Accident to OY-HIO (Bell 206L-4) at Tunulliarfik Fjord on 28-2-2024

The engine consisted of:

• a single stage centrifugal flow compressor
• a single can-type combustor
• a turbine assembly, which incorporated a two-stage gas producer turbine, a two-stage power turbine, an exhaust collector, and an accessory gearbox

The accessory gearbox consisted of:

• a gas producer gear train
• a power turbine gear train

The two-stage gas producer turbine drove:

• the centrifugal flow compressor
• the fuel pump
• the fuel control unit
• the oil pump
• the N1(gas producer turbine rpm (rotations per minute)) tacho-generator

A common shaft drove the oil pump and the N1 tacho-generator.

The two-stage power turbine drove:

• the main rotor
• the tail rotor
• the N2 tacho-generator
• the turbine governor

See Appendix 1.

See Appendix 2.

The airframe manufacturer installed the engine oil tank on the top aft section of the fuselage. From the oil tank, a hoseline routed the oil by gravity to the inlet port at the oil pump pressure pump in the accessory gearbox.

The engine manufacturer routed pressurized oil from the oil pressure pump to the bearings of the engine compressor, the engine gas producer turbine, the engine power turbine, the accessory gearbox bearings and the gears for lubrication and cooling. The oil pump scavenge pumps in the accessory gearbox returned the heated oil to an oil cooler and back into the oil tank. See Appendix 3.

The engine manufacturer installed an oil filter just after the engine oil pressure pump. The airframe manufacturer installed a second oil filter after the oil scavenge pumps and before the return oil went into the oil cooler and the engine oil tank. The oil pump installed in the accessory gear box consisted of a pressure pump and scavenge pumps. A common shaft drove all pumps.

Figure 5. Oil pump cross section.

The engine manufacturer installed a total of two magnetic chip detectors in the engine oil system, one at the bottom of the accessory gearbox, and one on the engine accessory gearbox at the oil return line from the engine to the oil cooler and the engine oil tank.

The engine manufacturer numbered the engine bearings as shown in the figure below.

Figure 6. Engine bearing locations and numbering.

 

The Gas Producer Turbine assembly consisted of a G/P support in which the 1st stage nozzle and G/P turbine rotor were installed.

Bearing no. 7 and bearing no. 8 in the G/P support supported the G/P turbine rotor.

The engine manufacturer installed a thrust plate with an anti-rotation tab to prevent the outer race of bearing no. 8 from rotating. This tab fitted into slots of the G/P support and the outer race of bearing no. 8.

Figure 7. Location of the thrust plate in the gas producer turbine.

The installation of floats and liferafts provided ditching capability. The emergency float system was installed on the landing gear skid.

The emergency float system consisted of six floats: one forward (fwd), one mid and one aft on the LH and RH sides.

Each float consisted of gas holding chambers separated by internal bulkheads to minimize the loss of buoyancy in the event of a puncture.

Each separate chamber contained an inlet check valve for inflation, a combination topping valve for manual inflation, and a pressure relief valve to prevent overpressure.

Each float had interconnect hoses between the inlet valves.

Each of the mid floats also included a six-person liferaft.

The system included two pressurized gas reservoirs, installed on the belly of the helicopter.  One reservoir supplied gas to the floats, and the other reservoir supplied gas to the liferafts.

On top of each of the reservoirs were discharge valves.

Hose-type supply lines connected the discharge valves to each of the floats and liferafts inlet valves.

Activating a lever handle on the cyclic stick in the cockpit activated the discharge valve of the gas reservoir through a cable, thus releasing the stored gas and inflating the floats. 

The liferaft release mechanism handles were in three locations. One inside the cockpit on the centre console and two outside the cockpit on the landing gear skid fwd cross tube on the LH and RH sides.

Activating a lever handle activated the discharge valve of the gas reservoir through a cable, thus releasing the stored gas and inflating the liferafts.

Each release mechanism simultaneously inflated both liferafts.

Figure 8. The floats and liferafts packed and installed on the landing gear skid.

Figure 9. All floats and liferafts inflated

Figure 10. Gas reservoirs and hose lines on the belly of the helicopter.

