TABLE 3. ICE ACCRETION DEFINITIONS (Continued)
Ice Accretion Normal, Utility and Acrobatic Categories
Commuter Category Final Takeoff Not applicable. Same as “takeoff” ice except ice accretion occurs between liftoff and 1,500 feet above the takeoff surface.
Enroute Ice accretion on the unprotected surfaces, and any ice accretion on the protected surfaces appropriate to normal ice protection system operation, during the en route phase, in part 25, Appendix C, continuous or intermittent maximum icing conditions. At the applicant’s option, “holding” ice may be used in showing compliance with requirements that specify “en route” ice.
Holding Ice accretion on the unprotected surfaces, and any ice accretion on the protected surfaces appropriate to normal ice protection system operation, during a 45-minute hold in part 25, Appendix C, continuous maximum icing conditions.
Pre- Activation The ice accretion prior to normal system operation is the ice accretion formed on the unprotected and normally protected surfaces prior to activation and effective operation of any ice protection system in continuous maximum atmospheric icing conditions. Ice detection procedures, ice detector system design and performance, and boot activation procedures should be considered.
Critical The ice accretion, applicable to the phase of flight that has the most adverse effect on performance and flying qualities. For protected surfaces, intercycle ice, residual ice, and runback ice should be accounted for. In order to reduce the number of ice accretions to be considered when demonstrating compliance:
(1) The more critical of takeoff ice and final takeoff ice may be used throughout the takeoff phase.
(2) Holding ice may be used for the en route, holding, approach, landing, and goaround flight phases.
(3) Holding ice may also be used for the takeoff phase provided it is shown to be more conservative than takeoff ice and final takeoff ice.
(4) The ice accretion that has the most adverse effect on handling characteristics may be used for climb performance tests provided any difference in climb performance is conservatively taken into account.
(b) Shape and Texture of Simulated Ice. The shape and texture of the simulated ice should be established and substantiated by agreed methods. Common practices include:
1. Use of computer codes;
2. Flight in measured natural icing conditions;
3. Icing wind tunnel tests; and
4. Flight in a controlled artificial icing cloud (e.g. airborne icing tanker).
(c) Ice Adhesion Inhibitors. For the determination or validation of intercycle and residual ice shapes and roughness on deicing boots, the application of any ice adhesion inhibitor such as ICEX is not permitted for natural icing flight tests, artificial icing flight tests or icing tunnel tests. This is because the use of ICEX cannot be controlled in operations and the effectiveness in operations degrades over time. (Intercycle ice is the ice accretion that exists on a deicing system surface just prior to an actuation of the deicing system; residual ice is the accretion remaining immediately after an actuation.) Other products that enhance appearance or life should also not be applied. Deicing boots can be cleaned at the start of a natural icing or artificial icing test program per recommended maintenance procedures.
(2) Propeller Icing. To date, climb performance analyses and climb flight testing with ice shapes have not taken into account propeller efficiency losses due to ice accretion on the propeller blades. Deicing boot manufacturers analyses show that residual ice does exist with propeller deicing systems and their analyses do not account for ice runback. The outer part of the blade, which normally is not protected, theoretically has sufficient centrifugal force to shed ice. Stop frame video of recent flight testing of a part 23 aircraft in Super-Cooled Large Droplet (SLD) conditions have shown ice accretions on the full span of the propeller blades. Although this condition was outside of part 25, Appendix C icing conditions, there is a possibility that this may occur within some portions of part 25, Appendix C. Airplane performance during natural icing flight testing should be qualitatively compared with performance during ice shape flight tests. On reciprocating and turboprop powered airplanes, if there is degradation in performance compared to the ice shape results, propeller efficiency losses due to propeller ice accretions should be investigated.
(3) Failure Ice Accretions. Flight tests with failure ice shapes representing failures not shown to be extremely improbable should be conducted to validate hazard classifications and to develop procedures for safe operation following a failure. For example, this testing may show that landing flap settings may have to be reduced following failure of the empennage ice protection system.
