Special manufacturer's symbols may be assigned when two manufacturers are jointly producing an engine. In these instances, the manufacturer's symbol is one letter from each of the manufacturers' symbols. Model Numbers The third part of the designation is a dash and a number indicating the model number. !    Navy numbers begin with 2, and they continue with consecutive even numbers. All even model  numbers  are  assigned  to  engines approved by the Naval Air Systems Command. !    Air Force numbers begin with 1 and continue with consecutive odd numbers. Each engine design has only one type and model designation for both the Air Force and Navy. For example, the Navy may wish to use an engine that has Air Force-approved type and model numbers. The Navy may use those numbers without change, provided there are no engine changes. If the Air Force wants to use a Navy-approved type engine, but requires minor engine production changes, the Air Force must use the Navy type designation. The Air Force then assigns its own model designation (which begins with the number 1 and progresses with consecutive odd numbers) to the modified engine, regardless of the Navy model number. This model number is actually a modification number. It tells which service made the last production change to the engine for a particular aircraft application. Special Designations The letter X or Y preceding the basic designation signifies a special designation. The prefix letter X is a basic engine designation signifying the experimental and service test of a particular engine. This prefix letter is removed after tests prove the engine can perform as it should under all operating conditions. The prefix letter Y indicates a Restricted Service designation. It indicates that the engine will not, or is not expected to, perform satisfactorily under all operating conditions. It is applied to an engine with a specific function or that has completed a 150-hour qualification test only. Upon satisfactorily completing the qualification testing, the Y designation is dropped. The engine is then approved for installation in a production aircraft. The following is an example of a complete ANA Bulletin No. 306M engine designation number: T56-A-14 !    T—Turboprop !    56—Navy developed !    A—Allison !    14—Navy model The ANA Bulletin No. 306M designation system is effective until each engine manufactured before the introduction of MIL-STD-1812 is modified or deleted from service. MIL-STD-1812 DESIGNATION SYSTEM This engine designation system is made up of three-digit numerals and model numbers. It is used on all newly developed gas turbine engines. Existing engines receive a new three-digit model number whenever there are major changes in engine configuration or design. In most instances the old two-digit indicator will be retained. The MIL-STD-1812 engine designation system applies to all the armed services—Air Force, Navy, and the Army. The  complete  designation  system  has  three parts—the type indicator, the manufacturer's indicator, and the model indicator. Special designations in this system are the same as those discussed under the ANA Bulletin No. 306M system (X or Y preceding the basic designation). Type Indicator The first part is the type indicator. It consists of the type letter symbol and the type numeral. Letter type symbols are shown in table 6-3: Table 6-3.—Engine Type Indicator INDICATOR ENGINE TYPE J Turbojet T Turboprop/Turboshaft F Turbofan 6-18