Timber for aircraft construction in converted form (finished form)

IS 1898: Scope and Key Specifications for Timber in Aircraft Construction

Scope

• Covers finished timber converted from baulks/scantlings for aircraft use.
• Ensures timber meets strength, density, and moisture content criteria for safety and performance.

Key Specifications

Density Requirements (Clause 7.1)

• Density at 15% moisture: 440 to 580 kg/m³
• Adjust density by ±4 kg/m³ for each 1% moisture content change from 15%.

Sample Preparation (Clause 1.3)

• Measure length, width, thickness; calculate volume ( V ) in m³.
• Weigh sample ( W ) in grams (accuracy 0.001 g).
• Density formula:

[ \text{Density} = \frac{W}{V} ]

Notes

• Sample size for compression test: 20 mm × 20 mm × 80 mm
• Moisture content significantly affects strength and density; always adjust accordingly.

flowchart TD
    A[Timber Sample] --> B[Measure Dimensions]
    B --> C[Calculate Volume (V)]
    A --> D[Weigh Sample (W)]
    C & D --> E[Calculate Density = W

Referenced Indian Standards in IS 1898 (Timber for Aircraft Construction)

• IS 707:1976 — Glossary of timber technology terms
• IS 1141:1973 — Code of practice for seasoning of timber
• IS 1329:1975 — Specification for aircraft timber (baulks and scantlings)
• IS 1708:1986 — Methods of testing small clear specimens of timber

Key Formula for Density (Clause A-1.3)

[ \text{Density} = \frac{W}{V} ]

• ( W ) = Weight of sample in grams (accurate to 0.001 g)
• ( V ) = Volume of sample in cubic centimeters (cm³)

Density Specification (Clause 7.1)

• Density range at 15% moisture content: 440 to 580 kg/m³
• Adjust density by ±4 kg/m³ for each 1% moisture content change from 15%

Sample Dimensions (Note 1 under Table 1)

• Sample size: 20 mm × 20 mm × 80 mm

Additional Notes

• Use IS 2:1960 for rounding off numerical values.
• BIS governs the use of Standard Mark ensuring quality and conformity.

flowchart TD
    A[Sample Preparation] --> B[Weigh sample (W) in grams]
    B --> C[Measure volume (V) in cm³]
    C --> D[Calculate Density = W/V]
    D --> E[Check density range: 440-580 kg/m³ at 15% moisture]
    E --> F[Adjust density by ±4 kg/m³ per 1% moisture variation]

This concise summary helps ensure compliance with IS 1898 and related standards for aircraft timber.

IS 1898: Definitions & Key Specifications for Timber for Aircraft Construction

Density Calculation (Clause 7.1 & A-1.3):

• Density, ρ = W / V
  • W = Weight of sample (grams, accuracy 0.001 g)
  • V = Volume of sample (cubic meters)
• Density limits at 15% moisture content:
  • Minimum: 440 kg/m³
  • Maximum: 580 kg/m³
• Adjust density by ±4 kg/m³ for each 1% moisture content change from 15%.

Sample Preparation (Clause 1.3 & A-1.2):

• Measure length, width, thickness multiple times; take arithmetic mean.
• Calculate volume (V) in cubic meters.
• Weight (W) in grams with 0.001 g accuracy.

Strength Requirements (Table 1, Clause 6.1)

Notes

• Sample size for testing: 20 mm × 20 mm × 80 mm (substituted from 25×25×100 mm).
• Moisture content affects strength and density; adjust values accordingly.
• Use IS 2:1960 for rounding off test results

IS 1898 - Species of Timber: Key Points & Tables

1. Species Reference

• Clause 3.1 refers to IS 707:1976 for timber definitions and species classification.
• Timber species must comply with IS 707 for terminology and classification.

2. Relevant Standards for Timber Properties

• IS 1708:1986: Testing methods for small clear timber specimens.
• IS 1141:1973: Seasoning of timber.
• IS 1329:1975: Specification for aircraft timber.

3. Strength Requirements for Aircraft Timber (from Table 1, Clause 6.1)

4. Notes:

• Moisture content is critical; strength values are at 15% moisture.
• Adjust strength values linearly for moisture above or below 15%.

Summary Diagram of Strength vs Moisture Content

graph LR
A[Moisture Content] -->|<15%| B[Increase Strength]
A -->|>15%| C[Decrease Strength]
B --> D[Add % Increment × Factor]
C --> E[Subtract % Decrement × Factor]

IS 1898 - Prohibited Defects & Strength Specifications for Aircraft Timber

1. Prohibited Defects (Clause 5.1)

Timber used must be clear and free from:

• Rot, dote, brashness
• Incipient decay
• Discoloration
• Shakes (splits along the grain)
• Knots
• Resin pockets
• Compression failures and compression wood

These defects compromise structural integrity and are strictly prohibited.

