### Docket no. FAA-2008-0107
Directorate Identifier 2007-NM-087-AD

RIN 2120-AA64 Airworthiness Directives; Boeing Mode l 747-100, 747-100B, 747-100B SUD, 747-200B, 747-200C, 747-200F, 747-300, 747-400, 747-400D, 747-400F, 747SR, and 747SP series airplanes.

References:

A/ Service Bulletin 737-53A1262

B/ FAA Airworthiness Directive 2006-07-12

C/ Service Bulletin 747-53A2563 rev 0 dtd march 29, 2007 (original issue).

D/ Service Bulletin 757-53A0092

E/ Service Bulletin 767-53A0193

F/ Service Bulletin 777-53A0054

G/ Service Bulletin 747

- 53A2 563 rev 1 dtd November 08, 2007 (Deleted)

H/ Service Bulletin 747-53A2563 rev 2 dtd January 03, 2008

I/ Service Bulletin 747-53A2563 rev 3 dtd 11 June, 2009

J/ Boeing M&PT Engineering report No: MS 22811 dtd March 27, 2008

K/ FAA Notice of Proposed Rulemaking 2007-NM-087-AD dtd 31 January, 2008

L/ Boeing Service Request 1-777780501, Difficulties in Inspecting and Measuring Scribe Lines

M/ Boeing Service Request 1-787298654, Scribe Line crack Indications

N/ Service Bulletin 747-53A2563 rev 4 dtd 6 May, 2010



### KLM would like to share the following con cerns with the FAA about NPRM 2007-NM-087- AD, and Boeing SB 747-53A2563;



## General:

In 2004 Boeing issued a Service Bulletin (ref /A/) that warns the Operator that scribing of the fuselage skin of the 737 fleet affects its fatigue life. Since than, inspections and fatigue analysis have revealed and confirmed that scribing of the fuselage skin, on the 737 fleet, has a negative effect on the fatigue life of said skins, and indeed leads to fatigue cracking. Because of the negative effect on flight safety the FAA issued an AD (ref /B/) to mandate inspections and corrective actions. Since 2004 Boeing also issued Service Bulletins for the 747, 757, 767 and 777 fleet (ref /C/, /D/, /E/, and /F/) expressing Boeings concern for scribe lines, and issuing initial surveys and repeat inspections. However, it seems that only Boeing models are affected by scribe lines; neither Airbus nor Douglas issued Service Bulletins for scribe lines.



## 15,000 total flight cycles threshold.

For aircraft younger than 17,500 total flight cycles, Boeing SB 747-53A2563, Revision 4, par E. Compliance, Table 1 – Area 1, requi res the operator to perform Exploratory Inspections by 15,000 total flight cycles, or 1,500 flight cycles after the issue date of the Bulletin, whichever occurs later.

In KLM’s experience with operation of the 747 since 1971 (> 24,000 FC, > 130,000 FH), the 767 since 1995 (>12,000 FC, > 45,000 FH), and the 777 since 2003 (> 4,400 FC, > 35,600 FH), no cracks have ever been found that originated from scribe lines at the locations as mentioned in the Service bulletins (ref /C/, /E/, /F/, /G/, and /H/).Boeing supports this by stating in the Service Bulletins that, although scribe lines have been found, no cracks originating from scribe lines have been found on the 747, 767 and 777 fleet. KLM has also heard of one operator with a 747 with > 30,000 flight cycles where scribe lines where found, but no crack findings.

KLM has only inspected one (1) aircraft on which scribe lines where found. Although crack indications where found, no cracks where pres ent (see ref /J/). After further analysis by KLM and Boeing the indications where caused by a incorrect NDT inspection procedure in the
NDT manual. KLM has no evidence that supports any exact “scribe line initiation” date for these particulare scribe lines, but can prove that the scribe lines fo und are at least 6 years old in which time the A/C has made 3,390 FC and 29,729 FH. The initial scribe opportunity had to be the first D-check when the A/C was 6 years old.

