Department Number

C8020

Northwest Airlines, Inc.

5101 Northwest Drive

St. Paul, MN 55111-3034

June 28, 2002

federal Aviation Administration

Transport Airplane Directorate, ANM-114

Attention: Rules Docket No. 2002-NM-12-AD

Seattle Aircraft Certification Office

1601 Lind Avenue SW

Renton, WA 98055-4056

NORTHWEST

AIRLINES

RECEIVED

JUN 282002

ANM -114

Subject:

Reference:

Comments to Docket No. 2002-NM-12-AD

(A) NPRM Docket No. 2002-NM-12-AD

(B) Boeing Service Letter 757-SL-24-069

(C) Boeing Service Letter 747-SL-24-058

(D) Avionic Instruments Inc. Service Bulletin 1-002-0102-1000-24-24

The FAA issued referenced (A) NPRM 2002-NM-12-AD proposing the adoption ofa new

airworthiness directive (AD) that would require the modification of specific static inverters to

prcvent an overheating condition caused by a resistor within the inverter. The NPRM identifies

an incident where the resistor overheated burning surrounding capacitors causing smoke in the

flight deck. Thc NPRM is proposing that the modification be accomplished within 18 months of

the effective date. NW A's Boeing 757-200/300 and 747-400 aircraft currently operate with the

referenced static inverters. The following are NW A's comments regarding the referenced (A)

NPRM Docket:

Item # I Discussion:

Under the discussion paragraph, the NPRM identifies that analysis conducted by the

manufacture of the static inverter identified that the proximity of the R 170 resistor to capacitors

C50 and C51 could lead to overheating and possible damage or failure of the referenccd

capacitors. Furthermore, the paragraph indicates that the manufacturc's analysis suggests that

the combination of obstructed convcction cooling and localized heat from RI70 resulted in

sufficient energy to damage the adjacent capacitors. NW A's engineering group contactcd the

static inverter manufacturer's repair dcpartmcnt and found that while the manufacturer agreed

with thc assessment that the R170 resistor was the source of energy that damaged the rcfcrenced

capacitors, the manufacturer was conccrned that morc recent evidence indicates that a liquid

suhstanee may have leaked on to the resistor and contributed to the R170 resistors' overheating

condition.

N\VA is concerned that the root cause of the resistor's overheat condition my not be accurately

addresscd, and that this NPRM may be more symptomatic in nature. NW A is requesting

confirmation as to the cause of the referenced Rl70 resistor's overheating condition. If the

cause of the Rl70 resistor's excessivc over heat condition is found to be due to a liquid

spill/lcak, NW A would recommend that the NPRM bc withdrawn, or at a minimum the

compliance time bc relaxcd to 4 ycars to avoid unduc hardsbip on the airlines.
Item #2 Cost Impact:

The NPRM Cost impact paragraph states that, " the FAA has been advised that manufacturer

warranty remedies are available for the cost of the modified replacement parts and labor

associated with accomplishing the actions specified by this AD. Therefore, the economic cost

impact of this proposed AD on U.S. operators may be minimal." Per the referenced (D) service

bulletin and confirmation from the manufacturer, all units driven into the manufacturer's repair

facility specifically for this modification will be responsible for re-certification charges. These

re-certification charges equate to approximately 5550.00 per unit. NW A operates approximately

forty of the suspect static inverters. The manufacturer has indicated that units returned for

normal repair or service will automatically be upgraded at no re-certification cost. NW A is

requesting that the referenced paragraph identify the re-certification charges for the

accomplishment of the AD.

Item #3 Applicability Table:

Per Table I under the Applicability paragraph, aircraft model and series information relative to

the specific static inverter part number is not consistent with referenced (B) & (C) Boeing

service letters. For example, Table I of the NPRM implies that part number 1-002-0 I02-0265 is

used on the Boeing 737 series aircraft only, while the referenced (B) & (C) Boeing service

letters indicate that the referenced part number is used on the 737 and 747-400 series aircraft.

NW A is requesting clarification as to which table has the correct applicability information.

Item #4 Applicability Table:

Per the manufacture, part number 1-001-0102-0265 also requires the referenced resistor

modification. This part number is not identified in any of the referenced documentation. NWA

is requesting clarification as to whether part number 1-001-0102-0265 is to be included into the

applicability tables.

Respectfully.

Mark Millam

Chief Engineer

cc: K. Hylander

R. Gleason

D. Lee

G. Budinger

R. Fox

D. Bowling

J. Marcolina

W. Branz