[Federal Register Volume 60, Number 227 (Monday, November 27, 1995)]
[Rules and Regulations]
[Pages 58204-58208]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-28842]



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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 33

[Docket No. 95-ANE-42; Notice No. SC-95-04-NE]


Special Conditions: Allison Engine Company Model 250-C40 
Turboshaft Engine

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final special conditions.

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SUMMARY: These special conditions are issued for the Allison Engine 
Company (AE) Model 250-C40 turboshaft engine. This engine will have 
novel or unique engine ratings that are not defined by the applicable 
airworthiness regulations. These special conditions contain the 
additional safety standards which the Administrator considers necessary 
to establish a level of safety equivalent to that established by the 

[[Page 58205]]
airworthiness standards of part 33 of the Federal Aviation Regulations 
(FAR).

EFFECTIVE DATES: December 27, 1995.

FOR FURTHER INFORMATION CONTACT:
Chung Hsieh, Engine and Propeller Standards Staff, ANE-110, Engine and 
Propeller Directorate, Aircraft Certification Service, FAA, New England 
Region, 12 New England Executive Park, Burlington, Massachusetts 01803-
5229; (617) 238-7115; Fax (617) 238-7199.

SUPPLEMENTARY INFORMATION:

Background

    On May 11, 1993, Allison Engine Company applied for an amendment to 
type certificate E1GL to include a new model 250-C40 turboshaft engine. 
On March 30, 1995, Allison Engine Company applied for 30-Second one 
engine inoperative (OEI) and 2-Minute OEI ratings for the engine. The 
AE Model 250-C40 turboshaft engine will be rated at 30-Second OEI, 2-
Minute OEI, 30-Minute OEI, Continuous OEI, Takeoff, and Maximum 
Continuous ratings.
    The applicable airworthiness requirements do not contain 30-Second 
OEI and 2-Minute OEI rating definitions, and do not contain adequate or 
appropriate safety standards for the type certification of these new 
and unusual engine ratings.

Type Certification Basis

    Under the provisions of section 21.101 of the FAR, Allison Engine 
Company must show that the AE Model 250-C40 turboshaft engine meets the 
requirements of the applicable regulations in effect on the date of the 
application. The applicable regulations for this engine are FAR part 
33, effective February 1, 1965, as amended by Amendments 33-1 through 
33-4.
    The Administrator finds that the applicable airworthiness 
regulations in part 33, as amended, do not contain adequate or 
appropriate safety standards for the AE Model 250-C40 turboshaft engine 
because of the new and unique engine ratings. Therefore, the 
Administrator prescribes special conditions under the provisions of 
section 21.16 to establish a level of safety equivalent to that 
established in the regulations.
    Special conditions, as appropriate, are issued in accordance with 
section 11.49 of the FAR after public notice and opportunity for 
comment, as required by sections 11.28 and 11.29(b), and become part of 
the type certification basis in accordance with section 21.101(b)(2).

Discussion of Comments

    Interested persons have been afforded the opportunity to 
participate in the making of these special conditions. Two commenters 
from a domestic company and a foreign airworthiness authority provided 
the Federal Aviation Administration (FAA) with comments to the special 
conditions, addressing numerous issues. The comments are grouped 
according to the applicable special condition paragraphs and are 
discussed below.

Section 33.4  Instructions for Continuous Airworthiness

    One commenter states that there is no requirement in the proposed 
special conditions stating the use of 2-Minute and 30-Second OEI 
ratings must be followed by mandatory inspections and maintenance 
actions.The commenter suggests that the proposed addition to section 
33.4 be changed by adding a sentence to state those requirements.
    The FAA agrees that the requirements for mandatory inspections and 
maintenance action after the use of 2-Minute and 30-Second OEI ratings 
need to be addressed in the proposed special conditions. However, it is 
more appropriate to set such requirements out in section 33.7, Engine 
Ratings and Operational Limitations, instead of in the instruction for 
continuous airworthiness section. The rating definition of 2-Minute and 
30-Second OEI is thereby modified by adding inspection and maintenance 
requirements.

