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