Narrative:

During the ground time of the aircraft; several workscopes and scheduled maintenance [tasks] were assigned to perform; one of them being the gas path cleaning of all 4 engines. While the company decided whether or not the assigned station would perform the gas path cleaning of the engines; the decision was made to perform the work the same day of departure. The aircraft had to be towed from the remote area to the mid-field blast fence; once at the blast fence the [technician] in charge of the engine run noticed his engine run-up qualifications were expired. One hour past our arrival time to the blast fence; a one-time authorization was issued by [the quality control department] to the mechanic in charge of the engine run. In the mean time all the equipment was set up for the work being accomplished; including covering all generators cooling ducts to prevent any internal damages due to the abrasive powder used in this process. After acquiring the one-time authorization for engine run-up; we proceeded to do the gas path cleaning. While at #3 engine; bad weather including lightning approached the airport; so we knew we have to be quick. Once the work was accomplished and all ground equipment removed; the towing company denied to tow the aircraft to the assigned gate due to financial issues with the airline. At this point the airport authority was questioning us about our long stay on the blast fence. [We are aware that the] authorities are able to fine the airlines due to so much unjustified time at this location. At the same time we were receiving calls from the rental company of the air compressor being used for the gas path cleaning; asking us to return it right away. The airline found another company to tow the aircraft to the assigned departure gate. While they set up the aircraft to be towed; we decided it was best to take the airplane first back to the gate and there remove the covers from the generators cooling ducts. Once at the gate; we decided to return the air compressor and then take a quick lunch break. One mechanic stayed with the aircraft for the towing from the blast fence to the assigned departure gate. While at lunch; we received a call from the load master taking the flight out; stating that the ceo of the airline stopped the operation and the airplane had to be towed back to the remote area of the airport as soon as possible. [When] we got to the airplane; the towing company was already awaiting for a brake rider. At this point; the mechanic that brought the airplane from the blast fence to the gate was released so he can take his lunch and another mechanic rode the aircraft under tow back to the remote area. Once at the remote area; it was already late; and per the company; the aircraft was not going anywhere; so we decided to finish our operation. Unfortunately; we forgot to remove all 4 of the engine generator cooling duct covers [installed] during the gas path cleaning process; also known as coke wash. Two and half days later; we were called to prepare the aircraft for departure. All of the systems on the aircraft were checked as a part of a pre-departure check policy. The paperwork was completed; the airworthiness [release was] signed and the aircraft was dispatched. We were informed this morning that the aircraft landed with all 4 generators failed; and deflated 13 tires. [Fortunately] the crew was unharmed. Apparently the covers installed in the cooling ducts of the generators did not allow any ram air to cool the generators. It was a terrible mistake from the maintenance crew that day; forgetting the removal of this covers.

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Original NASA ASRS Text

Title: Several Maintenance Technicians performed a gas path cleaning procedure on all four engines of a B747 under less than ideal conditions. Weather; equipment; pressure; changing plans and distractions plagued the process. The aircraft was released for service with the protective covers still installed over all of the generator cooling ducts. This caused all of the generators to fail in flight due to lack of cooling air. The flight crew safely landed the aircraft; but without the use of thrust reversers or anti-skid system. Several tires were blown.

Narrative: During the ground time of the aircraft; several workscopes and scheduled maintenance [tasks] were assigned to perform; one of them being the gas path cleaning of all 4 engines. While the company decided whether or not the assigned station would perform the gas path cleaning of the engines; the decision was made to perform the work the same day of departure. The aircraft had to be towed from the remote area to the mid-field blast fence; once at the blast fence the [Technician] in charge of the engine run noticed his engine run-up qualifications were expired. One hour past our arrival time to the blast fence; a one-time authorization was issued by [the Quality Control Department] to the mechanic in charge of the engine run. In the mean time all the equipment was set up for the work being accomplished; including covering all generators cooling ducts to prevent any internal damages due to the abrasive powder used in this process. After acquiring the one-time authorization for engine run-up; we proceeded to do the gas path cleaning. While at #3 engine; bad weather including lightning approached the airport; so we knew we have to be quick. Once the work was accomplished and all ground equipment removed; the towing company denied to tow the aircraft to the assigned gate due to financial issues with the airline. At this point the airport authority was questioning us about our long stay on the blast fence. [We are aware that the] authorities are able to fine the airlines due to so much unjustified time at this location. At the same time we were receiving calls from the rental company of the air compressor being used for the gas path cleaning; asking us to return it right away. The airline found another company to tow the aircraft to the assigned departure gate. While they set up the aircraft to be towed; we decided it was best to take the airplane first back to the gate and there remove the covers from the generators cooling ducts. Once at the gate; we decided to return the air compressor and then take a quick lunch break. One mechanic stayed with the aircraft for the towing from the blast fence to the assigned departure gate. While at lunch; we received a call from the Load Master taking the flight out; stating that the CEO of the airline stopped the operation and the airplane had to be towed back to the remote area of the airport as soon as possible. [When] we got to the airplane; the towing company was already awaiting for a brake rider. At this point; the mechanic that brought the airplane from the blast fence to the gate was released so he can take his lunch and another mechanic rode the aircraft under tow back to the remote area. Once at the remote area; it was already late; and per the company; the aircraft was not going anywhere; so we decided to finish our operation. Unfortunately; we forgot to remove all 4 of the engine generator cooling duct covers [installed] during the gas path cleaning process; also known as coke wash. Two and half days later; we were called to prepare the aircraft for departure. All of the systems on the aircraft were checked as a part of a pre-departure check policy. The paperwork was completed; the airworthiness [release was] signed and the aircraft was dispatched. We were informed this morning that the aircraft landed with all 4 generators failed; and deflated 13 tires. [Fortunately] the crew was unharmed. Apparently the covers installed in the cooling ducts of the generators did not allow any ram air to cool the generators. It was a terrible mistake from the maintenance crew that day; forgetting the removal of this covers.

Data retrieved from NASA's ASRS site as of July 2013 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.