(2) Handling Qualities. Handling qualities evaluation should include actual flight investigation of at least the following with the artificial ice shapes:
(a) Stall characteristics, stall warning and stall speeds;
(b) Trim;
(c) Lateral directional stability/control;
(d) Longitudinal stability/control;
(e) VMC;
(f) Landing approach speeds, maneuvering and landing characteristics; and
(g) Appropriate high speed characteristics up to VMO/MMO/VNE, or 250 knots continuous airspeed, whichever is less.
( h) Stall characteristics and adequacy of stall warning should also be evaluated when the airplane is stalled with the autopilot engaged, unless the design of the autopilot precludes its ability to operate beyond stall warning angle of attack. For these designs the controllability of the airplane at autopilot disconnect should be evaluated.
1. The stall warning should meet the requirements of § 23.207(a), (b), and
(c). The type of stall warning in icing conditions should be the same as in no icing conditions. Biasing of the stall warning and stall protection system, if installed, may be required to achieve acceptable margins to stall. The maneuver margin requirements § 23.207(d) should be demonstrated in icing conditions.
(i) The handling qualities test matrix for ice shapes can be reduced from the basic (no ice) matrix, with concurrence from the Administrator, to configurations and flight conditions that were deemed critical based on the no ice testing (basic aircraft certification).
(3) Ice Contaminated Tailplane Stall (ICTS). ICTS occurs due to airflow separation on the lower surface of the tailplane that is caused by the angle-of-attack of the horizontal tailplane being increased above the reduced stall angle-of-attack that can result when even small quantities of ice have formed on the tailplane leading edge. The increase in tailplane angle-of-attack can result from airplane configuration (for example, increased flap extension increasing the downwash angle or trim required for the center-of-gravity position) and/or flight conditions (for example, high approach speed resulting in an increased flap downwash angle and reduced angle-of-attack, gusts, maneuvering or engine power changes). ICTS is characterized by a reduction or loss, sometimes sudden, of pitch control or stability while operating in, or recently departing from, icing conditions. For 25
airplanes with longitudinal control systems that are not powered, the pressure differential between the upper and lower surfaces of the stalled tailplane may result in a high elevator hinge moment, forcing the elevator trailing edge down. This elevator hinge moment reversal can be of sufficient magnitude to draw the control column forward with a level of force that is beyond the combined efforts of the flightcrew to overcome. On some airplanes, ICTS has been caused by a lateral flow component coming from the vertical stabilizer, as may occur in sideslip conditions or due to a gust with a lateral component.
An evaluation should be made to determine if this unsafe flight condition is likely to occur. Susceptible airplanes are those having a near zero or negative stall margin with contamination. Flight test procedures for determining susceptibility to ICTS are included in the new AC 23.143-1, Ice Contaminated Tailplane Stall (ICTS).
e. Performance and Handling Qualities – § 23.1419 at Amendment 23-43 or later. In addition to the guidance of Paragraph 11.d., in accordance with § 23.1419(a), "capable of operating safely" means that airplane performance, controllability, maneuverability, and stability may be degraded from the non-iced airplane but must not be less than the requirements in part 23, Subpart B. This means that the aircraft must comply with the applicable requirements in § 23.45 through § 23.157 and § 23.171 through § 23.181, with the exception discussed in Paragraph 11.e.(1).
(1) Performance. Section 23.1419. Amendment 23-43, requires “…airplane performance…must not be less than that required in part 23, Subpart B.” The stall speed requirements of § 23.49 are included in Subpart B performance. For single engine aircraft that do not meet the emergency landing requirements of § 23.562(d), the stall speed at maximum weight must not exceed 61 knots. Multi-engine airplanes under 6,000 pounds are required to meet § 23.67(a)(1) in icing conditions and therefore would not be subject to the 61-knot stall speed requirement. Recent flight testing of deicing bootequipped aircraft with simulated intercycle/residual ice has shown stall lift coefficient losses of 17 percent to 23 percent with flaps extended. These lift losses were experienced on an airplane equipped with a stick pusher and on an airplane whose stall was defined by aerodynamic wing stall. This can represent a significant performance penalty for new single engine aircraft if they had to be designed to meet the 61-knot stall speed requirement with ice on protected surfaces. Recently certificated single engine part 23 airplanes would most likely not meet this requirement since their no-ice stall speed in landing configuration is at or near 61 knots.
