CS 25.1303 Flight and navigation instruments

In this AMC, ‘primary direction indicator’ refers to the direction indicator required by CS 25.1303(b)(6) and ‘standby direction indicator’ to the one required by CS 25.1303(a)(3). When designing and installing a standby direction indicator, the applicant should follow the guidelines below:

(a)Independence between the primary direction indicator and the standby direction indicator should be established in all foreseeable operating conditions. Failure conditions and subsequent switching to the backup source of direction should be carefully considered;

(b)The reliability of the standby direction indicator should be commensurate with the identified hazard level. Consideration should be given to CS 25.1333(b) and AMC 25-11, Chapter 4, Table 6;

(c)Additional availability assessments should be provided:

(1)Direction indications should be available immediately following the loss of the primary direction source without additional crew member action, and after any single failure or combination of failures. Consideration should be given to CS 25.1333(b);

(2)Direction indications should not be adversely affected following a loss of normal electrical power. Consideration should be given to CS 25.1351(d);

(3)Operation during and after exposure to a high-intensity radiated field (HIRF) environment should be demonstrated. Consideration should be given to CS 25.1317(a);

(4)Operation after exposure to indirect effects of lightning should be established. Consideration should be given to CS 25.1316(a).

[Amdt 25/21]

1 Attitude Displays

1.1 For turbo-jet aeroplanes each display should be usable over the full range of 360° in pitch and in roll. For propeller-driven aeroplanes the pitch range may be reduced to ± 75° provided that no misleading indication is given when the limiting attitude is exceeded.

1.2 Paragraph 1.1 is not intended to prohibit the use of vertical references having controlled gyro precession, or its equivalent in the case of a stable platform, but precession should not occur at a pitch attitude closer to the horizontal than 70°, and should be completed within an attitude change of 15°.

1.3 The display should take the form of an artificial horizon line, which moves relative to a fixed reference aeroplane symbol so as to indicate the position of the true horizon. NOTES: 1 It is acceptable for the fixed reference aeroplane symbol to be positioned so that it is aligned with the horizon line during cruising flight. 2 If a variable index is provided in addition to the fixed aeroplane symbol it should be so designed that it will not introduce any risk of misinterpretation of the display.

1.4 There should be no means accessible to the flight crew of adjusting the relationship between the horizon line and the reference aeroplane symbol.

1.5 The artificial horizon line should move in roll so as to remain parallel to the true horizon, i.e. when the aeroplane rolls through an angle of 30° the artificial horizon line should also rotate through 30° relative to the fixed index.

1.6 The artificial horizon line should remain in view over a range of pitch attitudes sufficient to cover all normal operation of the aeroplane plus a margin of not less than 2° in either direction. Additional ‘ghost’ horizon lines should be provided parallel to the main horizon line so that beyond this range at least one such line is in view at an attitude with the range of the display.

1.7 The pitch attitude scale should be sensibly linear while the main horizontal line is in view, but may become non-linear beyond this range. All the attitude displays in the aeroplane should have a similar presentation so as to prevent any risk of confusion in transferring attention from one display to another.

1.9 Sufficient pitch and bank angle graduations and markings should be provided to allow an acceptably accurate reading of attitude and to minimise the possibility of confusion at extreme attitudes.

1.10 A bank angle index and scale should be provided. The index may be on the fixed or moving part of the display.

1.11 The ‘earth’ and ‘sky’ areas of the display should be of contrasting colours or shades. The distinction should not be lost at any pitch or roll angle.

1.12 Any additional information (e.g. flight director commands) displayed on an attitude display should not obscure or significantly degrade the attitude information.

1.13 The display should be clearly visible under all conditions of daylight and artificial lighting.

1.14 Words that may be ambiguous (e.g. ‘climb’, ‘dive’, ‘push’, ‘pull’) should not be used. 2 Attitude Display Systems (Acceptable Means of Compliance)

2.1 The probability of indication of dangerously incorrect information without a warning being given should be Extremely Remote.

2.2 The warning may be provided by means of self- or comparison-monitoring and should be clear and unambiguous, e.g. a flashing light. Instrument flags are unlikely to be acceptable as a comparator warning unless they exclude a significant portion of the display in which case means should be provided to permit the removal of the flag from the display, which is not in error.

2.3 The definition of dangerously incorrect information depends to some extent on the characteristics of the aeroplane, but in general an error greater than 5° in pitch or 10° in roll will be considered to be dangerous.

In the absence of warning through the inherent aerodynamic qualities of the aeroplane (e.g. buffeting) it should be shown that no single faults can result both in misleading airspeed information and in operation of the warning system outside its tolerances, such as would be likely to lead to exceedance of VMO/MMO.