**Memorandum**

U.S. Department of Transportation

**Federal Aviation Administration**

+-------+------------------------------------------+--------+----------+
| Sub   | **INFORMATION:** Use of Structural       | Date:  | March 8, |
| ject: | Dynamic Analysis Methods for Blade       |        | 2001     |
|       | Containment and Rotor Unbalance Tests    |        |          |
+-------+------------------------------------------+--------+----------+
| From: | Manager, Engine and Propeller            | Reply  | Jay      |
|       | Directorate, Aircraft Certification      | to     | T        |
|       | Service                                  | Attn.  | urnberg, |
|       |                                          | of:    | (781)    |
|       |                                          |        | 238 7116 |
|       |                                          |        |          |
|       |                                          |        | or       |
|       |                                          |        | jay.     |
|       |                                          |        | turnberg |
|       |                                          |        | @faa.gov |
+-------+------------------------------------------+--------+----------+
| To:   | Manager, Aircraft Engineering Division,  | Policy | A        |
|       | AIR-100                                  | No.    | NE-2000- |
|       |                                          |        | 33.94-R0 |
|       | Manager, Brussels Aircraft Certification |        |          |
|       | Staff, AEU-100                           |        |          |
|       |                                          |        |          |
|       | Manager, Engine Certification Office,    |        |          |
|       | ANE-140                                  |        |          |
|       |                                          |        |          |
|       | Manager, Engine Certification Branch,    |        |          |
|       | ANE-141                                  |        |          |
|       |                                          |        |          |
|       | Manager, Engine Certification Branch,    |        |          |
|       | ANE-142                                  |        |          |
|       |                                          |        |          |
|       | Manager, Boston Aircraft Certification   |        |          |
|       | Office, ANE-150                          |        |          |
|       |                                          |        |          |
|       | Manager, New York Aircraft Certification |        |          |
|       | Office, ANE-170                          |        |          |
|       |                                          |        |          |
|       | Manager, Airframe and Propulsion Branch, |        |          |
|       | ANE-171                                  |        |          |
|       |                                          |        |          |
|       | Manager, Rotorcraft Directorate, ASW-100 |        |          |
|       |                                          |        |          |
|       | Manager, Rotorcraft Standards Staff,     |        |          |
|       | ASW-110                                  |        |          |
|       |                                          |        |          |
|       | Manager, Airplane Certification Office,  |        |          |
|       | ASW-150                                  |        |          |
|       |                                          |        |          |
|       | Manager, Rotorcraft Certification        |        |          |
|       | Office, ASW-170                          |        |          |
|       |                                          |        |          |
|       | Manager, Special Certification Office,   |        |          |
|       | ASW-190                                  |        |          |
|       |                                          |        |          |
|       | Manager, Small Airplane Directorate,     |        |          |
|       | ACE-100                                  |        |          |
|       |                                          |        |          |
|       | Manager, Small Airplane Standards        |        |          |
|       | Office, ACE-110                          |        |          |
|       |                                          |        |          |
|       | Manager, Atlanta Aircraft Certification  |        |          |
|       | Office, ACE-115A                         |        |          |
|       |                                          |        |          |
|       | Manager, Airframe and Propulsion Branch, |        |          |
|       | ACE-117A                                 |        |          |
|       |                                          |        |          |
|       | Manager, Chicago Aircraft Certification  |        |          |
|       | Office, ACE-115C                         |        |          |
|       |                                          |        |          |
|       | Manager, Propulsion Branch, ACE-118C     |        |          |
|       |                                          |        |          |
|       | Manager, Wichita Aircraft Certification  |        |          |
|       | Office, ACE-115W                         |        |          |
|       |                                          |        |          |
|       | Manager, Systems and Propulsion Branch,  |        |          |
|       | ACE-116W                                 |        |          |
|       |                                          |        |          |
|       | Manager, Anchorage Aircraft              |        |          |
|       | Certification Office, ACE-115N           |        |          |
|       |                                          |        |          |
|       | Manager, Transport Airplane Directorate, |        |          |
|       | ANM-100                                  |        |          |
|       |                                          |        |          |
|       | Manager, Transport Standards Staff,      |        |          |
|       | ANM-110                                  |        |          |
|       |                                          |        |          |
|       | Manager, Airframe and Propulsion Branch, |        |          |
|       | ANM-112                                  |        |          |
|       |                                          |        |          |
|       | Manager, Seattle Aircraft Certification  |        |          |
|       | Office, ANM-100S                         |        |          |
|       |                                          |        |          |
|       | Manager, Propulsion Branch, ANM-140S     |        |          |
|       |                                          |        |          |
|       | Manager, Denver Aircraft Certification   |        |          |
|       | Office, ANM-100D                         |        |          |
|       |                                          |        |          |
|       | Manager, Los Angeles Aircraft            |        |          |
|       | Certification Office, ANM-100L           |        |          |
|       |                                          |        |          |
|       | Manager, Propulsion Branch, ANM-140L     |        |          |
+-------+------------------------------------------+--------+----------+

