[Federal Register Volume 66, Number 161 (Monday, August 20, 2001)]
[Notices]
[Pages 43603-43606]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-20911]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
Proposed Revisions to Advisory Circular 25-7A, Flight Test Guide
for Certification of Transport Category Airplanes
AGENCY: Federal Aviation Administration, DOT.
ACTION: Notice of proposed advisory circular and request for comments.
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SUMMARY: This notice requests comments regarding proposed revisions to
Advisory Circular (AC) 25-7A, ``Flight Test Guide for Certification of
Transport Category Airplanes.'' AC 25-7A provides guidance on
acceptance means, but not the only means, of demonstrating compliance
with the airworthiness standards for transport category airplanes. The
proposed revisions harmonize, expand, and clarify existing advisory
material concerning certain airplane performance requirements to
address inconsistencies in the means of compliance with the existing
airworthiness standard and to reflect increased knowledge of airplane
and propulsion system performance modeling and test verification
practices since the standard was established. This notice provides
interested persons an opportunity to comment on the proposed revisions
to AC 25-7A.
DATES: Your comments must be received on or before October 19, 2001.
ADDRESSES: You should send your comments on the proposed AC revisions
to the Federal Aviation Administration, Attention: Don Stimson,
Airplane & Flight Crew Interface Branch, ANM-111, Transport Airplane
Directorate, Aircraft Certification Service, 1601 Lind Ave SW., Renton,
WA 98055-4056. You may examine comments at this address between 7:30
a.m. and 4:00 p.m. weekdays, except Federal holidays.
FOR FURTHER INFORMATION CONTACT: Don Stimson, Airplane & Flight Crew
Interface Branch, ANM-111, at the above address, telephone 425-227-
1129, or facsimile 425-227-1320.
SUPPLEMENTARY INFORMATION:
Comments Invited
Your are invited to comment on the proposed revisions to AC 25-7A
by submitting such written data, views, or arguments as you may desire.
You should identify the title of the AC and submit your comments in
duplicate to the address specified above. The Transport Airplane
Directorate will consider comments received on or before the closing
date for comments before issuing the revision to AC 25-7A. You may view
the complete text of AC 25-7A at the following Internet address: http://www.faa.gov/avr/air/airhome.htm at the link titled ``Advisory
Circulars'' under the ``Available Information'' drop-down menu.
Discussion
Harmonization of Standards and Guidance
The following proposed revisions to AC 25-7A are based on a
recommendation that the Aviation Rulemaking Advisory Committee (ARAC)
submitted to the FAA. The FAA tasked ARAC (63 FR 50954, September 23,
1998) to provide advice and recommendations on ``harmonizing'' certain
sections of part 25 (including 25.101(c)) with the counterpart
standards contained in Joint Aviation Requirements (JAR) 25. The goal
of ``harmonization tasks'' such as this is to ensure that:
Where possible, standards and guidance do not require
domestic and foreign parties to manufacture or operate to different
standards for each county involved; and
The standards and guidance adopted are mutually acceptable
to the FAA and the foreign aviation authorities.
What Are the Differences in the FAA and JAA Standards or Policy and
What Do These differences Result In?
In the case of Sec. 25.101(c), the FAA and JAA standards are the
same. The differences are in the policies and certification approval
practices relative to altitude/temperature extrapolation of takeoff
performance data.
In general, both FAA and JAA policy is to limit the unrestricted
extrapolation of takeoff data to 6,000 feet above the altitude at which
the takeoff performance data are obtained. For further extrapolations,
a takeoff distance penalty of 2 percent must be applied for each 1,000
feet of extrapolation beyond the 6,000-foot limit. For the FAA, a
further constraint is that engine data may only be extrapolated 3,000
feet above the altitude at which specific engine data have been
obtained to verify takeoff thrust models.
For the JAA, a 2 percent takeoff distance penalty must also be
applied for every 5 deg.C of temperature extrapolation beyond a
temperature that exceeds either:
A temperature 15 deg.C higher than the maximum temperature
tested; or
The amount by which the maximum temperature tested exceeds
the minimum temperature tested.
The FAA does not apply extrapolation limits for temperature.
Instead, the FAA policy is to require engine limits compliance to be
demonstrated by airplane testing at a sea level ambient temperature
near the highest temperature for which the engine is flat-rated. In
addition, to allow higher altitude data extrapolation, the use of
engine power setting overboost will generally provide higher
temperature conditions (i.e., closer to the flat-rated highest
temperature) at the simulated altitude.
