4-31
4.18 Stall Warning, § 25.207.
4.18.1 To show compliance with § 25.207, the applicant should assess stall warning in conjunction with stall speed testing and stall demonstration/characteristics testing (§§ 25.103, 25.201, and 25.203, and paragraphs 4.2 and 4.17 of this AC, respectively), and in tests with faster entry rates.
4.18.2 Normal Ice Protection System Operation.
The following specifications represent an example of an acceptable test program for stall warning in slow-down turns of at least 1.5 g and at entry rates of at least 2 knots per second:
Maneuvers
4.18.2.1 The holding ice accretion should be used.
4.18.2.2 The airplane should be loaded to a medium to light weight, aft center-of-gravity position, with symmetric fuel loading.
4.18.2.3 The test should be conducted at the normal stall test altitude.
4.18.2.4 In the configurations listed in paragraphs 4.18.2.4.1 through 4.18.2.4.3 below, trim the airplane at 1.3 VSR with the power or thrust necessary to maintain straight level flight. Maintain the trim power or thrust during the test demonstrations. Increase speed as necessary before establishing at least 1.5 g and a deceleration of at least 2 knots per second. Decrease speed until 1 second after stall warning and recover using the same recovery maneuver as for the uncontaminated airplane.
4.18.2.4.1 High lift devices retracted configuration.
4.18.2.4.2 Lowest lift takeoff configuration.
4.18.2.4.3 Highest lift landing configuration.
4.18.3 Stall Warning Prior to Activation and Operation of the Ice Protection System.
The following represents an acceptable means for showing compliance with § 25.207(h).
4-32
Maneuvers
4.18.3.1 In the configurations listed in paragraphs 4.18.3.1.1 and 4.18.3.1.2 below, with the ice accretion specified in the requirement, trim the airplane at 1.3 VSR.
4.18.3.1.1 High lift devices retracted configuration: Straight Flight/Power Off.
4.18.3.1.2 Landing configuration: Straight Flight/Power Off.
4.18.3.2 At deceleration rates of up to 1 knot per second, reduce the speed until 1 second past stall warning, and demonstrate that stalling can be prevented using the same recovery maneuver as used for the uncontaminated airplane, without encountering any adverse characteristics (for example, rapid wing roll-off). If stall warning for the airplane with ice accretion is provided by a different means than it is for the airplane without ice accretion, the following requirements must be met:
4.18.3.2.1 Section 25.207(h)(3)(ii) requires that stalling be preventable even if the pilot takes no recovery action for at least 3 seconds after stall warning.
4.18.3.2.2 Section 25.207(i) requires that satisfactory stall characteristics be demonstrated if stall warning is provided by a different means in icing conditions than for non-icing conditions.
4.19 Wind Velocities, § 25.237. To show compliance with this requirement:
4.19.1 Crosswind landings with the landing ice accretion should be evaluated on an opportunity basis.
4.19.2 Results of the steady heading sideslip tests with landing ice may be used to establish the safe crosswind component of this requirement. If the flight test data show that the maximum sideslip angle demonstrated is similar to that shown with the uncontaminated airplane, and the flight characteristics (control forces and deflections, for example) are similar, then the non-contaminated airplane crosswind component test result is considered valid for icing conditions.
4.19.3 If results of the comparison discussed in paragraph 4.19.2 above are not clearly similar, and in the absence of a more extensive analysis, the applicant may use a conservative analysis based on results of the steady heading sideslip tests to establish the safe crosswind component value. The crosswind value may be estimated from:
4-33 𝑉𝐶𝑊 = 𝑉𝑅𝐸𝐹 ∗ sin(𝑠𝑖𝑑𝑒𝑠𝑙𝑖𝑝 𝑎𝑛𝑔𝑙𝑒)1.5 Where:
VCW is the crosswind component, and VREF is the landing reference speed appropriate to a minimum landing weight, and Sideslip angle is that demonstrated at VREF (see paragraph 4.15 of this AC).
4.20 Vibration and Buffeting, § 25.251.
To show compliance with this requirement:
4.20.1 Combine qualitative evaluations with the other testing, including speeds up to the maximum speed obtained in the longitudinal stability tests. (See paragraph 4.14 of this AC).
4.20.2 Demonstrate that the airplane is free from harmful vibration due to residual ice accumulation. This may be done in conjunction with the natural icing tests.
4.20.3 Evaluate an airplane with pneumatic deicing boots up to VDF/MDF, both with the deicing boots operating (including deicing boot inflation cycles) and not operating. It is not necessary to do this demonstration with ice accretions on the airplane.
