Jun 30/01 51-72-00 Page 1 EFFECTIVITY: All z SD3-60 STRUCTURAL REPAIR MANUAL SRM 51-72-00 THERMOPLASTIC AND FIBREGLASS REPAIRS 1. General Thermoplastics by nature of their chemical structure, can be soft and pliable or semi-hard in the formed state depending on the nature and type of substance and the chemical additives added to the plastic for the desired properties. In the polyvinyl chloride range the plastic can be in the soft leather grained style used for furnishings, which never ages or hardens, or in the hard sheet form used to furnish the inside of the aircraft. Both types can be made more workable when a small amount of heat is applied, and therefore it can be reworked to a certain degree and small repairs can be carried out. Thermo-setting plastics are used for sealing, potting, and general bonding applications, and this type is applied as a mixture of resin and accelerator which mixed in the correct proportions hardens to the desired specification for its respective purpose. In the hardened condition heat has no effect on the mixture. Cured glass fibre/epoxy components, are used in several areas of the aircraft in various constructional forms. For example, the air conditioning system makes use of glasscloth ducting; the radome, and wing strut top fairing, are constructed from a glass fibre/honeycomb sandwich; the crew compartment sliding doors are made from a glass fibre/Nomex sandwich; the wing tip fairing, and several electrical control and distribution panels, etc, are either manufactured from, or provided with covers, of laminated glass fabric. The applications and use of pre-formed plastic components and the mouldable compounds are many, and the need for careful selection for a specific operation is of great importance for the safety and quality of product and repair of components. 2. Health and Safety A. Precautions must be taken to avoid skin contact with the resin systems. B. Clean white lint free gloves (or other approved gloves) must be worn when handling uncured materials. C. Smoking, drinking or eating is strictly prohibited in the Lay-Up area. D. When cured laminates are being machined an efficient vacuum collection and disposal system must be used to remove dust particles. E. A dust mask must also be worn such as RQ 100 Mask with RC 54 Filter. F. Scrap prepreg, off-cuts etc., must be collected together and sealed In a Polythene bag for safe disposal. G. Solvents must only be applied in a well ventilated area. Operators must use Neoprene Gloves. Solvents must be kept in an approved Safety Storage Can/Dispenser.
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SRM51-72-00 20.0.0.0THERMOPLASTIC AND FIBREGLASS REPAIRS
1. General
Thermoplastics by nature of their chemical structure, can be soft and pliable or semi-hard in the formed state depending on the nature and type of substance and the chemical additives added to the plastic for the desired properties. In the polyvinyl chloride range the plastic can be in the soft leather grained style used for furnishings, which never ages or hardens, or in the hard sheet form used to furnish the inside of the aircraft.
Both types can be made more workable when a small amount of heat is applied, and therefore it can be reworked to a certain degree and small repairs can be carried out. Thermo-setting plastics are used for sealing, potting, and general bonding applications, and this type is applied as a mixture of resin and accelerator which mixed in the correct proportions hardens to the desired specification for its respective purpose. In the hardened condition heat has no effect on the mixture.
Cured glass fibre/epoxy components, are used in several areas of the aircraft in various constructional forms. For example, the air conditioning system makes use of glasscloth ducting; the radome, and wing strut top fairing, are constructed from a glass fibre/honeycomb sandwich; the crew compartment sliding doors are made from a glass fibre/Nomex sandwich; the wing tip fairing, and several electrical control and distribution panels, etc, are either manufactured from, or provided with covers, of laminated glass fabric.
The applications and use of pre-formed plastic components and the mouldable compounds are many, and the need for careful selection for a specific operation is of great importance for the safety and quality of product and repair of components.
2. Health and Safety
A. Precautions must be taken to avoid skin contact with the resin systems.
B. Clean white lint free gloves (or other approved gloves) must be worn when handling uncured materials.
C. Smoking, drinking or eating is strictly prohibited in the Lay-Up area.
D. When cured laminates are being machined an efficient vacuum collection and disposal system must be used to remove dust particles.
E. A dust mask must also be worn such as RQ 100 Mask with RC 54 Filter.
F. Scrap prepreg, off-cuts etc., must be collected together and sealed In a Polythene bag for safe disposal.
G. Solvents must only be applied in a well ventilated area. Operators must use Neoprene Gloves. Solvents must be kept in an approved Safety Storage Can/Dispenser.
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H. Cloths which have been used for applying solvent must be disposed of in metal bins which must be emptied daily.
3. Storage and Handling of Materials
Storage and handling of glass fibre and thermoplastic materials etc., is to be in accordance with the manufacturers instructions.
4. Thermoplastic Furnishings
The furnishings are made from pre-formed thermoplastic material which is heat jointed together on assembly.