Section 33.27  Turbine, Compressor, Fan, and Turbo-Supercharger Rotors

    One commenter states that the first sentence of the proposed 
additional test requirement does not clearly state whether the 2-Minute 
and 30-Second OEI conditions are intended to be treated the same as 
other ratings, when complying with all or with only some parts of the 
current section 33.27 for non-failure mode cases. One commenter states 
that the 2-Minute and 30-Second OEI rating concept was originally 
initiated by industry as a means of safety utilizing the reserve power 
inherent in a turbine engine for a brief controlled period of time 
during a critical flight phase, OEI emergency situation. The commenter 
recommends that a 5 percent reduction in the test margin be made for 
the non-failure mode cases when section 33.27(c)(2) (i), (ii), (iii) or 
(iv) applies. The commenter argues that this recommendation has been 
adopted by the Aviation Rulemaking Advisory Committee's Harmonization 
Working Group (ARAC-HWG) on Rotor Integrity, and therefore, should be 
included in the proposed special conditions.
    The FAA does not agree. The 5 percent reduction in test margin 
compared to the current requirements for no-failure cases is still in 
the drafting stage of the ARAC-HWG deliberations, and has yet to be 
published for public comment. The FAA has determined that the same test 
and post-test inspection requirements that appeared in the Supplemental 
Notice of Proposed Rulemaking (SNPRM) No. 89-27A on the subject of OEI 
ratings for rotorcraft engines, should be applied to these special 
conditions for 33.27(c)(2) (i), (ii), (iii) or (iv).
    One commenter states that the additional test requirements in this 
section which impose a demonstration at 100 percent of the rotor speed 
under failure conditions when operating at 2-Minute and 30-Second OEI 
ratings are not warranted, and believes that the basis for such 
demonstration at the OEI conditions should be the probability of 
occurrence of failures that lead to the use of OEI ratings, in 
combination with the probability of a rotor failure involving the 
operating engine.
    One commenter states that a five-minute test should be conducted at 
a combination of the maximum 2-Minute OEI or 30-Second OEI operating 
temperature, and a speed equal to 105 percent of the highest overspeed 
that would result from a single failure when operating at 2-Minute OEI 
or 30-Second OEI conditions. The test results from the overspeed 
demonstration should be acceptable if the rotor having the minimum 
material properties and the most adverse dimensional tolerances does 
not burst.
    The FAA does not agree with the use of a probability of occurrence 
in lieu of a test for compliance of section 33.27 requirements, and 
does not agree that the test should be at a speed equal to 105 percent 
of the overspeed resulting from a single failure when operating at the 
2-Minute and 30-Second OEI ratings. The FAA bases its determination on 
the potential severity of failure conditions due to disk burst, the 
probability of occurrence of the failure condition because of the lower 
utilization rate of these ratings, and the mandatory post flight 
inspections and maintenance associated with 2-Minute OEI and 30-Second 
OEI that actually discourage use of those OEI ratings. In considering 
the lower combined probability of occurrence of failures that involve 
the use of 2-Minute OEI or 30-Second OEI rating and a failure 
occurrence in the operating engine, a 5 percent reduction in test 
speed, that is 

[[Page 58206]]
required for the traditional OEI ratings of 2\1/2\ minutes or longer, 
is therefore adopted in this special conditions. In addition, these 
special conditions require an acceptable growth criteria, in addition 
to no burst of a minimum strength rotor after it has been subjected to 
the combined effects of maximum operating temperature and 100 percent 
of the maximum overspeed resulting from the most critical single 
failure when operating at 2-Minute OEI or 30-Second OEI operating 
condition. The conditions imposed by the acceptable growth criteria 
would minimize the potentially hazardous conditions if the rotor has 
been operating in an engine. Therefore, the additional rotor test 
requirements for failure conditions in the final special conditions 
will remain the same as the proposed special conditions. In summary, 
the requirements for 2-Minute and 30-Second OEI ratings in the final 
special condition are appropriate standards for rotor integrity in the 
context of utilizing the inherent overspeed margin in this engine model 
without compromising safety. The ARAC's HWG may continue to discuss 
these issues and propose changes to rules of general applicability, as 
opposed to dealing with this particular engine design.
    One commenter asks the meaning of the term ``the structural 
integrity of the rotor is maintained'' for the post rotor test 
requirements.
    The FAA disagrees that these special conditions should provide a 
definition of that phrase for general application. For this engine 
model, however, the rotor should not burst and not develop, through 
damage or disk growth, a condition that would prevent safe operation of 
the engine. The ARAC-HWG continues to work on a proposal for a rule of 
general application for the 30-second and 2-minute OEI ratings, and on 
guidance material that would help future applicants in meeting the 
certification requirements for those ratings.