4.21 Natural Icing Conditions, § 25.1419(b).
To show compliance with this requirement, the applicant should perform additional flight testing.
4.21.1 Whether flight testing has been performed with artificial or simulated ice accretions or in natural icing conditions, additional limited flight testing described in this section should be conducted in the natural atmospheric icing conditions specified in part 25, Appendix C, and, if necessary, in the icing conditions described in Appendix O. (AC 25-28 provides guidance on when it is necessary to perform flight testing in the natural atmospheric icing conditions of Appendix O). If flight testing with simulated ice accretions is the primary means for showing compliance, the objective of the tests described in this section is to corroborate the handling characteristics and performance results obtained in the flight testing with simulated ice accretions. Validating all aspects of predicted ice accretions through natural icing flight tests is not required. The flight testing should confirm the general physical characteristics and location of the ice accretions and their effect on airplane performance and handling characteristics. The applicant should make at least a qualitative assessment to confirm that the artificial ice accretions are conservative relative to ice accretions resulting from natural atmospheric icing conditions, and that ice in natural atmospheric icing conditions does not accrete in unexpected places.
4-34
4.21.2 It is acceptable for some ice to be shed during the testing due to air loads or wing flexure, etc. However, the applicant should attempt to accomplish the test maneuvers as soon as possible after exiting the icing cloud in order to minimize atmospheric influences on ice shedding.
4.21.3 Acceptable Test Program.
During each of the maneuvers specified in paragraph 4.21.4 below, the behavior of the airplane should be consistent with that obtained with simulated ice accretions. There should be no unusual control responses or uncommanded airplane motions.
Additionally, during the level turns and bank-to-bank rolls, there should be no buffeting or stall warning.
Maneuvers
4.21.4 Ice Accretion Maneuvers.
4.21.4.1 Holding scenario.
4.21.4.1.1 The maneuvers specified in table 4-4. below should be carried out with ice accretions defined in paragraphs 4.21.4.1.2 and 4.21.4.1.3 below, which is representative of normal operation of the ice protection system:
4.21.4.1.2 Ice on unprotected parts. A target accretion thickness equivalent to the 45-minute dry air ice accretions on an unprotected part of the wing should be the objective. (A thickness of 2 inches is normally a minimum value, unless a lesser value is agreed to with the responsible ACO).
4.21.4.1.3 Ice on protected parts. The ice accretion thickness should be that resulting from normal operation of the ice protection system.
4.21.4.1.4 For airplanes with control surfaces that may be susceptible to jamming due to ice accretion (for example, elevator horns exposed to the air flow), the holding speed that is critical with respect to this ice accretion should be used.
4-35 Table 4-4. Holding Scenario—Maneuvers
4.21.4.2 Approach/Landing Scenario.
4.21.4.2.1 The maneuvers specified in table 4-5 of this AC should be carried out with successive accretions in different configurations on unprotected surfaces.
4.21.4.2.2 Each test condition should be accomplished with the ice accretion that exists at that point.
4.21.4.2.3 The final ice accretion (Test Condition 3) represents the sum of the amounts that would accrete during a normal descent from holding to landing in icing conditions.
4.21.5 For airplanes with unpowered elevator controls, unless the applicant can adequately substantiate the critical simulated ice accretion used to demonstrate compliance with the controllability requirement, the pushover test of paragraph 4.9.4.3.3 should be repeated with a thin accretion of natural ice.
4.21.6 Existing propeller speed limits or, if required, revised propeller speed limits for flight in icing, should be verified by flight tests in natural icing conditions.
Airplane Configuration Center-of- Gravity Position Trim Speed Maneuver Flaps up, Gear up Any position in the aft range Holding, except
1.3 VSR for the stall maneuver Level, 40° banked turn;
Bank-to-bank rapid roll, 30° - 30°;
Speedbrake extension, retraction;
Full straight stall (1 knot per second deceleration rate, wings level, power off) Flaps in intermediate positions, gear up Any position in the aft range
1.3 VSR Deceleration to the speed reached 3 seconds after activation of stall warning in a 1 knot per second deceleration Landing flaps, gear down Any position in the aft range VREF Level, 40° banked turn;
Bank-to-bank rapid roll, 30° - 30°;
Speedbrake extension, retraction (if approved);
Full straight stall (1 knot per second deceleration rate, wings level, power off)
4-36 Table 4-5. Approach/Landing Scenario—Maneuvers Test Condition Ice Accretion Thickness* Airplane Configuration Center-of- Gravity Position Trim Speed Maneuver — First 0.5 inch Flaps up, gear up Any position in the aft range Holding No specific test 1 Additional
0.25 inch
(0.75 inch total) First intermediate flaps, gear up Any position in the aft range Holding, except
1.3 VSR for the deceleration maneuver Level 40° banked turn;
Bank-to-bank rapid roll, 30°- 30°;
Speed brake extension and retraction (if approved);
Deceleration to the speed reached 3 seconds after activation of stall warning in a 1 knot per second deceleration.
