I. General. A normal category helicopter may not be type certificated for operation under the instrument flight rules (IFR) of this chapter unless it meets the design and installation requirements contained in this appendix.

II. Definitions. (a) VYI means instrument climb speed, utilized instead of VY for compliance with the climb requirements for instrument flight.

1. (b) VNEI means instrument flight never exceed speed, utilized instead of VNE for compliance with maximum limit speed requirements for instrument flight.

2. (c) VMINI means instrument flight minimum speed, utilized in complying with minimum limit speed requirements for instrument flight.

   III. Trim. It must be possible to trim the cyclic, collective, and directional control forces to zero at all approved IFR airspeeds, power settings, and configurations appropriate to the type.

   IV. Static longitudinal stability. (a) General. The helicopter must possess positive static longitudinal control force stability at critical combinations of weight and center of gravity at the conditions specified in paragraph IV (b) or (c) of this appendix, as appropriate. The stick force must vary with speed so that any substantial speed change results in a stick force clearly perceptible to the pilot. For single-pilot approval, the airspeed must return to within 10 percent of the trim speed when the control force is slowly released for each trim condition specified in paragraph IV(b) of the this appendix.

3. (b) For single-pilot approval:

   1. (1) Climb. Stability must be shown in climb throughout the speed range 20 knots either side of trim with—

      1. (i) The helicopter trimmed at VYI;
      2. (ii) Landing gear retracted (if retractable); and
      3. (iii) Power required for limit climb rate (at least 1,000 fpm) at VYI or maximum continuous power, whichever is less.

   2. (2) Cruise. Stability must be shown throughout the speed range from 0.7 to 1.1 VH or VNEI, whichever is lower, not to exceed ±20 knots from trim with—

      1. (i) The helicopter trimmed and power adjusted for level flight at 0.9 VH or 0.9 VNEI, whichever is lower; and
      2. (ii) Landing gear retracted (if retractable).

   3. (3) Slow cruise. Stability must be shown throughout the speed range from 0.9 VMINI to 1.3 VMINI or 20 knots above trim speed, whichever is greater, with—

      1. (i) the helicopter trimmed and power adjusted for level flight at 1.1 VMINI; and
      2. (ii) Landing gear retracted (if retractable).

   4. (4) Descent. Stability must be shown throughout the speed range 20 knots either side of trim with—

      1. (i) The helicopter trimmed at 0.8 VH or 0.8 VNEI (or 0.8 VLE for the landing gear extended case), whichever is lower;
      2. (ii) Power required for 1,000 fpm descent at trim speed; and
      3. (iii) Landing gear extended and retracted, if applicable.

   5. (5) Approach. Stability must be shown throughout the speed range from 0.7 times the minimum recommended approach speed to 20 knots above the maximum recommended approach speed with—

      1. (i) The helicopter trimmed at the recommended approach speed or speeds;
      2. (ii) Landing gear extended and retracted, if applicable; and
      3. (iii) Power required to maintain a 3° glide path and power required to maintain the steepest approach gradient for which approval is requested.

4. (c) Helicopters approved for a minimum crew of two pilots must comply with the provisions of paragraphs IV(b)(2) and IV(b)(5) of this appendix.

   V. Static Lateral Directional Stability. (a) Static directional stability must be positive throughout the approved ranges of airspeed, power, and vertical speed. In straight and steady sideslips up to ±10° from trim, directional control position must increase without discontinuity with the angle of sideslip, except for a small range of sideslip angles around trim. At greater angles up to the maximum sideslip angle appropriate to the type, increased directional control position must produce an increased angle of sideslip. It must be possible to maintain balanced flight without exceptional pilot skill or alertness.

5. (b) During sideslips up to ±10° from trim throughout the approved ranges of airspeed, power, and vertical speed, there must be no negative dihedral stability perceptible to the pilot through lateral control motion or force. Longitudinal cyclic movement with sideslip must not be excessive.

   VI. Dynamic stability. (a) For single-pilot approval—

   1. (1) Any oscillation having a period of less than 5 seconds must damp to 1/2 amplitude in not more than one cycle.
   2. (2) Any oscillation having a period of 5 seconds or more but less than 10 seconds must damp to 1/2 amplitude in not more than two cycles.
   3. (3) Any oscillation having a period of 10 seconds or more but less than 20 seconds must be damped.
   4. (4) Any oscillation having a period of 20 seconds or more may not achieve double amplitude in less than 20 seconds.
   5. (5) Any aperiodic response may not achieve double amplitude in less than 6 seconds.

   6. (b) For helicopters approved with a minimum crew of two pilots—

      1. (1) Any oscillation having a period of less than 5 seconds must damp to 1/2 amplitude in not more than two cycles.
      2. (2) Any oscillation having a period of 5 seconds or more but less than 10 seconds must be damped.

