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 Page MG 18-1 

This AC section change is approved for inclusion with AC 27-1B and will later be 

incorporated into the next published change or revision to AC 27-1B. 

Due to the significant changes of this guidance material, no change markings are 

provided. Review this section in its entirety as a replacement MG section to 

AC 27-1B, AC 27 MG 18. 

CHAPTER 3 

AIRWORTHINESS STANDARDS 

NORMAL CATEGORY ROTORCRAFT 

MISCELLANEOUS GUIDANCE (MG) 

AC 27 MG 18. HELICOPTER TERRAIN AWARENESS AND WARNING SYSTEM 

(HTAWS). 

 a. Background. 

 (1) HTAWS is a computer-based system that provides the flight crew with alerts 

(both aural and visual) of pending collision of the rotorcraft with the terrain, considering 

such items as crew recognition and reaction times. HTAWS evolved from earlier 

rotorcraft alerting systems to support specific helicopter operational requirements. 

 (2) HTAWS takes inputs from a horizontal position source, vertical position 

source, terrain database, and an obstacle database to provide enhanced terrain and 

obstacle awareness. The intended function of HTAWS is an alerting system, which 

presents terrain and obstacle aural and visual alerts within a chosen flight/alert 

envelope. Guidance for rotorcraft specific requirements and system performance is 

found in Technical Standard Order (TSO)-C194, Helicopter Terrain Awareness and 

Warning System (HTAWS). TSO-C194 was developed to support rotorcraft specific 

operational requirements and prescribes the minimum performance standards that a 

HTAWS must meet for approval and identification with the applicable TSO label. 

Note: The issuance of a technical standard order authorization (TSOA) against 

TSO-C194 (or further amendments) does not constitute an installation approval. 

 (3) HTAWS is required for operations under 14 CFR part 135 subpart L, 

Helicopter Air Ambulance Equipment, Operation, and Training Requirements; 

§ 135.605, Helicopter Terrain Awareness and Warning System (HTAWS). 

 b. Purpose. 

 (1) This guidance sets forth a method of compliance with the requirements of 14 

CFR part 27 pertaining to installations of HTAWS equipment. It is for guidance 

purposes and provides an acceptable method of compliance. This guidance covers the 

safety assessment, types of environmental testing that should be considered for such 
AC 27-1B CHANGE 2/27/2014 

Page MG 18-2 

installations, and identifies other installation considerations. The guidance does not 

change regulatory requirements and does not authorize changes in, or deviations from, 

regulatory requirements. The applicant may elect to follow an alternate method 

provided the FAA also finds the alternate method acceptable. It describes the 

airworthiness considerations for such installations as they apply to the unique features 

of the HTAWS and the interfaces with other systems on the helicopter. The HTAWS 

certification should address the complete certification process. There are five basic 

aspects for certification of HTAWS installations that are discussed throughout this 

document: equipment qualification, installation, system performance validation, testing 

considerations, and instructions for continued airworthiness (ICA). 

 (2) AC 27-1 provides general guidance for certification and compliance of 

systems and equipment installation on part 27 rotorcraft. TSO-C194 specifies HTAWS 

equipment requirements and prescribes, by reference to RTCA specification DO-309, 

the minimum performance standards that a HTAWS must meet for approval. RTCA 

DO-309 defines specific Minimum Operational Performance Standards (MOPS) for 

HTAWS equipment. Compliance with RTCA DO-309 provides a method of compliance 

for qualification of HTAWS equipment. A method of compliance other than described in 

this AC may be used provided it is determined to be acceptable to the Administrator. 

 (3) HTAWS required by operational regulation must comply with TSO-C194 and 

should be installed in accordance with this AC or other methods acceptable to the 

Administrator. Terrain and obstacle warning systems that do not comply with TSOC194 and are not installed according to this AC or other method acceptable to the 

Administrator may be installed as non-required equipment but may not be identified as 

HTAWS. The certification data, including the rotorcraft flight manual supplement 

(RFMS) and ICA, must state that the installed system does not comply with any 

operational regulation that requires HTAWS, and may require a placard. 

c. Related Regulations and Documents. 

