CS 25.1091 Air intake

1 Except where it is obvious by inspection or other means, that precipitation on the runway would not enter the engine air intake under the declared operating conditions, including the use of the thrust reverser, compliance with the requirements should be demonstrated by tests using tyres representative of those to be approved for operational use. These tests should clear the aeroplane for operation from runways which are normally clear and also for operation in precipitation up to 13 mm (0·5 in) depth of water or dense slush. The tests should be conducted with the minimum depth of 13 mm (0·5 in) and an average depth of 19 mm (0·75 in), or if approval is sought for a greater depth than 13 mm (0·5 in), the average depth should be 1·5 times the depth for which the take-offs are to be permitted, and the minimum depth should be not less than the depth for which take-offs are to be permitted. 2 It should be shown that the engines operate satisfactorily without unacceptable loss of power at all speeds from zero up to lift-off speed and in the attitudes likely to be used. Any special aeroplane handling techniques necessary to ensure compliance with the requirement should comply with the handling techniques assumed in establishing the scheduled performance of the aircraft. 3 The tests may be made in water or slush either by complete take-offs and landings as necessary in the specified precipitation conditions, or by a series of demonstrations in areas of precipitation sufficiently large to permit the spray pattern to become stabilised and to determine engine behaviour and response. Experience has shown that where a trough is used, a length of 70 to 90 m (230 to 295 ft) is usually satisfactory. If marginal results are obtained the effect of the difference between water and slush should be examined. 4 The effects of cross-winds should be examined and where necessary a cross-wind limitation established for inclusion in the Flight Manual for operation from precipitation covered runways. 5 It may be difficult to deduce the effect of low density precipitation (dry snow) from high density testing, but nevertheless clearance of the aeroplane for operation in dense precipitation up to 13 mm (0·5 in) will usually clear the aeroplane for operation in low density precipitation of depths greater than 10 cm (4 in) depth. If clearance is requested for operation in low density precipitation of depths greater than 10 cm (4 in) additional tests (in low density precipitation having a depth close to that for which approval is sought) will be necessary. 6 When auxiliary devices are fitted to prevent spray from being ingested by the engines it will be necessary to do additional tests in low density precipitation to permit operations in depths greater than 25 mm (1 in).

The parts or components to be considered are, for example, intake splitters, acoustic lining if in a vulnerable location and inlet duct-mounted instrumentation.