Turbine engine accidents caused by foreign object damage

Turbine Engine Accidents Caused by Foreign Object Damage

Course No - Turbine Engine Accidents

When materials go to the turbine engine core that is likely to degrade compressor stability, combustor flameout margin and fuel control run-down margins. It leads to a variety of troubles including surging of the engine, power loss and engine flameout. Studies have been conducted to look into these issues. We shall look at some cases of accidents so that we are better informed as to the reason for them. The accident that took place in Toowoomba aerodrome in Queensland of Australia by the Beech aircraft corporation King Air C90 on 27th November 2001 was found to be the failure of the left engine and that was viewed to be the critical engine under the conditions in which the aircraft was flying. Another related accident took place at 1630 with a Boeing 747 aircraft at Changi airport in Singapore which was due to the failure of the number-2 engine, or the left inboard engine. The paper shall finally look at an accident that took place at North Wilkesboro, North Carolina on October 22, 2001 wherein the aircraft collided with power lines. When the engine and transmission assemblies were examined, it was seen that the turbine was split apart at the power turbine support and almost half of the accessory gearbox was destroyed by fire due to the impact.

Introduction:

There is always trouble when outside matter is taken up into the turbine engines fixed in any type of aircraft. The general losses are of power and stability of the aircraft and finally even accidents. The problems arise in North America mainly from ingestion of water or hail.

Discussion:

Continuing the subject of ingestion of water or hail, it was seen that the concentration is increased due to the high speed of the aircraft and the effect is worse if the flight slows down. The material goes to the turbine engine core and that is likely to degrade compressor stability, combustor flameout margin and fuel control run-down margins. This leads to a variety of troubles including surging of the engine, power loss and engine flameout. The continuous difficulties posed by these led the Aerospace Industries Association to undertake a study in 1987 on the natural icing effects on high bypass ratios of turbofan engines with an emphasis on mechanical damage aspects. The study ended up with a conclusion that separate power loss and instability that were seen were not due to mechanical damage. This led to another study in 1988 to recommend changes if any were required. The final opinion of that study was that a potential threat to flight safety existed due to extreme rain and hail for turbine engines when the aircraft operated in such conditions. The study also agreed that the standards present before did not meet the required standards for safety from such threats. This led to another study. (FAR NPRM)

The earlier standards were suitable for mechanical damage from ingestion of water and large hailstones, but they did not suitably take care of engine power-loss and instability. This caused run down and flameout at lower than takeoff-rated power settings for turbine engines fixed on airplanes. Regarding the turbine engines fixed on rotorcraft or turbo-shaft engines, however the previous standards were viewed to be adequate. The decision was taken based on the review of service experience of rotorcraft turbine engines and did not find a single incidence where the earlier water ingestion standards were inadequate for their use. The difference between the two types of engines probably occur because rotorcraft turbine engines operate under different conditions that other turbine engines - they operate at higher power settings during descent than aircraft turbine engines and the crafts themselves operate at much lower speed when compared to aircraft. Probably the higher engine power and lower flight speed reduces the water concentration and amplification effects on these engines. (FAR NPRM)

At the same time, let us look at some cases of accidents so that we are better informed as to the reason for them. A report was released on 25th June 2004 about the accident that took place in Toowoomba aerodrome in Queensland of Australia. The plane was on an Instrument flight rules Charter flight to Goondiwindi, also in Queensland. The plane was Beech aircraft corporation King Air C90 aircraft taking off from runway 29 on 27th November 2001. On board the plane were the pilot and three passengers. Just before the flight took off or about the time the flight was in the air, the left engine failed, and a future examination showed that it had lost all power almost immediately. In the meantime the aircraft continued on its path and became airborne just before crashing and all the 4 persons on the aircraft lost their lives. (Accident and Incident Reports- Detail: Air Safety Occurrence Report)

The principal reason for the accident was found to be the failure of the left engine and that was viewed to be the critical engine under the conditions in which the aircraft was flying. The engine was checked and damage on it was found to be in line with damages due to the fracture and release of one or more compressor blades into the path of the gas of the engine. This was the reason why the engine had not given the power that was expected from it. There were no reasons to suspect that the engine defect was due to manufacturing defects, metal fatigue, and foreign object damage during the flight or the quantity or quality of fuel used during the flight. When the compressor blades were checked, they showed that they had been exposed to higher than normal operating temperatures during the period that finally led to the accident. The engine failure occurred at 3,556.0 hours after the last overhaul and this was within the 3,600 hours gap that is permitted within overhauls as specified by the service bulletins of the engine manufacturers.

But, the aircraft engines were on a life extension to 5,000 hours as per the provisions within the Australian civil Aviation Safety Authority Airworthiness Directive. This required that the engines which continued to operate under these conditions had to be checked for and engine condition trend monitoring program. When the data from the left engine through this test was checked, it was found that there was a potentially safety-critical problem in that engine for some weeks before the crash took place. The data had not been analyzed sufficiently for this flaw to come out in the open. According to the experts, if the study had been done, then the likelihood of the left engine having an in-flight emergency problem was likely to have been known. (Accident and Incident Reports- Detail: Air Safety Occurrence Report) This was thus a case of inadequate checking.

Let us look at another recent report which has come out on 22 June 2005. This is regarding an accident that took place at 1630 with a Boeing 747 aircraft at Changi airport in Singapore. The aircraft rotated during takeoff, the crew heard a loud bang and the aircraft just turned left. The indications on the aircraft instruments showed that there was a failure of the number-2 engine, or the left inboard engine. The crew then dumped fuel that they were carrying and returned immediately to the airport. The first inspection found that there was a failure within the engine's first stage high-pressure compressor assembly. Then the engine was returned to Australia for checking after disassembling at the maintenance facility and there the confirmation was that there was a failure due to the loss of one blade and a very high thermal and mechanical damage of the other blades within that disk. The blade which had become loose was recovered from within the engine. (Accident and Incident Reports- Detail: Occurrence Brief)

The failure of the number 2 engine was due to the slow release of one blade from within its setting in the first stage high pressure compressor disk. This is what led to the fire in the titanium metal within the compressor assembly. For the final break up of the blade from the compressor disc was a result of the cracking and consequent failure of the root corners of the blade. There was a large extent of damage to the dovetail root surfaces of even the blades which were retained. This suggests that fatigue cracking mechanism was initiated due to the stresses that came from uneven dovetail root bedding. Further evidence is established from the engine manufacturer through the initiation of blade dovetail root cracking. (Accident and Incident Reports- Detail: Occurrence Brief) Here again study showed that turbine engine failure was not due to the impact of foreign objects.