Aircraft-Produced Ice Particles (APIPs): Additional Results and Further Insights

Abstract

This paper presents new results from studies of aircraft-produced ice particles (APIPs) in supercooled fog and clouds. Nine aircraft, including a Beech King Air 200T cloud physics aircraft, a Piper Aztec, a Cessna 421-C, two North American T-28s, an Aero Commander, a Piper Navajo, a Beech Turbo Baron, and a second four-bladed King Air were involved in the tests. The instrumented King Air served as the monitoring aircraft for trails of ice particles created, or not created, when the other aircraft were flown through clouds at various temperatures and served as both the test and monitoring aircraft when it itself was tested. In some cases sulfur hexafluoride (SF6) gas was released by the test aircraft during its test run and was detected by the King Air during its monitoring passes to confirm the location of the test aircraft wake. Ambient temperatures for the tests ranged between ?5° and ?12°C. The results confirm earlier published results and provide further insights into the APIPs phenomenon. The King Air at ambient temperatures less than ?8°C can produce APIPs readily. The Piper Aztec and the Aero Commander also produced APIPs under the test conditions in which they were flown. The Cessna 421, Piper Navajo, and Beech Turbo Baron did not. The APIPs production potential of a T-28 is still indeterminate because a limited range of conditions was tested. Homogeneous nucleation in the adiabatically cooled regions where air is expanding around the rapidly rotating propeller tips is the cause of APIPs. An equation involving the propeller efficiency, engine thrust, and true airspeed of the aircraft is used along with the published thrust characteristics of the propellers to predict when the aircraft will produce APIPs. In most cases the predictions agree well with the field tests. Of all of the aircraft tested, the Piper Aztec, despite its small size and low horsepower, was predicted to be the most prolific producer of APIPs, and this was confirmed in field tests. The APIPs, when they are created, appear in aircraft wakes in concentrations up to several hundred per liter, which are initially very small and almost uniform in size but grow to larger nearly uniform sizes with time. APIPs production is most likely at low ambient temperatures when an aircraft is flown at maximum power with the gear and flaps extended, resulting in a relatively low airspeed under high-drag conditions. It is predicted that APIPs production of an aircraft can be decreased or eliminated altogether by using a propeller with a larger number of propeller blades, such that the engine thrust is distributed over more blades, thereby decreasing the cooling on each blade. Plans to test this hypothesis using three- and four-bladed King Airs as the test aircraft never came to fruition because of unsatisfactory weather conditions. It is likely that APIPs have confounded the results of some past cloud microphysical investigations, especially those in which repeat passes were made through individual clouds under heavy icing conditions by aircraft known now to be APIPs producers. Aircraft flying under such conditions are forced to use high power settings to overcome the drag of a heavy ice load. These are the conditions that field tests demonstrate are most conducive to the production of APIPs. In these situations, APIPs may have led investigators to conclude that there was more rapid development of ice, and higher concentrations of ice particles in clouds, than actually was the case.