Because adhesives play a critical role in the bonding of  aircraft structure, the mechanic must employ only those types  of adhesives that meet all of the performance requirements  necessary for use in certificated aircraft. The product must  be used strictly in accordance with the aircraft and adhesive  manufacturer’s instructions. All instructions must be  followed exactly, including the mixing ratios, the ambient and  surface temperatures, the open and closed assembly times,  the gap-filling ability, or glue line thickness, the spread of  the adhesive, whether one or two surfaces, and the amount  of clamping pressure and time required for full cure of  the adhesive. AC 43.13-1 provides information on the criteria for  identifying adhesives that are acceptable to the FAA. It  stipulates the following: New adhesives have been developed in recent years, and some  of the older ones are still in use. Some of the more common  adhesives that have been used in aircraft construction and  repair include casein glue, plastic resin glue, resorcinol glue,  and epoxy adhesives.  Casein glue should be considered obsolete for all aircraft  repairs. The adhesive deteriorates when exposed to moisture  and temperature variations that are part of the normal  operating environment of any aircraft.  NOTE: Some modern adhesives are incompatible with casein  adhesive. If a joint that has previously been bonded with  casein is to be reglued using another type adhesive, all traces  of the casein must be scraped off before a new adhesive is  applied. If any casein adhesive is left, residual alkalinity may  cause the new adhesive to fail to cure properly.  Plastic resin glue, also known as a urea-formaldehyde  adhesive, came on the market in the middle to late 1930s.  Tests and practical applications have shown that exposure  to moist conditions, and particularly to a warm humid  environment, under swell-shrink stress, leads to deterioration  and eventual failure of the bond. For these reasons, plastic  resin glue should be considered obsolete for all aircraft  repairs. Discuss any proposed use of this type adhesive on  aircraft with FAA engineering prior to use. Resorcinol glue, or resorcinol-formaldehyde glue, is a  two-component synthetic adhesive consisting of resin  and a catalyst. It was first introduced in 1943 and almost  immediately found wide application in the wood boat-building  and wood aircraft industry in which the combination of high  durability and moderate-temperature curing was extremely  important. It has better wet-weather and ultraviolet (UV)  resistance than other adhesives. This glue meets all strength  and durability requirements if the fit of the joint and proper  clamping pressure results in a very thin and uniform bond line.  The manufacturer’s product data sheets must be followed  regarding mixing, usable temperature range, and the open  and close assembly times. It is very important that this type  of glue is used at the recommended temperatures because  the full strength of the joint cannot be relied on if assembly  and curing temperatures are below 70 °F. With that in mind,  higher temperatures shorten the working life because of a  faster cure rate, and open and closed assembly times must  be shortened. Epoxy adhesive is a two-part synthetic resin product  that depends less on joint quality and clamping pressure.  However, many epoxies have not exhibited joint durability  in the presence of moisture and elevated temperatures and  are not recommended for structural aircraft bonding unless  they meet the acceptable standards set forth by the FAA in  AC 43.13-1, as referenced earlier in this chapter.  1.  Refer to the aircraft maintenance or repair manual for  specific instructions on acceptable adhesive selection  for use on that type aircraft. 2.  Adhesives meeting the requirements of a MIL- SPEC, Aerospace Material Specification (AMS), or  Technical Standard Order (TSO) for wooden aircraft  structures are satisfactory, provided they are found  to be compatible with existing structural materials  in the aircraft and fabrication methods to be used in the repair. •  Close contact adhesive—a non-gap-filling adhesive  (e.g., resorcinol-formaldehyde glue) suitable for use  only in those joints where the surfaces to be joined can  be brought into close contact by means of adequate  pressure, to allow a glue line of no more than 0.005- inch gap. •  Gap-filling adhesive—an adhesive suitable for use  in those joints in which the surfaces to be joined  may not be close or in continuous contact (e.g.,  epoxy adhesives) due either to the impracticability  of applying adequate pressure or to the slight  inaccuracies of fabricating the joint. •  Glue line—resultant layer of adhesive joining any two  adjacent wood layers in the assembly. •  Single spread—spread of adhesive to one surface only. •  Double spread—spread of adhesive to both surfaces  and equally divided between the two surfaces to  be joined. Definition of Terms Used in the Glue Process 6-10 Glues (Adhesives)