/g183/g32 In the notice of proposed rulemaking published in the Federal Register on October 3, 1990 (55 FR 40598), for the proposed rule that was to become Amendment 23-43, the FAA stated the background for imposing Subpart B requirements on part 23 airplanes versus part 25 transport airplanes: “The justification given was that normal and transport category airplanes must operate in about the same icing environment, but the normal category airplane is more likely to remain in icing conditions for longer periods of time because it may not have the performance capability to exit the icing environment as readily as transport category airplanes.” Normal category airplane 26
airfoils, being smaller than those of transport airplanes, are much more efficient collectors of ice and their percentage drag increase in icing conditions is larger than transport airplanes. The requirement to meet Subpart B performance was added to guarantee there would be a given level of excess power that could be used to exit icing conditions. An increase in stall speed in icing would not prevent an airplane from meeting the Subpart B performance requirements if it was accounted for in analyses and testing.
/g183/g32 For single engine airplanes that do not meet the 61-knot stall speed requirement with critical ice accretions, the applicant should consider the following compensating features to propose an exemption to the stall speed requirement of § 23.1419(a), Amendment 23-43. The exemption with the following compensating features would not adversely affect safety since it is safer to make a forced landing at higher speed than it is to inadvertently stall the airplane. There have been much more fatal accidents in icing conditions attributed to the latter.
(a) The airplane with no ice accretions meets the 61-knot stall speed requirement of § 23.49(c);
(b) The airplane with critical ice accretions complies with stall warning requirements of § 23.207.
1. For aircraft with artificial stall warning systems, item (b) though, would require a bias in the stall warning schedules to maintain adequate stall warning margins.
2. For aircraft without artificial stall warning systems, item (b) may require one be installed to meet minimum stall warning margin requirements. Meeting § 23.207(d) may require an increase in operational speeds in icing to preclude nuisance stall warnings.
(c) The AFM performance data in icing conditions reflects the higher stall and operating speeds.
(d) Most importantly, the airplane with critical ice accretions has acceptable stall characteristics and is safely controllable with normal piloting skill as required by §§ 23.201 and 23.203.
(e) The tire requirements of § 23.733 and brake requirements of § 23.735 are met with the higher stall and operating speeds.
(f) The ground handling requirements of §§ 23.231, 23.233 and 23.235 and nose/tail wheel steering system of § 23.745 (if applicable) are met with the higher landing speeds.
(g) All other airplane system or testing requirements that could be affected by higher operating speeds, such as autopilot and flight director gains are evaluated.
(h) Each seat/restraint system would have to include a safety belt and shoulder harness with a metal to metal latching device (this would address STCs on older airplanes that do not include § 23.785 in their certification basis).
(i) The airplane certification basis would have to include § 23.1091 at Amendment 23-51 and § 23.1093 at Amendment 23-51 to provide the latest regulations for engine operation in icing conditions.
(j) The airplane certification basis would have to include § 23.995 at Amendment 23-29. This regulation was promulgated as a result of an NTSB recommendation and a 1983 study which indicated at least half of off-field forced landings were a result of fuel mismanagement.
/g183/g32 Critical ice accretions include 45-minute shapes on unprotected surfaces; and preactivation, intercycle and residual shapes/roughness on protected surfaces.
/g183/g32 The above approach represents only the minimum consideration, other issues may have to be considered depending on the aircraft design.
/g183/g32 When analyzing climb performance data to determine if Subpart B climb performance requirements are met, it can be assumed that ice accretions will not be present on the airframe at outside air temperatures (OAT) at and above +5 °C. The minimum climb requirements on a standard day at sea level as required on the non-iced (icing systems off) are to be applied to the iced airplane at +5 °C OAT at sea level.
