Aviation Safety Corner
this month's topic, I would like to continue our discussion on, "The Deadly Deluge." Research has shown
that heavy rain can rob an aircraft of lift quickly and without much warning. This is Part III of IV.
Now, let's start our discussion with "Shear or Heavy Rain?" Two case studies were first examined in an article published in the Journal of Aircraft by Luers and Haines. Their rigorous studies suggested that heavy rain contributed more to the loss of performance than did the wind shear, which was blamed for the incidents.
Now, let's look at case Number 2:
2. Eastern Air Lines Flight 693, Boeing 727, William B. Hartsfield Atlanta International Airport, August 22, 1979: The aircraft encountered a small but intense rain shower with associated wind shears on final approach. The aircraft, with 71 passengers and six crew members aboard, came within 375 feet of crashing before it exited the shower and completed a missed approach.
According to the flight crew, the rain and turbulence increased when they were at about 1,000 feet above the ground. The rain became heavy enough to noticeably increase the noise level within the cockpit. The indicated airspeed began to fluctuate-decreasing from 135 knots to 120 knots, increasing to about 140 knots, then decreasing to 108-110 knots. The rate of descent increased to 1,000 fpm.
At 800 feet, the first officer, who was flying, rotated the aircraft to a 10-degree, nose-up pitch attitude, advanced the throttles, called for takeoff power and began a missed approach. According to the first officer, the pitch correction and added thrust had no effect, and the sink rate increased to 1,500 fpm, then to 2,100 fpm.
He then rotated the aircraft to 15 degrees nose-up and advanced the throttles to their forward stops. At 500-600 feet above ground level and 110 KIAS, the stall warning systems stick shaker activated.
The Captain estimated that stall warning system operated "for about 10 to 20 seconds" (which must have seemed like eternity). The First Officer then reduced the pitch angle from 15 to 12 degrees, and the stick shaker stopped shortly thereafter.
According to the flight crew, the aircraft flew out of the precipitation at 375 feet in a right-wing-down attitude and, only then, began to accelerate. The descent rate was arrested and a climb initiated.
It is interesting to note that this downburst was not observed by aircraft closely preceding or following Flight 693. It was not observed by any ground-measurement sensors, either, and did not activate the low level wind shear alert system (LLWAS) at the Atlanta Airport.
(The LLWAS configuration in place at that time had fairly coarse net comprising seven sensors placed far apart from each other. It was entirely possible for a microburst event to occur between the sensors and not be accurately detected in a timely manner).
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 I'll finish our discussion, "Brief Events." And "Avoiding the Phenomenon." This will be Part IV of IV.
Larry G. Harmon
FAA AVIATION SAFETY COUNSELOR
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