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3.2.2 Flight Testing Using Approved Simulated Ice Accretions.
3.2.2.1 Tests for airplane performance and handling characteristics in icing conditions may be based on flight testing performed in dry air using simulated ice accretions that have been agreed to by the responsible ACO.
3.2.2.2 Additional limited flight tests should be conducted in natural icing conditions, as discussed in paragraph 3.2.3.2 below.
3.2.3 Flight Testing in Natural Icing Conditions.
3.2.3.1 If the applicant uses flight testing with ice accretions obtained in natural atmospheric icing conditions as the primary means of showing compliance, all relevant meteorological conditions (including liquid water content (LWC) and median volume diameter (MVD) of the cloud drops) should be measured and recorded. (The Appendix C icing envelopes use the term mean effective diameter (MED) rather than MVD, but this equates to MVD for the instrumentation used and the drop size distribution assumed at the time those envelopes were established.) The tests should ensure good coverage of the atmospheric icing conditions of part 25, Appendices C and O (consistent with the extent of the certification approval sought for operation in Appendix O icing conditions) and, in particular, the critical conditions for the airplane. The conditions for accumulating ice (including the icing atmosphere, airplane configuration, speed, and duration of exposure) should be agreed to by the responsible ACO.
3.2.3.2 If the applicant uses flight testing with simulated ice accretions as the primary means of showing compliance, additional limited flight tests should be conducted with ice accretions obtained in natural icing conditions. The objective of these tests is to corroborate the simulated ice accretions, and the handling characteristics and performance results obtained in flight testing with the simulated ice accretions. It is not necessary to measure the atmospheric icing characteristics (for example, LWC and MVD) of these flight test icing conditions. For derivative airplanes with aerodynamic characteristics and ice protection systems similar to the ancestor airplane, it may not be necessary to carry out additional flight tests in natural icing conditions if such tests have already been performed with the ancestor airplane. Depending on the extent of the Appendix O icing conditions for which certification is being sought, and the means used for showing compliance with the performance and handling characteristics requirements, it may also not be necessary to conduct flight tests in the natural icing conditions of Appendix O. See AC 25-28 for guidance on when it is necessary to conduct flight tests in the natural atmospheric icing conditions of Appendix O.
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3.3 Wind Tunnel Testing and Analysis.
To help substantiate acceptable performance and handling characteristics, the applicant may use analysis of results of dry air wind tunnel testing of models with the simulated ice accretions defined in Appendices C and O.
3.4 Engineering Simulator Testing and Analysis.
Results of an engineering simulator airplane analysis that includes effects of ice accretions defined in Appendices C and O may be used to help substantiate acceptable handling characteristics. Data used to model the effects of ice accretions for the engineering simulator may be based on results of dry air wind tunnel tests, flight tests, computational analysis, or on engineering judgment.
3.5 Engineering Analysis.
To help substantiate acceptable performance and handling characteristics, the applicant may use an engineering analysis that includes effects of the ice accretions defined in Appendices C and O. Effects of the ice accretions used in this analysis may be determined by analyzing results of dry air wind tunnel tests, flight tests, computational analyses, engineering simulator analyses, or by engineering judgment.
3.6 Ancestor Airplane Analysis.
3.6.1 To help substantiate acceptable performance and handling characteristics, the applicant may use an analysis of an ancestor airplane that includes effects of ice accretions as defined in Appendices C and O. This analysis should consider the similarity of the airplane configuration, operating envelope, performance and handling characteristics, and ice protection system of the ancestor airplane to the one being certified.
3.6.2 The analysis may include flight test data, dry air wind tunnel test data, icing tunnel test data, engineering simulator analysis, service history, or engineering judgment.
4-17 CHAPTER 4. ACCEPTABLE MEANS OF COMPLIANCE—FLIGHT TEST PROGRAM
4.1 Overview.
4.1.1 This chapter provides guidance for developing an acceptable flight test program. Such a test program may be used if the applicant selects flight testing as the primary means for showing compliance with the requirements for airplane performance and handling qualities for flight in icing conditions and if that method of showing compliance is agreed to by the responsible ACO.
