Boeing 737-236 series 1, G-BGJL: Main document - Leonardo

1.6.2 Engines
1.6.2.1 General
The Pratt and Whitney JT8D-15 is a two-shaft turbofan engine.The combustion section is canannular
and comprises 9 combustorcans enclosed by a Combustion Chamber Outer Case (CCOC)
(Appendix5 Fig b). Compressor delivery air enters the CCOC, where a smallproportion is mixed
with fuel in the combustor cans and ignitedto produce the combustion flame. The remainder of the
compressorair flows around the inner and outer walls of the cans to providea cooling flow (note:
the combustion temperatures are above themelting point of the can materials and thus the cooling
flow isessential to maintain can integrity). Whilst the combustor canscontain the combustion
process, the CCOC must withstand the compressordelivery pressure (in the order of 240 psi at takeoff
conditions)and it is therefore essentially a pressure vessel. It is basicallya one-piece tube of
AMS 5603 steel alloy with flanges fore andaft which attach to the engine casing by two rings of
steel bolts.
The combustor cans themselves comprise a cast Stellite dome, orhead, and 11 liners of Hastelloy X
sheet material (Appendix 5Fig c). The dome incorporates swirl vanes which direct the
incomingcompressor delivery airflow into the can prior to mixing withfuel from the fuel nozzle
which is inserted into the centre ofthe dome. The fuel nozzle also provides radial location of
theforward end of the can. Axial location is achieved via an integrallug on the dome which picks up
on a mounting pin bolted to thediffuser case. The remainder of the can is constructed from 11rings
(liners) of sheet Hastelloy X material of varying diametersto achieve the desired profile of the can.
Liner 3 incorporatesthe flame transfer ports to adjacent cans. The liners are resistanceseam-welded
to each other. The aft end of liner 11 is a slidingfit in the transition duct bulkhead, which provides
radial supportfor the rear of the can but allows movement in an axial directionto accommodate
thermal expansion and contraction. Can numbers4 and 7 also have an igniter plug boss incorporated
in Liner 2.All cans are fitted with an "air scoop" over the topof liner 2, as part of a programme to
reduce the engine's smokeemission.
Cooling of the liners is achieved by directing the relativelycool compressor delivery air over the
outside surfaces of thecan and onto the inner surface through small film-cooling holesadjacent to
each liner joint. Since there is a pressure differentialof about +3% of compressor delivery pressure
from outside to insideof the can, cooling air will flow inwards. Larger holes in theliner also allow
larger volumes of air to flow in locally to cooland adjust the combustion gas flow pattern inside the
can.
The combustor cans fitted to G-BGJL's engines were to Pratt andWhitney modification standard
5192, ie. the latest standard applicableto the JT8D-15 at the time of the accident. The modification
wasintended to overcome various problems encountered on the previousstandard of can, including
cracking of the seam weld between linernumbers 2 and 3. This was felt to be particularly
undesirablebecause it occurred under the air scoop and could only be detectedby radiographic
techniques. It was stated by the manufacturerthat this modification standard would provide a
combustor canof "improved durability".
The combustion section is further enclosed by an aluminium alloyfan case which forms the by-pass
duct and is the externally visiblepart of the engine casing in this area.
Each engine was fitted with a thrust reverser system typical ofreverser systems fitted to this
category of aircraft, comprisinga pair of clam-shell doors which swung on linkages from
theirstowed position (around the exhaust duct) into a position aftof the engine, where they deflected
the exhaust gases sidewaysand slightly forwards to provide reverse thrust (Appendix 6 Figa).