 (1) Regulation Sections: 

Section Title 

§ 27.1301 Function and installation. 

§ 27.1303 Flight and navigation instruments. 

§ 27.1309 Equipment, systems, and installations. 

§ 27.1316 Electrical and electronic system lightning protection. 

§ 27.1317 High-intensity Radiated Field (HIRF) Protection. 

§ 27.1321 Arrangement and visibility. 

§ 27.1322 Warning, caution, and advisory lights. 

§ 27.1351 General. [Electrical Systems and Equipment] 

§ 27.1357 Circuit protective devices. 

§ 27.1381 Instrument lights. 

§ 27.1459 Flight data recorders. 
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Section Title 

§ 27.1529 Instructions for Continued Airworthiness. 

§ 27.1541 General. [Markings and Placards] 

§ 27.1581 General. [Rotorcraft Flight Manual and Approved 

Manual Material] 

§ 27.1585 Operating procedures. 

Part 91 General Operating and Flight Rules. 

Part 135 

Operating Requirements: Commuter and On Demand 

Operations and Rules Governing Persons on Board 

Such Aircraft. 

 (2) ACs, Orders, and TSOs: 

Publication Title 

AC 20-115 

RTCA, Inc. Document RTCA/DO-178B, Software 

Considerations in Airborne Systems and Equipment 

Certification. 

AC 20-136 Protection of Aircraft Electrical/Electronic Systems 

against the Indirect Effects of Lightning. 

AC 20-152 

RTCA, Inc. Document RTCA/DO-254, Design 

Assurance Guidance for Airborne Electronic 

Hardware. 

AC 20-153 Acceptance of Aeronautical Data Processes and 

Associated Databases. 

AC 20-158 

The Certification of aircraft Electrical and Electronic 

Systems for Operation in the High-Intensity Radiated 

Fields (HIRF) Environment. 

AC 20-174 Development of Civil Aircraft and Systems 

AC 21-16 

RTCA Document DO-160 versions D, E, F, and G, 

Environmental Conditions and Test Procedures for 

Airborne Equipment. 

AC 21-40 Guide for Obtaining a Supplemental Type Certificate. 

AC 27-1 Certification of Normal Category Rotorcraft. 

Order 8110.4 Type Certification. 

Order 8110.49 Software Approval Guidelines. 

Order 8110.54 Instructions for Continued Airworthiness 

Responsibilities, Requirements, and Contents. 

Order 8110.105 Simple and Complex Electronic Hardware Approval 

Guidance. 

TSO-C92c Ground Proximity Warning/Glide slope Deviation 

Alerting Equipment. 

TSO-C194 Helicopter Terrain Awareness and Warning System. 
AC 27-1B CHANGE 2/27/2014 

Page MG 18-4 

 (3) Industry documents. 

 (i) RTCA documents listed below are available from RTCA, Inc., 1140 

Connecticut Avenue N.W., Suite 1020, Washington, D.C. 20036-4001. 

Publication Title 

DO-160 Environmental Conditions and Test Procedures for 

Airborne Equipment. 

DO-161A Minimum Performance Standards - Airborne Ground 

Proximity Warning Equipment. 

DO-178B Software Considerations in Airborne System and 

Equipment Certification. 

DO-254 Design Assurance Guidance for Airborne Electronic 

Hardware. 

DO-200A Standards for Processing Aeronautical Data. 

DO-309 

Minimum Operational Performance Standard (MOPS) 

for Helicopter Terrain Awareness and Warning System 

(HTAWS) Airborne Equipment. 

 (ii) The Society of Automotive Engineers (SAE) Aerospace Recommended 

Practices documents listed below are available from SAE Customer Service, 400 

Commonwealth Drive, Warrendale, PA 15096-001. 