4.1.2 Section 25.21(a)(1) states that compliance with each requirement of subpart B must be shown by tests upon an airplane of the type for which certification is requested, or by calculations based on, and equal in accuracy to, the results of testing. In accordance with § 25.21(a)(1), calculations, if used, must provide at least the same degree of confidence of compliance as testing would provide.
4.1.3 Ice accretions for each flight phase are defined in part 25, part II of Appendices C and O. Additional guidance for determining the applicable ice accretions is provided in appendix A of this AC.
4.1.4 This example test program uses the holding ice accretion for the majority of the testing, assuming that it is the most conservative ice accretion. However, in accordance with part 25, Appendix C, part II(b), and part 25, Appendix O, part II(e), if the holding ice accretion is not as conservative as the ice accretion appropriate to the flight phase, then the ice accretion appropriate to the flight phase and icing conditions of Appendix C or O, as applicable, or a more conservative ice accretion, must be used. In general, the applicant may choose to use either a conservative ice accretion or the specific ice accretion appropriate to the particular phase of flight and icing conditions.
4.1.5 For the approach and landing configurations, in accordance with the guidance provided in paragraph 2.1.11 of this AC, the flight tests in natural icing conditions specified in table 4-5 of this AC are usually sufficient to evaluate whether ice accretions on trailing edge flaps adversely affect airplane performance or handling qualities. If these tests show that airplane performance or handling qualities are adversely affected, additional tests may be necessary to show compliance with the airplane performance and handling qualities requirements.
4.1.6 Unless otherwise specified, the speeds (for example, VSR, VREF, V2, etc.) referenced in the flight tests described in this AC refer to the speeds used with the appropriate ice accretion on the airplane.
4.2 Stall Speed, § 25.103.
4.2.1 The stall speed for intermediate high lift configurations (for takeoff configurations, for example) can normally be obtained by interpolation. However, additional tests may be necessary if—
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4.2.1.1 A stall identification system (for example, a stick pusher) activation point is set as a function of the high lift configuration,
4.2.1.2 The activation point is reset for icing conditions, or
4.2.1.3 Significant configuration changes occur with extension of trailing edge flaps (such as extension of wing leading edge high lift devices).
4.2.2 Acceptable Test Program.
The following specifications represent an example of an acceptable test program subject to the provisions outlined paragraph 4.2.1 of this AC.
Maneuvers
4.2.2.1 Load the airplane to a forward center-of-gravity position appropriate to the airplane configuration.
4.2.2.2 Conduct the test at the stall test altitude used in non-icing tests.
4.2.2.3 Trim at an initial speed of 1.13 to 1.30 VSR. Decrease speed at a rate not to exceed 1 knot per second until an acceptable stall identification is obtained. Perform this maneuver with the following ice accretions:
4.2.2.3.1 In high lift devices retracted configuration—final takeoff ice.
4.2.2.3.2 In high lift devices retracted configuration—en route ice.
4.2.2.3.3 In holding configuration—holding ice.
4.2.2.3.4 In lowest lift takeoff configuration—holding ice.
4.2.2.3.5 In highest lift takeoff configuration—takeoff ice.
4.2.2.3.6 In highest lift landing configuration—holding ice.
4.3 Accelerate–Stop Distance, § 25.109.
To comply with this requirement, the effect of any increase in V1 for taking off in icing conditions may be determined by a suitable analysis.
4.4 Takeoff Path, § 25.111.
In accordance with § 25.105(a), the applicant should conduct takeoff evaluations to substantiate the speed schedule and distances for takeoff in icing conditions if the following applies:
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4.4.1 VSR in the configuration defined by § 25.121(b) with the takeoff ice accretion exceeding VSR for the same configuration without ice accretions by more than the greater of 3 knots or 3 percent.
4.4.2 Any need for a takeoff speed increase, and any effects of thrust loss or drag increase on the takeoff path, may be determined by a suitable analysis.
4.5 Landing Climb: All–Engines–Operating, § 25.119.
The following specifications represent an example of an acceptable test program for showing compliance with § 25.119.