Section 33.29  Instrument Connection

    One commenter states that the proposed additional requirements do 
not match the relevant needs of FAR 29.1305(a) (24) and 25(l), and 
recommends the following additions to section 33.29: In addition to the 
requirements of section 33.29, the engine must provide for a means to:
    (a) Indicate and alert the pilot when the engine is at the 30-
Second and 2-Minute OEI power levels when any such event begins and 
when the permitted time interval expire;
    (b) Determine, in a positive manner after flight that the engine 
has been operated at either or both of these power levels; and
    (c) Determine, after flight, the elapsed time of operation at each 
of these power levels.
    The FAA agrees that the section should be modified to clarify the 
additional requirements to section 33.29. While not adopting the 
commenters recommendations word-for-word, the FAA has changed section 
33.29 of these special conditions to reflect the commenters' changes.

Section 33.67  Fuel System

    One commenter states the engine test runs must be performed to 
demonstrate the means for automatic control of 30-Second OEI ratings in 
addition to the automatic availability of those ratings.
    The FAA agrees. This section is modified as recommended.

Section 33.83  Vibration Test

    One commenter states that the last sentence of these proposed 
additional requirements is not satisfactory because the vibration 
survey is required to cover the 2-Minute OEI operation for speeds 
beyond the maximum permitted within the OEI flight envelope. The last 
sentence states that the survey may need to be extended to even higher 
speeds if there is any indication of a stress peak arising at the upper 
end of the survey speed, that conflicts with compliance with section 
33.63 which would not require the survey to cover maximum rotational 
speeds beyond the operating range. The commenter suggests that the last 
sentence of the proposed addition to section 33.83 be changed to read: 
``If there is any indication of a stress peak arising at the highest of 
those physical and corrected rotational speeds, the surveys shall be 
extended sufficiently to reveal the maximum stress values present 
except that the extension need not cover more than a further 2 percent 
point beyond those speeds.''
    One commenter states that vibration survey be tested to 100 percent 
of the 30-Second OEI and 2-Minute OEI rotor speeds and any further 
speed margin requirements beyond the test speeds be addressed based 
upon requirements to further evaluate any stress peak arising at the 
maximum rotor speed.
    The FAA does not agree with the test speed of 100 percent for the 
2-Minute OEI rating, but agrees with the recommended 2 percentage point 
extension beyond the required test speeds. The purpose of survey speed 
extension is intended to cover inherent variations in vibratory 
response due to engine manufacturing and build tolerances that can 
result in peak stresses occurring at slightly different rotor speeds 
between engines and engine parts. This section is therefore changed as 
recommended.

Section 33.85  Calibration Test

    One commenter states that the proposed additional requirement is 
not clear regarding the ``applicable endurance test'' definition under 
the proposed special conditions and recommends the following changes to 
read as: ``In addition to the requirements of section 33.85, tests 
performed at * * * during the applicable additional endurance test 
prescribed in section 33.87 as amended by these special conditions may 
be used. * *  *''
    The FAA agrees. For clarification, this section is changed as 
recommended.

Section 33.88  Engine Overtemperature Test

    One commenter questions the logic for allowing shorter test 
duration of four minutes instead of five minutes for engines that 
incorporate a means for temperature limiting, but not for the engines 
without such a device. The commenter recommends that the 
overtemperature test duration should be four minutes for all engine 
models having the 30-Second OEI and 2-Minute OEI ratings.
    The FAA does not agree. Since this engine has the added protection 
of a temperature limiter, an overtemperature condition of 75 degrees 
Fahrenheit and five-minute duration cannot reasonably be expected, and 
an overtemperature test at that level is considered excessively severe. 
However, the engines equipped and qualified to the 35 degrees 
Fahrenheit (19 degrees Celsius) and 4-minute test conditions will need 
provisions for predispatch operational status checking of the 
temperature limiters. The rational for 5 minutes and 75 degrees 
Fahrenheit overtemperature test conditions to engines not equipped with 
a temperature limiter is to apply the existing rule requirements.