Publication Title 

SAE ARP 

4754A 

Certification Considerations for Highly Integrated or 

Complex Aircraft Systems. 

SAE ARP 

4761 

Guidelines and Methods for Conducting the System 

Safety Assessment Process on Civil Airborne Systems 

and Equipment. 

 d. Definitions. 

 (1) Alert: A visual or aural stimulus presented either to attract attention or to 

convey information regarding system status or condition, or both. 

 (2) Aural Alert: A verbal statement used to annunciate a condition, situation, or 

event. 

 (3) Caution Alert: An alert requiring flight crew awareness. Subsequent 

corrective action will normally be necessary. 

 (4) Failure: The inability of the equipment or any subpart of that equipment to 

perform its intended function within previously specified limits. 
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 (5) False Alert: A warning or caution that occurs when the designed terrain or 

obstacle warning or caution threshold of the system is not exceeded. 

 (6) Hazard: A state or set of conditions that, together with other conditions in the 

environment, could result in an adverse safety impact. 

 (7) Hazardously Misleading Information (HMI): An incorrect depiction of the 

terrain or obstacle threat relative to the rotorcraft during an alert condition (excluding 

source data). This means that the HTAWS alert information presented in the cockpit is 

in error relative to information contained in the terrain or obstacle database. 

 (8) HTAWS: A generic term used to describe an alerting system that provides 

the flight crew with sufficient information and time to detect potentially hazardous terrain 

or obstacle. 

 (9) Integrity: Attribute or reliability of a system or a component that can be relied 

upon to function at a level that is commensurate with the criticality determined by the 

functional hazard assessment (FHA). 

 (10) Maneuver: A change in the flight path of the aircraft initiated by the flight 

crew in response to an HTAWS alert to include climbs, descents (inappropriate for most 

situations), and turning procedures. 

 (11) Nuisance Alert: An alert that occurs when there is no threat or is 

unnecessary for the intended operation. 

 (12) Obstacle: A human-made structure that is in the flight path of the rotorcraft. 

 (13) Reduced Protection Mode: A reduced warning algorithm state that allows 

operation closer to terrain and obstacles with minimal alerts. 

 (14) Terrain and Obstacle Database: Terrain and obstacle information stored 

within an HTAWS. 

 (15) Unannunciated Failure: A form of hazardous misleading information that is 

particular to warning systems, such as HTAWS. 

 (16) Visual Alert: The use of projected or displayed information to present a 

condition, situation, or event to the flight crew. 

 (17) Warning Alert: An alert for a detected terrain or obstacle threat that requires 

immediate flight crew attention and decision. 

 e. System Description. 
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 (1) The HTAWS will assist rotorcraft pilots in maintaining awareness of their 

proximity to terrain and obstacle hazards. HTAWS takes inputs from a horizontal 

position source, vertical position source, terrain database, and an obstacle database. 

The HTAWS is typically designed to provide the following high level functions: 

 (i) visual information depicting terrain, relative location of terrain, and terrain 

avoidance alerts; 

 (ii) visual information depicting obstacles, relative location of obstacles, and 

obstacle avoidance alerts; and 

 (iii) aural terrain and obstacle avoidance alerts. 

 (2) Although TSO-C194 and RTCA DO-309 do not require a reduced protection 

mode, TSO applicants should consider providing a mode that will account for off-airfield 

operations that will still provide the pilot with essential alerts regarding terrain while 

minimizing nuisance alerts. Without a reduced protection or similar mode, nuisance 

alerts may lead to pilots ignoring or inhibiting the HTAWS at inappropriate times. 

 (i) Reduced protection mode performance should be evaluated during the 

initial airworthiness certification. 

 (ii) Reduced protection mode should always provide an alert with sufficient 

time to avoid terrain or obstacles. 

 (3) Flight evaluations of systems have revealed that reduced protection or similar 

modes for terrain alerting functions are important in rotorcraft operations. Operations 

into off-airfield and unimproved landing zones usually trigger nuisance alerts if a 

reduced mode is not provided. These modes usually decrease the vertical and 

horizontal alerting envelope over terrain and obstacles thereby reducing time to collision 

alerts. TSO-C194 and RTCA DO-309 do not require a reduced protection mode. 