Maneuvers
4.5.1 The holding ice accretion should be used.
4.5.2 The airplane should be loaded to a forward center-of-gravity position appropriate to the airplane configuration.
4.5.3 The airplane should be in the highest lift landing configuration, at a landing climb speed no greater than VREF.
4.5.4 Stabilize the airplane at the specified speed and conduct two climbs or drag polar checks as agreed to by the responsible ACO.
4.6 Climb: One-Engine-Inoperative, § 25.121.
The following specifications represent an example of an acceptable test program for showing compliance with § 25.121.
Maneuvers
4.6.1 The airplane should be loaded to a forward center-of-gravity position appropriate to the configuration.
4.6.2 Stabilize the airplane at the specified speed with one engine inoperative (or simulated inoperative if all relevant effects of an inoperative engine can be taken into account) and conduct two climbs or drag polar checks in each of the following airplane configurations, with the specified ice accretions, to substantiate the asymmetric drag increment as agreed to by the responsible ACO.
4.6.2.1 High lift devices retracted configuration, final takeoff climb speed—final takeoff ice.
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4.6.2.2 Lowest lift takeoff configuration, landing gear retracted, V2 climb speed— takeoff ice.
4.6.2.3 Approach configuration appropriate to the highest lift landing configuration, landing gear retracted, approach climb speed—holding ice.
4.7 En Route Flight Paths, § 25.123.
The following specifications represent an example of an acceptable test program to show compliance with § 25.123.
Maneuvers
4.7.1 The en route ice accretion should be used.
4.7.2 The airplane should be loaded to a forward center-of-gravity position appropriate to the airplane configuration.
4.7.3 The airplane should be in the en route configuration at the en route climb speed.
4.7.4 Stabilize the airplane at the specified speed with one engine inoperative (or simulated inoperative if all relevant effects of an inoperative engine can be taken into account) and conduct two climbs or drag polar checks to substantiate the asymmetric drag increment as agreed to by the responsible ACO.
4.8 Landing, § 25.125.
The effect of any landing speed increase on the landing distance may be determined by a suitable analysis.
4.9 Controllability and Maneuverability—General, § 25.143.
4.9.1 A qualitative and quantitative evaluation is usually necessary to show compliance with the controllability and maneuverability requirements of § 25.143. Depending on how clearly compliance is demonstrated, or whether the force limits or stick force per g limits of § 25.143 are being approached, additional substantiation may be necessary to ensure that the airplane complies.
4.9.2 Acceptable Test Program.
The following specifications represent an example of an acceptable test program to evaluate general controllability and maneuverability, subject to the provisions outlined above.
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Maneuvers
4.9.2.1 The holding ice accretion should be used.
4.9.2.2 The airplane should be loaded to a medium to light weight, with an aft center-of-gravity position, and symmetric fuel loading.
4.9.2.3 In the configurations listed in table 4-1 below, trim the airplane at the specified speeds and conduct the following maneuvers:
4.9.2.3.1 Thirty-degree banked turns left and right with rapid reversals.
4.9.2.3.2 A pull-up to 1.5 g (except that this may be limited to 1.3 g at VREF) and a pushover to 0.5 g (except that the pushover is not required at VMO and VFE).
4.9.2.3.3 Include deployment and retraction of deceleration devices.
Table 4-1. Controllability and Maneuverability—Trim Speeds Configuration Trim Speed High lift devices retracted configuration 1.3 VSR and VMO or 250 KIAS, whichever is less Lowest lift takeoff configuration 1.3 VSR and VFE or 250 KIAS, whichever is less Highest lift landing configuration VREF and VFE or 250 KIAS, whichever is less VSR – Reference Stall Speed VMO – Maximum operating limit speed KIAS – Knots indicated airspeed VFE – Maximum flap extended speed VREF – Reference landing speed
4.9.2.4 In the lowest lift takeoff configuration, at the greater of 1.13 VSR or V2MIN, with one engine inoperative (simulated), conduct 30° banked turns left and right with normal turn reversals and, in wings-level flight, a 5-knot speed decrease and increase.
