[Federal Register Volume 65, Number 131 (Friday, July 7, 2000)]
[Notices]
[Pages 42053-42058]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 00-17243]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration


Advisory Circular 25-23, Airworthiness Criteria for the 
Installation Approval of a Terrain Awareness and Warning System (TAWS) 
for Part 25 Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of issuance of advisory circular.

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SUMMARY: This notice announces the issuance of Advisory Circular (AC) 
25-23, Airworthiness Criteria for the Installation Approval of a 
Terrain Awareness and Warning System (TAWS) for Part 25 Airplanes. The 
AC provides guidance for designing an acceptable installation for a 
TAWS that is compliant with Technical Standard Order (TSO) C151. The 
guidance provided is specific to installations of these systems on 
transport category airplanes.

DATES: Advisory Circular 25-23 was issued on May 22, 2000, by the 
Acting Manager of the Transport Airplane Directorate, Aircraft 
Certification Service, Federal Aviation Administration.

HOW TO OBTAIN COPIES: A paper copy of AC 25-23 may be obtained by 
writing to U.S. Department of Transportation, Subsequent Distribution 
Center, SVC-121.23, Ardmore East Business Center, 3341 Q 75th Avenue, 
Landover, Maryland 20785. The AC also will be available on the Internet 
at 
http://www.faa.gov/avr/air/airhome.htm, at the link titled ``Advisory 
Circulars'' under the ``Available Information'' drop-down menu.

FOR FURTHER INFORMATION CONTACT: For technical issues, contact J. Kirk 
Baker, FAA, Transport Airplane Directorate, Los Angeles Aircraft 
Certification Office, Systems & Equipment Branch, ANM-130L, 3960 
Paramount Boulevard, Lakewood, California 90712-4137; telephone (562) 
627-5345; fax (562) 627-5210.
    For other information contact: Jill DeMarco, FAA, Transport 
Airplane Directorate, Program Management Branch, ANM-114, 1601 Lind 
Avenue SW., Renton, Washington 98055-4056; telephone (425) 227-1313; 
fax (425) 227-1320.

SUPPLEMENTARY INFORMATION:

Discussion of Comments

    On September 23, 1999, the FAA issued a notice of the availability 
of proposed Advisory Circular (AC) 25.XX, ``Airworthiness Criteria for 
the Installation Approval of a Terrain Awareness and Warning System 
(TAWS) Approved under Technical Standard Order (TSO) C151.'' That 
notice was published in the Federal Register on September 30, 1999 (64 
FR 52820) and requested public comment on the proposed AC document. 
Five commenters submitted comments to the proposed AC. A discussion and 
disposition of each comment follows.

Format of AC

    Some commenters request that the format of the proposed AC be 
improved. One commenter requests that a table of contents or index be 
included.
    The FAA concurs and has added a table of contents to the final 
document.

Guidance for Part 23 Airplanes

    One commenter requests that the proposed AC be revised to include 
guidance on TAWS installations for airplanes certificated under 14 CFR 
part 23 (small airplanes).
    The FAA does not concur with the commenter's request. The FAA 
currently is developing a separate AC that provides guidance specific 
to TAWS installations for part 23 airplanes. By issuing with two 
separate AC's, the FAA anticipates that there will be less confusion 
for applicants.

Definition of Class B TAWS Equipment

    One commenter suggests that the definition of Class B TAWS 
equipment be revised to include a note indicating that Class B TAWS 
requires a GPS input but does not require a radio altitude input. The 
commenter states that the inclusion of such a note will help to clarify 
the composition of Class B TAWS equipment.
    The FAA agrees that clarification is appropriate. Paragraph 
11.b.(2), ``Radio Altimeter,'' of the final AC has been revised to 
specify that Class B equipment does not require a radio altitude input.

Regulatory References

    One commenter requests that the reference to the requirements of 
Sec. 91.223 in paragraph 5.a. be revised to add the words ``or 
certified'' in the phrase:

    ``Sec. 91.223 states that no person may operate a turbine-
powered U.S.-registered airplane configured or certified with 6 or 
more passenger seats * * *.''

    The FAA disagrees with the addition of these words. The text, as 
presented in the AC, is quoted directly from Sec. 91.223 of the 
regulations. An AC is not the vehicle for making changes to the text of 
current regulations.

