14 CFR §34.9

1. (a)Spare engines. Certain engines that meet the following description are excepted:
   1. (1)This exception allows production of an engine for installation on an in-service aircraft. A spare engine may not be installed on a new aircraft.
   2. (2)Each spare engine must be identical to a sub-model previously certificated to meet all applicable requirements.
   3. (3)A spare engine may be used only when the emissions of the spare do not exceed the certification requirements of the original engine, for all regulated pollutants.
   4. (4)No separate approval is required to produce spare engines.
   5. (5)The record for each engine excepted under this paragraph (c) must indicate that the engine was produced as an excepted spare engine.
   6. (6)Engines produced under this exception must be labeled “EXCEPTED SPARE” in accordance with § 45.13 of this chapter.
2. (b)On and after July 18, 2012, and before August 31, 2013, a manufacturer may produce up to six Tier 4 compliant engines that meet the NOX standards of paragraph (d)(1)(vi) of this section rather than § 34.23(a)(2). No separate approval is required to produce these engines. Engines produced under this exception are to be labeled “COMPLY” in accordance with § 45.13 of this chapter.