Federal Aviation Administration, DOT § 23.925

(2) The effects of airplane and pro- § 23.925 Propeller clearance. peller operating and airworthiness lim- Unless smaller clearances are sub- itations. stantiated, propeller clearances, with [Amdt. 23–59, 73 FR 63345, Oct. 24, 2008] the airplane at the most adverse combination of weight and center of grav- § 23.909 Turbocharger systems. ity, and with the propeller in the most (a) Each turbocharger must be ap- adverse pitch position, may not be less proved under the engine type certifi- than the following: cate or it must be shown that the tur- (a) Ground clearance. There must be a bocharger system, while in its normal clearance of at least seven inches (for engine installation and operating in each airplane with nose wheel landing the engine environment— gear) or nine inches (for each airplane (1) Can withstand, without defect, an with tail wheel landing gear) between endurance test of 150 hours that meets each propeller and the ground with the the applicable requirements of § 33.49 of landing gear statically deflected and in this subchapter; and the level, normal takeoff, or taxing at- (2) Will have no adverse effect upon titude, whichever is most critical. In the engine. addition, for each airplane with con- (b) Control system malfunctions, vi- ventional landing gear struts using brations, and abnormal speeds and fluid or mechanical means for absorb- temperatures expected in service may ing landing shocks, there must be posi- not damage the turbocharger com- tive clearance between the propeller pressor or turbine. and the ground in the level takeoff at- (c) Each turbocharger case must be titude with the critical tire completely able to contain fragments of a com- deflated and the corresponding landing pressor or turbine that fails at the gear strut bottomed. Positive clear- highest speed that is obtainable with ance for airplanes using leaf spring normal speed control devices inoper- struts is shown with a deflection cor- ative. responding to 1.5g. (d) Each intercooler installation, (b) Aft-mounted propellers. In addition where provided, must comply with the following— to the clearances specified in paragraph (a) of this section, an airplane (1) The mounting provisions of the intercooler must be designed to with- with an aft mounted propeller must be stand the loads imposed on the system; designed such that the propeller will (2) It must be shown that, under the not contact the runway surface when installed vibration environment, the the airplane is in the maximum pitch intercooler will not fail in a manner al- attitude attainable during normal lowing portions of the intercooler to be takeoffs and landings. ingested by the engine; and (c) Water clearance. There must be a (3) Airflow through the intercooler clearance of at least 18 inches between must not discharge directly on any air- each propeller and the water, unless plane component (e.g., windshield) un- compliance with § 23.239 can be shown less such discharge is shown to cause with a lesser clearance. no hazard to the airplane under all op- (d) Structural clearance. There must erating conditions. be— (e) Engine power, cooling character- (1) At least one inch radial clearance istics, operating limits, and procedures between the blade tips and the airplane affected by the turbocharger system in- structure, plus any additional radial stallations must be evaluated. Turbo- clearance necessary to prevent harmful charger operating procedures and limi- vibration; tations must be included in the Air- (2) At least one-half inch longitudinal plane Flight Manual in accordance clearance between the propeller blades with § 23.1581. or cuffs and stationary parts of the air- [Amdt. 23–7, 34 FR 13092, Aug. 13, 1969, as plane; and amended by Amdt. 23–43, 58 FR 18970, Apr. 9, 1993]

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(3) Positive clearance between other reversing condition expected in oper- rotating parts of the propeller or spin- ation. ner and stationary parts of the air- (b) For propeller reversing systems. (1) plane. Each system must be designed so that no single failure, likely combination of [Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as failures or malfunction of the system amended by Amdt. 23–43, 58 FR 18971, Apr. 9, 1993; Amdt. 23–51, 61 FR 5136, Feb. 9, 1996; will result in unwanted reverse thrust Amdt. 23–48, 61 FR 5148, Feb. 9, 1996] under any operating condition. Failure of structural elements need not be con- § 23.929 Engine installation ice protec- sidered if the probability of this type of tion. failure is extremely remote. Propellers (except wooden propellers) (2) Compliance with paragraph (b)(1) and other components of complete en- of this section must be shown by failure analysis, or testing, or both, for gine installations must be protected propeller systems that allow the pro- against the accumulation of ice as nec- peller blades to move from the flight essary to enable satisfactory func- low-pitch position to a position that is tioning without appreciable loss of substantially less than the normal thrust when operated in the icing con- flight, low-pitch position. The analysis ditions for which certification is re- may include or be supported by the quested. analysis made to show compliance with [Amdt. 23–14, 33 FR 31822, Nov. 19, 1973, as § 35.21 for the type certification of the amended by Amdt. 23–51, 61 FR 5136, Feb. 9, propeller and associated installation 1996] components. Credit will be given for pertinent analysis and testing com- § 23.933 Reversing systems. pleted by the engine and propeller (a) For turbojet and turbofan reversing manufacturers. systems. (1) Each system intended for [Doc. No. 26344, 58 FR 18971, Apr. 9, 1993, as ground operation only must be de- amended by Amdt. 23–51, 61 FR 5136, Feb. 9, signed so that, during any reversal in 1996] flight, the engine will produce no more than flight idle thrust. In addition, it § 23.934 Turbojet and turbofan engine must be shown by analysis or test, or thrust reverser systems tests. both, that— Thrust reverser systems of turbojet (i) Each operable reverser can be re- or turbofan engines must meet the re- stored to the forward thrust position; quirements of § 33.97 of this chapter or or it must be demonstrated by tests that (ii) The airplane is capable of contin- engine operation and vibratory levels ued safe flight and landing under any are not affected. possible position of the thrust reverser. [Doc. No. 26344, 58 FR 18971, Apr. 9, 1993] (2) Each system intended for in-flight use must be designed so that no unsafe § 23.937 Turbopropeller-drag limiting condition will result during normal op- systems. eration of the system, or from any fail- (a) Turbopropeller-powered airplane ure, or likely combination of failures, propeller-drag limiting systems must of the reversing system under any op- be designed so that no single failure or erating condition including ground op- malfunction of any of the systems dur- eration. Failure of structural elements ing normal or emergency operation re- need not be considered if the prob- sults in propeller drag in excess of that ability of this type of failure is ex- for which the airplane was designed tremely remote. under the structural requirements of (3) Each system must have a means this part. Failure of structural ele- to prevent the engine from producing ments of the drag limiting systems more than idle thrust when the revers- need not be considered if the prob- ing system malfunctions; except that it ability of this kind of failure is ex- may produce any greater thrust that is tremely remote. shown to allow directional control to (b) As used in this section, drag lim- be maintained, with aerodynamic iting systems include manual or auto- means alone, under the most critical matic devices that, when actuated

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