The Propulsion Systems Laboratory (PSL) altitude test facility at NASA Glenn Research Center, has been used to test a full scale Honeywell turbofan engine at simulated altitude operating conditions. The PSL has spray bars to create a continuous cloud of fully glaciated ice crystals. The tests successfully duplicated the icing events that were experienced by the Honeywell engine (ALF502R-5) during flight through ice crystal clouds. After the ice cloud was turned on key engine performance parameters such as the fan speed, air flow rate, fuel flow rate, and compressor exit pressure and temperature responded immediately to the ingestion of the ice crystals. For some of the test points, these performance parameters remained unchanged from the initial response to the ice crystals, while during other test points the engine performance began to deteriorate to the point where an uncommanded loss of thrust control (engine rollback) was judged by the test engineers to have been imminent. The data points where the performance deteriorated with time have been attributed to ice accretion in the low pressure compressor. To gain understanding of the transient engine performance, nine data points were analyzed with the Numerical Propulsion System Simulation engine system code and a compressor flow analysis code. The focus was to improve understanding of the effect of ice crystal ingestion on the transient engine performance, and to quantify the rate of growth of blockage and additional losses due to ice accretion. The engine responds immediately after the initiation of the ice cloud, but whether ice growth in the low pressure compressor will occur after a period of time, appears to be strongly dependent on the concentration of ice crystals ingested, the wet bulb temperature distribution, and the melt ratio.