g ***GE Aviation***

***CF6 Engineering***

*Phone/FAX: 513-552-3210*

*Mail Drop: S160*

**Docket No. FAA--2008--0402**

**Directorate Identifier 2007--NM--165--AD**

## GE Comments on Subject NPRM May 21, 2008

\(1\) regarding: Title block

Airworthiness Directives; Boeing

Model 747 Airplanes and Model 767

Airplanes Equipped With General

Electric CF6--80C2 and CF6--80A Series

Engines

GE Comments: GE suggests removing all reference to CF6-80A engine model
for the following reasons:

GE is not aware of any confirmed engine flameout events on the CF6-80A
due to ice crystal inclement weather. This may be due to several
factors, including:

1.  Significantly different type-design booster than the -80C2 (the
    --80A has fewer rotor and booster stages, with \~30% fewer airfoils
    resulting in significantly reduced potential accretion sites than
    the --80C2).

2.  The -80A also has a significantly different VBV system (especially
    the exit path).

3.  The -80A has a purely hydro-mechanical (PMC with MEC) fuel control
    system, whereas the -80C2 has predominately FADEC control with
    different fueling schedules and response characteristics.

\(2\) regarding: Discussion (1^st^ paragraph, 5^th^ sentence)

...

accumulate in the core flow path of the

engine during low-power conditions,

such as idle or idle descent. The

accumulated ice sheds during throttle

increase and is ingested into the engine,

GE Comments: GE suggests removal of the word "core", or if that is not
acceptable, at least changing from "core flow path", to: "booster and
core flowpath" since the term "core" can be interpreted to mean just the
high pressure spool portion of a turbofan.

\(3\) regarding: Discussion (1^st^ paragraph, last sentence)

...

compressor due to ice impact. The GE

CF6--80C2 and CF6--80A series engines

models have similar compressor

designs.

GE Comments: GE suggests removal of the entire sentence "The GE CF6-80C2
and CF6-80A series engines models have similar compressor designs.", for
the same reason as GE's earlier comments (item 1, above). However, if
the FAA disagrees with this comment, then GE proposes the follow
clarification from: "have similar compressor designs" to: "have
different booster and VBV system designs, but similar compressor
designs".

\(4\) regarding: Discussion (3^rd^ paragraph, first sentence)

The requirement to activate the

engine anti-ice prior to descent in

visible moisture with total air

temperature less than 10 °Celsius (C)

and greater than -40 °C already exists

GE Comments:

GE believes the reference to --40 should be clarified to indicate SAT
--40.

\(5\) regarding: Costs of Compliance

GE Comments:

GE suggests that there should be an operational cost of compliance
included for completeness. While increasing engine off-take or bleed
does provide additional margin against flameout, it does require
somewhat increased fuel burn, and as written, the proposed AD would
likely be required on a significant percentage of flights. An estimate
of the incremental fuel required is around 100 pounds of fuel per flight
for the 747, but less for the 767.

\(6\) regarding: Applicability, paragraph (c)

GE recommends removal of "or CF6-80A", for same reasons mentioned above.

\(7\) regarding: Unsafe Condition, paragraph (d)

GE acknowledges a small number of inclement weather or significant
convective weather systems encounters have resulted in short-duration
multiple engine powerloss in over 20 years with approximately 14 million
flights of total CF6-80C2 service experience, however GE's assessment is
that for these in-flight ice crystal conditions the risk of a forced
landing is extremely improbable (including demonstrated relight
performance) and thus doesn't meet the definition of an 'Unsafe
Condition' as defined by AC39-8 (Continued Airworthiness Assessments of
Powerplant and Auxiliary Power Unit Installations of Transport Category
Airplanes).

# *David C. Zigan*

David C. Zigan

CF6 Systems Engineer