Applicants with systems that have a reduced protection mode with terrain and obstacle 

alerting envelopes different from those in the normal mode, should provide for sufficient 

alerting and clearance from terrain and obstacles when conducting visual 

meteorological conditions operations.

 f. Airworthiness Considerations. 

 (1) The scope of the applicant’s program should be directed toward airworthiness 

approval through the type certification (TC), amended TC, or supplemental type 

certificate (STC) processes. Installation of the HTAWS when integrated with other 

systems and equipment may result in a significant change under the changed product 

rule, 14 CFR 21.101. Installation of HTAWS in legacy aircraft may require meeting the 

current regulations that address installation of these newer technologies. 
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 (2) The remainder of this document provides airworthiness considerations that 

applicants should consider as part of the certification process. 

 g. Certification Requirements. Compliance with RTCA DO-309, along with the 

following certification guidance material and clarifications, is an acceptable means, but 

not the only means, to secure FAA approval of HTAWS equipment qualification and 

installation. 

 (1) General. For initial approvals, the applicant should provide a detailed 

systems description and design features that can be verified by certification engineers 

and flight test pilots. Flight-testing should concentrate on the adequacy of the interface, 

basic functionality of the system, location and visibility of the display, adequacy of the 

visual and aural alerts, day and night lighting, ease of use, understanding of the terrain 

and obstacle display, and potential interference with other installed equipment. In 

general, each mode of operation of the system should be evaluated in flight. Obstacles 

are frequently treated as a single point object, but in reality, obstacles (particularly tall 

obstacles) may have significant length and width due to guy wires. Obstacle alerting 

functions need to ensure that alerts are provided at sufficient distances and times to 

prevent flight into guy wires. 

 (2) System Safety Assessment. The applicant should perform an FHA and 

system safety assessment to establish the HTAWS criticality and hazards associated 

with the proposed installation. The reliability level of the system must be commensurate 

with the assessed criticality, and compliance with this criticality level must be 

demonstrated during certification. These assessments should consider the probability 

of such failures as: unannunciated failures, false caution or warning alerts due to 

undetected (or latent) failures, failure of the system to provide the required alerting 

functions due to undetected (or latent) failures, effects of HTAWS failures on other 

aircraft systems, nuisance alerts, etc. 

 (3) Installations of Required HTAWS. Rotorcraft that operate under regulations 

requiring HTAWS must conform to minimum design assurance levels (DAL) to meet 

operational reliability and functional requirements. The annunciated loss of all HTAWS 

functions is classified as a failure condition “minor.” Failure of the HTAWS to provide 

accurate terrain and obstacle aural and visual alerts, on rotorcraft that operate under 

rules that require HTAWS, is classified as a failure condition “major” by the TSO-C194. 

The HTAWS installation must satisfy the following requirements: 

 (i) The probability of an annunciated failure that would lead to the loss of all 

HTAWS functions that are described in paragraph e. above must be less than or equal 

to 10-3 per flight hour. 

 (ii) The probability of the system to provide HMI to the HTAWS display due to 

undetected or latent failures must be less than or equal to 10-5 per flight hour. 
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Page MG 18-8 

 (A) This may be a false caution or warning alert due to undetected or 

latent failures. 

 (B) This may be an unannunciated failure of the system to provide the 

required alerting functions due to undetected or latent failures. 

 (iii) Failure of the installed HTAWS must not degrade the integrity of any 

essential or critical system installed in the rotorcraft with which the HTAWS interfaces. 

 (iv) Installed equipment must meet all requirements of TSO-194. 