Scribe line crack growth propagation in thicker 747 skins is not linear,and can not be extrapolated from 737. Since 747 skins are thicker crack growth propagation theory can not be easily “scaled” to fit 747, 

### fracture toughness has a dir ect (non-linear) correlation with the thickness of the material ( Kc ). Crack growth propagation in aluminium alloys has a complex mathematical mechanism which can only be verified by doing coupon, detail, semi-scale, and full scale testing. So far, reports of scribe lines have never included crack findings. This supports the above

statements.

Boeing announces in the new revision of the Service Bulletin par E. Compliance includes alleviation of the LRTS repeat inspection intervals. This implies that Boeing has found new substantiating data to do so, previous Service Bulletin revisions where published prematurely, and Boeing has found new data to aleviate the Exploratory Inspection Threshold.

Action items;

o Based on the renewed insight of Boeing in operator findings, and relief in repeat inspection intervals, a new (extended) fatigue based threshold can be determined and incorporated, before the current threshold is ammended, also

o The threshold for exploratory inspections is currently based on total flight cycles. However, critical crack length originating from scribe lines is related to cycles since these scribe lines where originated. The operator as owner of the aircraft is the only one that has insight in when the first scribe opportunity occurred. KLM proposes that the initial inspection threshold be based, like 737, on scribe opportunity.



## Scribe lines limited to the CLAD-layer.

According ref /N/ Service Bulletin scribe lines should be measured for depth, relative to the material next to the scribe line. However, that material usually consists of the CLAD-layer of the skin. Although the thickness of the CLAD-layer changes with the thickness of the skin, theService Bulletin does not take this into account. Even more, although the skin (and thus CLADlayer) thickness between the 737 and 747 is not compareable, the typical scribe line criteria are exactly the same. This is not correct. Scribe line crack growth initiation/propagation fro m the (relative thick 747) CLAD-layer has, in KLM’s opinion, never or insufficiently been investigated. On the aircraft that KLM inspected, it was concluded that none of the scribe lines that were found, were beyond the thickness of the CLAD-layer. Boeing also found the scribes not protruding the CLAD layer as confirmed by Boeing in ref /J/. 

### From KLM’s understanding of fatigue crack growth propagation models (NLR Arij de Koning, NASGRO, Boeing Structural Repair Courses, etc) scri be lines in the CLAD layer

### only are not a structural concer n; due to slipband formati on, and/or crack tip shielding mechanisms, such as plasticity- or roughness-indu ced crack closure, crack propagation rate ( da 

### /

dN



### ) in the CLAD-layer is retarded, or even stopped. To initiate crack growth from an initial scribe line, a load has to be applied that exceeds the plasticity of the CLAD laye r (stress intensity factor K ). Loads in CLAD layers are relatively small / negligible since CLAD layer is non-struct ural and has much higher plasticity than the structural base material underneath; the plas tic deformation of the base material does not induce high stresses in the CLAD-layer. Norma lly the stress in the net section near the scribe/crack tip increases to infinite, however plasticity in CLAD layers causes blunting of the scribe/crack tip.
Cracks from scribe lines are Surface Crack Configuration model SC03 and SC04 (Ref NASGRO, de Koning NLR).
KLM believes that cracks can not initiate from scribe lines limited to the CLAD layer, and the potential to extend into the base material.

- Sheet and Plate of Alclad 2024 over a thic kness of 0.063 inch have a nominal clad thickness of 2.5%, a min. of 2% and a max. of 3%. KLM is currently in the process of developing a method to measure the CLAD layer thickness on fuselage skins. Such a method should be incorporated in either the SB or the NDT manual.

Action items;

o Include a paragraph in the proposed AD that takes into account that scribe lines limited to the CLAD-layer of the skin (up to a certain depth/percentage, to be determined by the TC holder), are not critical for continued operation and do not require repeat inspections as per the LRTS program.

o 

### Investigate a single case of a fatique crack evolving from a scribe line in clad layer
and NOT basematerial.

o 

### Based on the above rational, blending of scri be lines limited to CLAD layers should be allowed by the SRM, SB and AD.

Lastly KLM would like to add, that repetitive removal of sealant degrades anti-corrosion capacity of primer, sealant, and paint system.

Thomas van Schaik KLM Wide Body Structures Engineering.