2/27//2014 CHANGE AC 27-1B

Page MG 18-1 This AC section change is approved for in clusion with AC 27-1B and will later be incorporated into the next publishe d change or revision to AC 27-1B.

Due to the significant changes of this gui dance 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 TERRAI N AWARENESS AND WARNING SYSTEM (HTAWS).

a. Background.

(1) HTAWS is a computer-based system t hat provides the fli ght crew with alerts (both aural and visual) of pending collision of the rotorcraft with the terrain, considering such items as crew recognition and reac tion times. HTAWS evolved from earlier rotorcraft alerting systems to support spec ific helicopter operati onal requirements.

(2) HTAWS takes inputs from a horizont al position source, vertical position source, terrain database, and an obstacle da tabase to provide enhanced terrain and
obstacle awareness. The intended functi on of HTAWS is an alerting system, which presents terrain and obstacle aural and visual alerts within a chosen flight/alert envelope. Guidance for rotorcraft specif ic requirements and system performance is found in Technical Standard Order (TSO)-C 194, Helicopter Terrain Awareness and Warning System (HTAWS). TSO-C194 was developed to support rotorcraft specific operational requirements and prescribes t he minimum performance standards that a HTAWS must meet for approval and identif ication with the applicable TSO label.

Note: The issuance of a technical st andard order authorizati on (TSOA) against TSO-C194 (or further amendments) does no t constitute an installation approval.

(3) HTAWS is required for operat ions under 14 CFR part 135 subpart L, Helicopter Air Ambulance Equipment, O peration, and Training Requirements; § 135.605, Helicopter Terrain Awarene ss and Warning System (HTAWS).

b. Purpose.

(1) This guidance sets forth a method of compliance with the re quirements 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 te sting that should be considered for suchAC 27-1B CHANGE 2/27/2014

Page MG 18-2 installations, and identifies ot her installation considerations. The guidance does not change regulatory requirements and does not authorize changes in, or deviations from, regulatory requirements. The applicant ma y elect to follow an alternate method provided the FAA also finds the alternat e method acceptable. It describes the
airworthiness considerations for such instal lations as they apply to the unique features
of the HTAWS and the interfaces with other systems on the helicop ter. The HTAWS certification should address the complete cert ification process. There are five basic aspects for certification of HTAWS insta llations that are discussed throughout this document: equipment qualificatio n, installation, system perfo rmance validation, testing considerations, and instructions fo r continued airworthiness (ICA).

(2) AC 27-1 provides general guidanc e for certification and compliance of systems and equipment installation on part 27 rotorcraft. TSO-C194 specifies HTAWS equipment requirements and prescribes, by re ference to RTCA specification DO-309, the minimum performance standards that a HTAWS must meet for approval. RTCA DO-309 defines specific Minimum Operati onal Performance Standards (MOPS) for HTAWS equipment. Compliance with RTCA DO -309 provides a method of compliance for qualification of HTAWS equipment. A met hod of compliance other than described in this AC may be used provided it is determi ned to be acceptable to the Administrator.

(3) HTAWS required by operational r egulation must comply with TSO-C194 and
should be installed in accordance with this AC or other methods acceptable to the
Administrator. Terrain and obstacle warn ing 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-requi red 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 Regula tions and Documents.

(1) Regulation Sections:

Section Title § 27.1301 Function and installation. § 27.1303 Flight and navi gation instruments. § 27.1309 Equipment, system s, and installations. § 27.1316 Electrical and electron ic system lightning protection. § 27.1317 High-intensity Radiated Field (HIRF) Protection. § 27.1321 Arrangement and visibility. § 27.1322 Warning, cauti on, and advisory lights. § 27.1351 General. [Electri cal Systems and Equipment] § 27.1357 Circuit protective devices. § 27.1381 Instrument lights. § 27.1459 Flight data recorders.2/27//2014 CHANGE AC 27-1B

Page MG 18-3 Section Title § 27.1529 Instructions for Continued Airworthiness. § 27.1541 General. [Markings and Placards]

§ 27.1581 General. [Rotorcraft F light Manual and Approved Manual Material] § 27.1585 Operating procedures. Part 91 General Operati ng 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 Airb orne Systems and Equipment
Certification.

AC 20-136 Protection of Aircraft Elec trical/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 ve rsions D, E, F, and G,
Environmental Conditions and Test Procedures for Airborne Equipment.

AC 21-40 Guide for Obtaining a Suppl emental 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, Requir ements, and Contents.

Order 8110.105 Simple and Complex Electr onic 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 Proce ssing Aeronautical Data.

DO-309 Minimum Operational Pe rformance 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 ava ilable from SAE Cust omer 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 attr act attention or to convey information regarding system status or condition, or both.

(2) Aural Alert: A verbal statement us ed to annunciate a conditi on, situation, or

event.

(3) Caution Alert: An alert requiri ng flight crew awareness. Subsequent corrective action will normally be necessary.