2. Strength Requirements (Clause 6.1, Table 1)

3. Moisture Content

• Strength values are specified at 15% moisture content.
• Adjust strength values by the given increments/decrements for moisture content above or below 15%.

Summary Diagram: Prohibited Defects and Strength Checks

graph TD
    A[Timber for Aircraft] --> B[Check for Defects]
    B --> C{Defects Present?}
    C -->|Yes| D[Reject Timber]
    C -->|No| E[Perform Strength Tests]
    E --> F

IS 1898 Key Points on Moisture Content and Seasoning

1. Moisture Content (Clause 6.1 & 7.1)

• Timber must be seasoned to 10% - 17% moisture content (dry weight basis) as per IS 1708:1986.
• Moisture content is measured by taking a 25 mm disc from the failure point or center of test specimens.
• Density at 15% moisture content:
  4400 N/m³ ≤ Density ≤ 5800 N/m³
• Adjust density for moisture content changes:
  [ \rho = \rho_{15%} \pm 40 \times (M - 15) ]
  where ( \rho ) = density at moisture content ( M % ).

2. Seasoning (Clause 6.1 & 5.2)

• Season timber by approved methods per IS 1141:1973.
• Growth rate must be ≥ 6 annual rings per 25 mm in radial direction.

3. Sampling (Annex A)

• Cut 25 mm long full cross-section samples from both ends, avoiding weathered parts.

Summary Table: Density Adjustment with Moisture Content

flowchart TD
    A[Timber Sample] --> B[Cut 25 mm Disc]
    B --> C[Measure Moisture Content (IS 1708)]
    C --> D{Moisture Content 10-17%?}
    D -- Yes --> E[Proceed with Mechanical Tests]
    D -- No --> F[Re-season Timber (IS 1141)]
    E --> G[Calculate Density Adjustment]

This ensures reliable timber quality for structural use per IS 1898.

IS 1898 - Density Requirements for Timber (Aircraft Construction)

Key Specifications:

• Density Range at 15% Moisture Content:

  • Minimum: 440 kg/m³
  • Maximum: 580 kg/m³

• Adjustment for Moisture Content:

  • Density changes by ±4 kg/m³ for each 1% increase or decrease in moisture content from 15%.

Density Calculation (Clause A-1.3)

[ \text{Density} = \frac{W}{V} ]

• W = Weight of sample in grams (accuracy 0.001 g)
• V = Volume of sample in cubic centimeters (cm³)

Sample Preparation (Clause 1.3)

• Sample size: 20 mm × 20 mm × 80 mm
• Weight measured to 0.001 g accuracy
• Volume in cubic centimeters (cm³)

Summary Table

flowchart TD
    A[Sample Preparation] --> B[Measure Weight (W) in grams]
    A --> C[Measure Volume (V) in cm³]
    B & C --> D[Calculate Density = W / V]
    D --> E[Check Density Range 440-580 kg/m³ at 15% Moisture]
    E --> F[Adjust Density for Moisture Content ±4 kg/m³ per 1%]

This ensures timber used in aircraft meets strict density and moisture standards for structural reliability.

IS 1898: Grades of Timber & Strength Requirements

Grades of Timber

• Grade I and Grade II based on mechanical properties (Clause 8.1).
• Maximum slope of grain:
  • Grade I: ≤ 1 in 15
  • Grade II: ≤ 1 in 12
    (Clause 9.1, per IS 1329:1975 Appendix B)

Density Requirements (Clause 7.1)

• Density at 15% moisture content: 440 to 580 kg/m³
• Adjust density by ±4 kg/m³ per 1% moisture content change from 15%.

Strength Requirements (Table 1, Clause 6.1)

Notes

• Moisture content adjustment applies for values above or below 15%.
• Finished timber sizes are as agreed between supplier and purchaser (Clause 10.1).
• Test specimen size for Izod impact: 20 mm × 20 mm × 80 mm (Note 1).

flowchart TD
    A[Timber Grades] -->|Grade I| B[Higher Strength & Lower Grain Slope

Grain Slope Limitations (IS 1898: Clause 9.1)

• The maximum resultant slope of grain is governed by IS 1329:1975 Appendix B.
• Limits:
  • Grade I timber: Maximum slope = 1 in 15
  • Grade II timber: Maximum slope = 1 in 12

This means the grain deviation angle θ from the longitudinal axis should satisfy:

[ \tan \theta \leq \frac{1}{15} \quad \text{(Grade I)}, \quad \tan \theta \leq \frac{1}{12} \quad \text{(Grade II)} ]

Strength Parameters for Aircraft Timber (IS 1898: Clause 6.1 & 8.1)

Summary

• Grain slope ensures structural integrity; exceeding limits reduces strength.
• Use Grade I timber for critical components requiring higher strength and lower grain slope.
• Always refer to IS 1329 Appendix B for detailed grain slope evaluation methods.

graph LR
A[Grain Slope] --> B{Grade}
B --> C[Grade I: ≤ 1/15]
B --> D[Grade II: ≤ 1/12]
C --> E[Higher Strength]
D --> F[Moderate Strength]

IS 1898: Sizes of Finished Timber - Key Points

• Clause 10.1: Sizes of finished timber are not fixed by the code; they are to be mutually agreed upon by supplier and purchaser.

• Grain Slope Limits (Clause 9.1):

  • Grade I timber: max slope of grain = 1 in 15
  • Grade II timber: max slope of grain = 1 in 12
    (As per Appendix B of IS 1329:1975)

• Density (Clause 7.1):

  • Density range at 15% moisture content: 440 to 580 kg/m³
  • Adjust density by ±4 kg/m³ for each 1% moisture content change from 15%.

• Reference Sizes (from Table 1 Note 1):

  • Minimum finished size example: 20 mm × 20 mm × 80 mm (substituted from 25×25×100 mm)

Density Adjustment Formula (Clause A-1.3)

[ \text{Density} = \frac{W}{V} ]

• (W) = weight of timber specimen
• (V) = volume (in cubic centimeters)

Summary Table: Grain Slope Limits

Practical Notes:

• Finished timber sizes are flexible but should meet agreed dimensions and grain slope limits for strength.
• Density must be checked and corrected for moisture content to ensure compliance.
• Refer IS 1329 for aircraft timber baulks and scantlings specifications.

flowchart TD
    A[Timber Supplier] -->|Agreed Sizes| B[Finished Timber]
    B --> C{Check Grain Slope}
    C -->|<=1:15 (Grade I)| D[Accept Grade I]
    C -->|<=1:12 (Grade II)| E[Accept Grade II]
    C -->|Exceeds limits| F[Reject or Reprocess]

For detailed strength tables and seasoning, refer to IS 1329 and IS 1141 respectively.

IS 1898 - Marking of Timber: Key Points

Clause 11.1: Marking Requirements

Timber shall be legibly and indelibly marked with:

• Species name
• Grade or quality
• Moisture content
• Manufacturer's identification
• Date of manufacture or seasoning
• Any special treatment or certification

Strength & Test Data (from Table 1, Clause 6.1)

Additional References

• IS 707: Glossary of timber terms
• IS 1141: Code of practice for seasoning
• IS 1329: Specification for aircraft timber
• IS 1708: Testing methods for small clear specimens

Summary Diagram: Timber Marking Process

flowchart LR
    A[Timber Production] --> B[Identification of Species & Grade]
    B --> C[Measurement of Moisture Content]
    C --> D[Marking with Indelible Ink]
    D --> E[Include Manufacturer & Date]
    E --> F[Special Treatment Details (if any)]

Note: Marking ensures traceability and quality control essential for structural and aircraft timber applications.

IS 1898: Method of Determination of Density of Wood

Key Specifications:

• Density range at 15% moisture content:
  [ 440 \text{ kg/m}^3 \leq \rho \leq 580 \text{ kg/m}^3 ]
• Density correction for moisture content:
  Increase/decrease density by 4 kg/m³ for each 1% change in moisture content from 15%.

Sample Preparation (Annex A):

• Timber moisture content: 10% to 17% (per IS 1141 & IS 1708).
• Test specimen sizes:
  • Compression parallel to grain: 20 mm × 20 mm × 80 mm
  • Static bending: 50 mm × 50 mm × 1000 mm

Density Calculation Formula:

[ \text{Density} = \frac{W}{V} ]

• (W) = weight of the wood sample (in kg)
• (V) = volume of the wood sample (in m³)

Moisture Content Adjustment:

[ \rho_{corrected} = \rho_{15%} \pm 4 \times (M - 15) ]

• (M) = actual moisture content (%)
• (\rho_{15%}) = density at 15% moisture content

Summary Table:

flowchart TD
    A[Wood Sample] --> B[Measure Weight (W)]
    A --> C[Measure Volume (V)]
    B & C --> D[Calculate Density (ρ = W/V)]
    D --> E[Adjust for Moisture Content]
    E --> F[Final Density]

This method ensures consistent density measurement for timber used in aircraft construction.