Aviation Safety Corner
this Month's Topic, I would like to Discuss, "The Deadly Deluge." Research has shown that heavy rain
can rob an aircraft of lift quickly and without much warning. This is Part I of IV.
Now, Let's start our Discussion with some accidents. As you read the brief descriptions of the following accidents, a common thread will emerge:
1. On July 27, 1970, a Flying Tigers DC-8 was on approach to Okinawa. Witnesses on the ground observed the aircraft emerge from heavy rain shower at a very low altitude just before it struck the water.
2. On July 23, 1973, an Ozark Fairchild FH-227B was approaching St. Louis, MO. At 1742:31, the local controller said, "Ozark 809, it looks like a heavy rain shower moving right across the approach end of the runway now." The first officer replied, "Roger, we see it." That was the last transmission from the flight. The cockpit voice recording stopped at 1743:24.
3. On November 27, 1973, a Delta DC-9 on approach to Chattanooga, TN developed an excessive rate of descent despite two verbal reports of increasing sink rate by the first officer. The NTSB's accident report concluded that the Captain disregarded the report by the First Officer because of the possible "influence of a visual illusion created by the refraction of light through the heavy rain on the windshield."
4. On November 12, 1975, Eastern AirLines Flight 576, a Boeing 727, was flying through light to moderate rain during an ILS approach to Raleigh, NC. At 200 feet, the aircraft encountered heavy rain. The Captain noticed the aircraft go below the glide slope and applied power. He described the situation as encountering "a wall of water" and as having "the bottom fall out" as he added thrust. He further recalled that the windshield became "opaque" and the external light glare became brilliant." The aircraft struck the ground about 282 feet short of the runway and slid to a stop. The Flight Data Recorder showed descent rates as high as 1,400 fpm during the last 3.6 seconds of flight. (The recorded rate of rainfall was more than two inches per hour).
5. On June 23, 1976, an Allegheny Air Lines Flight 121, a DC-9, on approach to Philadelphia, PA, was observed by another air carrier Captain holding on a taxiway. This Captain said that the rain was heavy and that he saw Flight 121 when the aircraft emerged from the rain 75 to 125 feet above the ground. He stated further that the aircraft appeared to "stop flying" and "descended to the ground with the nose up". The DC-9 crashed on the runway 4,000 feet beyond the threshold.
Now, Let's Discuss, "Common Thread". The common thread in all of these accidents is the presence of heavy rain. This does not mean that the heavy rain caused the accidents (correlation is not causation): but it certainly merits attention as a possible factor. Like other pilots who have been caught in an in-flight deluge, I have wondered about the effects of heavy rain on the aircraft's aerodynamics and contemplated what type of safety margin, pilots should add when operating in heavy rain. Unfortunately, there are no ready answers.
Several teams of researchers have examined a number of commercial aircraft accidents, and they have raised the possibility of heavy rain as a causal factor. When we looked at the number of accidents ascribed to wind shear, we found that only a small number of the investigations considered the effects of heavy rain on aircraft performance.Yet, scientific findings suggest that heavy rain creates a very distinct hazard to flight operations. You may wonder why no light aircraft accidents have been attributed to the effects of heavy rain.
The answer is that while accident investigation teams comprise very motivated individuals, their resources are limited. The NTSB doesn't have the resources to thoroughly and painstakingly pour hundreds and thousands of man-hours into the crash of a light aircraft. Additionally, accident investigators can't be expected to know everything. The research on heavy rain effects to date has been limited to a rather small group of very talented researchers, and their work has received small notice by the industry.
Now, in conclusion, Remember that All Pilots should be alert to the possibility of a significant increase in descent rate and decrease in airspeed when penetrating a heavy-rain cell.
Don't miss next month's, Aviation Safety Corner, when we'll discuss, "Shear or Heavy Rain?" This will be Part II of IV.
Larry G. Harmon
FAA AVIATION SAFETY COUNSELOR
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