U.S. Deportment of Transportation Federal Aviation Administration 

# Advisory Circular

Subject: Compliance with Requirements Date: 08/04/11 AC No: 121.321-1 of§ 121.321, Operations in Icing Initiated By: ANM-100 1. Purpose. This advisory circular (AC) describes an acceptable means for showing compliance with the requirements ofTitle 14, Code ofFederalRegulations (14 CFR) 121.321, Operations in Icing. Part 121 contains the aircraft operatingrequirements applicable to domestic, flag, and supplemental operations. This AC provides guidance for:
a. Using visible moisture andtemperature as a means for the flightcrew to know when the airframe ice protection system (IPS) must be activated. b. Developing acceptable procedures for activating and deactivating the airfi·ame IPS.
c. Installing a primary or advisory ice detection system. 2. Applicability.
a. General. This AC provides guidance to part 121 operators, airplane manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) airplane type certification engineers, their designees, and inspectors. This guidance applies to the operation, inconditions conducive to inflight airframe icing, ofpart 23 (small) and part 25 (transport category) airplanes with a maximum certified takeoffweight less than 60,000 pounds that are used in part 121 operations. b. Limitations. This material is neither mandatory nor regulatory in nature and does not constitute a regulation. Itdescribes acceptable means, but not the only means, for demonstrating compliance with the applicable regulations. TheFAA will consider othermethods of demonstrating compliance that an applicant may electto present. While these guidelines are not mandatory, they are derived from extensive FAA and industry experience in determining compliance with the pertinent regulations. On the other hand, ifwe become aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the tem1s ofthis AC, and we mayrequire additional substantiation ordesign changes as a basis for finding compliance.
c. Disclaimer. This material does not change, create any additional, authorize changes in, orpem1it deviations from, regulatory requirements.08/04/11 AC 121.321-1
3. Definition ofTerms. Forpurposes ofthis AC, the following definitions should be used. These definitions are intended for use only with respect to§ 121.321.
a. Advisory ice detection system. A system that advises the flightcrew ofthe presence of ice accretion or icing conditions. Bothprimary ice detection systems and advisory ice detection systems can either direct the pilot to manually activate the IPS orprovide a signal that automatically activates the IPS. However, because it has lowerreliabilitythan a primary system, an advisory ice detection system can only be used in conjunction with other means (most commonly, visual observation bythe flightcrew) to detem1ine the need for, or timing of, activating the anti-icing or deicing system. With an advisory ice detection system, the flightcrew is responsible for monitoring icing conditions or ice accretion as defined in the airplane flight manual (AFM), typically using total air temperature and visible moisture criteria orvisible ice accretion. With an advisory ice detection system, the flightcrew is responsible for activating the anti-icing ordeicing system(s). b. Airframe icing. Ice accretion on the airplane, except for on the propulsion system.
c. Anti-icing. Prevention ofice accretions on a protected surface eitherby evaporating the impinging water, or allowing the impinging water to run back and offthe protected surface or freeze onnon-critical areas. d. Automatic cycling mode. A mode ofoperation ofthe airframe de-icing system that provides repetitive cycles ofthe system without the need for the pilot to select each cycle. This is generally done with a timer, and there maybe more than one timing mode.
e. Conditions conducive to airframe icing. Visible moisture at or below a static air temperature of5

# oc ( 41 

### °f) ortotal air temperature of 10

# oc (50 

### °f), unless otherwise substantiated. f. Deicing. The removal or the process ofremoval ofan ice accretion after ithas formed on a surface. g. Ice protection system (IPS). A system that protects certain critical aircraft parts from ice accretion. To be an approved system, it must satisfy the requirements of§ 23.1419 or § 25.1419 and other applicable requirements. h. Primary ice detection system. A detection system used to determine when the IPS must be activated. This system announces the presence ofice accretion or icing conditions, and it may also provide information to other aircraft systems. A primary automatic system automatically activates the anti-icing ordeicing IPS. A primary manual system requires the flightcrew to activate the anti-icing ordeicing IPS upon indication from the pr imary ice detection system. i. Reference surface. The observed surface used as a reference for the presence ofice on the monitored surface. The reference surface may be observed directly or indirectly. Ice must occur on the reference surface before--or at the same time as- it appears onthe monitored surface. Examples ofreference surfaces include windshield wiper blades orbolts, windshield

208/04/11 AC 121.321-1 posts, ice evidence probes, the propeller spitmer, and the surface ofice detectors. The reference surface may also be the monitored surface. 