2 Additional
0.25 inch
(1.00 inch total) Further intermediate flaps, gear up (as applicable) Any position in the aft range
1.3 VSR Bank-to-bank rapid roll, 30° - 30°;
Speed brake extension and retraction (if approved);
Deceleration to the speed reached 3 seconds after activation of stall warning in a 1 knot per second deceleration.
3 Additional
0.25 inch
(1.25 inch total) Landing flaps, gear down Any position in the aft range VREF Bank-to-bank rapid roll, 30° - 30°;
Speed brake extension and retraction (if approved), Bank to 40°;
Full straight stall (1 knot per second deceleration rate, wings level, power off).
* The indicated thickness is that accumulated on the parts of the unprotected airfoil most likely to accumulate ice.
4-37
4.22 Failure Conditions, § 25.1309.
To show compliance with this requirement:
4.22.1 For failure conditions that are annunciated to the flightcrew, the applicant may take credit for flightcrew action to follow the established operating procedures provided in the AFM.
4.22.2 Acceptable Test Program.
In addition to a general qualitative evaluation, the applicant should carry out the following test program for the most critical, probable failure condition for which the associated procedure requires the airplane to exit the icing condition. The test program should be modified as necessary to reflect the specific operating procedures.
Maneuvers
4.22.2.1 The ice accretion is defined as a combination of the following:
4.22.2.1.1 Ice on unprotected surfaces. The holding ice accretion described in paragraph A.2.1.3 of appendix A of this AC.
4.22.2.1.2 Ice on normally protected surfaces that are no longer protected. The failure ice accretion described in paragraph A.3.2 of appendix A of this AC.
4.22.2.1.3 Ice on normally protected surfaces that are still protected following segmental failure of a cyclical deice system. The ice accretion that will form during the rest time of the deice system following the critical failure condition.
4.22.2.2 The airplane should be loaded to a medium to light weight, at aft center-of-gravity position, with symmetric fuel loading.
4.22.2.3 In the configurations listed in paragraphs 4.22.2.3.1 through 4.22.2.3.3 below, trim the airplane at the specified speed. Conduct 30° banked turns left and right with normal reversals. Conduct a pull-up maneuver to 1.5 g and a pushover maneuver to 0.5 g.
4.22.2.3.1 High lift devices retracted configuration (or holding configuration if different): Holding speed, power or thrust for level flight. In addition, deploy and retract the deceleration devices.
4.22.2.3.2 Approach configuration: Approach speed, power or thrust for level flight.
4.22.2.3.3 Landing configuration: Landing speed, power or thrust for landing approach (limit pull-up to 1.3 g). In addition, conduct steady heading sideslips to the angle of sideslip appropriate to the airplane type and the AFM landing procedure.
4-38
4.22.2.4 In the configurations listed in paragraphs 4.22.2.4.1 and 4.22.2.4.2 below, trim the airplane at the estimated 1.3 VSR. Decrease speed at approximately 1 knot per second until 1 second after stall warning, and demonstrate prompt recovery using the same recovery maneuver as for the uncontaminated airplane. It is acceptable for stall warning to be provided by a different means (for example, by the behavior of the airplane rather than by stick shaker) for failure cases not considered probable.
4.22.2.4.1 High lift devices retracted configuration: Straight Flight/Power Off.
4.22.2.4.2 Landing configuration: Straight Flight/Power Off.
4.22.2.5 Conduct an approach and go-around with all engines operating using the AFM approach and go-around procedure.
4.22.2.6 Conduct an approach and landing with all engines operating (unless the one-engine-inoperative condition results in a more critical probable failure condition) using the appropriate AFM approach and landing procedure.
4.22.3 For improbable failure conditions, flight testing may be required to demonstrate that the effect on safety of flight (as measured by degradation in flight characteristics) supports the system safety analysis, or to verify results of analyses or wind tunnel tests. The extent of each required flight testing should be similar to that described in paragraph 4.22.3 above, or as agreed to by the responsible ACO for the specific failure condition.