4.9.2.5 Conduct an approach and go-around with all engines operating using the AFM approach and go-around procedure.
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4.9.2.6 Conduct an approach and go-around with one engine inoperative (simulated) using the AFM one-engine-inoperative approach and go-around procedure.
4.9.2.7 Conduct an approach and landing using the AFM approach and landing procedure. In addition, you should demonstrate satisfactory controllability during a landing at VREF minus 5 knots. These tests should be done at heavy weight and forward center-of-gravity.
4.9.2.8 Conduct an approach and landing with one engine inoperative (or with one engine simulated to be inoperative if all effects associated with an engine failure can be taken into account) using the procedure provided in the AFM.
4.9.3 Evaluation of Lateral Control Characteristics.
Aileron hinge moment reversal and other lateral control anomalies have been identified as causal factors in icing accidents and incidents. The following maneuvers, along with the following two evaluations, are intended to determine susceptibility of the airplane to aileron hinge moment reversals or other adverse effects on lateral control characteristics due to ice accretion.
E valuations
4.9.3.1 Evaluate lateral controllability during deceleration to the stall warning speed (covered in paragraph 4.17.2.5 of this AC), and
4.9.3.2 Evaluate static lateral-directional stability (covered in paragraph 4.15 of this AC).
Maneuvers
4.9.3.3 For each of the test conditions specified in table 4-2 below, Perform the following maneuvers:
4.9.3.3.1 Establish a 30° banked level turn in one direction.
4.9.3.3.2 Using a step input of approximately 1/3-full lateral control deflection, roll the airplane in the other direction.
4.9.3.3.3 Maintain the control input as the airplane passes through a wings level attitude.
4.9.3.3.4 At approximately 20° of bank, apply a step input in the opposite direction to approximately 1/3-full lateral control deflection.
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4.9.3.3.5 Release the control input as the airplane passes through a wings level attitude.
4.9.3.3.6 Repeat this test procedure with 2/3- and up-to-full lateral control deflection unless the roll rate or structural loading is judged excessive. It should be possible to readily arrest and reverse the roll rate using only lateral control input, and the lateral control force should not reverse with increasing control deflection.
Table 4-2. Test Conditions—Lateral Control Characteristics Test Condition 1 Holding configuration, holding ice accretion, maximum landing weight, forward center-of-gravity position, at minimum holding speed (highest expected holding angle-of-attack) 2 Landing configuration, holding ice accretion, medium to light weight, forward center-of-gravity position, at VREF (highest expected landing approach angle-of-attack)
4.9.4 Low g Maneuvers and Sideslips.
The maneuvers in paragraph 4.9.4.3 of this AC represent an example of an acceptable test program for showing compliance with controllability requirements in low g maneuvers and in sideslips to evaluate susceptibility to ice-contaminated tailplane stall.
4.9.4.1 Section 25.143(i)(2).
4.9.4.1.1 The regulation states: “It must be shown that a push force is required throughout a pushover maneuver down to a zero g load factor, or to the lowest load factor obtainable if limited by elevator power or other design characteristic of the flight control system. It must be possible to promptly recover from the maneuver without exceeding a pull control force of 50 pounds….”
4.9.4.1.2 An example of another design characteristic of the flight control system that would limit the lowest obtainable load factor would be a g-limiting envelope protection system.
4.9.4.2 Section 25.143(i)(3).
4.9.4.2.1 For sideslips, the regulation requires that: “Any changes in force that the pilot must apply to the pitch control to maintain speed with increasing
4-24 sideslip angle must be steadily increasing with no force reversals, unless the change in control force is gradual and easily controllable by the pilot without using exceptional piloting skill, alertness, or strength.” (See paragraph 4.15.1 of this AC for lateral-directional aspects).
4.9.4.2.2 Abrupt changes in the control force characteristic, unless so small as to be unnoticeable, would not be considered compliant with the requirement that the force be steadily increasing. A gradual change in control force is a change that is not abrupt and does not have a steep gradient. It can be easily managed by a pilot of average skill, alertness, and strength. Control forces in excess of those permitted by § 25.143(d) would be considered excessive.