### j. Static air temperature. The air temperature that would be measured by a temperature sensor that is not in motion in relation to that air. This temperature is also referred to in other documents as "outside air temperature," "true outside temperature," or "ambienttemperature." k. Total air temperature. The static air temperature plus the rise in temperature due to the air being brought to rest relative to the airplane. I. Visual cues. Ice accretion on a reference surface that the flightcrew observes. The visual cue is used to detect the first sign ofairframe ice accretion. 4. Compliance with 

### § 121.321: Determining Static Air Temperature and Total Air Temperature.
a. Regulation compliance. In the absence ofmore specific guidance provided bythe manufacturer and approved by the fAA, § 121.321 allows use ofvisible moisture and either a total air temperature at or below 10 °C (50 °f), or a static air temperature at orbelow 5 °C (41 °f), for detennining conditions conducive to airframe icing. Ifthis provision is used, the flightcrew should be able to easily detennine total airtemperature or static airtemperature. If the engine and airframe IPS are both activated based on visible moisture and temperature, a common conservative temperature for operation ofboth systems shouldbe used. This will ease flightcrew workload and increase the probability ofprocedural compliance. b. TheFAA anticipates that most types ofairplanes to which 

### § 121.321 applies already incorporate a display oftotal and/or static air temperature available to the pilot. Existing displays that have been previously certificated need not be re-certificated. Ifthe display is a new installation, the responsible aircraft certification office must approve the modification through an amended type certificate or supplemental type certificate (STC). Some aircraft, however, may display temperature information that is not appropriately calibrated to reflect either static or total temperature. Ifthat is the case, a placard should be provided showing the relationship of whatever temperature information is available and the current true air speed, or altitude and other displayed airspeed ifnot true, to total and/or static air temperature. Temperatures should be shown to the nearest degree Centigrade inthe region ofinterest (for example, around 0 degrees).
c. Hand-held charts and calculators
. Requiring pilots to access hand-held charts or calculators instead ofa placard should not be used for compliance with§ 121.321 because ofthe increased flightcrew workload that would result. 5. Compliancewith 

### § 121.321(a). This section ofthe rule requires compliance with paragraph (a)(l), (a)(2), or (a)(3) of§ 121.321(a). Each option provides the flightcrew with a means to know when the airframe ice protection must be activated; requires activation based on that means; and requires adherence to otherAFM operational procedures for flight in icing when the airframe IPS is activated. Section 121.321 defines conditions conducive to airframe icing as visible moisture at or below a static air temperature of5 °C ( 41 °f), or a total air temperature of 10 °C (50 °f), unless the approved AFM provides another definition.

308/04/11 AC 121.321-1

a. Section 121.321(a)(l). This option requires a primary airframe ice detection system. One ofthe following provides an acceptable means ofcompliance with § 121.321(a)(l): (1) A primary manual ice detection system that provides an alert that the airframe IPS mustbe activated. (2) A primary automatic ice detection system. b. Section 121.321(a)(2). This option requires both an advisory ice detection system and visual cues to the flightcrew ofthe first sign ofice formation anywhere on the aircraft. The rule requires the flightcrew to activate the airframe IPS when any ofthe visual cues are observed or the advisory ice detection system indicates activation is necessary, whichever occurs first.
c. Ice Detection Systems and Amended Type Certificates/STCs. An amended type certificate or STC is necessary for installation ofan ice detection system. The applicant should present an ice detection system certification plan to the responsible aircraft certification office. The requirements in§ 121.321(a)(l) and (a)(2) for ice detection and visual cues are identical to the requirements of§ 25.1419(e)(1) and ( e)(2), respectively. Guidance for complying with § 25.1419(e)(l) and (e)(2) is provided in AC 25.1419-2, 