(a) Failure ice accretion is defined as:
1. “Holding” ice as defined in Table 3 for unprotected surfaces; and
2. For protected surfaces, one-half the accretion specified for unprotected surfaces (22.5 minutes) unless another value is agreed to by the responsible aircraft certification office.
(b) If the failure is annunciated, the applicant may propose an ice accretion based on a realistic exit scenario in lieu of the 22.5 minutes ice accretion. This failure scenario should account for the time it takes:
1. for the system to annunciate the failure (e.g., one deicing boot cycle);
2. for the pilot to decide on a course of action and notify Air Traffic Control (e.g., two minutes); and
3. to exit the icing conditions.
(c) The time to exit should include a 180-degree standard rate turn and transiting a 17.4-nautical mile, part 25, Appendix C continuous maximum cloud. Besides the design standard 17.4 nautical mile horizontal cloud extent, a cloud extent of 46-nautical miles (adjusted for liquid water content per part 25, Appendix C), which is expected for 10 percent of icing encounters, should also be considered in the safety analysis. The exit scenario shall include the possibility that the airplane may have to climb 4,000 feet out of icing if it results in a longer time than traversing the part 25, Appendix C cloud.
c. Natural Icing Flight Tests. Whether the performance and handling qualities flight testing has been performed with simulated ice shapes or in natural icing conditions, additional limited flight testing described in this section should be conducted in natural icing conditions.
Where flight testing with simulated ice shapes 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 flight testing with simulated ice shapes. It is acceptable for some ice to be shed during the testing due to air loads or wing flexure, etc., or during transit to a higher altitude or test area for safety reasons. However, an attempt should be made to accomplish the test maneuvers as soon as possible after exiting the icing cloud to minimize the atmospheric influences on ice shedding. During any of the maneuvers specified in Table 4, the performance and behavior of the airplane should be consistent with that obtained with simulated ice shapes.
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.
TABLE 4. NATURAL ICING PERFORMANCE
AND HANDLING QUALITIES TESTS
Configuration Ice Accretion Trim Speed
Maneuver Flaps up, gear up Equivalent to 45-minute hold at critical conditions.
Minimum Holding
• Level, 40-degree banked turns;
• Bank-to-bank rapid rolls, 30 degrees – 30 degrees;
• Climb or level performance evaluation;
• Autopilot tests
• Straight stall.
Landing flaps, gear down
1.25 inches on unprotected part of wing tip VREF
• Level, 40-degree banked turns;
• Bank-to-bank rapid rolls, 30 degrees – 30 degrees;
• Climb or level performance evaluation;
• Straight stall.
Flaps up, gear up Defined preactivation ice Optional
• Level, 40-degree banked turns;
• Bank-to-bank rapid rolls, 30 degrees – 30 degrees;
• Climb or level performance evaluation;
• Autopilot tests
• Straight stall.
d. Tolerances. The same airplane and system production tolerances used in the non-icing tests should be used when evaluating performance and handling qualities with ice accretions. Ice protection system production tolerances should be addressed during flight testing in natural icing conditions. Examples are provided in Table 5. Stall speed and warning system tolerance are critical when establishing tolerances for production acceptance flights.
TABLE 5. FLIGHT TEST TOLERANCES
Test Tolerances Stall speed
• Elevator to minimum trailing edge up if stall defined by aft control stop
• Stick pusher, if equipped, set for minimum angle of attack
• Flap travels should be set to minimum allowable settings Stall warning • Set for maximum angle of attack.
Stall characteristics • Elevator to maximum trailing edge up. • Stick pusher, if equipped, set for maximum angle of attack Maneuver margin • Stall warning set for minimum angle of attack.
Natural icing flight tests • Pneumatic boots set for minimum pressure • Electrothermal systems set at minimum current 38
e. 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 CG 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 airplanes with longitudinal control systems that are not powered (reversible control systems), 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 AC 23.143-1, “Ice Contaminated Tailplane Stall (ICTS)”.
(1) Maximum landing flaps may be limited to the “takeoff/approach” configuration due to 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.