(2) Handling Qualities. The handling qualities test matrix with ice shapes can be reduced, with Administrator concurrence, from the no-ice basic test matrix to critical flight conditions and configurations as determined by the no-ice basic certification testing. Controllability, maneuverability and stability can be degraded from the non-iced airplane but must still meet Subpart B pass/fail criteria.
(3) Ice Contaminated Tailplane Stall (ICTS). Maximum landing flaps may be limited to the “takeoff/approach” configuration due to ice contaminated tailplane stall (ICTS) characteristics, either with normal operation of the ice protection systems or with a failed horizontal tail system. This is true regardless of the certification basis. Literal interpretation of § 23.1419, Amendment 23-43 means that the aircraft would have to comply with the 61-knot stall speed in icing at the takeoff/approach flap setting if the flap setting was limited to preclude ICTS with normal ice protection system operation. The FAA would also consider the exemption approach described above for the 61-knot rule to address higher stall speeds for those aircraft that limit flaps to takeoff/approach setting with ice accretions to preclude ICTS.
f. Ice Shedding. Ice shed from forward airplane structure could result in damage or erode engine or powerplant components, as well as lifting, stabilizing, and flight control surface leading edges. Fan and compressor blades, impeller vanes, inlet screens and ducts, as well as propellers, are examples of powerplant components subject to damage from shedding ice. For pusher propellers that are very close to the fuselage and well back from the airplane's nose, ice shed from the forward fuselage and from the wings may cause significant propeller damage. Control surfaces such as elevators, ailerons, flaps, and spoilers are also subject to damage, especially those that are thin metallic, non-metallic, or composite constructed surfaces. Trajectory and impingement analysis may not adequately predict such damage. Unpredicted ice shedding paths from forward areas such as radomes and forward wings (canards) have been found to negate the results of this analysis. For this reason, flight tests should be conducted to supplement analysis or a damage analysis should consider the worst case ice shed event. Video or motion pictures are excellent for documenting ice shedding trajectories and impingements, while still photography may be used to document the extent of damage. Any damage should be evaluated for acceptability.
g. Pneumatic Deicer Boots. Many AFMs specify a minimum ice accumulation thickness prior to activation of the deicer boot system. This practice has been in existence due to the belief that a bridge of ice could form if the boots are operated prematurely. Flight testing and icing tunnel testing of several “modern” boot designs have not shown evidence of “ice bridging”, and no degradation in ice shedding performance, when the boots were activated at the first sign of ice accretion. Although the ice may not shed completely with one cycle of the boots, this residual ice will be removed during subsequent boot cycles. Tunnel testing is documented in FAA Technical Report DOT/FAA/AR-02/68, "Effect of Residual and Intercycle Ice Accretions on Airfoil Performance" (May 2002), and recommends that activating the deicing boots “early and often” be given more consideration as a means of limiting the size of intercycle and residual ice accretions. “Modern” boots are defined as high operating pressure (nominal greater than 15 pounds per square inch gauge (psig)) and fast inflation and deflation times. Both one-inch diameter tube designs operating at a nominal 18 psig, and 1.75- inch diameter tube designs, operating at a nominal 15 psig, have been evaluated. The recommended AFM procedure for boot operation should be to operate the boots in an appropriate continuous mode at the first sign of ice and not to wait for a specific amount of ice to accumulate. The boots should be operated until icing conditions are exited and ice no longer adheres to the airframe.
/g183/g32 For applicants that choose to recommend a measurable ice accumulation prior to activation of the boots, flight tests in simulated or natural icing conditions should be accomplished to verify that the crew can detect and recognize the specified ice accumulation thickness. The following test criteria have been accepted for previous flight test programs:
(1) The pilot or a crewmember should be provided a means to detect from his crew position, under both day and night operation, the accumulation level the applicant has specified for activation of the boot system for proper ice removal.
(2) The applicant should show that an ice accumulation margin exists that allows for errors in crew recognition of the ice accumulation level.
/g183/g32 In addition, for applicants that choose to recommend a measurable ice accumulation prior to boot activation, this pre-activation ice accretion must be considered when determining critical ice accretions for performance, stability, control, and stall testing.