### Compliance with the Ice Protection Requirements of§ 25.1419(e), 

### (/),(g), and (h). That AC should also be consulted for compliance with § 121.321(a)(l) and (a)(2). The guidance in that AC is equally applicable to part 23 and part 25 airplanes with the exceptions identified in paragraphs 5c(l) and (2) ofthis AC. Additional guidance on certification ofice detectors and visual cues is located in AC 23.1419-2D, 

### Certification of Part 23

### AirplanesforFlight in Icing Conditions, and AC 20-73A,

### Aircraft Ice Protection. (1) Part 25 airplanes should comply with§§ 25.1301, 25.1309, and 25.1419. (2) Pati 23 airplanes should comply with§§ 23.1301, 23.1309, and 23.1419. d. Section 121.321(a)(3). This option requires the flightcrew to operate the airframe IPS when the airplane is in conditions conducive to airframe icing (in other words, visible moisture and applicable temperature), during certain phases offlight, and at the first sign ofice formation anywhere on the aircraft during other phases offlight. Itis not acceptable to use crew assessment ofice depth as the first sign ofice formation. The intent is to petmit current certitied manual systems to be used in such a way that they replicate the effectiveness ofan automatic system without depending on the crew to establish ice depths. 6. Fluid Ice Protection Systems. Certain IPS use fluids that lower the freezing point ofwater. Unlike other IPS, fluid systems have a limited duration ofice protection that is related to the capacity offluid that the airplane can carry. Therefore, these systems need additional evaluation. When considering compliance with§ 121.32 1, for airplanes equipped with fluid IPS , two issues should be addressed:
a. System design
. Older fluid systems may have been certificated based on an assumption ofactivation at the first sign ofice accretion orafter certain accumulations ofice, not activation upon entering conditions ofvisible moisture and temperature in accordance with§ 121.321(a)(3).

408/04/11 AC 121.321

- 1 Therefore, the system design should be re-evaluated and shown to have adequate fluid capacity to ensure that the airplane/flightcrew can comply with § 121.32l(a)(3), ifthat option is elected. b. AFM dispatch instructions. (1) The minimum dispatch quantity ofIPS fluid in the AFM Limitations section should be re-evaluated and shown to be adequate to ensure that the airplane /flightcrew can comply with § 121.321(a)(3), ifthat option is elected. (2) The AFM Limitations section should contain pre-flight procedures to verify adequate fluid quantity, both at the tank itselfand on the cockpit display, and contain procedures for the system to be operationally checked before flight. 7. Airplane Flight Manual or Manual Required by§ 121.133.
a. Section 121.321(a)(1) or (a)(2) compliance. For airplanes that comply with § 121.321(a)(1) or (a)(2), § 121.321(c) requires that procedures for safe operation ofthe airframe IPS be established and documented in the AFM. For airplanes certified without§ 25.1419 at amendment 25-129 in their certification basis, these procedures will be established as part ofthe STC process for adding equipment. For airplanes certified with § 25.1419 at amendment 25-129 or later, these procedures will be established in the AFM as part ofthe type certification process. b
. Section 121.321(a)(3) compliance. For airplanes t hat comply with§ 121.321(a)(3), procedures for safe operation ofthe airframe IPS must be included in either the AFM or in the manual required by § 121.133. The location ofthese procedures will likely depend upon the certification basis ofthe airplane. For airplanes that were certified without§ 25.1419 at amendment 25-129 in its certification basis, the procedures for operation ofairframe IPS could be documented in the manual required by § 121.133. For airplanes certified with § 25.1419 at amendment 25-129, these procedures must be established during type certification and, therefore, mustbedocumented in the AFM. They may also bedocumented in the manual required by § 121.133
. c. Section 121.321(d) compliance. Section 121.32l(d)requires specific procedures for safe operation that mustbe included in the AFM, or in the manual required by§ 121.133. Refer to Appendix A ofthis AC for acceptable AFM procedures for operation ofthe airframe IPS for compliance with § 121.321(a)(2). Refer to Appendix B for acceptable AFM procedures for operation ofthe airframe IPS for compliance with§ 121.321(a)(3)
.