System Criticality/Probability

    One commenter requests clarification of the descriptions of failure 
probability that appear in paragraph 5.b., ``System Criticality,'' of 
the proposed AC. The commenter suggests that the proposed text:

    ``* * * the applicant must demonstrate that the TAWS possesses a 
level of reliability commensurate with systems that have a failure 
probability of 10 -4 or less per flight hour * * *.''

be changed to:

    ``* * * have a failure probability due to undetected failures 
(latent failures) of 10-4 or less per flight hour.''

    The commenter also suggests similar changes in the probability 
descriptions that appear in paragraphs 7.c.(2) and 7.g.(1)(b).
    On this same issue, another commenter points out that the 
reliability value of 10-3, as stated in paragraph 7.c. of 
the proposed AC, is not consistent with the value of 10-4, 
specified in paragraph 5.b.
    The FAA concurs with these commenters' requests and has revised the 
final AC accordingly. [Although paragraph 5.b. (as it appeared in the 
proposed AC) has been eliminated in the final AC, the item noted by the 
commenter has been clarified and corrected in the final AC in paragraph 
9., ``System Safety Assessment.'']

Project Specific Certification Plan (PSCP)

    One commenter suggests that paragraph 7.b. of the proposed AC, 
concerning the Project Specific

[[Page 42054]]

Certification Plan (PSCP), be revised to include ``switches and 
annunciators'' in the list of items that should be included in a 
comprehensive system description. Additionally, this commenter requests 
that this paragraph provide guidance on acceptable or desirable 
locations for the installation of TAWS controls, control panels, 
annunciators, displays, etc.
    The FAA concurs and has revised paragraph 8.b.(1), ``System 
Description,'' of the final AC accordingly.
    Another commenter suggests revising paragraph 7.b. of the proposed 
AC by deleting the inclusion of information regarding software aspects 
of any certification and any certification-specific integrated circuits 
(ASIC). The commenter states that this issue is TSO-related and should 
be addressed as part of the TSO approval of the Line Replaceable Unit 
(LRU), not as part of the approval of the TAWS.
    The FAA does not agree that the material should be deleted. 
Instead, the FAA has revised the final AC to clarify this issue. This 
information appears in paragraph 8.b.(3) of the final AC.

System Safety Assessment

    One commenter requests that the text of the lead-in phrase in 
paragraph 7.c.(2) of the proposed AC, which concerns the System Safety 
Assessment, be revised to include the phrase highlighted below:

    * * * Given that TSO-C151 requires 10 -\5\ for unannunciated 
failure, HMI, and false alerts at the box level as a result of a 
TAWS computer failure, the box as installed must meet the following 
criteria * * *''

    The FAA does not concur. Restricting the annunciations to only 
those indicated as a result of a TAWS computer failure would be too 
limiting. The FAA's intent is for this criterion to be more 
comprehensive. The language as used the proposed AC is more open to the 
inclusion of other types of avionics architectures, such as modular 
types, that are being proposed today. This language has been retained 
in the final AC.
    Another commenter suggests that paragraph 7.c.(2)(a) of the 
proposed AC be revised in the text concerning probability of failure of 
the availability of the TAWS function. The text in the proposed AC 
states:

    ``The probability of failure of the installed system to perform 
its intended function from a reliability/availability perspective * 
* * shall be less than or equal to 10-\3\ per flight 
hour.''

    However, this commenter requests that the reference to 
``availability'' be deleted. The commenter considers this request 
reasonable because:

     The FAA in the past has never had any availability 
requirements for a GPWS system.
     Availability is more dependent on the inputs to the 
TAWS system than on the TAWS computer itself.

     If an aircraft's only source of air data to the TAWS 
system is not designed to meet this requirement, then the TAWS 
system will not meet this requirement.

