Advisory
Circular
Subject: AIRCRAFT ICE PROTECTION Date: 8/16/06 AC No: 20-73A Initiated by: AIR-120
FOREWORD This advisory circular (AC) describes an acceptable means, but not the only means of showing compliance with the ice protection requirements of Title 14 of the Code of Federal Regulations (14 CFR) parts 23, 25, 27, 29, 33, and 35. It also offers applicants guidance on how to maintain the aircraft’s airworthiness when operating in an icing environment.
/s/ Susan J. M. Cabler Acting Manager, Aircraft Engineering Division Aircraft Certification Service
CONTENTS
Section Page
1. PURPOSE 1
2. CANCELLATION 1
3. APPLICABILITY 1
4. RELATED ADVISORY CIRCULARS, DOCUMENTS, READING MATERIAL, AND NOMENCLATURE 2
5. ICE PROTECTION REGULATORY DEVELOPMENT BACKGROUND 2
6. APPLICABLE AIRFRAME, ENGINE, AND PROPELLER ICE PROTECTION REQUIREMENTS 2
6.1 Ice Protection Requirements: Unintended Exposure to Icing Conditions 3
6.2 Ice Protection Requirements: Intentional Operations in Known and Forecast Icing Conditions 4
6.3 Ice Protection Requirements: Operating Safely in Icing Conditions 5
7. CERTIFICATION PROCEDURES 5
7.1 Certification Procedures: General 5
7.2 Certification Procedures: Airframe Manufacturer 6
7.3 Certification Procedures: Engine Manufacturer 7
8. COMPLIANCE MEANS 7
8.1 Compliance Means: Analysis 9
8.1.1 Compliance Means: Airframe Analysis 10
8.1.2 Compliance Means: Engine and Propeller Systems Analyses 13
8.1.3 Compliance Means: Engine Inlets Analysis 14
8.1.4 Compliance Means: Ice and Icing Conditions Detection Systems
Analyses 15
8.1.5 Compliance Means: Windshield Analysis 15
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8.1.6 Compliance Means: Air Data System Sensors and Probes Analyses 16
8.1.7 Compliance Means: Ice Protection System Failure Analysis 16
8.1.8 Compliance Means: Flutter Analysis 16
8.1.9 Compliance Means: Power Sources Analysis 17
8.2 Compliance Means: Tests 17
8.2.1 Compliance Means: Natural Icing Flight Tests 17
8.2.2 Compliance Means: Dry Air Flight Tests 20
8.2.3 Compliance Means: Artificial Icing Flight Tests 24
8.2.4 Compliance Means: Wind Tunnel Tests 25
8.3 Compliance Means: Surfaces Without Ice Protection 25
8.4 Compliance Means: Ice Inspection Lights and Cues 25
8.5 Compliance Means: Reference to Previously Accomplished Tests 26
8.6 Compliance Means: Perform Intended Function in Icing Conditions 27
8.6.1 Compliance Means: Engines and Equipment Performance of Intended Function in Icing Conditions 27
8.6.2 Compliance Means: Engine Alternate Induction Air Sources Performance of Intended Functions in Icing Conditions 27
8.6.3 Compliance Means: Fuel System Venting Performance of Intended Function in Icing Conditions 27
8.6.4 Compliance Means: Landing Gear Performance of Intended Function in Icing Conditions 27
8.6.5 Compliance Means: Stall Warning and Protection Performance of Intended Function in Icing Conditions 27
8.6.6 Compliance Means: Icing Conditions 28
8.6.7 Compliance Means: Primary and Secondary Flight Control Surfaces Performance of Intended Function in Icing Conditions 28
8.6.8 Compliance Means: Ram Air Turbine Performance of Intended Function in Icing Conditions 28
8.6.9 Compliance Means: Pilot Compartment View Performance of Intended Function in Icing Conditions 28
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9. AIRPLANE FLIGHT MANUAL (AFM) 28
9.1 AFM: Limitations 28
9.2 AFM: Operating Procedures 29
9.3 AFM: Performance Information 30
9.4 AFM: Airworthiness Directives for Severe Icing Conditions 30
10. ROTORCRAFT 31
10.1 Rotorcraft: General 31
10.2 Rotorcraft: Compliance 32
10.2.1 Rotorcraft: Compliance: Analyses 35
10.2.2 Rotorcraft: Compliance: Tests 36
10.3 Rotorcraft Flight Manual 37
11. AIRCRAFT FROST AND CLEAR ICE CONSIDERATIONS 38