Section 33.93  Teardown Inspection

    One commenter states that the proposed additional requirements lack 
a requirement equivalent to the proposed section 33.93(b)(1) of SNPRM 
89-27A, and suggests that the second sentence of the proposed additions 
to section 33.93 be changed to read: ``The engine must comply with 
section 33.93(a), but it may exhibit deterioration in excess of that 
permitted in section 33.93(b) and may * * *''.
    The FAA agrees. The proposed change is an implied requirement of 
the additional requirements in this section, 

[[Page 58207]]
and the recommendation is adopted accordingly.
    After careful review of the available data and the comments noted 
above; the FAA determined that air safety and the public interest 
require the adoption of the special conditions with the changes 
described previously.

Conclusion

    This action affects only certain novel or unusual design features 
on one model engine. It is not a rule of general applicability and 
affects only the manufacturer who applied to the FAA for approval of 
these features on the engine.

List of Subjects in 14 CFR Part 33

    Air Transportation, Aircraft, Aviation safety, Safety.

    The authority citations for these special conditions continues to 
read as follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704, 14 CFR 
21.16, 14 CFR 11.49.

The Special Conditions

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the following special conditions are issued as part of 
the type certification basis for the Allison Engine Company (AE) Model 
250-C40 turboshaft engine:


Sec. 33.4  Instructions for Continued Airworthiness.

    In addition to the requirements of section 33.4, the mandatory 
inspection and maintenance actions required following the use of the 
30-Second or 2-Minute OEI rating must be included in the airworthiness 
limitations section of the appropriate engine manuals.


Sec. 35.7  Engine Ratings and Operating Limitations.

    In addition to the requirements of section 33.7, the following 
ratings are defined as:
    (a) Rated 30-Second one engine inoperative (OEI) power: The 
approved brake horsepower developed under static conditions at 
specified altitudes and temperatures within the operating limitations 
established for the engine under part 33 and this special conditions, 
for continued one-flight operation after the failure of one engine in 
multi-engine rotorcraft, limited to three periods of use, no longer 
than 30 seconds each, in any one flight, and followed by mandatory 
inspection and prescribed maintenance action.
    (b) Rated 2-Minute OEI power: The approved brake horsepower, 
developed under static conditions at specified altitudes and 
temperatures, within the operating limitations established for the 
engine under part 33 and this special conditions, for continued one-
flight operation after the failure of one engine in multi-engine 
rotorcraft, limited to three periods of use, of no longer than 2 
minutes each in any one flight, and followed by mandatory inspection 
and prescribed maintenance action.


Sec. 33.27  Turbine, Compressor, Fan, and Turbo-supercharger Rotors.

    In addition to the requirements of section 33.27(a) and (b), the 
following tests must be conducted for the most critically stressed 
rotor component of each turbine and compressor including integral drum 
rotors and centrifugal compressor, as determined by analysis or other 
acceptable means for 2-Minute and 30-Second OEI conditions:
    (a) Test for a period of two and one-half minutes--
    (1) At the maximum operating temperature except as provided in 
paragraph (c)(2)(iv) of this section; and
    (2) At the highest speed determined, in accordance with section 
33.27(c)(2)(i) through (iv).
    (3) This test may be performed using a separate test vehicle as 
desired.
    (b) The following additional test requirements must be considered 
under 33.27(e)(2)(v) and (vi):
    (1) Test for a period of 5 minutes--
    (i) At 100 percent of the highest speed that would result from 
failure of the most critical component of each turbine and compressor 
or system in a representative installation of the engine when operating 
at 30-Second and 2-Minute OEI rating conditions.
    (ii) The test speed must take into account minimum material 
properties, maximum operating temperature, and the most adverse 
dimensional tolerances.
    (c) Following the test, rotor growth and distress beyond 
dimensional limits for an overspeed condition is permitted for 30-
Second and 2-Minute OEI rating only, provided the structural integrity 
of the rotor is maintained, as shown by a procedure acceptable to the 
Administrator.


Sec. 33.29  Instrument Connection.

    In addition to the requirements of section 33.29, the engine must 
have a provision for a means to:
    (a) Alert the pilot when the engine is at the 30-Second OEI and a 
2-Minute OEI power levels;
    (b) Determine, in a positive manner, that the engine has been 
operated at each rating; and
    (c) Determine the elapsed time of operation of each rating.