4.9.4.3 Test maneuvers used to show compliance with the regulatory requirements quoted in paragraphs 4.9.4.1.1 and 4.9.4.2.1 above should be conducted using the following ice accretions, configurations, and procedures:
Maneuvers
4.9.4.3.1 The holding ice accretion should be used. For airplanes with unpowered elevators, these tests should also be performed with sandpaper ice.
4.9.4.3.2 The airplane should be loaded to a medium to light weight, at the most critical center-of-gravity position, with symmetric fuel loading, and should be in the highest lift landing configuration.
4.9.4.3.3 Test maneuver for showing compliance with § 25.143(i)(2) is provided in table 4-3 below.
4-25 Table 4-3. Test Maneuver—§ 25.143(i)(2) Step Maneuver 1 Start with the airplane in trim, or as near as possible to being in trim, at the most critical of the following trim speeds:
(1) Trim speed 1.23 VSR, target speed not more than 1.23 VSR.
(2) Trim speed VFE, target speed not less than VFE - 20 knots.
2 Pull up to a suitable pitch attitude, then—
(1) Push over in a continuous maneuver (without changing trim) to reach zero g normal load factor or, if limited by elevator control authority (or other design characteristic), the lowest load factor obtainable at the target speed, as the airplane’s pitch attitude passes through approximately level flight (that is, through the horizon), and
(2) Perform this maneuver with idle power or thrust and with go-around power or thrust.
4.9.4.3.4 Test maneuver for showing compliance with § 25.143(i)(3): Conduct steady heading sideslips to full rudder input, 180 pounds rudder force, or full lateral control authority (whichever comes first) at a trim speed of
1.23 VSR and with the power or thrust needed for a -3° flight path angle.
4.9.5 Controllability Prior to Activation and Operation of the Ice Protection System.
The following is an example of an acceptable test program for showing compliance with § 25.25.143(j).
Maneuvers
4.9.5.1 For the configurations, speeds, and power settings listed below, with the ice accretion specified in the requirement, trim the airplane at the specified speed, conduct a pull-up maneuver to 1.5 g and pushover maneuver to
0.5 g, and show that longitudinal control forces do not reverse.
4.9.5.1.1 High lift devices retracted configuration (or holding configuration if different), holding speed, power or thrust for level flight.
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4.9.5.1.2 Landing configuration, VREF for non-icing conditions, power or thrust for landing approach. If necessary, limit the pull-up maneuver to the point at which stall warning occurs.
4.10 Longitudinal Control, § 25.145.
4.10.1 No specific quantitative evaluations are required for demonstrating compliance with§ 25.145(b) and (c). Qualitative evaluations should be combined with the other testing. Review results of tests on the uncontaminated airplane for any cases of marginal compliance. All tests showing marginal compliance should be repeated with ice accretions on the airplane.
4.10.2 Acceptable Test Program.
The following specifications represent an example of an acceptable test program for compliance with § 25.145(a).
Maneuvers
4.10.2.1 The holding ice accretion should be used.
4.10.2.2 The airplane is at a medium to light weight, aft center-of-gravity position, with symmetric fuel loading.
4.10.2.3 In the configurations listed below, trim the airplane at 1.3 VSR. Reduce speed approximately 1 knot per second using elevator control to 1 second past stall warning and demonstrate prompt recovery to the trim speed using elevator control.
4.10.2.3.1 High lift devices retracted configuration, maximum continuous power or thrust.
4.10.2.3.2 Maximum lift landing configuration, maximum continuous power or thrust.
4.11 Directional and Lateral Control, § 25.147.
To show compliance with § 25.147, applicants should combine qualitative evaluations with the other testing (for example, the roll tests described in paragraphs 4.9.2.3.1 and
4.9.2.4 of this AC). Review the results of the tests on the uncontaminated airplane for any cases of marginal compliance. All tests showing marginal compliance should be repeated with ice accretions on the airplane.