(4) Failure: The inability of the equipm ent or any subpart of that equipment to perform its intended func tion within previously specified limits.2/27//2014 CHANGE AC 27-1B

Page MG 18-5 (5) False Alert: A warning or caution that occurs when the designed terrain or obstacle warning or caution threshol d 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 t he rotorcraft during an alert condition (excluding source data). This means that the HTAWS al ert information presented in the cockpit is
in error relative to information contai ned in the terrain or obstacle database.

(8) HTAWS: A generic term used to de scribe an alerting syst em 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 commens urate with the critica lity determined by the functional hazard assessment (FHA).

(10) Maneuver: A change in the flight path of the aircraft init iated by the flight crew in response to an HTAWS alert to incl ude 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 M ode: 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 syst ems, such as HTAWS.

(16) Visual Alert: The use of projec ted or displayed information to present a condition, situation, or ev ent to the flight crew.

(17) Warning Alert: An alert for a detect ed terrain or obstacle threat that requires immediate flight crew attention and decision.

e. System Description.AC 27-1B CHANGE 2/27/2014

Page MG 18-6 (1) The HTAWS will assist rotorcraft p ilots 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 provi de the following high level functions:

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

(ii) visual information depicting obst acles, 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 providi ng a mode that will account for off-airfield operations that will still provide the pilot with essentia l alerts regarding terrain while minimizing nuisance alerts. Without a r educed protection or similar mode, nuisance alerts may lead to pilots ignoring or inhi biting 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 impor tant in rotorcraft operations. Operations into off-airfield and unimproved landing zones usually trigger nuisance alerts if a reduced mode is not provided. These m odes usually decrease the vertical and horizontal alerting envelope over terrain and obs tacles 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 c onditions 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. Installati on of the HTAWS when integrated with other systems and equipment may result in a sign ificant change under t he changed product rule, 14 CFR 21.101. Installation of HTAWS in legacy aircraft may require meeting the current regulations that address insta llation of these newer technologies.2/27//2014 CHANGE AC 27-1B

Page MG 18-7 (2) The remainder of this document prov ides airworthiness considerations that applicants should consider as part of the certification process.

g. Certification Requir ements. Compliance with RT CA DO-309, along with the following certification guidance material and cl arifications, is an a cceptable means, but not the only means, to secure FAA approv al of HTAWS equipment qualification and
installation.

(1) General. For initial approvals, the applicant should provide a detailed systems description and design f eatures 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 disp lay, adequacy of the visual and aural alerts, day and night lightin g, ease of use, understanding of the terrain and obstacle display, and potent ial 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 widt h due to guy wires. Obstacle alerting functions need to ensure that alerts are pr ovided 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 t he HTAWS criticality and hazards associated with the proposed installation. The reliability level of the system must be commensurate with the assessed criticality, and complianc e with this criticality level must be demonstrated during certification. These a ssessments should consi der the probability of such failures as: unannunciated failures, false caution or warning alerts due to undetected (or latent) failures, failure of t he system to provide the required alerting functions due to undetected (or latent) failu res, effects of HTAWS failures on other aircraft systems, nuisance alerts, etc.

(3) Installations of Required HTAWS. Rotorcraft that operat e under regulations requiring HTAWS must conform to minimum design assurance levels (DAL) to meet operational reliability and functional requirem ents. 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 sa tisfy the followi ng requirements:

(i) The probability of an annunciated fa ilure that would lead to the loss of all HTAWS functions that are described in parag raph 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 mu st be less than or equal to 10 -5 per flight hour.AC 27-1B CHANGE 2/27/2014

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 HT AWS must not degrade t he integrity of any essential or critical system installed in t he rotorcraft with which the HTAWS interfaces.

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

(4) Software and Airborne Electronic Ha rdware (AEH) Qualification. The software for the HTAWS should be develop ed in accordance with RTCA DO-178B,
Software Considerations in Airborne S ystems and Equipment Ce rtification, or equivalent. Applicants from the European Un ion (EU) applying for FAA letter of design approval (LODA) through European Aviati on Safety Agency (EASA) may use the European Organization for Civil Aviation E quipment (EUROCAE) document EUROCAE ED-12, Software Considerations in Airbor ne Systems and Equipment Certification, in
lieu of RTCA DO-178. AEH should be dev eloped in accordance with RTCA DO-254, Design Assurance Guidance for Airborne Electr onic 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 helic opters should be commensurate with the following assigned failure condition classifications:

(i) All rotorcraft using HTAWS, whet her or not required by regulation, must conform to the minimum DAL prescribed below to meet operational reliability and functional requirements. The loss of all HT AWS 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 cla ssified as a failure condition “major” by the
TSO-C194. The minimum DAL are:

(A) The system software and AEH DA L for failures that lead to the loss of all HTAWS functions, described in par agraph e. above, must be level D.

(B) The system software and AEH DA L for failures that lead to HMI to the HTAWS display due to undetected or la tent failures must be level C.