    This commenter further states that the FAA Master Minimum Equipment 
List (MMEL) policy allows the TAWS function to be inoperative for up to 
10 flight days. The commenter questions why the AC guidance would 
require availability numbers when the MMEL does not require the same.
    The FAA does not concur with this commenter's requests. Controlled 
flight into terrain (CFIT) continues to be a major contributor to 
commercial jet airplane hull losses and, therefore, constitutes a 
failure value of at least 10-\3\ for all of the functions of 
a TAWS system. Furthermore, this value is consistent with the expected 
value of the inputs to TAWS. If the sensor input does not meet this 
failure level, it should be replaced with one that does.
    The FAA's Flight Standards Service determines the MMEL requirements 
established for GPWS. The determination is made separate from the 
determination of hazard level. The process used to establish relief 
under the MMEL does not consider the use of probabilities. Instead, it 
is based on the FAA's evaluation of the risks associated with the 
system's effect on the aircraft and, along with guidance from air 
traffic control, the pilot's ability to operate the aircraft.

Self-Test Functions

    One commenter points out that the proposed AC does not consider 
self-test functions, and suggests that criteria for activation or 
inhibition of self-test functions in flight should be provided. [For 
example, if a self-test function results in a failure to alert, then 
this should be inhibited during certain (or all) phases of flight.] The 
commenter suggests that the System Safety Assessment, as described in 
the AC, should consider whether credit may be taken for self-test 
functions in meeting the reliability rates provided.
    The FAA concurs that self-test functions should be considered and 
has included reference to them in the final AC in paragraph 19., 
``Ground Test Considerations.''

Software

    One commenter suggests that paragraph 7.d. of the proposed AC, 
which addresses the verification of software requirements, is 
unnecessary. The commenter states that software requirements are 
already addressed in TSO-C151a, and requests that either the paragraph 
be deleted or revised to state that it applies to systems that do not 
meet TSO-C151a.
    The FAA does not agree that the information should be deleted. 
However, the FAA has revised this information in the final AC to 
clarify the issue raised. Paragraph 10., ``Software,'' of the final AC 
now states:

    ``The applicant should provide evidence that the TAWS software 
meets the requirements of TSO-C151a and that it meets the 
appropriate software levels for any added feature(s).''

Position Source

    One commenter points out that vertical accuracy is also a 
consideration for the correct function of TAWS for various phases of 
flight. However, paragraph 7.e. of the proposed AC, which discusses 
position source, does not reference any criteria for required vertical 
accuracy for the correct function of TAWS. The commenter suggests that 
the AC should include such criteria.
    The FAA concurs and has expanded the final AC to include the 
requested criteria for both vertical and horizontal position sources. 
This information is located in the final AC in paragraph 11., 
``Position Source.''

[[Page 42055]]

Terrain Database

    One commenter requests revision to the guidance in paragraph 
7.f.(2) of the proposed AC, which addresses verification of the TAWS 
terrain database. The commenter states that the requirement to 
``include terrain and airport information of the area of intended 
operation'' may result in additional and undesired STC limitations. For 
example, if an applicant certifies TAWS equipment on an airplane in the 
United States (and uses a ``U.S. database'') there is nothing to keep 
the operator from moving the airplane to another region of the world 
and the STC would still be valid. The commenter states that limiting an 
STC to a region of operation would be non-productive and undesirable. 
Therefore, the commenter suggests that the AC be revised to require 
that applicants verify that an obvious TAWS failure condition will be 
indicated to the pilot whenever an airplane is operated outside of the 
region that is covered by the terrain/airport database installed on the 
airplane.
    The FAA concurs that clarification is necessary for this item. The 
final AC now directs applicants to the Instructions for Continued 
Airworthiness, which should identify the procedures for determining the 
status of the terrain database. The AC states that operators should use 
this information to determine if the current terrain database is 
appropriate for the area of intended operation. This information can be 
found in the final AC in paragraph 12.b., ``Valid Regional Data.''

Terrain Display

    One commenter notes that paragraph 7.g.(1)(a) of the proposed AC 
states that any device approved under TSO-C151 must be capable of 
providing terrain, obstacle, and alerting data to display hardware that 
is already onboard the airplane. The commenter states that a TAWS 
designed to use its own proprietary display must also support existing 
display hardware in the aircraft. The commenter requests that this 
requirement not exclude dedicated displays.
    The FAA concurs and has revised paragraph 13.a.(1), ``Terrain 
Display,'' of the final AC accordingly.
    Another commenter suggests that paragraph 7.g. of the proposed AC, 
which concerns display presentation criteria, be revised to address 
situations where the means of display is used for more than only the 
presentation of the terrain display. The commenter suggests the 
addition of a new paragraph that states:

    ``The applicant should consider the selection of terrain display 
where the display is utilized for multiple functions. In these 
cases, a means should be provided to select and deselect the display 
of terrain.''