(2) If sandpaper ice results in ICTS susceptibility and limited flap deflection for landing, the AFM procedure for limiting flap should be based on visible moisture and temperature rather than airframe ice accretions if the flight crew cannot see the tail.
(3) For pre-activation ice, when the activation of the ice protection system does not require the flight crew to wait for a specified ice accumulation, the pushover maneuver can be accomplished to 0.5g rather than 0g.
(4) For failure ice accretions, when the failure is annunciated to the flight crew and the procedures are to exit icing conditions, the pushover maneuver can be accomplished to 0.5g rather than 0g.
f. 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 39
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” are 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.
(1) 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 could detect and recognize the specified ice accumulation thickness. The following test criteria have been accepted for previous flight test programs:
(a) 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.
(b) The applicant should show that an ice accumulation margin exists that allows for errors in crew recognition of the ice accumulation level.
(2) 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 2
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.
(3) 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. A recent part 23 applicant 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.
g. 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. The tests in Table 6 represent a sample matrix for a part 23 airplane with failure ice shapes defined in paragraph 12b(3).
TABLE 6. FAILURE ICE ACCRETION FLIGHT TESTS
Ice Shape Configuration Configuration and Trim Speed
Maneuver One wing zone failure Flaps up and minimum holding
• Level, 40-degree banked turns;
• Bank-to-bank rapid rolls, 30 degrees – 30 degrees;
• Deceleration to stall warning (natural acceptable), recover after one second.
Empennage zone failure Full landing flaps and VREF • ICTS evaluation;
• Sideslip s Total wing and empennage zone failure
• ICTS evaluation
• Level, 40-degree banked turns;
• Bank-to-bank rapid rolls, 30 degrees – 30 degrees;
• Deceleration to stall warning (natural acceptable), recover after one second.
• Approach and go-around demonstration Pilot's windshield ice protection failure Full landing flaps and VREF • Approach and landing demonstration
h. Roll Control in Supercooled Large Droplet Conditions. The following guidance, with some modifications, is contained in a July 23, 1997 Policy Memorandum issued by the Transport Directorate and coordinated with the Standards Office. This policy memorandum was applicable to certain part 25 and part 23 airplanes. The FAA and EASA are currently harmonizing rulemaking and guidance for flight in icing conditions that exceed part 25, Appendix C. This policy in this paragraph is interim and will be replaced when the harmonized regulations and guidance are issued for part 23 airplanes. In October 1994, an accident involving a transport category airplane occurred in which severe icing conditions were reported in the area. During extensive testing, the accident profile was replicated by ice shapes developed from testing in an icing cloud having droplets in the size range of freezing drizzle at a temperature near freezing.
This condition created a ridge of ice aft of the deicing boots and forward of the ailerons, which resulted in uncommanded motion of the ailerons and rapid roll of the aircraft. The NTSB recommended that the FAA develop a test procedure to identify the unsafe aileron hinge moment characteristics. The procedure described herein is the procedure used during an FAA program to screen certain airplanes for susceptibility to aileron control anomalies. The FAA has identified the susceptibility to loss of control following exposure to supercooled large droplets as an unsafe condition that may exist on other aircraft. The FAA is particularly concerned with airplanes that are equipped with non-powered roll control systems, since non-powered roll flight controls do not have the physical advantage of hydraulic or electrical power to assist the pilot in overcoming the large control forces that may exist from differential pressure resulting from flow separation over the roll control surfaces. Therefore, airplanes certified for flight in icing equipped with non-powered roll control systems and without fully evaporative wing anti-ice systems should be evaluated for susceptibility to roll upset in the event the airplane is exposed to certain freezing drizzle conditions. The following paragraphs provide an acceptable means of compliance with the evaluation.