To repair certain fixed items on the aircraft a special repair kit to aid the replacement of the damaged item is required, and the kit consists of thermo active adhesive material and the necessary heating unit or heat activating tool.
A suitable kit for furnishing repair may be obtained from:
- John Schneller and Associates Inc.,Kent,Ohio 44240, U.S.A.
5. Fibreglass Components
A. General
A large smooth topped work bench in a clean air zone is required when repairing fibreglass components, and care should be taken to avoid contamination by dirt and foreign matter.
The choice and use of materials is most important and care is needed when deciding on the use of certain resins and polyesters when repairing components.
Care is also needed in planning the method and in the carrying out of the repair, to obtain the required strength and finish of the component, before assembly on the aircraft, and typical methods and repairs are quoted for guidance in this Chapter.
B. Typical Equipment for Fibreglass Repair
- Small hand saw- Coarse file- Paint brush 3" size- Slitting knife- Pneumatic power tool (with slitting saw attachment)- Form block (manufacture to suit repair)- Resin (polyester type) and hardener CIBA GEIGY LTD or DEXTER CORPORATION,
- M.E.K. Solution (Methyl-ethyl-ketone).- Heat source (heat lamp infra red).
C. Preparation of Moulds and Fromers
When manufacturing moulds and temporary formers care and attention is needed both in the manufacture and in the surface preparation of the moulds and formers prior to lay up of fibreglass for the repair.
The formers can be shaped from the profiles of undamaged ducts or components and filed to the required shape, and should be coated with a catalyst resin to assist in obtaining a highly polished finish. The finish is obtained by coating the surface with a silicon wax (e.g. TRAFFIC WAX) and polishing to a high gloss using a Silicon Based Automobile type wax.
Before laying up the fibreglass on the former, the use of a good parting agent is desirable to give a clean surface break. There are several types of parting agents which are marketed for this purpose and a suitable type is Crystic No.2 Release Agent, which is obtainable from
Moulds for lay ups can be roughly made using marine plywood and coated with a mix of Araldite LW559 and hardener HY559 and smoothed to the desired shape for lay up of fibreglass component. The surface is then finished with CIBA GEIGY Gellcoat resin. Formers are made from marine plywood sections and glued and screwed together for the desired profile, and coated after smoothing and polished with catalyst resin for the desired surface finish. When the desired surface finish is obtained the mould or former is sprayed with a suitable parting agent prior to lay up of fibreglass material.
When a repair to a profiled portion of the aircraft has to be made the need for templates and a form block to match the profile is required. These templates are easily manufactured by taking profiles from an undamaged aircraft or component, and they can be transferred to a hard wood or tufnol block whose profile is finished to the correct shape. The pores of the surface profile of the block are sealed with silicon wax and sprayed with a parting agent or covered with a small sheet of cellophane to act as a prating agent and surface finishing device.
Temperature proof cellophane or parting agents of a silicone nature can be supplied by
The 3M Company, St Paul Minnesota U.S.A., PO Box 5800 CODE 55101
The 3M Company U.K.Limited, 3M House, Wigmore Street, London W1A 1ET
Monsanto Corporation U.S.A.
Canadian Tape Company Canada
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6. Glasscloth ducting and pipes
A. General
Glasscloth ducting is used in various areas and repairs can be carried out effectively with the minimum of equipment and without difficulty.
The instructions given in the following paragraphs and illustrations are a guide to the methods of preparation and layup procedure to obtain the standard and required specifications of the repaired component and this information can be applied to any component of a regular profile which has to be repaired.
B. Typical Repair of Low Pressure Glasscloth Ducts
(1) Remove the damaged area and dress to a regular shape. To prevent fraying and internal peeling of the duct, seal the edges of the cut with resin.
(2) Making allowance for the requisite overlap cut a patch from a duct of similar section of ducting and apply 3 layers of glasscloth, impregnated with resin. When set hard remove the patch and peel off the cellophane.
(3) Abrade the outside surface of the damaged duct completely over the area to be covered by the glasscloth tape to give a uniform matt finish. Remove all dust from the duct.
(4) Apply resin to both surfaces, and apply the patch to the duct and allow to cure.
(5) Wind 2.0" wide (50 mm) glasscloth tape impregnated with resin around the duct as shown. Refer to Figure 1. Allow each turn of the tape to overlap the previous one by 2/3 of its width. One winding of tape is sufficient for this repair. Allow to cure.
(6) Remove irregularities in the contour of the duct by brushing on a thick but uniform layer of resin.
(7) Pressure test duct to 4 PSI.
C. Typical Repair of Box Section Ducts
(1) Cut out, dress and seal the damaged area. Refer to Figure 2.
(2) Allowing for the requisite overlap, prepare a suitable patch from a similar duct section or make up as specified in para B (2).