    The FAA concurs with this suggestion and has revised paragraph 
13.a.(4) of the final AC accordingly.
    One commenter requests a revision to paragraph 7.g.(2) of the 
proposed AC, which addresses the terrain display presentation. The 
commenter points out that paragraph 7.g.(2)(g) states that the terrain 
display should be ``viewable in direct sunlight, and at least one 
display must be viewable by each pilot.'' The commenter requests that 
consideration be given for aircraft with only one display. Single 
displays are common in older aircraft, where weather radar PPI displays 
may be installed on only one side of the flightdeck. The commenter 
suggests that the wording be changed to ``* * * at least one display 
must be viewable by one of the pilots.''
    The FAA concurs and has revised paragraph 13.b.(7), ``Terrain 
Display Presentation,'' of the final AC accordingly.
    One commenter suggests a revision to paragraph 7.g.(2)(h) of the 
AC, which concerns the display of a ``failed and/or inoperative TAWS'' 
indication to the flight crew. The commenter suggests that an 
``inhibited TAWS'' should be indicated as well, and suggests that the 
text of the AC be changed to include this.
    The FAA concurs and has revised paragraph 13.b.(8) of the final AC 
accordingly.
    One commenter points out an inconsistency between guidance provided 
in paragraph 7.g.(2)(n) of the AC, concerning the selected colors of 
the terrain display, and the requirements of Sec. 25.1322 (``Warning, 
caution, and advisory lights''). The commenter notes that Sec. 25.1322 
calls for indication lights to be red (for warnings) or amber (for 
cautions); however, paragraph 7.g.(2) of the AC appears to contradict 
this where it states, ``TAWS equipment shall be designed to interface 
with a terrain display, either color or monochromatic.'' The commenter 
is concerned that the use of a monochromatic display would violate the 
regulations and, therefore, suggests that acceptability of 
monochromatic displays must be deleted.
    The FAA concurs that clarification of this item is needed. The 
final AC has been revised to specify that the terrain display feature 
applies only to color displays. This information is found in paragraph 
13.b.(14) of the final AC.

Pop-Up Mode Switching Functionality

    One commenter requests that paragraph 7.g.(3)(a) of the proposed 
AC, which concerns pop-up mode switching functionality, be revised to 
address display systems that cannot accommodate annunciating the 
terrain display mode. The commenter suggests that this paragraph be 
revised to state:

    ``The terrain display mode should be annunciated on the display 
or, if not feasible on the display, with a mode annunciation light 
near the terrain display, or equivalent.''

    The FAA concurs with the commenter's suggestion and has revised 
paragraph 13.c.(1)(d), ``Pop-Up Mode-Switching Functionality,'' of the 
final AC accordingly.
    Another commenter requests that the AC provide guidance on 
acceptable default display ranges for this mode of operation.
    The FAA concurs and has revised the final AC to include a new 
paragraph 13.c.(1)(f) to provide this guidance.
    Another commenter requests that the criteria for automatic pop-up 
function be revised to state that the pop-up feature should 
automatically display TAWS-related information when a TAWS caution or 
warning alert occurs. The proposed AC, however, only mentions the TAWS 
caution alert.
    The FAA disagrees. The required pilot response to a TAWS warning is 
an immediate climb to clear the terrain/obstacle. Pop-up of the display 
with a ``warning'' may distract the pilot. With a ``caution'' alert, 
there will be enough time for the pilot to evaluate the information on 
the display and determine if either a level-off is adequate or the 
initiation of a climb is necessary.

Alerts Within the Flightcrew's Primary Field of View

    One commenter requests that paragraph 7.h.(2) of the proposed AC, 
which calls for the alerting system to be ``within the flightcrew's 
primary field of view,'' be revised to include a definition of the 
``flightcrew's primary field of view.'' The commenter states that, on 
many retrofit installations, there is simply not any room available to 
install lights in today's generally accepted primary field of view. The 
commenter also requests that, if an applicant is retrofitting an 
airplane that has previously-installed alert lights, then the applicant 
should be allowed to use those previously-installed light locations.
    The FAA agrees that clarification is appropriate. Paragraph 14, 
``Alerts,'' of the final AC has been modified to include the 
previously-installed light

[[Page 42056]]

locations, and to reference the human factors considerations in the 
applicant's certification plan as a method of determining the flight 
crew's primary field of view.

