Timber for aircraft construction in converted form (finished form)
1990 Edition

Under the IS 1898:1990 standard, the permitted timber species for aircraft construction are primarily Picea smithiana (Spruce) and Abies species, excluding Abies densa. These species must be processed into Grade I or Grade II timber based on their mechanical attributes, ensuring conformity with the strength and quality requirements stipulated in the standard. Only these species and grades are acceptable to guarantee the structural integrity and performance of aircraft components.

IS 1898 mandates that timber intended for aircraft use should be seasoned to contain moisture levels ranging from 10% to 17% by dry weight. The moisture measurement must comply with the procedures outlined in IS 1708:1986. Seasoning should be conducted following the approved techniques described in IS 1141:1973 to avoid defects and ensure optimal timber quality. Moisture content is verified on a 25 mm disc taken from the specimen’s failure point or center after mechanical testing.

According to IS 1898, Grade I timber exhibits superior mechanical strength compared to Grade II. Specifically, at 15% moisture content, Grade I timber must have a minimum compressive strength parallel to the grain of 42.5 N/mm², Izod impact energy absorption of 8200 N-mm, modulus of elasticity in bending of 1.05 × 10⁵ N/mm², and a modulus of rupture of 66.5 N/mm². In contrast, Grade II timber has lower minimum values for these properties. Additionally, grain slope limitations are tighter for Grade I (maximum 1 in 15) than for Grade II (maximum 1 in 12), reflecting its use in more critical structural areas.

IS 1898 prohibits timber exhibiting any of the following defects to ensure safety and durability: decay forms such as rot and dote, brittleness (brashness), early-stage decay (incipient decay), discoloration, grain splits known as shakes, knots, resin pockets, compression failures, and compression wood. Such imperfections compromise the wood’s strength and are unacceptable in the manufacture of aircraft components, which must be clear and structurally sound.

Timber density measurement under IS 1898 is conducted according to the procedure in Annex A, where the density at 15% moisture content must lie between 440 kg/m³ and 580 kg/m³. The density is calculated by dividing the mass of the sample by its volume (Density = Weight/Volume). For moisture content deviations from 15%, the density is adjusted by ±4 kg/m³ for each 1% change in moisture. Timber samples used for testing typically measure 20 mm × 20 mm × 80 mm for compression tests, ensuring adherence to these density and moisture criteria for reliable aircraft timber performance.