Sec. 33.67  Fuel System.

    In addition to the requirements of section 33.67, the engine must 
provide for a means for automatic availability and automatic control of 
the 30-Second OEI power; and engine test runs must be performed to 
demonstrate automatic functioning of both of these means.


Sec. 33.83  Vibration Test.

    In addition to the requirements of section 33.83, the following 
additional test requirements must be considered under 33.83(a):
    For 30-Second and 2-Minute OEI rating conditions, the vibration 
survey shall cover the ranges of power, and both the physical and 
corrected rotational speeds for each rotor system, corresponding to 
operations throughout the range of ambient conditions in the declared 
flight envelope, from the minimum rotor speed up to 103 percent of the 
maximum rotor speed permitted for 2-Minute OEI rating, and up to 100 
percent of the maximum rotor speed permitted for 30-Second OEI rating 
speed. If there is any indication of a stress peak arising at the 
highest physical or corrected rotational speeds, the surveys shall be 
extended sufficiently to reveal the maximum stress values present 
except that the extension needs not cover more than a further 2 percent 
beyond those speeds.


Sec. 33.85  Calibration Test.

    In addition to the requirements of section 33.85, tests performed 
at the 30-Second and 2-Minute OEI ratings, during the applicable 
additional endurance test prescribed in section 33.87 as amended by 
these special conditions, may be used to show compliance with the 
requirements of section 33.85.


Sec. 33.87  Endurance Test.

    In addition to the requirements of section 33.87, an engine test 
must be conducted four times, using the following test sequence, for a 
total of not less than 120 minutes:
    (a) Takeoff Power--three minutes at rated takeoff power.
    (b) 30-Second OEI power--thirty seconds at rated 30-Second OEI 
power.
    (c) 2-Minute OEI Power--two minutes at rated 2-Minutes OEI Power.
    (d) 30-Minute OEI, Continuous OEI, or Maximum Continuous power--
five minutes at rated 30-Minute OEI power, or rated Continuous OEI 
power, or rated Maximum Continuous power, whichever is greatest, except 
that during the first test sequence this period shall be 65 minutes.
    (e) 50 Percent takeoff power--one minute at 50 percent takeoff 
power.

[[Page 58208]]

    (f) 30-Second OEI power--thirty seconds at rated 30-Second OEI 
power.
    (g) 2-Minute OEI power--two minutes at rated 2-Minute OEI power.
    (h) Idle power--one minute at idle power.


Sec. 33.88  Engine Overtemperature Test.

    In addition to the requirements of section 33.88, the following 
must be performed:
    (a) For engines that do not provide a means for temperature 
limiting; conduct a test for a period of five minutes at the maximum 
permissible power-on RMP, with the gas temperature at least 75 degrees 
fahrenheit higher than the 30-Second OEI rating operating temperature 
limit.
    (b) For engines that provide a means for temperature limiting; 
conduct a test for a period of four minutes at the maximum permissible 
power-on RPM, with the gas temperature at least 35 degrees fahrenheit 
higher than the 30-Second OEI rating operating temperature limit.
    (c) A separate test engine may be used for each test.
    (d) Following the test, rotor assembly growth and distress beyond 
serviceable limits for an overtemperature condition is permitted, 
provided the structural integrity of the rotor assembly is maintained, 
as shown by a procedure that is acceptable to the Administrator.


Sec. 33.93  Teardown Inspection.

    In addition to the requirements of section 33.93, this special 
condition requires that the engine be completely disassembled after 
completing the additional testing of section 33.87. The engine must 
comply with section 33.93(a), but it may exhibit deterioration in 
excess of that permitted in section 33.93(b), and may include some 
engine parts and components that may be unsuitable for further use. It 
must be shown by procedures approved by the Administrator that the 
structural integrity of the engine, including mounts, cases, bearing 
supports, shafts and rotors, is maintained.

    Issued in Burlington, Massachusetts, on November 16, 1995.
Jay J. Pardee,
Manager, Engine & Propeller Directorate, Aircraft Certification 
Service.
[FR Doc. 95-28842 Filed 11-24-95; 8:45 am]
BILLING CODE 4910-13-M