Alert Prioritization

    One commenter points out what seems to be a conflict in the 
guidance concerning the number of aural alerts permitted at one time. 
The commenter notes that paragraph 7.i.(1)(a) of the proposed AC states 
that, on aircraft equipped with a TAWS, Predictive Windshear System 
(PWS), and a Traffic Collision Avoidance System (TCAS), ``only one 
aural alert is given at any one time.'' However, the NOTE at the bottom 
of Table 2 states, ``Voice callouts are allowed simultaneously with 
TCAS.'' The commenter requests clarification of this issue.
    The FAA concurs that clarification is necessary. The FAA's intent 
is to prioritize callouts so that only one is given at any time. 
Therefore, Table 2 (``Recommended Voice Prioritization between the TAWS 
and Other Systems Installed'') of the final AC has been revised to 
delete the references to simultaneous callouts.
    Another commenter questions the material contained in Table 2 of 
the proposed AC. The commenter asks if the ``PDA (`Too Low Terrain') 
Caution,'' shown as Priority Level 9 on the table, is considered a 
separate function or sub-function of the forward-looking terrain-
avoidance (FLTA) function. In the table, the term ``(FLTA)'' follows 
the term ``PDA * * * Caution.'' The commenter finds the meaning of this 
unclear. Additionally, this commenter questions whether Table 2 is 
intended to consider both Class A and Class B TAWS.
    The FAA concurs that clarification of the information presented in 
Table 2 is appropriate. First, the FAA has deleted the term ``(FLTA)'' 
that appears in the Priority Level 9 column, as it is inappropriate in 
that section of the table. Second, the FAA has revised Table 2 to 
include indications in each Priority Level as to whether the guidance 
applies to Class A TAWS, Class B TAWS, or both systems.

System Inhibit

    One commenter requests that paragraph 7.j. of the proposed AC, 
concerning system inhibit, be revised to clarify the system inhibit 
provisions for Class B TAWS systems. The commenter notes that TSO-C151 
does not require Class B systems to inhibit the FLTA and PDA functions 
while maintaining basic GPWS functions, and requests that the AC 
specify this.
    The FAA agrees and has clarified this information in the final AC 
in paragraph 16., ``System Inhibit.''

Flight Data Recorders

    One commenter suggests deletion of paragraph 7.k. of the proposed 
AC, which addresses considerations relevant to flight data recorders. 
The commenter states that this is essentially an LRU issue and should 
be addressed via the TSO approval process. Since there are no FAA 
requirements to record TAWS FLTA alerts, this does not need to be 
addressed in the AC.
    Similarly, another commenter notes that paragraph 7.k. implies that 
flight data recorders certificated under part 91 (and potentially all 
JAA-certificated flight data recorder installations) should have a 
means to record FLTA alerts as well as FLTA and/or PDA inhibits. The 
commenter states that this information is not recorded today in these 
installations and the format is not defined. The commenter requests 
that this paragraph be clarified as to its specific installation 
application.
    The FAA does not consider that any change to the information 
concerning flight data recorders is necessary. The AC clearly specifies 
that the guidance applies only to aircraft that are defined by the 
requirements of Sec. 25.1459 (``Flight recorders''). Furthermore, it 
specifies the recording requirements for the various alerts associated 
with TAWS. (This information is found in the final AC in paragraph 17, 
``Flight Data Recorder.'')

Human Factors

    One commenter requests clarification of paragraph 7.l. of the 
proposed AC, which addresses inclusion of human factors considerations 
as part of the certification program. The commenter suggests that the 
paragraph clarify the requirements for a first-time certification of a 
display design versus follow-on installations or minor display 
improvements. The commenter considers that follow-on certifications 
(i.e., same display design in different aircraft) and minor changes 
should not require the rigorous human factor review that a new display 
design should require.
    The FAA agrees that clarification on this issue is necessary. The 
FAA has revised the final AC by eliminating the separate paragraph 
addressing human factors, and including the pertinent information in 
paragraph 8.f., ``Testing Plan.''