**1. Purpose**

This policy provides guidance for evaluating the use of structural
dynamic analysis methods to show compliance with the requirements of
§33.94 of Title 14 of the Code of Federal Regulations (14 CFR 33.94),
\"Blade containment and rotor unbalance tests.\" This policy
specifically addresses paragraph (a) of §33.94 for engine design and
configuration changes. This policy is derived from extensive Federal
Aviation Administration (FAA) and industry experience in evaluating
compliance with the pertinent regulations. This policy does not create
any new requirements.

**2. Background**

Engine manufacturers are developing and using various types of
structural dynamic analysis methods to support both engine certification
activities and the aircraft manufacturers\' certification activities.
These structural dynamic analysis methods include various types of
mathematical models, such as two dimensional and three dimensional
finite element models of the engine and installation. These models are
used to determine loads and perform structural dynamic analyses on the
engine rotating components, static structures, mounts, and other
components as needed to simulate the engine test stand or aircraft
installation. One such aircraft level assessment examines windmilling
imbalance using the methods described in the Aviation Rulemaking
Advisory Committee (ARAC) report, \"Engine Windmilling Imbalance Loads -
Final Report,\" dated July 1, 1997, and the associated advisory circular
(AC) 25.24 \"Sustained Engine Imbalance,\" dated August 2, 2000.
Although the ARAC report and AC state that these dynamic analysis models
should be validated by data obtained during a §33.94 engine test, the
validation associated with these airplane requirements does not
substantiate the use of these dynamic analyses as a substitute for
conducting the blade out tests required by §33.94.

Section 33.94 requires an engine test to demonstrate failure of the most
critical fan, compressor, or turbine blade. Typically, the engine test
is run on the first engine of an engine type to establish an acceptable
baseline configuration. When there is a major change to the engine, the
effect of the change on the §33.94 requirements is evaluated to
determine if the results from the baseline test are applicable. This is
referred to as reconciling the change with the baseline configuration.
Dynamic analysis is just one method used in the reconciliation process.
When engine changes cannot be appropriately reconciled with the baseline
configuration, a new baseline test may be necessary. An analysis cannot
substitute for a baseline engine test. However, the results of a
baseline engine test can be used to develop, validate, and calibrate an
engine dynamic analysis. Thus, the engine dynamic analysis, when shown
to be acceptable, may be used to support future §33.94 compliance
findings for some engine design and configuration changes.

**3. General**

When changes are made to an existing engine type certificate or
derivatives are added to the type certificate, these changes must be
reconciled with the baseline engine test. The analysis that reconciles
the new configuration with the baseline engine test may vary from a
qualitative engineering evaluation to the use of a complex dynamic
model, depending on the extent of the modifications. In all cases the
analysis must be validated with respect to the baseline engine test
before it can be used for extrapolation to the new configuration. The
analysis is usually validated by showing that it can reliably predict
applicable results from engine tests or rig tests. The analysis should
consider the period from just before blade loss through the deceleration
rundown for 15 seconds or until a self-induced shutdown.