12. AIRPLANE DE/ANTI-ICING BEFORE TAKEOFF 38
13. EXTENDED RANGE OPERATION WITH TWO-ENGINE AIRPLANES (ETOPS) ICING CONSIDERATIONS 39
14. REFERENCES 41 APPENDIX A. RELATED REGULATIONS AND DOCUMENTS A-1 A.1 Regulations A-1 A.2 Advisory Circulars A-6
A.3 Related Reading Material A-8
APPENDIX B. NOMENCLATURE B-1 B.1 Terms B-1
B.2 Acronyms and Symbols B-11 B.3 References B-15
APPENDIX C. BACKGROUND OF ICE PROTECTION REGULATORY DEVELOPMENT C-1
C.1 Transport Category Airplanes (14 CFR Part 25) C-1
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C.2 Normal, Utility, Acrobatic, and Commuter Category Airplanes (14 CFR Part 23) C-5
C.3 Aircraft Engines (14 CFR Part 33) C-10
C.4 Rotorcraft (14 CFR Parts 27 and 29) C-11 C.5 References C-12
APPENDIX D. COMPONENT ICE PROTECTION SYSTEM DESIGN AND CERTIFICATION PROCESS FLOWCHARTS D-1
APPENDIX E. METEOROLOGICAL CONDITIONS E-1
E.1 Icing Cloud Specifications E-1
E.2 Using 14 CFR Parts 25 and 29, Appendix C E-6 E.3 References E-7
APPENDIX F. OPERATIONAL FACTORS F-1 F.1 Airplane Operational Factors F-1 F.2 Rotorcraft Operational Factors F-1
F.3 Engine Operational Factors F-1
APPENDIX G. ICING CONDITIONS EXPOSURE TIME G-1
APPENDIX H. ICE PROTECTION SYSTEMS H-1
H.1 Ice Protection Technologies and Operating Modes H-1 H.1.1 Ice Protection Technologies H-2
H.1.2 Ice Protection System Operation H-9 H.2 References H-11
APPENDIX I. DROP IMPINGEMENT AND WATER CATCH I-1
I.1 Drop Impingement and Water Catch I-1 I.2 References I-5
APPENDIX J. PROPELLER ICE PROTECTION J-1 J.1 Propeller Ice Protection J-1 J.2 References J-3
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APPENDIX K. ICE AND ICING CONDITIONS DETECTION K-1
K.1 Ice and Icing Conditions Detection K-1 K.1.1 AFIDS K-2 K-2 K-2 K-3 K-4 K.1.2 PFIDS K.1.3 Ice Detector Sensor Characteristics K.1.4 Freezing Fraction (n) Considerations K.1.5 Drop Splashing, Runback, and Shedding K.2 Installation Location K-4 K.3 System Considerations K-5
K.4 Certification of Ice Detectors K-6
K.4.1 Certification of an AFIDS K-6
K.4.2 Certification of a PFIDS K-7 K.5 References K-8
APPENDIX L. WINDSHIELD ICE PROTECTION L-1
APPENDIX M. FINDING NATURAL ICING CONDITIONS FOR TEST PURPOSES M-1 M.1 Background M-1 M.2 Statistical Guidance M-1
M.2.1 Natural Probabilities for LWC Averages in Stratiform Icing Conditions M-1 M.2.2 Natural 99 Percent LWC Limits vs. Altitude M-2
M.2.3 Probability of Finding Large MVDs M-3 M.3 Experience M-4 M.3.1 Continuous Maximum Conditions M-4 M.3.2 Intermittent Maximum Conditions M-4 M.3.3 Glaciated Conditions M-5 M.3.4 Mixed-Phase Conditions M-5 M.3.5 Large Drop Conditions M-5
M.4 Using Forecasts of Icing Conditions M-6 M.5 Professional Guidance M-6 M.6 References M-7
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APPENDIX N. INSTRUMENTATION N-1 N.1 Instrumentation N-1 N.2 References N-2
APPENDIX O. WAYS TO EVALUATE ICING EXPOSURES RELATIVE TO APPENDIX C O-1 O.1 Background O-1
O.2 Appendix C Converted to Distance- or Time-Based Envelopes O-1 O.3 References O-3
APPENDIX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES P-1 P.1 Background P-1
P.2 Data From an Icing Rate Sensor P-1
P.3 Converting Icing Rate to Icing Intensity P-2
P.4 Data Sheet for Documenting Icing Exposures P-3 P.5 References P-8
APPENDIX Q. ICE SHEDDING Q-1 Q.1 Ice Shedding Q-1 Q.2 References Q-2
APPENDIX R. ICE SHAPES R-1 R.1 Protected Surfaces R-1 R.1.1 Preactivation Ice Roughness R-1 R.1.2 Intercycle Ice Roughness R-6 R.1.3 Runback Ice R-12 R.2 Unprotected Surfaces R-14
R.3 Ice Protection System Failure Ice Shapes R-14 R.4 Critical Ice Shapes R-15
R.4.1 Considerations for Critical Ice Shapes R-15
R.4.2 Aerodynamic Considerations for Determining Critical Ice Shapes R-21 viii
R.5 Methods for Determining (Predicting) Ice Shapes R-31 R.5.1 Natural Icing Flight Tests R-31