Flight Test Requirements

    One commenter suggests that paragraph 7.m.(2), of the proposed AC, 
which concerns flight test requirements, be revised to clarify that the 
``cases'' listed are intended only to assist in determining flight test 
guidelines for potential TAWS configurations; however, the actual 
requirement for a flight test needs to be evaluated for each 
installation. The commenter is concerned that, with the cases written 
as they are, someone unfamiliar with TAWS may gain the impression that 
a flight test is required for every possible TAWS installation, which 
is not the case. To clarify this issue further, the commenter suggests 
that all of the ``cases'' listed be changed to ``examples.'' 
Additionally, the commenter requests that a note be added to state that 
some installations may be a combination of these examples and, as such, 
all of the guidance should be considered when determining flight test 
requirements.
    The FAA agrees with this commenter's suggestions and has revised in 
paragraph 20.a, ``Flight Test Considerations,'' of the final AC 
accordingly.
    One commenter requests clarification concerning the flight test 
requirements for follow-on installations of a previously approved TAWS 
in which a required sensor input has not been previously approved. The 
commenter assumes that bench testing of each model of sensors would be 
an acceptable means of compliance for these (thus, not requiring flight 
testing during installation of follow-ons).
    The FAA agrees that clarification is required and has modified 
paragraph 20., ``Flight Test,'' to include guidance in determining the 
scope of flight testing required.
    One commenter requests clarification in paragraph 7.m.(2) 
concerning Case 5, which pertains to a follow-on installation of a 
previously-approved TAWS in which the radio altitude to the TAWS 
equipment has not been previously approved. The commenter states that 
this case should not be applicable to Class B TAWS, because Class B 
TAWS does not require a radio altitude input. The commenter suggests 
that the AC specify this.
    The FAA agrees with the commenter and has revised the final AC to 
specify that this example is not for Class B equipment.
    One commenter notes that the proposed AC does not consider systems 
able to account for altitude variations in cold weather (function 
similar to the Allied Signal Geometric Altitude), other than a note in 
the AFMS requirements. The commenter suggests that the AC provide 
guidance that describes this

[[Page 42057]]

function and any testing associated with it.
    The FAA agrees with the commenter and has revised the final AC to 
add this guidance in new paragraph 20.g., ``Pressure Altitude 
Variations in Cold Weather.''

Ground Tests

    One commenter requests clarification of whether the airworthiness 
considerations described in paragraph 7. of the proposed AC are to be 
construed as requiring verification by ground or flight test. The 
commenter notes that the AC does not explicitly consider any ground 
testing required for evaluation of an installation. The commenter 
suggests that some items that should be considered for ground testing 
are:

     An acceptable location of TAWS controls, displays, an 
annunciators;
     Exercise of self-test functions;
     Evaluation of identified failure modes;
     Evaluation of all discretes and TAWS interfaces;
     EMI/EMC testing, and
     Electrical transient testing.

    The FAA concurs with the commenter's suggestion and has revised the 
final AC to include a new paragraph 19., ``Ground Test 
Considerations,'' to include this information.

Forward Looking Terrain Avoidance (FLTA) Flight Test Requirements

    One commenter suggests that the text of paragraph 7.m.(3)(a) of the 
proposed AC, which addresses FLTA flight test requirements, be changed 
from:

    ``Two tests are recommended * * * ,''

to:

    ``Test runs are recommended to be level flight at approximately 
500 feet above the terrain/obstacle of interest. The test runs 
should verify that all alerts (cautions and warnings) are given at 
an appropriate point in the test run; that all pop-up, auto range, 
or other display features are working; and that the display depicts 
the terrain accurately.''

    This commenter states that experience has shown that all FLTA 
features can be tested with level flight and there is no need to add 
the risk of descending toward the terrain/obstacle during the test 
flight.
    The FAA concurs with the commenter's suggestion and has revised 
paragraph 20.b., ``FLTA Flight Test Considerations,'' of the final AC 
accordingly.

