My guess is that it's the exhaust for the Auxiliary Power Unit [1]. The APU is essentially a small gas turbine generator, so the exhaust temperature is approxim

My guess is that it's the exhaust for the Auxiliary Power Unit [1]. The APU is essentially a small gas turbine generator, so the exhaust temperature is approximately 1000°C. This is way above the melting temperature of composites, and even aluminum. This site [2] has a very good discussion about the Boeing 737 APU and APU exhaust (which they call an "Educator"). [1] http://en.wikipedia.org/wiki/Auxiliary_power_unit http://en.wikipedia.org/wiki/Auxiliary_power_unit [2] http://www.b737.org.uk/apu.htm http://www.b737.org.uk/apu.htm