(1) Tests and analyses should show that the airplane characteristics meet the criteria specified in paragraph (2) following a 20-minute icing encounter characterized by:
(a) Supercooled droplets having maximum diameters of approximately 400 microns;
(b) A Liquid Water Content (LWC) of approximately 0.6 grams per cubic meter;
NOTE 3
For this condition, the LWC strongly affects the rate at which the ice feature develops. A higher LWC results in more rapid formation of the ice feature, while a lower LWC results in a slower formation of the ice feature. The LWC should be adequate to produce an ice feature during the exposure interval that will start to shed on its own accord and then reform.
(c) A median volumetric diameter of approximately 170 microns,
NOTE 4
The cloud physics instrumentation, calibration, and data processing methodologies should be presented for acceptance by the FAA.
(d) Temperatures near freezing such that runback conditions exist at the stagnation line; and
NOTE 5
For this test, temperature is a critical factor. Not only is the temperature critical to the development of the ice shape and dimension, static air temperature excursions above freezing, although short in duration, can reverse the ice accretion process.
(e) Operation at holding speeds and holding configurations.
(2) When manually flying the airplane:
(a) The pilot roll force to counter any uncommanded roll control surface deflection should not exceed 50 pounds with two hands available for control; and
(b) The airplane should not exhibit a hazardous degradation of flying qualities.
Rapid control force onset, and unsteady and oscillatory forces must be considered carefully as these dynamic conditions may be hazardous even though the peak force may be less than the static limit.
(3) The tests and analyses described in paragraph (1) should consider the effects of asymmetric shedding of the ice.
(4) There should be a means for the flightcrew to determine when the airplane has entered into a supercooled large droplet environment, to enable the crew to take appropriate action.
(5) There should be appropriate crew information provided in the AFM that describes the limitations and procedures to be observed while exiting the supercooled large droplet environment. The FAA has found that the limitations and procedures specified in Appendix H are an acceptable means of providing this information.
(6) One means of compliance with paragraphs (1), (2), and (3) is to perform a high speed taxi test to evaluate control wheel force characteristics that may result from flow separation over the roll control surfaces induced by an simulated ice shape as described below.
The testing should include the following:
(a) Install a one-inch high, quarter-round molding, flat side forward, located on the upper surface of the wing, at the chord position aft of the active portion of the boots and 43
forward of the non-powered roll control surfaces (i.e., ailerons and/or inflight spoilers) that produce the most adverse lateral wheel force.
(b) Locate this shape in front of the roll control surfaces on one wing only. As a minimum, the shape should cover the entire span of the roll control surface.
(c) Plan and configure the airplane for flight. Perform high-speed taxi tests with the flaps retracted and at various angles of attack. The maximum angle of attack should be obtained at the highest takeoff weight such that the airplane does not become airborne.
(d) Measure the forces required to maintain the wings level.
(e) Extrapolate the maximum forces obtained from the high-speed taxi tests to the maximum speeds expected while in holding conditions. In most cases the maximum forces will occur at the maximum angle of attack achieved during the high-speed taxi tests.
(f) The extrapolated forces should not exceed 50 pounds with two hands available for control.
(g) Inflight spoilers may be mechanically locked out while the airplane is on the ground. Therefore, airplanes equipped with non-powered inflight spoilers may need tunnel or flight testing to evaluate the effect on airplane control and handling characteristics.
13. 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 trajectory and impingement analysis. If flight tests are not conducted, 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. In flight testing the airplane should be exposed to an icing condition of magnitude and duration sufficient to create the expected worst-case ice accretion, including the 45-minute hold. Flight test evaluation should also account for critical, predicted trajectories in terms of normal operational angle of attack and sideslip.
14. 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 degrees)”.
(3) Speed restrictions (for example, "Maximum Speed for Boot Operation—175 Knots Indicated Airspeed (KIAS) inches)”.
(4) Fluid filler⎯inlets 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 and operate in icing conditions. 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, maximum altitude for boot operation, and maximum outside air temperature for operation of thermal ice protection systems).
(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 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 45
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 operations in icing conditions.
(i) Configuration limitations, if any (for example, a reduced flap setting for approach and landing, and flaps up for holding).
(j) For exceedance icing conditions 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 H 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.
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.
(k) 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 46