The initial tests were conducted in an altitude chamber. It was found that each port of the static plate entrained water by capillary action, and pressure differentials equivalent to about 35 feet in altitude, at sea level, were required to expel the water. A series of runs verified that any increase in the diameter of the orifice decreased the magnitude of the "jumps." Variations in the rate of descent affected the rate of "jumps," but not the magnitude.
Flow visualization tests were then conducted in the wind tunnel with 1/50-scale DC-9-10 and DC-8-55 models to identify any mechanism that might tend to concentrate water in the vicinity of the static ports. The testing covered both no-flap and 50°-flap configurations at angles of attack ranging from -8° to +8°. The observed flow was orderly, and the only deviation was around a high velocity region on the nose of the DC-9. This was later found to be due to model asymmetry.
Actual flight tests were conducted in light-to-heavy rain with a DC-9-30 in the following flight conditions: descent in the landing configuration for both the DC-9-10[1] and DC-9-30, and descent in the clean configuration that was representative of both aircraft. Nine simulated ILS approaches were flown in the DC-9-30 landing configuration.[2] Both the normal and alternative static systems were monitored throughout, and no instances of sticky altimeter operation were observed. Additionally, five typical descents were made in the clean configuration, at 2,500 to 3,000 feet/minute, through light to sporadically heavy rain. No evidence of sticky altimeter operation was detected on any system.
At the request of the Safety Board and the FAA, the National Aeronautics and Space Administration has taken a long term Static Pressure Measurements Project at the Lewis Research Center. This exploratory research project includes flight and ground testing to determine the flight and weather conditions which may lead to altitude misinformation. A secondary objective is to compare the water ingestion resistance of existing static ports with static ports being considered for future aircraft. The flight test portion of this project has begun, and ground tests will be predicated on the results of the flight test.
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