Note 3: Usually, selection of the deicing boots to operate causes one cycle of inflation and deflation of all boots, but not necessarily at the same time. Some systems are designed such that all the boots do not complete the cycle if the deicing boots are selected off during the middle of one cycle. For these systems, there should be an AFM warning to the flight crew to select the ice protection on for at least one complete cycle of the deicing boots. This note is equally applicable to any deicing system.
/g183/g32 For deicing systems that do not have a timer to cycle the system automatically once activated, the additional task of manually cycling deicing systems on pilot workload should be evaluated. Recent part 23 applicants found that definition of airframe deicing boot intercycle and residual ice steered them toward one-minute boot cycles and the workload evaluation dictated an automatic timer for the boots.
h. Emergency and Abnormal Operating Conditions. Flight investigations should be conducted to verify that, after pilot recognition of emergency and abnormal operating conditions, the airplane handling qualities have not deteriorated to the extent that the AFM procedures for the condition are ineffective. These demonstrations should be conducted with anticipated residual ice accumulation on normally protected surfaces.
12. PLACARDING AND AFM. This AC provides guidance on airplanes for which the certification basis requires an AFM. Guidance for AFMs in this AC also applies to AFM Supplements.
a. Placarding. Any placarding necessary for the safe operation of the airplane in an icing environment must be provided in accordance with § 23.1541. Examples of such placards are:
(1) Kinds of operation approved (for example, "Flight in Icing Conditions Approved if Ice Protection Equipment is Installed and Operational”).
(2) Equipment limits (for example, "Operation of Windshield Anti-Ice May Cause Compass Deviation in Excess of 10/g176").
(3) Speed restrictions (for example, "Maximum Speed for Boot Operation—175 KIAS").
(4) Fluid filler/g190inlets for fluid freezing point depressants should bear a placard showing approved fluid type and quantity.
b. AFM. The AFM should provide the pilot with the information needed to operate the ice protection system. Information should include:
(1) Operating Limitations Section. Suggested areas to be addressed are as follows:
(a) Limitations on operating time for ice protection equipment if these limitations are based on fluid anti-ice/deice systems capacities and flow rates.
(b) Speed limitations (if any).
(c) Environmental limitations for equipment operations as applicable (for example, minimum temperature for boot operation or maximum altitude for boot operation).
(d) A list of all equipment required for flight in icing conditions. Section
23.1583(h) (CAR § 3.778) requires that this list be included in the Kinds of Operation Equipment List (KOEL).
(e) Minimum engine speed if the engine ice protection system does not function properly below this speed.
(f) A list of required placards.
(g) For commuter category airplanes, the balked landing climb weight, approach climb weight, and landing weight limitations for flight in icing should be presented. The variation in weight limitations may be presented in the Performance section of the manual and included as limitations by specific reference in the Limitations section of the AFM.
(h) Minimum and maximum (as appropriate) airspeed that should be maintained during sustained operations in icing conditions.
(i) For severe icing that may result from environmental conditions outside the icing envelope established as the basis of the approval defined in part 25, Appendix C, information should be provided as follows (see Appendix 2 for an example):
1. A means to identify an icing condition that exceeds the limits of the ice protection system for which the airplane is certificated.
2. Recommended procedures and configurations when exiting the exceedance icing conditions.
3. Procedures to follow during and after flight in these conditions in the event of degraded performance or handling characteristics. Information should include recommended use of flight controls, configuration of high lift devices, drag devices, automatic flight guidance system, engine power/propeller settings (as appropriate), and ice-protection system operation.
/g183/g32 Exceedance icing conditions may be primarily water content related for thermal ice protection systems, primarily droplet diameter related for mechanical ice protection systems or some combination thereof.
(j) Autopilot operation should be prohibited if any of the following conditions in icing flight are experienced:
1. Severe icing;
2. Unusual control force or control deflection, or unusually large control forces to move flight controls when the autopilot is disconnected periodically; or
3. Indications of frequent autopilot retrimming during straight and level flight.
(2) Operating Procedures Section.
(a) Section 23.1585(a) requires the pilot be provided with the necessary procedures for safe operation. This should include any preflight action necessary to minimize the potential of enroute emergencies associated with the ice protection system.