508/04/11 AC 121.321-1 

### 8. Flightcrew Training. Training in the use ofand procedures for the equipment required 

### by § 121.321 should be included in the air carrier's initial andrecurrent approved training program for pilots.

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# .----~
John 

### M. Allen Director, Flight Standards Service

6

### 08/04111 Appendix A AC 121.32 1-1 Appendix A

1. Acceptable Airplane Flight Manual (AFM) Procedures for Compliance with 

### § 121.321 (a)(2). Paragraphs 2 and 3 ofthis appendix provide acceptable AFM procedures for airframe ice protection system (IPS) operation in airplanes that comply with§ 121.321(a)(2). 2. The Airframe IPS Mustbe Turned ON-At the first sign ofice formation anywhere on the aircraft, or upon annunciation from an ice detector system, whichever occurs first.
a. For anti-icing systems. The system must be operated continuously. b. For deicing systems. (1) 

### Ifthere is an automatic cycling mode with more than one rate ofcycling available, the system must be operated continuously at the cycle rate appropriate to minimize ice accretions. Ifthere is any uncertainty, the fastest cycling rate should be used. (2) Ifan automatic cycling mode is not available, the system must be operated at short intervals (not to exceed three minutes) to minimize ice accretions. In addition, the system must be operated for at least one complete cycle immediately: (a) Before decreasing airspeed for holding or for maneuvering for approach and landing. (b) Before beginning the turn intended to intercept the final approach course inbound, including the procedure turn. (c) Before selecting orchanging any flap configuration. (d) After gear and flap retraction on a go-around climb. 3. The Airframe IPS May be Turned OFF
. a. For anti-icing systems. Five minutes after: (1) The visual cues and the advisory ice detection system no longer indicate ice formation or accretion (accumulation), or (2) Leaving conditions conducive to airframe icing. b. For deicing systems. Following completion ofthree deicing cycles after the substantiated visual cues and the advisory ice detection system no longer indicate ice accretion.



### A-I08/04/11 Appendix B AC 121.321-1 Appendix B

1. Acceptable Airplane Flight Manual (AFM) Procedures for Compliance with 

### § 121.321(a)(3). Paragraphs 2 and 3 ofthis appendix provide acceptable AFM procedures for airframe ice protection system (IPS) operation in airplanes that comply with § 121.32l(a)(3). These procedures may also be used in the manual required by§ 121.133. 2. The Airframe IPS Must be Turned ON-Whenoperating in visible moisture at orbelow a static air temperature of5 

# oc ( 41 °F) or a total airtemperature of 10 °C (50 °F) during:

• Takeoffclimb after second segment, en route climb, and go-around climb,
• Holding,
• Maneuvering for approach and landing, and

• Any other operation at approach orholding speeds. During any otherphase offlight the 

### IPS must be activated and operated at the first sign ofice formation anywhere on the aircraft except where the AFM specifies that the iceprotection should not be used.
a. For anti-icing systems. The system must be operated continuously. b. For deicing systems. (1) Ifthere is an automatic cycling mode with more than one rate ofcycling available, the system must be operated continuously at the cycle rate appropriate to minimize ice accretions. Ifthere is any uncertainty, the fastest cycling rate should be used. (2) Ifan automatic cycling mode is not available, the system must be operated at short intervals (not to exceed three minutes) to minimize ice accretions. In addition, the system must beoperated for at least one complete cycle immediately: (a) Beforedecreasing airspeed for holding or for maneuvering for approach and landing. (b) Beforebeginningthe turn intended to intercept the final approach course inbound, including the procedure tum. (c) Before selecting orchanging any flap configuration. (d) After gear and flap retraction on a go-around climb.

B-108/04111
3. TheAirframe IPS May beTurned OFF. AC 121.321-1 Appendix B

a. For anti-icing systems. Five minutes after leaving conditions conducive to airframe lCillg. b. For deicing systems. Fol1owing completion ofthree deicing cycles after leaving conditions conducive to airframe icing.

B-2