Narrative:

I arrived at the aircraft about 45 minutes prior to departure. I turned the available power on thinking the safety check had been done as the crew was already on the aircraft. I did my normal checks and proceeded to do the walk around and preflight inspection on the aircraft. I did not notice the power fuel switch had been left on the APU. When I got to the left side of the aircraft I noticed black smoke coming from what I thought was the cowl. I immediately ran to get the captain and notify him that something had caught fire. The captain and I walked to the back of the airplane where we noticed the APU exhaust had smoke coming out from it and knew something was wrong. I immediately stopped the fueler from fueling the aircraft and had him unhook the fuel and told the ground crew to turn the ground air off. Meanwhile the captain went to the cockpit and called for a fire truck as a precaution. This was when he noticed the power fuel switch on the APU had been left on from the previous flight. I stayed outside of the aircraft and watched to see if the smoke had stopped. It actually led to a small fire inside the APU exhaust area. The fire and emergency vehicles arrived about 8 minutes later and the fire went out when the captain turned off the APU power fuel switch. An inspection was done by the fire and emergency crew and nothing was detected. The exhaust area had just been hot and burned up the fuel that was entering the APU. Maintenance did an inspection on the APU; the bottles were not discharged. We were then towed to the run up pad where maintenance performed an inspection of the APU and turned it on. Nothing was damaged on the aircraft and nothing was wrong with the APU. We then brought the aircraft back to the gate and it was returned to service for a revenue flight.

Related reports:

Google
 

Original NASA ASRS Text

Title: The First Officer of a commercial fixed wing aircraft noted smoke emanating from the APU exhaust and took appropriate action to investigate and eliminate its source.

Narrative: I arrived at the aircraft about 45 minutes prior to departure. I turned the available power on thinking the safety check had been done as the crew was already on the aircraft. I did my normal checks and proceeded to do the walk around and preflight inspection on the aircraft. I did not notice the power fuel switch had been left on the APU. When I got to the left side of the aircraft I noticed black smoke coming from what I thought was the cowl. I immediately ran to get the Captain and notify him that something had caught fire. The Captain and I walked to the back of the airplane where we noticed the APU exhaust had smoke coming out from it and knew something was wrong. I immediately stopped the Fueler from fueling the aircraft and had him unhook the fuel and told the ground crew to turn the ground air off. Meanwhile the Captain went to the cockpit and called for a fire truck as a precaution. This was when he noticed the power fuel switch on the APU had been left on from the previous flight. I stayed outside of the aircraft and watched to see if the smoke had stopped. It actually led to a small fire inside the APU exhaust area. The fire and emergency vehicles arrived about 8 minutes later and the fire went out when the Captain turned off the APU power fuel switch. An inspection was done by the fire and emergency crew and nothing was detected. The exhaust area had just been hot and burned up the fuel that was entering the APU. Maintenance did an inspection on the APU; the bottles were not discharged. We were then towed to the run up pad where Maintenance performed an inspection of the APU and turned it on. Nothing was damaged on the aircraft and nothing was wrong with the APU. We then brought the aircraft back to the gate and it was returned to service for a revenue flight.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.