 (4) Software and Airborne Electronic Hardware (AEH) Qualification. The 

software for the HTAWS should be developed in accordance with RTCA DO-178B, 

Software Considerations in Airborne Systems and Equipment Certification, or 

equivalent. Applicants from the European Union (EU) applying for FAA letter of design 

approval (LODA) through European Aviation Safety Agency (EASA) may use the 

European Organization for Civil Aviation Equipment (EUROCAE) document EUROCAE 

ED-12, Software Considerations in Airborne Systems and Equipment Certification, in 

lieu of RTCA DO-178. AEH should be developed in accordance with RTCA DO-254, 

Design Assurance Guidance for Airborne Electronic Hardware. Applicants from the EU 

applying for FAA LODA through EASA may use EUROCAE ED-80, Design Assurance 

Guidance for Airborne Electronic Hardware, in lieu of RTCA DO-254. The software and 

AEH DAL for HTAWS installed in helicopters should be commensurate with the 

following assigned failure condition classifications: 

 (i) All rotorcraft using HTAWS, whether or not required by regulation, must 

conform to the minimum DAL prescribed below to meet operational reliability and 

functional requirements. The loss of all HTAWS functions is classified as a failure 

condition “minor” by the TSO-C194. Failure of the HTAWS to provide correct terrain 

and obstacle aural and visual alerts is classified as a failure condition “major” by the 

TSO-C194. The minimum DAL are: 

 (A) The system software and AEH DAL for failures that lead to the loss of 

all HTAWS functions, described in paragraph e. above, must be level D. 

 (B) The system software and AEH DAL for failures that lead to HMI to the 

HTAWS display due to undetected or latent failures must be level C. 

 (i) This may be a false caution or warning alert due to undetected or 

latent failures. 

 (ii) This may be an unannunciated failure of the system to provide the 

required alerting functions due to undetected or latent failures. 

 (5) Environmental Qualification. Since a TSO is not an installation approval, the 

HTAWS installation should be shown to be capable of operating in its expected airborne 
2/27//2014 CHANGE AC 27-1B 

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environment. One method to show environmental qualification of equipment is set forth 

in RTCA DO-160. RTCA DO-160 provides a suite of tests from which tests appropriate 

for the expected environment are chosen. For example, the vibration test should be for 

the rotorcraft environment and anticipated installation location, such as cockpit or 

avionics bay. Similar decisions must be made for other tests, such as temperature and 

electromagnetic interference (EMI) susceptibility. If the TSO environmental 

considerations do not adequately represent the actual installation environment, the 

differences must be considered and evaluated, and a course of action must be taken to 

correct deficiencies. Procedures provided by AC 27-1, section 27.1309, associated with 

temperature testing, should be followed to determine whether the equipment design is 

appropriate for the specific installation environment. 

 (6) System Performance Validation. The applicant should demonstrate that the 

performance of the HTAWS, with regard to the position of the rotorcraft relative to the 

terrain or obstacle, is adequate to prevent hazardously misleading information. The 

integrity of the navigation source has a significant effect on acceptable performance of 

the system. The applicant should demonstrate that the performance of the HTAWS is 

suitable for each phase of flight (en route, terminal, approach, and low altitude mode) 

for which approval is sought. Flight evaluations are normally required to assess reduced 

protection modes, operation in the vicinity of airfields, operations into and out of 

unimproved landing zones and off-airfield operations (helipads or other destinations not 

coded into the HTAWS database as aerodrome or helipad). HTAWS status and mode 

configuration should be easily seen. Mode selection (e.g., inhibit, reduced protection) 

should be easily accomplished without undue concentration on the pilot’s part. All 

visual indications should be readable in all lighting conditions. Refer to RTCA DO-309, 

paragraph 3.4, Test Procedures for Installed Equipment Performance, for more 

information. 

 h. Installation Considerations. 