Figure 11. Aft float with inlet valves and interconnect hoses. Fwd and mid floats only had two inlet valves.

The pilot wore ordinary clothing.

The pilot’s feet and legs got wet during the evacuation.

The pilot used the shoulder and lap seat belt harnesses.

Inflation of the yellow floats and liferafts improved the visibility of the helicopter from the air.

The onboard ELT was not under water.

Model ME406 - Part No: 453-6603 Rev: D.

The ME406 was a type AF (Automatic Fixed) beacon, transmitting on 121.5 and 406 mega -hertz (MHz).

The location of the ELT was in the cockpit on the LH side of the pedestal fwd and below the instrument panel.

The ELT was armed and did automatically activate during the sequence of events.

Each liferaft contained a PLB and a survival kit.

The sequence of events did not trigger activation of the PLBs.

At 16:16 hrs, the onboard ELT automatically activated the International Satellite System for Search and Rescue Services (COSPAS- SARSAT), relaying the distress signal to Nuuk FIC.

The pilot audibly noted the activation of the ELT.

Via Joint Arctic Command, Nuuk FIC scrambled a SAR helicopter from BGBW to the accident site.

Approximately 20 minutes after the accident occurred, Nuuk FIC directed a helicopter from the same operator from BGBW to the accident site.

The pilot of the company helicopter had difficulties locating the accident site, and due to low fuel reserves, had to stop the search after a short period of time.

At 17:17 hrs, the SAR helicopter started the search and located the accident site at 17:29 hrs.

At the time of rescue, the pilot was conscious but had a low body temperature.

The SAR helicopter flew the pilot to the hospital in the city of Qaqortoq.

The following day, the hospital released the pilot.

Parallel to the rescue operation by air, the local police in Narsaq started a SAR operation by boat from Narsaq harbour.

Onboard the boat was a doctor, a nurse, a police officer, and three fire fighters.

At 18:53 hrs, they located the accident site.

At a warehouse in Narsaq, a thorough technical engine inspection, with the engine installed on the helicopter revealed the following abnormalities:

• The compressor impeller could not be turned.
• Removal of the chip detector at the bottom of the engine accessory gearbox and at the engine accessory gearbox oil outlet to the oil cooler and engine oil tank revealed ferrous material.
• The engine oil hoselines from the engine accessory gearbox to the oil cooler/the engine oil tank and from the engine oil tank to the engine accessory gearbox did not contain any oil. However, the engine oil tank oil level did not indicate that oil was be added.
• The engine exhaust pipe had engine oil stains.
• Upon removal of the engine accessory gearbox chip detector at the bottom of the engine accessory gearbox, no engine oil came out.
• The first-stage turbine wheel had blade tip damages.
• A greenish powdery substance could be seen inside the accessory gearbox.

Removal of the complete engine from the helicopter for further technical inspections gave no rise to remarks.

An approved engine repair and overhaul facility performed a teardown inspection in cooperation with and under supervision of Rolls-Royce and the AIB.

Figure 15. The engine at the engine repair and overhaul facility.

Inspections with remarks:

• The outer combustion case, the inner combustion chamber and the fuel nozzle had no damages. However, the two latter displayed black soot.

Inspections without remarks:

• The external accessories on the engine accessory gearbox (rotor brake, freewheel unit, fuel control unit, fuel pump, power turbine governor) revealed no obvious abnormalities other than saltwater intrusion and corrosion.
• A flow check for obstructions with low pressure oil of the engine oil supply and scavenge lines for the engine bearings revealed no abnormalities. There were no blockages of the engine oil lines.
• The engine oil line filter screen at the T-fitting for bearing no. 6 and 7 was clean.

Turbine inspection:

• The drive shafts from the gas producer turbine and from the free turbine could not be turned.
• Upon removal of the bearing no. 8 oil sump cover, the thrust plate at bearing no. 8 lacked an anti-rotation tab.
• The outer race slot of bearing no. 8 had turned approximately 10° clockwise in relation to the gas producer support hub slot.

Figure 16. Bearing no. 8 outer race slot did not line up with the gas producer support hub slot.