(i) This may be a false cauti on 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 Qualific ation. Since a TSO is not an installation approval, the HTAWS installation should be shown to be capabl e of operating in its expected airborne2/27//2014 CHANGE AC 27-1B

Page MG 18-9 environment. One method to s how environmental qua lification of equipm ent 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 inst allation location, such as cockpit or avionics bay. Similar decisions must be m ade for other tests, such as temperature and electromagnetic interference (EMI) suscept ibility. If the TSO environmental considerations do not adequately represent t he 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 whet her the equipment design is appropriate for the specific installation environment.

(6) System Performance Validation. The applicant s hould 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 navigat ion source has a significant effe ct on acceptable performance of the system. The applicant should demonstrate that the performance of the HTAWS is suitable for each phase of flight (en rout e, terminal, approach, and low altitude mode) for which approval is sought. Flight evaluatio ns are normally required to assess reduced protection modes, operation in the vicinity of airfields, operations into and out of unimproved landing zones and off-ai rfield 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 wi thout 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 Insta lled Equipment Performance, for more
information.

h. Installation Considerations.

(1) Selecting a display where multiple f unctions 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 shoul d be exercised in sele cting a multifunction implementation, to ensure that the display sharing is a ppropriate for the specific functions. The use of the HTAWS display should not unacceptabl y detract from the usability of required f unctions that share the display with HTAWS. Since the HTAWS display is not to be used for navigation, t he use of the display should not impair the ability of the pilot to perform required nav igation functions. An example of such impairment would be an installa tion that forces the pilot to choose between the HTAWS display and the needed navigation informati on in situations where both could be effectively used simultaneously and continuous ly (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 acceptabl e, the design should facilitate simple switching between the functi ons, with minimal time dela ys, so both functions are sufficiently accessible in realistic flight scenarios.AC 27-1B CHANGE 2/27/2014

Page MG 18-10 (2) Locate visual alerts in the pilot’s primary field of view . HTAWS status and mode selection annunciation (i.e., inhibit, re duced protection mode, or other pilot selectable mode) should be as cl ose to the pilot’s primary fi eld of view as possible to enable rapid assessment of HTAW S status and config uration. The terrain and obstacle display should be installed in a location that provides monitoring by the pilot(s) for identification of potentia l flight path conflicts. The terra in and obstacle display should be
in a location similar to other multifunc tion displays, such as electronic maps and
weather radar.

(3) The installation should ensure that aur al alerts are distinct and audible in all flight conditions.

(4) The certification plan should incl ude tests and analyses to assure that the
visual and aural alerts are cons istent with the alerting configur ation of the rotorcraft flight deck in which the HTAWS equipment is installe d. This is particularly important with retrofit installations, which may use previous ly installed alerting annunciations. The plan
should consider that visual alerts are:

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

(ii) consistent with their a ssociated 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 installati on of a previously installed HTAWS model that was modified). Some items to c onsider 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 HT AWS equipment lighting with previous night vision imaging system (NVIS) lighting modifications and night vision goggle (NVG) compatibility. Ensure the NVIS STC-appr oved data for the rotorc raft is updated to reflect the installation of any annunciators or displays related to the HTAWS; and

(vi) EMI and electromagnetic compat ibility (EMC) testing, and very high frequency (VHF) harmonic tests for HTAWS with internal or external GPS receivers.2/27//2014 CHANGE AC 27-1B

Page MG 18-11 (2) Evaluate on the ground all in-flight disp lay characteristics and interfaces that are available during flight and t hat can be evaluated on the ground.

(3) Determine testing that can not be a ccomplished 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 architectu re. 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. ST C follow-on installations that in troduce 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 characterist ics and the model rotorcraft are compatible with the current sensor param eters, and will not affect the performance of the HTAWS.

(2) Flight testing to veri fy 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 operat ion of HTAWS over a va riety 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 disp lays and audio in all flight and lighting conditions,

(iii) performance of the reduced prot ection mode flown against obstacles and terrain, and

(iv) evaluation of terrain scale, which:

(A) should be performed during the init ial airworthiness ce rtification of the HTAWS system;

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

(C) should have the capability of bei ng displayed if selected by the pilot.AC 27-1B CHANGE 2/27/2014

Page MG 18-12 Note: Operations into off-ai rfield 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 obstacl es 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 li ghting with previous NVIS lighting modifications and NVG compatib ility 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 coul d 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 syst em 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 app licant should provide instructions in the
Limitations Section of the RFM or RFMS that include the follo wing, as appropriate:

(1) Limitations. The following instruct ions should be included in the Limitations
section of the RFM or RFMS:

(i) Navigation must not be pr edicated 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 dat abase may not provide the accuracy or fidelity on which to base routine navigat ion 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 mu st not be selected when operating under IMC conditions except as required w hen 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 proce dures, consider the following:

(i) Terrain or Obstacle Caution Al ert. 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 warn ing alerts cease.

(iii) Inhibit. For those installations t hat 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 Airworthine ss. 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 indica te when and how the terrain and obstacle databases need to be updated.