 (1) Selecting a display where multiple functions are presented. In these cases, a 

means to select or de-select the display of terrain and obstacle information should be 

provided. However, the means to select or deselect the display should not void or alter 

terrain and obstacle aural alerts. Care should be exercised in selecting a multifunction 

implementation, to ensure that the display sharing is appropriate for the specific 

functions. The use of the HTAWS display should not unacceptably detract from the 

usability of required functions that share the display with HTAWS. Since the HTAWS 

display is not to be used for navigation, the use of the display should not impair the 

ability of the pilot to perform required navigation functions. An example of such 

impairment would be an installation that forces the pilot to choose between the HTAWS 

display and the needed navigation information in situations where both could be 

effectively used simultaneously and continuously (e.g., instrument approach in the 

vicinity of hazardous terrain and obstacles). If the timesharing of the display between 

HTAWS and other functions is deemed acceptable, the design should facilitate simple 

switching between the functions, with minimal time delays, so both functions are 

sufficiently accessible in realistic flight scenarios. 
AC 27-1B CHANGE 2/27/2014 

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 (2) Locate visual alerts in the pilot’s primary field of view. HTAWS status and 

mode selection annunciation (i.e., inhibit, reduced protection mode, or other pilot 

selectable mode) should be as close to the pilot’s primary field of view as possible to 

enable rapid assessment of HTAWS status and configuration. The terrain and obstacle 

display should be installed in a location that provides monitoring by the pilot(s) for 

identification of potential flight path conflicts. The terrain and obstacle display should be 

in a location similar to other multifunction displays, such as electronic maps and 

weather radar. 

 (3) The installation should ensure that aural alerts are distinct and audible in all 

flight conditions. 

 (4) The certification plan should include tests and analyses to assure that the 

visual and aural alerts are consistent with the alerting configuration of the rotorcraft flight 

deck in which the HTAWS equipment is installed. This is particularly important with 

retrofit installations, which may use previously installed alerting annunciations. The plan 

should consider that visual alerts are: 

 (i) located in pilots’ primary field of view, and 

 (ii) consistent with their associated voice or aural call out. 

 i. Ground Test Considerations. 

 (1) A ground test should be conducted for each HTAWS installation. The level of 

testing required will be determined by the scope of the installation (i.e., initial installation 

of a HTAWS model vs. a follow-on installation of a previously installed HTAWS model 

that was modified). Some items to consider for ground test should include: 

 (i) location of HTAWS controls, displays, and annunciators; 

 (ii) readability of HTAWS displays, annunciators, and alerts in all lighting 

conditions; 

 (iii) evaluation of identified failure modes; 

 (iv) evaluation of all HTAWS interfaces; 

 (v) compatibility evaluation of HTAWS equipment lighting with previous night 

vision imaging system (NVIS) lighting modifications and night vision goggle (NVG) 

compatibility. Ensure the NVIS STC-approved data for the rotorcraft is updated to 

reflect the installation of any annunciators or displays related to the HTAWS; and 

 (vi) EMI and electromagnetic compatibility (EMC) testing, and very high 

frequency (VHF) harmonic tests for HTAWS with internal or external GPS receivers. 
2/27//2014 CHANGE AC 27-1B 

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 (2) Evaluate on the ground all in-flight display characteristics and interfaces that 

are available during flight and that can be evaluated on the ground. 

 (3) Determine testing that can not be accomplished on the ground and that must 

be accomplished in flight. 

 j. Flight Test Considerations. 

 (1) The level of flight test required to validate a particular HTAWS installation will 

be based on the rotorcraft system architecture. Credit may be given for previously 

certificated installations, simulations, and ground tests. The requirement for a flight test 

needs to be evaluated for each installation. Initial installations and new sensor inputs 

will require flight tests. STC follow-on installations that introduce changes in flight deck 

configurations may require flight test. The evaluation of new sensor models or rotorcraft 

models may require flight tests, unless it can be shown through a sensitivity analysis 

that the new sensor’s dynamic characteristics and the model rotorcraft are compatible 

with the current sensor parameters, and will not affect the performance of the HTAWS. 