• Upon installation of the thrust plate, the outer part of the anti-rotation tab fitted into the slot at the gas producer support hub.
• The inner part of the thrust plate anti-rotation tab fitted into the bearing no. 8 outer race slot.
• The anti-rotation tab, per design, prevented the outer race of the bearing from rotating.

Figure 17. Bearing no. 8 thrust plate. The LH side thrust plate found in the engine without an anti-rotation tab. The RH side thrust plate was a new thrust plate with the anti-rotation tab.

Figure 18. Angled new thrust plate showing the anti-rotation tab contour.

• The no. 8 bearing sump had a film of oil. The bearing looked normal but could not rotate.
• The blade tips of the first-stage turbine wheel were damaged.
• There were no damages to the second-stage turbine wheel, the nozzles or the gas producer hub.
• The power turbine support hub, which contained bearings no. 6 and 7, was corroded.
• Bearing no. 6 looked normal with intact rollers, but could not rotate.
• Bearing no. 7 looked normal, and the rollers appeared undamaged.
• An inspection of the third- and fourth-stage turbine wheels and nozzles gave no rise to remarks.
• Bearing no. 5 looked normal but could not rotate.

Figure 19. First stage turbine wheel with blade tip damages.

Compressor inspection:

• The compressor could not be turned.
• Bearing no. 2 was damaged.
• The impeller blade leading edges showed heavy material transfer from contact with the impeller shroud.
• Bearing no. 1 looked normal.

Figure 20. Damage to the impeller blade leading edges.

Figure 21. Impeller shroud rub marks caused by contact with the impeller blades.

Accessory gearbox inspection:

• The gas producer turbine and the power turbine input geartrains could not be rotated.
• The accessory gearbox was full of a greenish powdery substance, consistent with deterioration of the accessory gearbox magnesium housing after exposure to salt water.
• Bearings no. 2 ½, 3 and 4 were normal in appearance but could not rotate.
• A drive shaft coupling from the fuel control gearshaft to the oil pump gearshaft was fractured.

Figure 22. One half of the split opened accessory gearbox with a greenish powdery substance and a fractured drive shaft coupling to the oil pump gearshaft.

Figure 23. Opposite half of the accessory gearbox with the fractured drive shaft coupling to the oil pump input gearshaft.

 

• The oil pump input gearshaft could not be rotated. The oil pump was disassembled.
• The oil pump shaft and the pressure and scavenge gear teeth were intact.
• Within the scavenge cavity of the pump scavenging oil from the gas and power turbine bearings, a piece of debris lodged between the gear and the pump housing prevented rotation of the gear.

Figure 24. Engine oil scavenge pump disassembled with a piece of debris (gear had been displaced to show the piece of debris).

The engine manufacturer forwarded the thrust plate from the engine, without the anti-rotation tab and the piece of debris found in the scavenge pump, to a laboratory for further analysis.

The laboratory concluded that the chemistry and morphology of the piece of debris was consistent with the anti-rotation tab of the thrust plate at bearing no. 8.

The anti-rotation tab of the thrust plate was brazed on the thrust plate ring.

The laboratory discovered that the brazing of the lost anti-rotation tab to the thrust plate ring only had a coverage of 30%, which did not meet the specification requirement of minimal 80% coverage.

At a warehouse in Narsaq a thorough technical float inspection, with the floats installed on the helicopter, revealed the following findings:

• The float activation handle on the cyclic stick had been activated.
• All floats apart from the LH aft float had been inflated.
• All float gas supply lines and their connections from the gas reservoirs bottle to each float inlet valves were visually inspected without remarks, and all connections were tightened.
• Float and liferaft gas reservoir pressure gauges indicated zero pounds per square inch (psi), and the discharge valves were inspected without remarks.
• The RH fwd, mid, and aft float inlet valves and inlet valves interconnect hoses were inspected without remarks, and all connections were tightened.
• The LH fwd float inlet valves, including the interconnect hose, were inspected. The interconnect hose connection to the aft inlet valve were not properly tightened.
• The connection was one-sixth (1/6^th) of a turn loose, and torque witness marks did not line up. Upon removal of the interconnect hose from the inlet valve, several drops of water came out.
• There was salt debris in the aft inlet valve.

Figure 25.  LH fwd float interconnect hose connection to the aft inlet valve was not tightened.