Since these policies represent only one means of compliance with
the regulatory standards, the criteria noted above have not always been
strictly applied. For example, experience from previous certification
programs, combined with thorough substantiation of an acceptable model
of engine thrust and lapse rate characteristics, has been used to allow
extrapolations beyond 6,000 feet above the highest altitude tested
without applying a conservative factor. In the same vein, the 3,000-
foot extrapolation limit on engine data has not always been applied.
Considerably more experience has since been gained both in terms of
modeling airplane and propulsion system (turbine engines and
propellers, where appropriate) performance and in verifying the
accuracy of these models for determining high (and low) altitude
takeoff and landing performance. This experience has shown that the
soundness of the extrapolation is primarily a function of the accuracy
of the propulsion system performance model and its integration with the
airplane drag model. The basic aerodynamic characteristics of the
airplane do not change significantly with altitude or ambient
temperature, and any such effects are readily taken into account by
standard airplane performance modeling practices.
The effect of the proposed changes to the acceptable means of
compliance that is proposed to replace the current guidance material in
AC 25-7A would be to allow extrapolation of airplane takeoff and
landing performance data to higher and lower altitudes without applying
an arbitrary distance penalty if the following criteria are met:
A comprehensive propulsion system model is developed
covering the entire operational envelope and
[[Page 43604]]
substantiated by inflight thrust measurement
Lapse rate takeoff testing to characterize the behavior of
power setting, rotor speeds, propeller effects (i.e., torque, RPM, and
blade angle), or gas temperature as a function of time, thermal state,
or airspeed, as appropriate, is performed at an altitude within 3,000
feet of the maximum approved takeoff airport altitude.
The combination of the propulsion system performance model
an the airplane performance model is validated by the takeoff
performance test data, climb performance tests, an tests used to
determine airplane drag.
Proper operation of other systems dependent on altitude is
considered for the highest takeoff and landing altitude for which
approval is sought.
This proposed methodology is consistent with, but more stringent
than, some of the means of compliance that have been accepted in past
certification programs. In some previous certification programs, the
validation of lapse rate characteristics by takeoff demonstrations has
not always been performed at an airport altitude within 3,000 feet of
the maximum approved takeoff airport altitude.
This proposed revision to the AC 25-7A guidance material should act
as a catalyst to provide more consistency throughout the industry for
applying ``best practices'' in determining and substantiating airplane
and propulsion system performance models throughout the operating
envelope. Instead of applying an arbitrary takeoff and landing distance
penalty for large extrapolations in altitude above the test altitude,
this means of compliance encourages applications to develop and verify
an accurate model of the propulsion system performance and substantiate
its integration with the airplane drag model.
Since AC 25-7A only provides one acceptable means of compliance
with the regulatory standard, applicants will continue to have the
option of proposing the use of another means of compliance.
Dissenting Opinion
One member of the ARAC working group registered the following
dissenting position regarding paragraph 3a(8)(v) of the proposed
advisory material.
``It is recognized that starting capability for the engines and APU
may be relevant to operations sat high altitude airports. However,
there are no specific FAR/JAR requirements for engine or APU starting
capability on the ground, so it is not appropriate to list ground
starting capability as relevant to FAR/JAR compliance. It is requested
that the references to engine and APU starting capability to deleted
from paragraph 3a(8)(v).''
The FAA does not agree with the dissenting opinion. The lack of a
``specific'' FAR/JAR requirement for engine or APU starting on the
ground does not mean that engine and APU starting need not be addressed
prior to granting airworthiness approval. Section 25.1309(a) requires
that ``equipment, systems, and installations whose functioning is
required . . . must be designed to ensure that they perform their
intended functions under any foreseeable operating condition.''
Regardless of this or any other ``non-specific'' requirement related to
engine and APU starting, starting capability for the engines and APU is
a consideration, as the working group member notes, that is relevant to
operations at high altitude airports. Also, the wording of the AC
paragraph of concern, ``consideration should be given to any other
systems whose operation may be sensitive to, or dependent upon airport
altitude, such as: engine and APU starting, passenger oxygen,
autopilot, autoland, autothrottle system thrust set/operation,''
identifies these items as items that should be considered in the
context of approval to operate from high altitude airports, not in
reference to any specific part 25 requirement. Therefore, the
references in paragraph 3a(8)(v) to engine and APU starting have been
retained in the proposed revision to AC 25-7A.