      3. (3) Any oscillation having a period of 10 seconds or more may not achieve double amplitude in less than 10 seconds.

         VII. Stability Augmentation System (SAS).

      4. (a) If a SAS is used, the reliability of the SAS must be related to the effects of its failure. Any SAS failure condition that would prevent continued safe flight and landing must be extremely improbable. It must be shown that, for any failure condition of the SAS that is not shown to be extremely improbable—

         1. (1) The helicopter is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved IFR operating limitations; and

         2. (2) The overall flight characteristics of the helicopter allow for prolonged instrument flight without undue pilot effort. Additional unrelated probable failures affecting the control system must be considered. In addition—

            1. (i) The controllability and maneuverability requirements in Subpart B of this part must be met throughout a practical flight envelope;
            2. (ii) The flight control, trim, and dynamic stability characteristics must not be impaired below a level needed to allow continued safe flight and landing; and
            3. (iii) The static longitudinal and static directional stability requirements of Subpart B must be met throughout a practical flight envelope.

      5. (b) The SAS must be designed so that it cannot create a hazardous deviation in flight path or produce hazardous loads on the helicopter during normal operation or in the event of malfunction or failure, assuming corrective action begins within an appropriate period of time. Where multiple systems are installed, subsequent malfunction conditions must be considered in sequence unless their occurrence is shown to be improbable.

         VIII. Equipment, systems, and installation. The basic equipment and installation must comply with §§ 29.1303, 29.1431, and 29.1433, with the following exceptions and additions:

      6. (a) Flight and Navigation Instruments.

         1. (1) A magnetic gyro-stablized direction indicator instead of a gyroscopic direction indicator required by § 29.1303(h); and
         2. (2) A standby attitude indicator which meets the requirements of §§ 29.1303(g)(1) through (7) instead of a rate-of-turn indicator required by § 29.1303(g). For two-pilot configurations, one pilot's primary indicator may be designated for this purpose. If standby batteries are provided, they may be charged from the aircraft electrical system if adequate isolation is incorporated.

      7. (b) Miscellaneous requirements.

         1. (1) Instrument systems and other systems essential for IFR flight that could be adversely affected by icing must be adequately protected when exposed to the continuous and intermittent maximum icing conditions defined in appendix C of Part 29 of this chapter, whether or not the rotorcraft is certificated for operation in icing conditions.
         2. (2) There must be means in the generating system to automatically de-energize and disconnect from the main bus any power source developing hazardous overvoltage.
         3. (3) Each required flight instrument using a power supply (electric, vacuum, etc.) must have a visual means integral with the instrument to indicate the adequacy of the power being supplied.
         4. (4) When multiple systems performing like functions are required, each system must be grouped, routed, and spaced so that physical separation between systems is provided to ensure that a single malfunction will not adversely affect more than one system.

         5. (5) For systems that operate the required flight instruments at each pilot's station—

            1. (i) For pneumatic systems, only the required flight instruments for the first pilot may be connected to that operating system;
            2. (ii) Additional instruments, systems, or equipment may not be connected to an operating system for a second pilot unless provisions are made to ensure the continued normal functioning of the required instruments in the event of any malfunction of the additional instruments, systems, or equipment which is not shown to be extremely improbable;
            3. (iii) The equipment, systems, and installations must be designed so that one display of the information essential to the safety of flight which is provided by the instruments will remain available to a pilot, without additional crewmember action, after any single failure or combination of failures that is not shown to be extremely improbable; and

            4. (iv) For single-pilot configurations, instruments which require a static source must be provided with a means of selecting an alternate source and that source must be calibrated.

               IX. Rotorcraft Flight Manual. A Rotorcraft Flight Manual or Rotorcraft Flight Manual IFR Supplement must be provided and must contain—

            5. (a) Limitations. The approved IFR flight envelope, the IFR flightcrew composition, the revised kinds of operation, and the steepest IFR precision approach gradient for which the helicopter is approved;
            6. (b) Procedures. Required information for proper operation of IFR systems and the recommended procedures in the event of stability augmentation or electrical system failures; and

            7. (c) Performance. If VYI differs from VY, climb performance at VYI and with maximum continuous power throughout the ranges of weight, altitude, and temperature for which approval is requested.

               X. Electrical and electronic system lightning protection. For regulations concerning lightning protection for electrical and electronic systems, see § 27.1316.

[Amdt. 27-19, 48 FR 4389, Jan. 31, 1983, as amended by Amdt. 27-44, 73 FR 11000, Feb. 29, 2008; Amdt. 27-46, 76 FR 33135, June 8, 2011; Amdt. 27-51, 88 FR 8739, Feb. 10, 2023]