(3) Abrade and clean bonding surfaces, apply resin to both surfaces and assemble the patch to the duct.
(4) Impregnate glass tape with resin and wind over the patch edges as dictated by working conditions.
NOTE: The minimum distance between repairs to be 12 inches.
(3) Impregnate glass tape, minimum width 1.5 in. and wind around the duct in overlapping layers to equal the existing duct wall thickness.
E. Butt Joints at Insertions or Severed Ends
(1) Trim the duct ends to provide a good butt joint, abrade and clean the bonding area and apply a brush coating of resin. Refer to Figure 4.
(2) Heat the glass tape at a temperature of approximately 50°C (112°F) for at least ten minutes to eliminate all moisture and wind around the joint, applying a brush coating of resin at and after each turn.
(3) Lay on sufficient laminations to equal the existing duct wall thickness.
(4) Pull the tape taut and smooth down whilst winding to eliminate as far as possible all air bubbles in the resin. Apply a final overall coat of resin and leave to cure.
(5) If working facilities permit this repair may also be applied to curved ducts or pipes in which case the use of 3 in. wide glass tape is recommended. Wind the tape around the joint bonding area, brush coat in layers as above and arrange for a 2/3 overlap at each turn, fairing down to the duct level at the extremities of the wound area, and apply a final thick, uniform coat of resin to even out irregularities.
7. Repair of Radome and/or Wing To Strut Fairing
A. General
Examine the component in question carefully to ascertain the overall extent of the damage to the fibreglass and honeycomb structure. Damage will amost always be irregular in pattern and the first operation is to mark out a regular and uniform shape in soft pencil on the radome. The amount to be rebated off the outer and inner fibreglass skins is also marked with a soft pencil. The new aperture should be of uniform shape on sound fibreglass and honeycomb structure around the damaged area.
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Refer to Figure 5. Refer to Figure 7. These show the preparation of such a repair, and the step-by step procedure is detailed at B.
NOTE: (1) As the radome and the fairing are constructed of honeycomb and glassfibre, the epoxy resin is in the totally cured state and if the same type of resin is applied again the repair material will not adhere to the existing structure, and it is therefore important to use a polyester compound when carrying out repairs.
A range of adhesives and materials for repair can be obtained from
Ciba Geigy (UK) Limited (Plastics Division) Duxford, Cambridge, England
The Dexter Corporation (Hysol Division) Main Plant, Division Headquarters, Olen New York USA
(2) Before selecting a resin check that it conforms to DTD.5537 (MIL-R-9300B) or BS3396 (MIL-C-9084C) specification. Check also that the type of accelerator is suitable as different types may require different cure rates and temperature conditions.
(3) Modern resins will cure naturally at room temperature but the curing cycle may be accelerated by using warm air covers or a small heat lamp. The method used will vary according to the shape of component and extent of repair.
B. Preparation for repair
(1) Examine the radome or fairing to ascertain the full extent of the damage.
(2) Decide upon a regular pattern to be cut around damaged area.
(3) Decide upon regular rebate for inner and outer skins around the damaged area.
(4) Remove paint locally in way of the damaged area to be repaired and totally clean with M.E.K. solution.
(5) Mark out in pencil to a regular shape the pattern to be cut and the amount of rebate to be employed on the outer and inner surfaces.
(6) Proceed as shown to remove damaged and ragged honeycomb debris and to rebate the surfaces.
(7) Remove swarf and clean area with MEK (methyl ethyl ketone) solution and dry with warm air.
(8) Cut away both faces of damaged area by two laminations to prepare for the new glasscloth patches.
(9) Shape the hole to a clean profile and clean area with M.E.K. and dry in warm air.
(1) Clamp form block covered with cellophane parting agent and inner glasscloth patch to inside surface of the radome, or fairing.
(2) Cut out new honeycomb insert to the profile of the hole in the radome or fairing.
(3) When honeycomb is in position, totally impregnate with resin to prepare for top cover patch.
(4) Cut out new top cover patch and impregnate with resin before application.
(5) Finish coat the resin soaked patch with a finish coating of resin to match the parent surface profile.
(6) Heat cure for 24 hours under an infra-red heat lamp until hard, and smooth off to finish profile.
D. Finish
(1) Preparation, priming, and finish painting, of repaired area. Refer to 51-23-00, pb1.
NOTE: Refer to 51-23-00, pb1. A portion of the radome is finished using an erosion resisting coating. This area is visually identifiable and should be treated in accordance with Paragraph 12 of the reffered section..
E. Radome
Radomes fitted to SD3-60 A/C are designed, manufactured and supplied by:
- Norton International Inc., Akron, Ohio 44309
Any repairs carried out must be such as to ensure conformance with radome performance as originally supplied.
