Importance Of Turbine Engines Overhauls And Repair

Published Date: 02 Nov 2017

Vidal Sanchez

Mr. Kahanek

PP1.702

Importance of turbine engines overhaul and repair

Engine overhaul is the process of maintaining and restoring the engine and its components to serviceable conditions. An engine overhaul consists of disassembly, inspection, reassembly and testing prior to return to service.

Turbine engines have to be overhauled after certain period of servicing. This period of servicing is called Time between Overhaul (TBO). Time between overhaul is stated by the engines manufacture. For a turbine engine TBO is not always specified, instead some manufactures establish a recommended TBO for individual components or sections. Even though an engine is not being operated for a long period of time it needs to be overhauled before returning to service. This is because if the engine is used less frequently it will become corroded because of moisture and oil corrosives. An engines time between overhaul can vary due to the exposure to extreme conditions such as dust, salt water, and high or low climate temperature.

One of the most common reasons for engine failure is FOD (Foreign object damage). FOD can occur any time that an external object is drawn into a turbine. The level of inspection in case of FOD depends on the amount of damage. If little damage is made in the turbine, only a borescope inspection is necessary. If a lot of damage is clearly visible the engine has to be removed and overhauled.

FOD can occur anywhere anytime. On January 15, 2009 US Airways Flight 1549 scheduled a commercial passenger flight from New York City to North California. About 3 minutes after takeoff multiple bird strikes occurred damaging both engines. This caused an immediate and complete loss of thrust. Being unable to reach an airfield, the crew of the aircraft decided to turn southbound and do an emergency landing over the Hudson River. Landing on water is a very difficult maneuver to achieve. The aircraft landed successfully and all 155 occupants safely evacuated the aircraft. If it wasn’t for the crews ability and experience this event would end in a catastrophic accident. This is just one example of what FOD can cause. (1)

Other reasons for engine failures are thermal stress, hot start, internal oil leakage and excessive vibration. All of these are causes for engine repair and overhaul. If inadequate or no maintenance is done on an engine when its needed the engine can fail during flight. Improper aircraft maintenance contributes to most of aviation accidents. A small portion of maintenance tasks are performed wrong or not performed due to human error. These errors can be missing parts, parts installed incorrectly or skipping necessary checks. Maintenance work requires clerical skills and attention to detail. From 1994 to 2004 42% of fatal airline accidents in the United States are because of maintenance problems. These accidents are caused by a breakdown organization process, culture and decisions. But human error keeps showing as the weakness on maintenance problems due to lack of motivation, fatigue and stress, time pressure, misperception of hazards, inadequate skills, lack of communication, personal life problems, boring repetitive jobs, poor instructions, poor training, design of tools, equipment and workplace, etc. (1.5)

On November 4, 2010 Qantas A380 had to do an emergency landing due to multiple engine failures. The failure was the first of its kind for this type of aircraft, the world’s largest passenger aircraft. It is still the only aviation accident involving this kind of plane. Several minutes after takeoff, the crew reported they heard two simultaneous loud bangs. After the investigation the ATSB (Australian Transport Safety Bureaus) found several failures on Number two engine which caused that many of the flight controls were unusable. Some sections of the intermediate pressure turbine disc bored the leading edge of the left wing inboard of engine number two. This caused damage to the leading edge structure, front and upper surface of the wing. It also damaged several system components, fuselage and electrical wiring. Electrical wiring damage affected the operation of hydraulic system, flight controls and landing gear. Also during the examination, Rolls-Royce found a fatigue cracking within a pipe that feeds oil to a bearing structure causing an oil leak which led to engine fire. The inspectors believe that the cracking was caused for a misaligned region counter-boring within the stub pipe outlet. (2)

Another accident due to poor maintenance took place on December 8, 1998. A Hughes 369D Helicopter was forced to land emergently due to loss of engine power, autorotation. A review of the maintenance records indicated that the last overhaul included the replacement of all turbine rotors. During the investigation it was noted that there was blending in the first stage rotor that was deeper and larger than allowed. This blending caused engine failure that forced the rotor craft to have an emergency landing. Like this case there is many other cases that because of distractions or fatigue a little scratch end up on a catastrophe. (3)

On January 20, 2007 in Gatwick, UK, a Dassault Falcon 900B was forced to make an emergency landing when experienced an engine problem. The UK’s Air Accidents Investigation Branch (AAIB) believe that the probable cause of this event was the fracturing of a stage two low pressure turbine blade due to a casting defect, ant that this was probably caused by a defect on a third stage blade. Just before the incident the crew heard a loud noise coming from the rear of the aircraft and engine fire was present and the number three engine fire warning light was illuminated. The engine had suffered an uncontained breach in the combustion chamber and ejected wreckage from the turbine assembly. This caused slight but significant damage to the horizontal stabilizer. Detailed examination of the rest of the turbine showed that one of the low pressure stage two blades had evidence of damage before the accident. This defect was identified as "hot tear" that was formed during casting process. This is an example of a poor technique for maintenance of lack of concentration. As said earlier, human error is always the ending point at aircraft accident investigations. (4)

Another famous catastrophe is the American Airlines Flight 191. On May 25, 1979 the DC-10-10 operating the flight crashed moments after takeoff from Chicago. Investigators found that just before takeoff, engine number one had detached and flipped over the top of the wing. The engine detachment caused damage to the hydraulic fluid lines. This caused the aircraft control systems to retract and therefore the airplane was out of control and nothing could be done. The National Transportation Safety Board (NTSB) believes that what could cause the accident was the asymmetrical stall because of the uncontrollable reaction of the left wing. The separation of the engine from the wing was caused by damage by improper maintenance procedures. American airlines had a quicker way to mount and unmount the engine by skipping a lot of important steps and using a forklift to lift the engine. When mounting the engine the shift was made at the most critical moment. Excessive force was applied on the engine mounts causing damage to the airframe. This caused cracks on the engine mounts which led to detachment of the engine. A few minutes after takeoff the airplane had crashed. No one survived to this accident. 273 people died. There were 258 passengers form all ages, 13 crew people and 2 people who were on the ground. The human error in this case was skipping important procedure steps, lack of communication and inadequate equipment. (5)

On July 6, 1996 at Pensacola Delta Airlines Flight 1228 experienced a catastrophic engine failure during takeoff. This event caused wreckage from the compressor hub of the left engine to damage the fuselage. The rear cabin passengers heard a very loud explosion and had blast sensation. The pilot stopped takeoff by bringing the throttle control to the idle positions and engaging the brake which stopped the aircraft with no use of the reverse spoilers. After a total and complete investigation, the NTSB found out that the most probable cause of the engine failure was a fluorescent dye penetrant inspection that was not done correctly and a crack on the lefts engine’s compressor fan hub was not detected. And as always the error ends being a human factor. (6)

On July 19, 1989 United Airlines Flight 232 crashed in Sioux City, Iowa as a result of a catastrophic failure on its tail engine. The loss of such engine led to loss of all flight controls.

Sources

http://en.wikipedia.org/wiki/US_Airways_Flight_1549 (1)

http://www.dviaviation.com/maintenance-error.html (1.5)

http://www.ainonline.com/aviation-news/2010-12-03/report-recounts-harrowing-experience-crew-stricken-qantas-a380 (2)

http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20001211X11630&ntsbno=SEA99FA018&akey=1 (3)

http://www.ainonline.com/aviation-news/aviation-international-news/2009-10-02/engine-failure-forced-emergency-landing(4)

http://en.wikipedia.org/wiki/American_Airlines_Flight_191 (5)

http://en.wikipedia.org/wiki/Delta_Air_Lines_Flight_1288 (6)