Improper Deicing Procedures
The Boeing 737 was deiced with a mixture of heated water and monopropylene glycol by American Airlines, under a ground service agreement with Air Florida. That agreement specified that covers for the pitots/static ports and engine inlets had to be used, but the American Airlines employees did not comply with those rules. Two different operators — who chose widely different mixture percentages — deiced the left and right sides of the plane. Subsequent testing of the deicing truck showed that "the mixture dispensed differed substantially from the mixture selected" (18% actual vs. 30% selected). The inaccurate mixture was the result of the replacement of the standard Trump nozzle, "…which is specially modified and calibrated, with a nonmodified, commercially available nozzle." The operator had no means to determine if the proportioning valves were operating properly because no "mix monitor" was installed on the nozzle.
Read more about this topic: Air Florida Flight 90
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