Figure 26. Salt debris in the LH fwd float aft inlet valve.

• LH mid float inlet valves, including inlet valve interconnect hoses, were inspected without remarks, and all connections were tightened.
• The LH aft float, uninflated, was inspected including its gas supply line hose, without remarks. However, the supply hose was not straight and had bends.

Figure 27. Bends on the supply line to the LH aft float.

• The liferaft release handle at the pedestal in the cockpit was partly pulled.
• The external RH side liferaft release handle was fully pulled.

The float manufacturer service centre performed all inspections and testing in cooperation with and under supervision of the AIB.

• A visual inspection of the float assembly, including the packing, gave no rise to remarks.
• A visual inspection of the float inlet valves, including inlet valve interconnect hoses and supply line hose, gave no rise to remarks.
• All hose connections to the inlet valves were found tightened.
• All inlet valves were fully functional without any blockages.
• An internal check for blockages of all inlet valves, interconnect hoses, and supply hose, including port fitting, revealed no blockages.
• Approximately 1 millilitre (ml) of water came out of the aft inlet valve interconnect hose upon removal from the middle inlet valve.

Upon reinstallation and connection of all float parts, a float inflation was successful.

Figure 28. Approximately 1 millilitre (ml) of water came out when the inlet valve interconnect hose at the LH aft float was disconnected.

Figure 29. Fully inflated LH aft float during test.

The float manufacturer service centre performed the tests in close cooperation with the AIB.

The service centre received all parts that made up the complete float system of OY-HIO, except the supply hose to the RH aft float. For the following tests, the service centre used a  serviceable supply hose similar to that of the RH aft float. 

Upon receipt, the RH aft float from OY-HIO was punctured. For the following tests, the service centre used a similar, serviceable RH aft float.

The service centre performed two freezing tests of the complete float system to determine how much water was needed in the supply hose to the LH aft float to prevent it from inflating when frozen.

Prior to testing, the service centre loosened the LH fwd float inlet valve interconnect hose connection to the aft inlet valve by one sixth (1/6^th) of a turn from the tightened position.

Freezing test no. 1:

• They injected 15 ml of water into the LH aft float supply hose while the hose was slightly bent in a bucket.
• They stored the complete float system, including the supply hose, in a -21 °C freezing chamber for 24 hours.
• Upon removal of the complete float system from the freezing chamber, and upon removal of the LH aft supply hose from the bucket, they connected it to the complete float system.
• They activated the complete float system and all floats, except the LH aft float, inflated.

 
Figure 30. Test with 15 ml of frozen water in the LH supply hose. The LH aft float did not inflate.

Freezing test no. 2:

• The service centre performed a test similar to freezing test no. 1. However, they only injected 7 ml of water into the LH aft float supply hose.
• They activated the complete float system, and all floats inflated. However, the forward chamber of the LH aft float did not inflate, because of ice from the supply hose froze up the fwd inlet valve.

Figure 31. Second test with 7 ml of water in the LH supply hose. The LH aft float partially inflated.

Figure 32. LH aft float fwd inlet valve froze up by ice from the supply hose.

Water ingress test of the LH fwd float interconnect hose connection:

• The service centre loosened the LH fwd float inlet valve interconnect hose connection to the aft inlet valve one-sixth (1/6^th) of a turn from the tightened position.
• They submerged the LH fwd float, with the loose interconnect hose connection at the aft inlet valve with the connected supply hose, into water for 18 hours.
• Upon completion of the test, they disassembled the inlet valves, interconnect hoses, and supply hose.
• The test revealed no ingress water.

Water ingress test of the remaining floats:

• The service centre submerged the remaining floats with interconnect hoses, inlet valves, and supply hoses into a water tank for approximately 18 hours before checking for any water ingress.
• Upon completion of the test, they disassembled the inlet valves, interconnect hoses, and supply hoses.
• The test revealed no ingress water.

The float system maintenance requirement included inspections every 18 and 36 months according to the float manufacturer’s “Instruction for Continued Airworthiness” document.

For extracts of the revision AG of the float manufacturer’s “Instruction for Continued Airworthiness” document, see Appendix 4.