Appendix A A-1 APPENDIX A. AIRFRAME ICE ACCRETIONS A.1 Overview.
A.1.1 In accordance with § 25.1419, each airplane certified for flight in icing conditions must be capable of safely operating in the continuous maximum and intermittent maximum icing conditions of part 25, Appendix C. Therefore, at a minimum, certification for flight in icing conditions must include consideration of ice accretions that can occur in Appendix C icing conditions.
A.1.2 In accordance with § 25.1420(a)(1), each airplane with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls certified for flight in icing conditions must, at a minimum, be capable of safely operating: (1) in the atmospheric icing conditions of part 25, Appendix C, and (2) after encountering the atmospheric icing conditions of part 25, Appendix O, and while subsequently exiting all icing conditions. Therefore, for airplanes with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls, certification for flight in icing conditions must consider, at a minimum, ice accretions that can occur during flight in Appendix C icing conditions and during detection and exiting of Appendix O icing conditions.
A.1.3 In accordance with § 25.1420(a)(2), an airplane with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls may also be certified for operation in a portion of the atmospheric icing conditions of part 25, Appendix O. In that case, the airplane must also be capable of operating safely after encountering, and while exiting, atmospheric icing conditions in the portion of Appendix O for which operation is not approved. Ice accretions used for certification must consider— A.1.3.1 Operations in Appendix C icing conditions, A.1.3.2 Operations in the Appendix O icing conditions for which approval is sought, and A.1.3.3 Detection and exiting of the Appendix O icing conditions beyond those for which approval is sought.
A.1.4 In accordance with § 25.1420(a)(3), in addition to being certified for flight in Appendix C conditions, an airplane with a maximum takeoff weight less than 60,000 pounds or with reversible flight controls may be certified for operation throughout the atmospheric icing conditions of part 25, Appendix O. Certification for flight throughout the atmospheric icing conditions of Appendix O must consider ice accretions resulting from— A.1.4.1 Operations in Appendix C icing conditions, and A.1.4.2 Operations in Appendix O icing conditions.
Appendix A A-2 A.1.5 The part 25, subpart B, airplane performance and handling characteristics requirements identify the specific ice accretions that apply in showing compliance. In accordance with Appendix C, part II(b), and Appendix O, part II(e), to reduce the number of ice accretions used for demonstrating compliance, the applicant may use any of the applicable ice accretions (or a composite accretion representing a combination of accretions) to show compliance with a particular subpart B requirement if that accretion is either the ice accretion identified in the requirement or is shown to be more conservative than the ice accretion identified in the requirement. In addition, the ice accretion with the most adverse effect on handling characteristics may be used for compliance with the airplane performance requirements if any difference in performance is conservatively taken into account. Ice accretion(s) used to show compliance should take into account the speeds, configurations (including configuration changes), angles-of-attack, power or thrust settings, etc., for the flight phases and icing conditions they are intended to cover.
A.1.6 The applicant should determine the most critical ice accretion in terms of handling characteristics and performance for each flight phase. Parameters to be considered include— A.1.6.1 Flight conditions (for example, airplane configuration, speed, angle-of-attack, altitude), and A.1.6.2 Atmospheric icing conditions for which certification is desired (for example, temperature, liquid water content (LWC), drop mean effective diameter (MED), and drop median volume diameter (MVD)).
A.1.7 For each phase of flight, the shape, chordwise and spanwise, and the roughness of the shapes considered in selection of a critical ice shape should accurately reflect the full range of atmospheric icing conditions for which certification. Justification and selection of the most critical ice shape for each phase of flight should be agreed to by the responsible ACO.
A.1.8 See Appendix R of AC 20-73A, Aircraft Ice Protection, dated August 16, 2009, for additional detailed information about determining the applicable critical ice accretion (shape and roughness).
A.2 Ice Accretions With an Operative Ice Protection System.
A.2.1 All Flight Phases Except Takeoff.
A.2.1.1 For unprotected parts, the ice accretion to be considered should be determined in accordance with part 25, Appendices C and O.
A.2.1.2 Unprotected parts consist of the unprotected airfoil leading edges and all unprotected airframe parts on which ice may accrete. The effect of ice accretion on protuberances such as antennas or flap hinge fairings need
Appendix A A-3 not normally be investigated. However, airplanes characterized by unusual unprotected airframe protuberances, for example fixed landing gear, large engine pylons, or exposed control surface horns or winglets may experience significant additional effects, which should therefore be taken into consideration.
