Sections of the U.S. Airworthiness Standards which contribute to rotor integrity are explored. Reports published under NASA's Rotor Burst Protection program are included in current FAA studies to determine the weight penalty for two different levels of increased containment, and the penalty associated with protecting critical structure and systems, the passenger cabin, and the flight deck by strategic location of armor shields or deflector plates. Findings of the two studies will be used to propose revisions to regulations to reduce uncontained rotor failures

Forney, A. K.

English

NASA Technical Reports Server

FEDERAL AVIATION ADMINISTRATION'S
APPROACH TO ENGINE ROTOR INTEGRITY
A. K. FORNEY*
INTRODUCTION
Close review of the record shows that aircraft engine failures are not
a major contributor to commercial aircraft accidents today. This
position has been arrived at by the concerted effort and resources of
the entire industry, and the results are such that everyone in the
industry can rightly have a feeling of pride and a sense of accomplish- 4)
t
ment. That is not to say, however, that now is the time to relax
the effort. Engine problems, while not major, do occur occasionally.
These include engine surge at or soon after rotation on takeoff_ the
need for engine shutdown after ingesting birds and an occasional rotor
i
disk failure.
_.-y •
*Chief, Engine Section
Propulsion Branch
Flight Standards ServLce
FAA_ Washington_ D.C.
._ t ' ,
] 978002 ] 25-009
https://ntrs.nasa.gov/search.jsp?R=19780002126 2020-03-20T12:23:18+00:00Z
Even though engine failures are not a major contributor to airline
accidents, the FAA is intensely interested in engine rotor integrity.
This interest is demonstrated by the fact that there are several active
programs under FAA sponsorship relating to this subject. Of par-
ticular interest to the FAA is the NASA Rotor Burst Protection
Program. It is hoped that this program will result in a significant
reduction in engine rotor failures.
The approach of the FAA to the protection of aircraft from uncontained
engine rotor fragments is threefold. First, design and test require-
ments are imposed on engines for the purpose of ensuring to the maxi-
mum extent practicable the integcity of the engine rotor. Second,
because the possibility always exists that the rotor will fail, design
and test requirements are imposed on the engine to ensure some con-
tainment capability. Finally, b_cause complete containment of all
high energy fragments has heen considered inpracticable up to now,
design requirements are imposed on transport type aircraft to mini-
_ mize the hazard to the aircraft from uncontained engine rotor fragments.
r/
ENGINE DESIGN AND TEST REQUIREMENTS
The engine design and test requirements are covered in the United
States Code of Fede=al Regulations, Title 14, Aeronautics and Space,
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1978002125-010
Part 33, Airworthiness Standards; Aircraft Engines. There are several
sections of these Airworthiness Standards that contribute to rotor
,?i
integrity.
Section 33.14, entitled "Start-Stop Cyclic Stress (Low-cycle Fatigue),"
presents the low-cycle fatigue requirements. At the present time the
engine manufacturer is required by this section to determine the
predicted safe life of each rotor disk and spacer in the engine.
An initial service life is then established at one-third of the pre- j
dicted safe life. The section also describes the procedure to be
used if it is desired to extend the initial service life to some
higher value. To do so, three disks and spacers of each part number
that have reached the initial life in service must undergo an additional
• number of cycles equal to at least twice the number of cycles comprising i
the increase in the limit desired.
Section 33.27, entitled "Turbine, Compressor, and Turbo-Supercharger
Rotors," presents the overspeed design and test requirements for these
engine components. The overspeed required is 120 percent of the
k_
maximum limiting rpm if the rotor is tested on a rig, or 115 percent
of its maximum limiting rpm if it is tested on the engine.
4 '
1978002125-011
.°
Section 33.62, entitled "Stress Analysis," requires that a stress
analysis be performed on each turbine engine showing the design safety
margin of each turbine engine rotor disk, spacer and rotor shaft.
Section 33.75, entitled "Safety Analysis," requires it to be shown
by analysis that any probable malfunction or any probable single or
multiple failure, or any probable improper operation will not cause
the engine to:
(a) catch fire;
(b) burst (penetrate its case);
(c) generate loads greater than those specified in §33.23; or
(d) lose the capability of being shutdown. 0
Section 33.83, entitled "Vibration Test," requires that each engine
must undergo a vibration survey to determine the vibration stresses.
" This section further requires that these stresses may not exceed the I
endurance limit stress of the material from which these parts are made.
f
CONTAINMENT REQUIREMENTS
"_ ' Section 33.19, entitled "Durability" requires that "the design ol _.he
" compressor and turbine rotor cases must provide for the containment of
damage from rotor blade failure." Traditionally, the demonstration
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1978002125-012
of compliance with this requirement was accomplished in an evacuated
spin pit. In October 1974, as part of a major revision of the engine
airworthiness requirements, a change was made to require that demon-
stration of compliance with the containment requirements be accomplished
o_ an engine.
AIRCRAFT DESIGN REQUIREMENTS
The requirements relating to transport type aircraft are covered in
the United States Code of Federal Regulations_ Title 14, Aeronautics
i
and Space_ Part 25, Airworthiness Standards; Transport Category Air-
planes. Section 25.903(d)(I) reads: "For turbine engine ins.._-
lations design precautions must be taken to minimize the h_zards to
the airplane in the event of an engine rotor failure...."
This requirement is very general and gives no guidance on what to do i
to comply with the requirement. In such cases, the general practice
is for FAA Headqua=ters to prepare and distribute what is called
: "guidance material" describing one or more ways of complying with
, a general requirement. Guidance material has been distributed for '
., this requirement and includes such considerations as:
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1978002125-013
1. Location of the engines relative to each other and to critical
portions and systems of the airplane.
2. Location and separation of critical components and redun-
dant systems.
3. The strategic location of protective armor and de/lector
shields.