R.5.2 Use of Icing Tankers to Predict Unprotected Surfaces Ice Shapes R-31 R.5.3 Use of Icing Wind Tunnels to Predict Unprotected Surfaces Ice Shapes R-31 R.5.4 Drop Impingement and Ice Accretion Computer Codes and Other Analytical Methods R-35
R.5.5 Prediction of Landing Gear and Strut Ice Shapes R-38
R.5.6 Prediction of Protected Surfaces Ice Shapes R-38
R.6 Correlation of Predicted (Simulated) and Natural Ice Shapes R-38 R.7 References R-40
APPENDIX S. ICE-CONTAMINATED HORIZONTAL STABILIZER (TAILPLANE) STALL S-1
S.1 Ice-Contaminated Horizontal Stabilizer (Tailplane) Stall S-1 S.2 References S-3
APPENDIX T. ARTIFICIAL ICING FLIGHT TESTS: ICING TANKERS AND
SPRAY RIG TESTS T.1 Icing Tanker Tests T-1
T.2 Spray Rig Tests T-1 T.3 References T-1
APPENDIX U. ICING WIND TUNNEL TESTS U-1
U.1 Icing Wind Tunnel Tests U-1 U.2 References U-1
APPENDIX V. AIRCRAFT FROST AND CLEAR ICE CONSIDERATIONS V-1 V.1 Aircraft Frost and Clear Ice Considerations V-1 V.2 References V-4
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List of Figures Figure Page B-1 Gridding of a NACA 0012 Wing Into a Mesh of Finite Volumes for Flow Field Analysis, Drop Impingement Computation, and Ice Accretion Analysis (See Reference B1.) B-3
B-2 Definition of Local Impingement Efficiency Parameter, E B-6 B-3 Definition of Horn Maximum Thickness Angle (See Reference B1.) B-6
D-1 Typical Aircraft Inflight Icing Certification Process D-2
D-2 Typical Inflight Icing Certification Processes for Wings, Horizontal and Vertical Stabilizers, and Rotor Blades D-3
D-3 Typical Inflight Icing Certification Process for Engine Induction Systems D-6 D-4 Typical Inflight Icing Certification Process for Propellers D-10
D-5 Typical Inflight Icing Certification Process for Windshields D-13
D-6 Typical Inflight Icing Certification Process for Air Data Probes and Sensors D-14 D-7 Typical Inflight Icing Certification Process for Antenna, Tanks, Fairings, Masts, Struts, and Other Miscellaneous Protuberances D-15
E-1 14 CFR Parts 25 and 29 Appendix C Continuous Maximum (Stratiform Clouds) Atmospheric Icing Conditions, Liquid Water Content vs. Mean Effective Drop Diameter (14 CFR Parts 25 and 29 Appendix C, Figure 1) E-2
E-2 14 CFR Parts 25 and 29 Appendix C Continuous Maximum (Stratiform Clouds) Atmospheric Icing Conditions, Ambient Temperature vs. Pressure Altitude (14 CFR Parts 25 and 29, Appendix C, Figure 2) E-3
E-3 14 CFR Parts 25 and 29 Appendix C Continuous Maximum (Stratiform Clouds) Atmospheric Icing Conditions, Liquid Water Content Factor vs. Cloud Horizontal Distance (14 CFR Parts 25 and 29, Appendix C, Figure 3) E-3
E-4 14 CFR Parts 25 and 29 Appendix C Intermittent Maximum (Cumulus Clouds) Atmospheric Icing Conditions, Liquid Water Content vs. Mean Effective Drop Diameter (14 CFR Parts 25 and 29, Appendix C, Figure 4) E-4
E-5 14 CFR Parts 25 and 29 Appendix C Intermittent Maximum (Cumulus Clouds) Atmospheric Icing, Ambient Temperature vs. Pressure Altitude (14 CFR Parts 25 and 29, Appendix C, Figure 5) E-4
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E-6 14 CFR Parts 25 and 29 Appendix C Intermittent Maximum (Cumulus Clouds) Atmospheric Icing Conditions, Liquid Water Content Factor vs. Cloud Horizontal Distance (14 CFR Parts 25 and 29, Appendix C, Figure 6) E-5
J-1 Propeller Ice Accretion During an SLD Encounter With the Propeller Ice Protection System Operating (See Reference J1.) J-3
M-1 Natural Probabilities for LWC Averages in Stratiform Cloud Icing Conditions at 32oF to 14oF, With an Average MVD of 15 Pm M-1 M-2 Natural 99 Percent LWC Limit Envelopes at the Highest Icing Temperatures Available at Indicated Altitude as a Function of Cloud Horizontal Extent (for all clouds containing supercooled water drops with MVD = 15-20 Pm) M-2