A.2.1.3 For holding ice, the applicant should determine the effect of a 45-minute hold in continuous maximum icing conditions. The analysis should assume that the airplane remains in a rectangular “race track” pattern, with all turns being made within the icing cloud. Therefore, no horizontal extent correction should be used for this analysis. For some previous airplane certification programs, the maximum pinnacle height was limited to 3 inches. This method of compliance may continue to be accepted for follow-on products if service experience has been satisfactory, and the designs are similar enough to conclude that the previous experience is applicable. The applicant should substantiate the critical MED, LWC, and temperature resulting in the formation of an ice accretion critical to the airplane’s performance and handling qualities. The shape and texture of the ice are important and should be agreed to by the responsible ACO.
A.2.1.4 In accordance with the definitions of holding ice, approach ice, and landing ice in Appendices C and O, these flight segments are considered contiguous and sequential. Unless there is a procedure to deice surfaces that have previously accreted ice in the holding flight phase, approach ice is normally the same accretion as the holding ice accretion plus any additional ice accreted during the transition to the approach configuration.
Similarly, unless there is a procedure to deice surfaces before landing, the landing ice accretion is normally the same accretion as the approach ice accretion plus any additional ice accreted during transition to the final landing configuration. A total time of 45 minutes can be used for the combination of hold, approach, and landing ice.
A.2.1.5 For protected parts, the ice protection systems are normally assumed to be operative. However, the applicant should consider the effect of ice accretion on or behind the protected surfaces that results from the rest time of a deicing cycle. Performance may be established on the basis of a representative intercycle ice accretion for normal operation of the deicing system. (The applicant should also consider effects of each residual ice accretion that is not shed). The average drag increment determined over the deicing cycle may be used for performance calculations. Consider the effect of— A.2.1.5.1 Runback ice that occurs on or downstream of the protected surface (including deicing boots), and A.2.1.5.2 Ice accretion prior to activation and operation of the ice protection system.
(See paragraph A.2.3 of this appendix.)
Appendix A A-4 A.2.2 Takeoff Phase.
A.2.2.1 For both unprotected and protected parts, the ice accretion identified in part 25, Appendices C and O, for the takeoff phase may be determined by calculation, assuming the following:
A.2.2.1.1 Airfoils, control surfaces, and propellers if applicable are free from frost, snow, or ice at the start of the takeoff.
A.2.2.1.2 The ice accretion starts at the end of the takeoff distance.
A.2.2.1.3 The airplane is at the critical ratio of power/thrust-to-weight.
A.2.2.1.4 Failure of the critical engine occurs at VEF.
A.2.2.1.5 The flightcrew activates the ice protection system in accordance with an AFM procedure; except that after beginning the takeoff roll, no flightcrew action to activate the ice protection system should be assumed to occur until the airplane is 400 feet above the takeoff surface.
A.2.2.2 The ice accretions identified in part 25, Appendix C, for the takeoff phase are takeoff ice and final takeoff ice.
A.2.2.2.1 Takeoff ice: The most critical ice accretion on unprotected surfaces, and any ice accretion on protected surfaces appropriate to normal ice protection system operation, occurring between the end of the takeoff distance and 400 feet above the takeoff surface, assuming accretion starts at the end of the takeoff distance in the takeoff maximum icing conditions of part 25, Appendix C, part I(c).
A.2.2.2.2 Final takeoff ice: The most critical ice accretion on unprotected surfaces, and any ice accretion on the protected surfaces appropriate to normal ice protection system operation, between 400 feet and the height at which the transition to the en route configuration and speed is completed, or 1,500 feet above the takeoff surface, whichever is higher, assuming accretion starts at the end of the takeoff distance in the takeoff maximum icing conditions of part 25, Appendix C, part I(c).
A.2.2.3 The final takeoff ice accretion includes the takeoff ice accretion since these flight segments are contiguous and sequential. For both flight path segments, ice begins accreting at the end of the takeoff distance. The reference to a height of 400 feet above the takeoff surface in these definitions is only used to separate the two takeoff segments. It does not mean that ice accreted before reaching 400 feet above the takeoff surface may be ignored when determining the final takeoff ice accretion.
A.2.2.4 The takeoff ice and final takeoff ice accretions for Appendix O are the same as those for Appendix C, except that the icing conditions are defined in part 25, Appendix O, part I.