Proposed Revisions to AC 25-7A
The guidance provided in the following proposed revision to AC 25-
7A has been harmonized with that of the JAA, and provides a method of
compliance that has been found acceptable to both the FAA and JAA.
This proposed revision should not be confused with other proposed
revisions to AC 25-7A for which the FAA may currently be seeking
comments. The revisions proposed in this notice address guidance
material associated with the polices and certification approval
practices relative to altitude temperature extrapolation of takeoff
performance data.
1. Replace Existing Paragraph 3a(8) Through 3a(9) With the Following
3. Proof of Compliance.
(8) Expansion of Takeoff and Landing Data for a Range of Airport
Elevations.
(i) These guidelines are applicable to expanding Airplane Flight
Manual takeoff and landing data above and below the altitude at which
the airplane takeoff and landing performance tests are conducted.
(ii) Historically, limits have been placed on the extrapolation or
takeoff data. In general, takeoff data could be extrapolated 6,000 feet
above and 3,000 feet below the test field elevation when proven testing
and data reduction methods were used. For extrapolations beyond these
limits, a 2 percent takeoff distance penalty was to be applied for
every additional 1,000 feet extrapolation. Such limitations were
generally not applied to extrapolation of landing data, provided the
effect of the higher true airspeed on landing distance was taken into
account.
(iii) Considerably more experience has since been gained both in
terms of modeling airplane and propulsion system (i.e., turbine engines
and propellers, where appropriate) performance and in verifying the
accuracy of these models for determining high (and low) altitude
takeoff and landing performance. This experience has shown that the
soundness of the extrapolation is primarily a function of the accuracy
of the propulsion system performance model and its integration with the
airplane drag model. The basic aerodynamic characteristics of the
airplane do not change significantly with altitude or ambient
temperature, and any such effects are readily taken into account by
standard airplane performance modeling practices.
(iv) As a result, with installed propulsion system performance
characteristics that have been adequately defined and verified,
airplane takeoff and landing performance data obtained at one field
elevation may be extrapolated to higher and lower altitudes within the
limits of the operating envelope without applying additional
performance conservatisms. It should be noted, however, that
extrapolation of the propulsion system data used in the determination
and validation of propulsion system performance characteristics is
typically limited to 3,000 feet above the highest altitude at which
propulsion system parameters were evaluated for the pertinent power/
thrust setting. (See paragraph 9 of this AC for more information on an
acceptable means of establishing and verifying installed propulsion
system performance characteristics.)
(v) Note that certification testing for operation at airports that
are above 8,000 feet should also include functional tests of the cabin
pressurization system in accordance with paragraph 87b(3) of this AC.
Consideration should be given to any
[[Page 43605]]
other systems whose operation may be sensitive to, or dependent upon
airport altitude, such as: engine and APU starting, passenger oxygen,
autopilot, autoland, autothrottle system thrust set/operation.
2. Replace Paragraph 9 in Its Entirety With the Following
9. General--Sec. 25.101
a. Explanation--Propulsion System Behavior. Section 25.101(c)
requires that airplane ``performance must correspond to the propulsive
thrust available under the particular ambient atmospheric conditions,
the particular flight conditions, * * * '' The propulsion system's
(i.e., turbine engines and propellers, where appropriate), installed
performance characteristics are primarily a function of engine power
setting, airspeed, propeller efficiency (where applicable), altitude,
and ambient temperature. the effects of each of these variables must be
determined in order to establish the thrust available for airplane
performance calculations.
b. Procedures.
(1) The intent of this testing is to develop a model of propulsion
system performance that covers the approved flight envelope.
Furthermore, it should be shown that the combination of the propulsion
system performance model and the airplane performance model are
validated by the takeoff performance test data, climb performance
tests, and tests used to determine airplane drag. Installed propulsion
system performance characteristics can be established via the following
tests and analyses:
(i) Steady-state engine power setting vs. thrust (or power)
testing. Engines should be equipped with adequate instrumentation to
allow the determination of thrust (or power). Data should be acquired
in order to validate the model, including propeller-installed thrust,
if applicable, over the range of power settings, altitudes,
temperatures, and airspeeds for which approval is sought. Although it
is not possible to definitively list or foresee all of the types of
instrumentation that might be considered adequate for determining
thrust (or power) output, two examples used in past certification
programs are: (1) engine pressure rakes, with engines calibrated in a
ground test cell, and (2) fan speed, with engines calibrated in a
ground test cell and the calibration data validated by the use of a
flying test bed. In any case, the applicant should substantiate the
adequacy of the instrumentation to be used for determining the thrust
(or power) output.
