Spirit Airlines (NK, Fort Lauderdale International) Chairman William Franke, who helped design the carrier's successful low-cost strategy, is selling his investment firm Indigo Partners' 16.6% stake in the airline, valued at USD427million, and resigning from its board on August 7. Mr John R. Wilson will also be joining him. The Wall Street Journal reports that the announcement comes days after Republic Airlines (Indianapolis International) parent firm, Republic Airlines Holdings Inc, said it has signed an exclusive, nonbinding term sheet to sell Frontier Airlines (F9, Denver International) to an undisclosed third party, speculated, though not confirmed, to be Indigo Partners. Indigo is an LCC investor specialists with investments including Spirit, Tigerair (Singapore Changi) in Singapore, Wizz Air (W6, Budapest) in Hungary, and Volaris (Y4, México City International) in Mexico.