Accident Analysis

How Precipitation Precipitates Accidents

Nothing seems more innocuous than a light rain shower. While visibility may be reduced, aircraft performance doesn't generally suffer, and you might expect to fly through the precipitation as if nothing had happened. In reality, rain showers can foreshadow more dire circumstances and create unanticipated problems.

The pilot of a Cessna 177 was on a cross-country flight from Concord, New Hampshire, to his home base in Westerly, Rhode Island. For the first 50 miles or so, the weather forecast indicated marginal VFR conditions with ceilings below 3,000 feet and occasional light rain. South of Nashua, New Hampshire, conditions were expected to improve to VFR with scattered clouds and occasional scattered precipitation.

Passing Nashua and clearing the overcast area, the pilot and his passenger anticipated smooth flying for the remainder of the trip. But as they approached southern Rhode Island, visibility deteriorated in an area of scattered showers. The pilot noted that engine power was decreasing and pushed the throttle forward several times to maintain the desired manifold pressure.

Struggling to navigate by pilotage in the decreased visibility caused by the rain, the distracted pilot finally noticed that the engine was running rough. He applied carburetor heat, and the roughness increased. Fearing the engine might quit completely, he began searching for an open field in which to land. The engine roughness subsided, and the pilot landed safely at Westerly State Airport.

Although the pilot was able to complete the flight safely, this report highlights two important considerations. First, even light rain showers can markedly decrease visibility, increasing the pilot's workload and making it difficult to navigate. Second, the increase in relative humidity associated with rain often predisposes the engine to develop carburetor ice. If carburetor heat is not applied, an engine failure may occur, requiring an off-field landing.

While pilots sometimes feel more confident navigating around showers, the conditions that result in showery precipitation are often ripe for the development of thunderstorms. While rain showers themselves may not represent an unmanageable hazard, thunderstorms may. And, because these storms often are embedded in areas of clouds and rain showers, they may be impossible to see and difficult to avoid.

The pilot of a Cessna 172 and his instructor had departed Connecticut's Hartford-Brainard Airport on a cross-country training flight to Albany, New York. The weather for the August day included scattered rain showers and the possibility of thunderstorms - a common forecast for the region on summer afternoons. The flight was being conducted as an instrument training flight under an IFR flight plan. However, the weather briefing suggested that most of the flight would be VFR, and thus the student would be under the hood.

About 40 miles from Albany, the aircraft encountered rain showers. The pilot maintained his assigned heading and altitude and proceeded toward the Albany area. As the aircraft continued, turbulence increased, and the pilot was advised of areas of heavy precipitation and possible thunderstorms. The instructor requested vectors around the heavier precipitation, and the controller suggested a new heading.

By this time, the turbulence was becoming worse, and the aircraft encountered strong updrafts and downdrafts. The instructor advised air traffic control (ATC) that he was having trouble maintaining altitude, and the controller requested that the pilot report deviations from his assigned altitude. As the aircraft entered a strong updraft, the student retarded the throttle to idle and pitched the nose down to maintain maneuvering speed. With the aircraft still climbing at a rate of 1,000 feet per minute in that configuration, the instructor advised ATC that they were unable to maintain altitude and were climbing. The aircraft had now entered an area of torrential rain and thunderstorm activity, and ATC advised the pilots that radar contact was lost.

A Piedmont airliner in the area had been listening to the exchange between ATC and the C-172. The flight crew was able to locate the aircraft on their radar and offered assistance in the form of vectors around the cells in the area. A short while later, ATC radar contact was re-established. The C-172 emerged from the precipitation undamaged and landed without incident in Albany.

Pilots are well advised to avoid areas of precipitation, especially if thunderstorms may be embedded within them. This is particularly true at night when clouds and precipitation may be impossible to see.

Even when you can see and stay out of rain, it's worth remembering that the effects of rain and thunderstorms can extend far beyond the area in which rain is falling.

It was an early afternoon in July when the private pilot of a Piper Cherokee departed Aretz Airport in Lafayette, Indiana, on a VFR flight to Meigs Field in Chicago. Shortly after departure, the pilot was advised of heavy rain along his planned route of flight. When the pilot saw the heavy rain looming along the path to his destination, he decided to return to Lafayette before the weather closed in.

Arriving back in the Aretz Airport area, the pilot commenced a full-flap landing on the field's Runway 25. As the airplane neared touchdown, a gust of wind pushed the it off the left side of the runway. The pilot added full power and retracted one notch of flaps, but was unable to maintain directional control. The airplane flew across a soybean field and crashed in a line of trees.

The gusty wind conditions were probably related to the other inclement weather in the area. And while gusty winds by themselves can make for a tricky landing, a wet runway can complicate a situation further by reducing tire friction or inducing hydroplaning. On a wet runway, stopping distance increases and crosswind capability is diminished.

Passing through a light rain shower may seem to be of little consequence, and often it is. But before you launch into the rain, be sure you have the whole weather picture. Potential hazards like reduced visibility, turbulence, and gusty winds, may be lurking near those rain clouds.

What Precipitation Echoes Reveal

Air traffic control will often provide pilots with in-flight information about precipitation. If you've ever used flight following, you may have heard references to precipitation intensities. This information is based on the echoes identified by ATC radar. Precipitation intensity is measured by the Video Integrator and Processor (VIP) and reported as a numerical value from one to six. While these values primarily classify the rate of precipitation, they also correlate to other phenomena, including turbulence, thunderstorms, and hail, as indicated below.

Based on this information, pilots without weather radar should avoid areas where precipitation echoes are greater than Level 2. Even in Levels 1 and 2, there is a possibility of lightning. Also, remember that cumulonimbus clouds can develop quickly and precipitation levels can increase very rapidly.

Although there is no good direct correlation between visibility and precipitation echoes, it is well known that precipitation can severely reduce visibility. In anything other than light precipitation, pilots should anticipate that visibility may at any time drop to near zero.

Precipitation Pointers

Here are a few important reminders about the potential hazards of precipitation:

1. Even light rain can reduce visibility substantially and may be associated with turbulence.
2. Areas of rain may contain embedded thunderstorms. Check the forecasts and get up-to-date information from flight service or ATC.
3. The decrease in temperature/dewpoint spread associated with rain is conducive to the formation of carburetor ice.
4. If the temperature is at or near freezing, rain may quickly form clear ice, dramatically reducing lift.
5. The area surrounding heavy rain showers and thunderstorms may be subject to gusty wind conditions, making landing a tricky proposition.
6. Wet runways increase landing distance and may diminish the crosswind capability of the aircraft.
7. Always check the maximum elevation figures (MEFs) listed on the sectional chart for your route of flight, and don't paint yourself into a corner trying to navigate around showers.
8. Don't rely on ATC to keep you out of threatening weather conditions. ATC radar is not designed specifically to observe weather.
9. Although you may be able to fly around isolated rain showers in the daytime, remember that you won't be able to see them at night. Avoid potentially rainy conditions when planning a night flight.