Premature Descent Alert (PDA) Flight Test Requirements

    One commenter requests that paragraph 7.m.(4) of the proposed AC, 
which concerns PDA flight test requirements, be revised to specify 
that, depending on the system design, PDA flight tests may only need to 
be accomplished during a system's initial certification. The commenter 
states that, for the Honeywell Enhanced Ground Proximity Warning System 
(EGPWS) specifically, all of the inputs driving the PDA function can be 
tested via FLTA testing and altitude callout testing. Thus, once the 
function has been proven during the initial certification, follow-on 
certifications can be proven by alternative tests. The commenter 
requests that text be added to this section to allow such 
``alternative'' means of testing for this system. This same commenter 
states that, with the Honeywell EGPWS, PDA testing can be performed 
most safely if the aircraft is more than 15 nautical miles (NM) from 
the nearest airport. The commenter requests that the AC be revised to 
allow PDA testing outside of 15 NM from the nearest runway to allow for 
testing at safer altitudes.
    The FAA does not agree with the commenter's suggested changes. The 
FAA considers the commenter's requests to be too specific to one 
particular system design, and may not be appropriate for other designs. 
As for ``alternative means of testing,'' the FAA points out that the 
guidance provided in this AC applies to one acceptable means, but not 
the only means, of compliance; therefore, applicants have the 
prerogative to apply for FAA approval of alternative means of 
compliance, which may include alternative testing methods.

Basic GPWS Flight Test Requirements

    One commenter requests a change in the text of the first sentence 
of paragraph 7.m.(5)(c) of the proposed AC, which concerns negative 
climb rate or altitude loss after takeoff. The commenter requests that 
the text be changed from:

    ``This test is conducted immediately after takeoff before 
climbing above 700 AGL or above runway elevation.''

to

    ``This test is conducted immediately after takeoff.''

    The commenter states that Honeywell has been able to develop tests 
of its EGPWS systems for this scenario that allow for higher altitude 
gain, thus reducing the risk of this test flight.
    The FAA does not concur. The FAA considers the commenter's requests 
to be too specific to one particular system design, and may not be 
appropriate for other designs.
    Another commenter suggests paragraph 7.m.(5)(f) of the proposed AC, 
which concerns the Voice Callout ``Five Hundred Feet'' function, be 
revised to include the following note:
    ``If selected or utilized in the installation, this test should be 
conducted.'' The commenter states that not all operators elect the use 
of the 500-foot callout and, therefore, it is not possible to test this 
callout in those installations. As an alternative, the commenter 
suggests that this section could be changed simply to address 
``altitude callouts,'' and thus test all of the selected altitude 
callouts.
    The FAA does not concur with this commenter's request. The Voice 
Callout ``Five Hundred Feet'' function is required by TSO C-151 for 
both the Class A and B TAWS.

Terrain Display Flight Test Requirements

    One commenter states that, in some new display systems, it is 
possible to evaluate the new display via ground tests. Therefore, this 
commenter suggests that paragraph 7.m.(6) of the proposed AC be revised 
by adding a note to indicate that ground test evaluation of a display 
may be acceptable if it can be shown that all aspects or performance of 
the display can be evaluated via ground tests.
    The FAA does not concur with the commenter's request. The guidance 
in this section of the AC is only for first-time evaluations of 
displays. The commenter's concerns are covered in the guidance 
contained in paragraph 18. (``System Evaluations with Simulators'') and 
paragraph 19. (``Ground Test Considerations'') of the final AC, which 
provide additional guidance for other types of evaluations.
    Another commenter questions whether flight testing is necessary to 
verify terrain display requirements. This commenter states that the 
Allied Signal EGPWS allows readout, via an external PC and a terminal 
program, of all interfaces to the EGPWS LRU. This test will determine 
that all signals interface properly with the system. This system also 
will allow simulation of all parameters inside the EGPWS unit, so that 
different scenarios can be simulated. During this simulation, a check 
of all aural and visual alerts can be made, and the presentation of 
terrain on the display can be checked. The commenter considers it 
unnecessary to require verification of the TSO-ed software with real 
parameters. Therefore, the commenter requests that paragraph 7.m.(6) of 
the proposed AC be revised to include the acceptability of ground 
testing for the evaluation of displays.

[[Page 42058]]

    The FAA does not concur. The FAA considers the commenter's request 
to be too specific to one particular system design, and may not be 
appropriate for other designs.

    Issued in Renton, Washington, on June 29, 2000.
D.L. Riggin,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service
[FR Doc. 00-17243 Filed 7-6-00; 8:45 am]
BILLING CODE 4910-13-U