M-3 Natural Cloud Horizontal Extent Limits and 99 Percent LWC Limits for Selected, Sustained Cloud Drop MVDs in Stratiform Clouds at Ambient Temperatures of 32oF to 14oF M-3 O-1 14 CFR Parts 25 and 29, Appendix C Icing Envelopes Converted to a Distance- Based Format (for MVD=15 Pm) O-2
P-1 Actual Example of the Analog Output Voltage From a Model 871FA Ice Detector During an Icing Encounter P-1
P-2 High Resolution Example of Analog Output Voltage From an 871FA Ice Detector P-2
R-1 Hybrid NACA 23012 2D (Simulating a 72-inch Chord Airfoil) Model Leading Edge Ice Roughness Before Activation of the IPS. Exposure time included 11 seconds for an ice detector alert and 30 seconds for the flight crew to activate the IPS. The test was performed in 14 CFR part 25, Appendix C Continuous Maximum Icing Conditions. (Static temperature = 14qF, LWC = 0.45 g/m3,
MVD = 20 micrometers, spray time = 41 sec., tunnel airflow speed = 195 mph, Model AOA = 4q.) (See Reference R1.) R-2
R-2 Hybrid NACA 23012 2D (Simulating a 72-inch Chord Airfoil) Model Leading Edge Ice Roughness Before Activation of the IPS. Exposure time included allowing 3 seconds for an ice detector alert and 30 seconds for the flight crew to activate the IPS. The test was performed in 14 CFR part 25, Appendix C Intermittent Maximum Icing Conditions. (Static temperature = 14qF, LWC =
1.95 g/m3, MVD = 20 micrometers, spray time = 33 sec., tunnel airflow speed = 195 mph, model AOA = 4q.) (See Reference R1.) R-4
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R-3 Hybrid NACA 23012 2D Model (Simulating a 72-inch Chord Airfoil) Leading Edge Ice Roughness Before Activation of the IPS. Exposure time included the time required for an observation of ¼ inch of ice and 30 seconds for the flight crew to activate the IPS. The test was performed in 14 CFR part 25, Appendix C Continuous Maximum Icing Conditions. (Static temperature = 14qF, LWC =
0.45 g/m3, MVD = 20 micrometers, tunnel airflow speed = 195 mph, model AOA = 4q.) (See Reference R1.) R-5
R-4 Typical Surface Roughness on the Leading Edge of a NACA 23012 Resulting from Normal Operation of Wing Pneumatic Deicing Boots. (See Reference R1.) R-7 R-5 Intercycle Ice Roughness for a 2D 36-inch NACA 0012 Airfoil Wind Tunnel Model R-9 R-6 Wing Deicing Boots Residual and Intercycle Ice Buildups R-10
R-7 Aerodynamic Effects Resulting From Deicing Intercycle Ice Shapes for a 2D 36-inch NACA 23012 Airfoil Wind Tunnel Model at a Reynolds Number of
7.5x106 R-11 R-8 Aerodynamic Effects of Distributed Roughness Along the Leading Edge of a NACA 23012 Airfoil (c = 36 inches, Re = 7.5x106, M = 0.21). (See Reference R2.) R-12 R-9 Lower Surface Runback Ice on a 36-inch NACA 0012 Airfoil Model. The model was equipped with an electro-thermal IPS. The IPS ran wet with a constant protection surface temperature of about 50qF. The mixed-phase icing cloud exposure time was 5 minutes. (Static Temperature = 0qF, LWC = 0.35 g/m3,
MVD = 20 micrometers, IWC (shaved ice) = 0.35 g/m3, tunnel airflow speed = 120 mph, model AOA = 0q.) R-13 R-10 Aerodynamic Effects of Continuous and Segmented 0.25-inch, Forward-Facing, Quarter-Round Simulated Ice Shapes on the Upper Surface of an 18-inch Chord NACA 23012m Airfoil 2D Model (Re = 1.8x106, M = 0.185). (See Reference R5.) R-20
R-11 Comparison of NACA 23012m and NLF 0414 Clean Airfoil Model Experimental Pressure Distributions (Cl ~ 0.6, Re = 1.8x106, M = 0.185). See Reference R5.) R-21 R-12 Effect of an Upper Surface Simulated Quarter-Round Ice Shape at Various Chordwise Positions on Lift for Forward- (NACA 23012m) and Aft-Loaded (NLF 0414) Airfoils (k/c = 0.0139, Re = 1.8x106, M = 0.185). (See Reference R5.) R-22
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