 (2) Flight testing to verify the proper operation of the terrain and obstacle display 

should be conducted while verifying all the other required HTAWS functions. Terrain 

databases vary significantly in resolution, quality, and treatment of permanent features, 

such as forests, which may be significantly different in elevation from the underlying 

terrain. It is necessary to evaluate the operation of HTAWS over a variety of topological 

conditions to ensure that protection is provided. 

 (3) Specific flight test points should be flown to assess: 

 (i) function performance in off-airfield operations, 

 (ii) performance of alerting displays and audio in all flight and lighting 

conditions, 

 (iii) performance of the reduced protection mode flown against obstacles and 

terrain, and 

 (iv) evaluation of terrain scale, which: 

 (A) should be performed during the initial airworthiness certification of the 

HTAWS system; 

 (B) should not change based on selected mode of operation, and 

 (C) should have the capability of being displayed if selected by the pilot. 
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Note: Operations into off-airfield locations should have a minimum of nuisance alerts. 

Obstacle alerts should provide sufficient time to allow for pilot scan, identification, 

decision making, and action. Additionally, flight test experience has shown that 

reducing spatial envelopes around obstacles and the resulting warning times 

may lead to flight unnecessarily close to obstacles. 

 (4) The applicant should perform sustained standard rate turns, climbs, and 

descents to evaluate: 

 Symbol stability. 

 Flicker. 

 Jitter. 

 Display update rate. 

 Color cohesiveness. 

 Readability. 

 The use of color to depict relative elevation data. 

 Caution and warning alert area depictions. 

o Normal mode. 

o Reduced Protection mode if installed. 

 Map masking. 

 Overall suitability of the display. 

 (5) Perform compatibility evaluation of HTAWS equipment lighting with previous 

NVIS lighting modifications and NVG compatibility that could not be evaluated during 

ground test. 

 (6) Perform EMI and EMC testing, and VHF harmonic tests for HTAWS with 

internal GPS receivers that could not be evaluated during ground test. 

 k. Rotorcraft Flight Manual (RFM) or RFMS. The applicant should make an 

evaluation to determine if there are any limitations of the system and, if so, how they will 

affect rotorcraft operations. Any limitations affecting operations should be included in 

the RFM or RFMS. As a minimum, the applicant should provide instructions in the 

Limitations Section of the RFM or RFMS that include the following, as appropriate: 

 (1) Limitations. The following instructions should be included in the Limitations 

section of the RFM or RFMS: 

 (i) Navigation must not be predicated upon the use of the HTAWS 

information. 

Note: The terrain and obstacle display is intended to serve as a terrain and obstacle 

awareness tool only. The display and database may not provide the accuracy or 

fidelity on which to base routine navigation decisions and plan routes to avoid 

terrain or obstacles. 
2/27//2014 CHANGE AC 27-1B 

 Page MG 18-13 

 (ii) The status of the inclusion of power lines in the obstacle database must be 

stated. 

 (iii) Reduced protection mode must not be selected when operating under 

IMC conditions except as required when performing offshore platform IFR approach 

procedures or other special IFR procedures. 

 (2) Operational Considerations for Normal and Abnormal Procedures. In addition 

to the HTAWS operational procedures, consider the following: 

 (i) Terrain or Obstacle Caution Alert. When this alert occurs, verify the 

rotorcraft flight path and correct it, if required. 

 (ii) Terrain or Obstacle Awareness Warning Alert. When this alert occurs, 

immediately initiate a maneuver that will provide maximum terrain or obstacle 

clearance, until all warning alerts cease. 

 (iii) Inhibit. For those installations that include the ability to inhibit all or some 

of the HTAWS audio alerts, the RFM (or RFMS) should address: 

 (A) When should the audio inhibit function be used? 

 (B) What alerts are inhibited? 

 (C) How long the alerts are inhibited? 

 (D) How to re-establish the alerts? 

 l. Instructions for Continued Airworthiness. ICAs are required by 14 CFR 27.1529, 

as appropriate and in accordance with part 27 Appendix A. In addition to Appendix A 

requirements, the applicant should indicate when and how the terrain and obstacle 

databases need to be updated.