To reconcile an engine change with the baseline configuration, the
objectives of §33.94 must be met. Therefore, the reconciliation must
show that, following failure of the most critical fan, compressor, or
turbine blade and when operated for 15 seconds or until a self-induced
shutdown, the engine:

-   Will not fail its mounting attachments;

-   Will not catch fire; and

-   Is capable of containing the damage.

**4. Loads Evaluation**

The reconciliation of an engine change with the baseline configuration
using analysis methods should involve an evaluation of loads. The
baseline engine test loads should include the effects of the test
configuration used to support the engine during the baseline blade out
test; the changed engine loads should be evaluated against the baseline
engine test results. This evaluation should verify that the changed
engine loads are consistent with any applicable limitations (for
example, engine or thrust reverser mount structure loads) noted within
the installation instructions required under §33.5 and the engine mount
load limits established under §33.23.

**5. Mount Evaluation**

The mount evaluation generally focuses on loads and the vibratory
response of the engine. The analysis should show that proposed changes
to the engine do not significantly modify mount loads from the baseline
engine test results. If the resultant mount loads are higher, further
evaluation should be conducted to show that the mounts have sufficient
capacity. Mount loads and load distributions that are significantly
higher than the baseline engine test results generally indicate that the
new configuration cannot be reconciled to the baseline engine test. In
this case a new blade out engine test may be required to show compliance
with §33.94.

**6. Fire Evaluation**

The fire evaluation generally focuses on the loads and vibratory
response of components that carry flammable fluids, such as: fuel lines,
oil lines, oil tanks, gearboxes, or integrated drive generators. The
analysis should show that the change to the engine does not increase the
loads and response of these components. If the loads and response are
increased, further evaluation should be conducted to show that the
components will not fail or catch fire. An evaluation should also be
conducted when changes are made to those components that carry flammable
fluids, to assess that the changes are acceptable.

**7. Blade Loss and Containment Evaluation**

Containment capability is dependent on the containment structure and
blade loss interaction, including blade fragmentation and the resulting
interaction between the blades and the case. The applicant should show
by test, validated analysis, or both that changes to the blade or
containment structure can be reconciled with the baseline engine test.
Containment rig testing, combined with dynamic analysis, is one method
that may provide the appropriate data to reconcile with the baseline
engine test. When the unbalance loads due to blade loss are
significantly higher than demonstrated during the baseline engine test,
reconciliation of the change with the baseline test may not be possible.
In those cases a new engine test is required by §33.94.

Changes to the fundamental method of containment, such as changing from
hardwall containment to softwall containment or introducing design
changes to the same case, may affect the containment capability of the
engine as well as the load transfer from blade fragmentation and the
resulting interaction between the blades and the case. Therefore, it
should be shown that, in addition to containment capability, the changes
do not significantly affect the overall engine loads. This can be shown
by test or by dynamic analysis that has been validated by showing that
it can reliably predict event outcomes for changed products.

**8. Static Structure**

The analysis method should include the loads and the load transfer
through the engine structure for 15 seconds or until a self-induced
shutdown. Changes to the static structure may significantly alter the
loads. For example, increasing the engine case stiffness and strut
stiffness may result in higher loads to the engine mounts. The dynamic
analysis should be sufficiently detailed to address these changes.

**9. Applicable Changes**

The use of analysis to reconcile major engine changes with a baseline
§33.94 engine test is a decision that requires substantial engineering
judgement. Major engine changes include, but are not limited to, changes
to the following:

-   Blade count, blade mass, blade platform or blade design;

-   Materials in the blade, containment, or attachment structure;

-   Maximum rotational speed;

-   Containment structure;

-   Static structure;

-   Mount locations;

-   Mount structure; and

-   Location or design of a component that carries flammable fluid.

**10. Concluding Remarks**

When analysis is proposed to reconcile major engine changes, the
applicant and the Engine Certification Office (ECO) or cognizant
Aircraft Certification Office (ACO) should agree on the method of
compliance early in the certification program. They should include the
Engine Dynamics National Resource Specialist and the Engine and
Propeller Standards Staff in the decision process when necessary.

Original signed by J. J. Pardee

Jay J. Pardee

DRAFT: This document does not represent final agency action on this
matter and should not be viewed as a guarantee that any final action
will follow in this or any other form.