[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