More details on what is actually done in any given airplane to meet
the FAR 25.903(d)(1) requirement can best be obtained from the air- -
plane manufacturer. 4
CURRENT FAAACTIVITY &
The aircraft industry is not a static industry. Consequently, the ,
FAAregulations are rot static. We have underway, therefore, study
contracts that will help improve the regulations. One of these
contracts is with an engine company and will determine the weight Z
penalty for two different levels of increased containment. The
other contract is with an airplane company and is studying the penalties
associated with protecting critical structure and systems, the pas-
., senger cabin and the flight deck by strategic location of armor shields 4
or deflector plates. Results of these two contracts are not yet avail-)
able, but they will be used to propose revisions to the regulations
as appropriate. _
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1978002125-014
CONCLUSION
The FAA has watched the NASA Rotor Burst Protection Program with
interest for several years. In fact, the two contracts mentioned
above require the contractor to evaluate and use to the degree practicable
all the reports published under the Rotor Burst Protection Program.
It is our hope that this workshop will somehow provide what is needed
to make a significant reduction in uncontained rotor failures.
¢
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1978002125-015
DISCUSSION
J.C. Wallin, British Aircraft Corporation
I am interested in your comments on the studies that you had done on
looking at the weight penalty for increased levels of containment on the engine
as against improved methods of protection on the airframe. In the UK, we were
getting together a similar study with the CAA and Rolls Royce to put to our
government for some funding. I'd be interested to know, how soon you expect
to get some results from your present study, whether in fact you already have
some results which are leading you to conclusions, and whether it is worthwhile
trying to press my goverrdnent for additional funding to do a study of this sort.
Intuitively, I think we believe that the aircraft protective methods are likely
(in the majority of the present day aircraft configurations) to result in the
lightest form for improving protection. We think that increased protection in
the engine is likely to lead to weight increases which are unjustified, on the
whole, on typical North American subsonic configurations. I think perhaps in
the case of special configurations, particularly of the Concorde kind, increased
containment on the engine might be the lightest way of doing it. We had to add
on something like about a thousand pounds of additional weight on the Concorde
to look after non-containment problems.
A.K. Forney, FAA /'
Well, we should have reports available for distribution within three to
six months on both of our projects. I'm not sure of the exact schedule. ForL
one of them we do have the draft final report for review now, so the report
should be available fairly soon. Our objective is the same as yours, it's to
determine the trade-offs between increased containment on the engine and doing
it on the airplane. And we just had no studies in hand that gave us any indica-
tion at all; that was the purpose of undertaking them. We confined the study
to a wide-bodied jet with a high bypass ratio engine. You are fully aware of
the current status of SST activity in the United States so we did not _ddress
that question. The draft report that I've seen (which was from the air °ame _study) does recoEmend continued effort. The results of the work to dat_
uncovered fruitful areas for further work. So I would heartily endorse your
continuing to try to press for effort in that area because, apparently it would
be fruitful. Does that answer all your questions?
Guy Mangano, Naval Air Propulsion Test Center
7 / I'd like to comment on Ken's statements regarding the conclusions that
i we drew in the cited report.
It's a matter of interpretation. The report means to imply that of all
the different types of fragments generated at burst (disk and fan blade fragments)
constitute the major threat to the welfare and safety of passengers. Taken out
< of context, as Mr. For,ey has, this can of course be misconstrued to mean that
"... disk and blade fragments (are a) major threat to welfare and safety of
passengers". The intent wan simply to identify which fragments present the
worst threat. Whether or not rotor burst fragments or the incidence of rotor
burst is a major threat in commercial aviation is a judgment that is clearly
beyond the stated purpose or scope of the report being discussed.
8
]978002]25-0]6
A.K. Forne_, FAA
The problem, Guy, of course, is that that report got some distribution
and there was reaction, and we had to respond, you see. I don't know how much
you got but I know we did, and it's difficult to respond to. Your statement
claims to be based on FAA data, and so it is kind of difficult for us to answer
the criticisms that we got. Clearly if your intent is what you have just stated,
you can't tell it by reading that report; that's my point.
G.J. Mangano, NAPTC
All right, we'll take these recommendations under advisement and perhaps
be more explicit in our future statements. Again, the intent of the report was
to identify the extent of the problem, that's all. The conclusions made are
so generally stated that they are subject to different interpretations by
different people. I just wanted to explain what was meant by a major threat.
By considering all of the fragments that were generated, the disk and the fan
blades were the major threats, not that rotor burst per se is a major threat.
I think we'll leave it up to the FAA to make that judgment. All that we do is
to present the statistics of the situation. If I interpret Ken's words correctly,
rotor burst is not a major threat. Again, I'll leave that judgment to the
regulating agencies.
J.H. Enders, FAA
One might note that in these comments on the first paper we've exposed a
very basic and common problem in research: human factors. Communication
difficulties between human beings are increasingly recognized as a culprit in
our business; one group trying to understand the other, whether it's the lay
public vs. the technical community, or segments within the technical community
itself.
Gordon Gunstone, CAA-UK
I just wanted to say that perhaps to save time on that particular point,
if y_u'd comport yourselves with patience for ten minutes, I have some figures
which I hope will illustrate exactly what the problem is or isn't.
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1978002125-017


Federal Aviation Administration's approach to engine rotor integrity

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