number of ways, and it is a very good plan to understand the challenges presented. Visibility is reduced and distorted. Take off and landing rolls are increased. Control problems may occur. Opportunities for the formation of carburetor ice are greatly increased. One option, of course, is to leave the aircraft tied down whenever any sign of rain threatens. This may well be a good option for many recreational, particularly low time pilots. It is important to know your personal limits and to stick by them. Another option is to develop the necessary base of knowledge, skills and experience to reduce the risks of operating in the potentially difficult conditions rain produces. Rain affects a pilot’s vision in three major ways, all of them negatively. The most obvious way is to limit both the quantity of light available to the pilot’s eyes and the forward distance a pilot can see. Rain is almost always associated with cloud layers that reduce the overall amount of ambient light available. Before commencing any flight, confirm that the required minimum visibility conditions exist and are forecast to exist for an appropriate time period. Temperature and dew point spreads can be very important information for flight planning in marginal conditions. Rain on the windscreen of our aircraft also produces the two very interesting effects of refraction and diffusion, both of which can have serious consequences if not properly understood. All of us have, at one time or another, stuck our hand or a stick into water and seen the hand or stick “bend”. This phenomenon is called refraction. Light, passing through the medium of water, changes direction, giving an illusion that the object we are looking at is displaced from its actual position. Refraction of light through the medium of water can result in our seeing objects outside at a different altitude than they would normally appear. Tops of hills, other obstacles and the horizon itself will appear lower than they actually are. The general rule of thumb is that objects half a mile ahead of our aircraft may appear to be as much as 200 feet lower than actual, roughly a 4.5º refraction (tan ? = 200/2640). If you want to really appreciate this illusion, drop a penny in a sink full of water and try to pick it up with one grab, first try. This phenomenon is important to understand both for terrain clearance and during landings and take offs. I always try to take student pilots flying in the rain at some point during their training so they can experience first hand the illusions resulting from rain on the windscreen. It’s one thing to talk about things in a theoretical way; it’s another entirely to experience first hand at 100 kts. Light passing through water as well as being refracted is diffused, or spread apart. This results in two interesting effects: lighted objects in the distance will appear less bright. Diffusion of the light energy reduces the apparent intensity of the source, and the lighted object will appear farther away than it actually is. Lighted objects, such as runway lights seen at close range, because of the diffusion of their light, will appear larger than they actually are; our poor, little brains will be tempted to interpret this information to mean that the lights are closer than they actually are. Take off and landing distances are affected by standing water on the runway. We are all familiar with the take off and landing performance charts or graphs or tables for the aircraft we normally fly. Landing on a grass runway, for example, can increase our landing roll by 45% or more. While specific numbers are probably not provided in your POH for calculating landing distances on wet runways, the James Brake Index (JBI) gives us valuable, performance information. The JBI is a measurement system used to indicate how slippery a runway is as a result of water, snow or ice. A clear example, with explanation, of a JBI chart is provided in section E of your Canadian Flight Supplement. A JBI reading of 1.0 indicates maximal effective braking; readings between 1.0 and 0.8 indicate the best possible braking conditions: a bare, dry runway. Equivalent braking action would be described as Good. Damp conditions with 0.01” to 0.03”of water on the runway surface would give us a JBI of 0.6 to 0.3. Equivalent braking action would be described as Fair to Poor: an increase in landing roll of between 45%-99% can be anticipated. Heavy rain with 0.03” to 0.1” of standing water will result in a JBI reading of 0.3 to 0.0: an increase in landing roll of 100% or more can be anticipated. Equivalent braking action would be described as Poor to Nil. Control problems can result from the presence of water on the take off and landing surface through the process known as hydroplaning. There are three types of hydroplaning and it is useful to be familiar with all three. Dynamic hydroplaning may occur when there is standing water on the runway surface. The standing water exerts a pressure between the aircraft tires and the runway surface causing the tires to lift. They lose their contact with the surface. When dynamic hydroplaning occurs, steering is not effective, braking may be ineffective and may, indeed, lead to worse difficulties and strong crosswinds may simply toss an aircraft off the runway. Until the aircraft has slowed below the hydroplaning speed we may resemble a pig on ice more than we might ideally choose. Even though we are in contact with the ground our control of the aircraft will be produced by use of our flying surfaces: aileron, elevator and rudder rather than the landing gear steering system. A rule of thumb for predicting dynamic hydroplaning speed in knots is given by the formula 8.6 x vtire pressure, psi. A tire pressure of 25 lbs psi will result in a minimum dynamic hydroplaning speed of 43 knots (1). At any speed above 43 knots, we can expect to experience dynamic hydroplaning with this aircraft, both on landing and taking off. Viscous hydroplaning can occur on areas of the runway where it has been painted or where there are rubber deposits from landing aircraft. Paint or rubber deposits make the runway slick and moisture creates a film over the slick surface resulting in loss of tire traction. Skidding may occur, resulting in loss of control. Viscous hydroplaning may occur at a much lower speed than dynamic hydroplaning and can cause control problems even while taxiing. Reverted rubber hydroplaning is a condition that is created through improper use of brakes. If we apply brakes at touchdown on a wet runway, the aircraft will enter dynamic hydroplaning: the wheels will lock. As the aircraft slides down the runway, water between the tires and the runway surface will heat up and a layer of steam and molten rubber will develop between the tire and the surface. We are now riding down the runway on a layer of very slippery material we create as we go. A serious danger of reverted rubber hydroplaning is that the condition can continue down to virtually zero speed and seriously affect our ability to control the aircraft. Carburetor icing in rainy conditions can be a safety factor if you fly an aircraft with a carbureted fuel system. I think we might leave an in depth coverage of this topic for another time, but suffice it to say, carburetor icing is something to be very, very alert for when flying in damp and rainy conditions. Learn the symptoms and the appropriate procedures for your particular aircraft. Know them well. Since we do get our share—some would say more than our fair share—of rain, it’s a good plan to either learn to deal with it effectively or to stay on the ground when it is or threatens to be rainy. The decision to remain on the ground in rain conditions can be a very good one, particularly if you have not developed the necessary base of knowledge, skills and experience. If you do choose or must fly during periods of rain, tackle the problem: learn the illusions, performance limitations and challenges and find a safe way to explore the reality first hand so you can gain the necessary experience. Enjoy the challenge. End Notes 1. Kershner, William, The Advanced Pilot’s Flight Manual sixth Edition, Iowa State University Press, 1994
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