(ii) Lapse rate takeoff testing to characterize the behavior of
power setting, rotor speeds, propeller effects (i.e., torque, RPM, and
blade angle), or gas temperature as a function of time, thermal state,
or airspeed, as appropriate. These tests should include the operation
of an Automotive Takeoff Thrust Control System (ATTCS), if applicable,
and should cover the range of power settings for which approval is
sought.
(A) Data for higher altitude power settings may be acquired via
overboost (i.e., operating at a higher than normal power setting for
the conditions) with the consent of the engine and propeller (when
applicable manufacturer(s). When considering the use of overboost on
turbopropeller propulsion system installations to stimulate higher
altitude and ambient temperature range conditions, the capability to
achieve an appropriate simulation should be evaluated based on the
engine and propeller control system(s) and aircraft performance and
structural considerations. Engine (gearbox) torque, rotor speed, or gas
temperature limits, including protection devices to prohibit or limit
exceedances, may prevent the required amount of overboost needed for
performance at the maximum airport altitude sought for approval.
Overboost may be considered as increased torque, reduced propeller
speed, or a combination of both, in order to achieve the appropriate
blade angle for the higher altitude and ambient temperature range
simulation. Consideration for extrapolations will depend on the
applicant's substantiation of the proper turbopropeller propulsion
system simulated test conditions.
(B) Lapse rate charactertics should be validated by takeoff
demonstrations at the maximum airport altitude for which takeoff
approval is being sought. Alternatively, if overboost (See paragraph
(A) above) is used to stimulate the thrust setting parameters of the
maximum airport altitude for which takeoff approval is sought, the
takeoff demonstrations of lapse rate characteristics can be performed
at an airport altitude up to 3,000 feet lower than the maximum airport
altitude.
(iii) Thrust calculation substantiation. Installed thrust should be
calculated via a mathematical model of the propulsion system, or other
appropriate means, adjusted as necessary to match the measured inflight
performance characteristics of the installed propulsion system. The
propulsion system mathematical model should define the relationship of
thrust to the power setting parameter over the range of power setting,
airspeed, altitude, and temperature for which approval is sought. For
turbojet airplanes, the propulsion system mathematical model should be
substantiated by ground tests in which thrust is directly measured via
a calibrated load cell or equivalent means. For turboproller airplanes,
the engine power measurement should be substantiated by a calibrated
dynamometer or equivalent means, the engine jet thrust should be
established by an acceptable enginer model, and the propeller thrust
and power characteristics should be substantiated by wind tunnel
testing or equivalent means.
(iv) Effects of ambient temperature. The flight tests of paragraph
9b(l)(i) above will typically provide data over a broad range of
ambient temperatures. Additional data may be obtained from other flight
or ground tests of the same type or series of engine. The objective is
to confirm that the propulsion system model accurately reflects the
effect of temperature over the range of ambient temperatures for which
approval is being sought (operating envelope). Because thrust (or
power) data can usually be normalized versus temperature using either
dimensionless variables (e.g., theta exponents or a thermodynamic cycle
model, it is usually uneccessary to obtain data over the entire ambient
temperature range. There is no needed to conduct additional testing if:
(A) The data show that the behavior of thrust and limiting
parameters versus ambient temperature can be predicted accurately and
(B) Analysis based upon the test data shows that the propulsion
system will operate at rated thrust without exceeding propulsion system
limits.
(2) Extrapolation of propulsion system performance data to 3,000
feet above the highest airport altitude test (up to the maximum takeoff
airport altitude to be approved) is acceptable, provided the supporting
data, including flight test and propulsion system operations data
(e.g., engine and propeller control, limits exceedance, and surge
protection devices scheduling), substantiates the proposed
extrapolation procedures. Considerations for extrapolation depend upon
an applicant's determination, understanding, and substantiation of the
critical operating modes of the propulsion system. This understanding
includes a determination and quantification of the effects that
propulsion system installation and variations in ambient conditions
have on these modes.
[[Page 43606]]
Issued in Renton, WA on August 9, 2001.
Ali Bahrami,
Acting Manager, Transport Airplane Directorate, Aircraft Certification
Service.
[FR Doc. 01-20911 Filed 8-17-01; 8:45 am]
BILLING CODE 4910-13-M