The Accident Report Of Helicopter SE-JME
Registered name SE-JME, EC130 B model helicopter took off at 8:25 a. m to control several antennas in the mountains. There were two passengers and one pilot in the helicopter. The weather was around -2/-30 C. After a few stops, while they were going to next inspection point, the pilot heard a loud sound and felt strong vibrations in the helicopter. They were around 700 ft above the ground. The pilot understood that something was wrong about main rotor speed. He saw a moor and landed on gently after 30 seconds the noise was heard. Nobody injured.
Before the accident, the helicopter was inspected every 100 flight hours but it was only about lubrication and greasing of the components. The company also conducted a detailed inspection where they checked for rotor hub’s condition and possible cracks every 500 flight hours according to aircraft maintenance manual. After several 100 flight hour inspections, they didn’t find any problems on the vibration damper and on the mass. The last 100 flight hours inspection was six days before the accident happened and nothing unusual was found. In 2004, the main rotor hub was corroded and it happened under guarantee terms and the manufacturer company fixed it. After the accident, Swedish Accident Investigation Board (SHK) ran the investigation with the help of French Air Accident Investigation Commission (BAE) because the helicopter was manufactured by France company called Eurocopter in 2000. A large crack in the main rotor hub was found. The crack was open about 5 mm and was visible. Eurocopter company cooperated with them.
The main rotor hub has six holes that also called yokes and bushings are located inside this yokes. The bushings have blue smooth surfaces that reduce friction, corrosion and improve fatigue life under tough conditions. The crack started when the blue surface on the bushings degraded because of mechanical movements and it caused higher tensions and stresses during flights.
The lack of high amount of blue protective layer on the bushing which were mounted at the yoke that crack occurred. Once the crack occurred, it continued to advance due to stress concentration at the tip of the crack during flights. The crack started from the top of the hub because tensile stress was the greatest there. There was a brown area in the yoke and it showed that the crack was there for a long time and where the crack started from. The crack propagated without sensing any and detecting anything. The helicopter took off so many times after the crack started. The time between the crack invitation and final fracture were about 112 hours. When the crack reached the critical level, the final fracture happened suddenly and fast. The main rotor hub is made of Ti 10. 2. 3 alloy which means it has % 10 V, % 2 Fe, % 3 Al. It is metastable and V and Fe are betastabilizers. Al is alfa stabiliser. It is one of the most common titanium alloys for helicopter hubs. They have high strength, high fatigue strength, high fracture toughness.
Titanium alloys are usually used in aircraft parts and have the highest specific strengths. There are three structural types of titanium alloys. They are Alpha, Alpha – Beta, Beta or near Beta. Ti 10. 2. 3 is in near beta group which means they are easily heat treatable, very weldable, have good creep resistance and high strength capacity. It is metastable and Vanadium and iron are beta stabilizers. They are used for solution treated and aging to improve the strength of the alloy.
The Eurocopter company ran some tests on the hub. They used Brinell Hardness and the unit was HBW2, 5/187, 5. 2, 5 mm tungsten carbide ball was used and 187, 5 kgf load applied. The main criteria is it should be between 320 and 390. They measured five different points on the hub and the average number was 365, 7 so it was conforming. They also checked the chemical composition of the hub which is made of Ti 10. 2. 3. The result complied with the general features of Ti 10. 2. 3. They used X-ray diffraction microanalyses to define it. They counted macrographic marks by using binolar magnifier and the number of marks were 210. It means the helicopter was run approximately 210 times before the failure. They counted the fatigue striations. Propagation rate increased every time the helicopter flew.
The hub had lower and upper sides. The company examinated all the yokes on them. After the examination of fretting damage levels of the all yokes, four of them had significant damages which one of them had the failure. All of the holes had missing blue coat, missing sulphuric acid anodizing and fretting corrosion. There was no evidence that caused the accident because of poor material properties.