The system components should be described with sufficient clarity and depth that the pilot can understand their function. Unless flight crew actions are accepted as normal airmanship, the appropriate procedures should be included in the FAA-approved AFM, AFM revision, or AFM supplement. These procedures should include proper pilot response to cockpit warnings, a means to diagnose system failures, and the use of the system(s) in a safe manner.
(b) Procedures should be provided to optimize operation of the airplane during penetration of icing conditions, including all flight regimes. The AFM should include procedures that advise upon which conditions the ice protection equipment should be activated.
(c) Emergency or abnormal procedures, including procedures to be followed when ice protection systems fail and/or warning or monitor alerts occur, should be provided.
(d) For fluid anti-ice/deice systems, information and method(s) for determining the remaining flight operation time should be provided.
(e) For airplanes that cannot supply adequate power for all systems at low engine speeds, load-shedding instructions should be provided to the pilot for approach and landing in icing conditions.
(f) For aircraft equipped with an autopilot, the autopilot should be disconnected periodically to check for unusual control force or deflection, and to move the flight controls to check for evidence of ice accreting in control surface gaps or frozen actuators.
(3) Performance Information Section. A brief statement that supercooled cloud test environment and freezing rain, freezing drizzle, or mixed conditions (as appropriate) have not been tested. These severe icing environmental conditions outside the icing envelope of part 25, Appendix C, may exceed the capabilities of the ice protection system, and it may result in a serious degradation of performance or handling characteristics.
(a) Normal, Utility, and Acrobatic Category Airplanes. For these airplanes, general performance information should be provided to give the pilot knowledge of allowances necessary while operating in ice or with residual ice on the airframe. The following items are only examples that provide some guidelines and are not requirements. These guidelines may be revised for specific airplanes as appropriate:
1. An accumulation of inch of ice on the leading edges can cause a loss in rate of climb up to FPM, a cruise speed reduction of up to KIAS, as well as a significant buffet and stall speed increase (up to knots). Even after cycling the deicing boots, the ice accumulation remaining on the boots and unprotected areas of the airplane can cause large performance losses. With residual ice from the initial inch accumulation, losses up to FPM in climb, KIAS in cruise, and a stall speed increase of knots can result. With inch of residual accumulation, these losses can double.
2. Airspeed—MAINTAIN BETWEEN KIAS AND KIAS with ____ inch or more of ice accumulation for appropriate configuration..
3. Prior to a landing approach, cycle the wing and stabilizer deice boots to shed any accumulated ice. Maintain extra airspeed on approach to compensate for the increased stall speed associated with ice on unprotected areas. Use caution when cycling the boots during an approach, since boot inflation with no ice accumulation may cause 33
mild pitching and increase stall speeds by knots. It may also decrease stall warning margin by the same amount; and it may cause or increase rolling tendency during stall.
4. Holding in icing conditions for longer than 45 minutes may reduce margins and could result in inadequate handling and control characteristics.
5. Maintain engine speeds of r.p.m. higher to ensure proper operation of the ice protection system.
(b) Commuter Category Airplanes. Data should be provided so that the balked landing climb limited weight and approach climb limited landing weight can be determined. These data should include the effect of drag due to residual ice on protected and unprotected surfaces, power extraction associated with ice protection system operation, and any changes in operating speeds due to icing. Also, the effect on landing distance due to revised approach speeds, and/or landing configurations, should be shown.
(4) For airplanes with a certification basis at Amendment 23-14 or higher, the AFM should contain a statement similar to “This airplane is approved for flight in icing conditions as defined in FAR 25, Appendix C.” For these airplanes, there should not be references to operational terms such as “light” or “moderate” ice or “known icing.”
(5) The AFM should reference the maintenance manual for ice protection surface cleaning procedures if the flight crew can be expected to perform this function.
c. Prior to AFM Requirement. If the airplane was certificated prior to the effective date of the requirement for an AFM, then the combination of manuals, markings, and placards should adequately address the placard and AFM subjects previously discussed in this AC.
S/
James Jackson Acting Manager, Small Airplane Directorate Aircraft Certification Service 34
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