Case Study #1 - Power Loss and Forced Landing
Transportation Safety Board Report Number A94A0180
Bell Helicopter Textron 206L
Long Ranger
Porcupine Point, Labrador
15 September 1994
Synopsis
About 12 minutes into the flight, the engine chip light illuminated. The pilot made a precautionary landing and shut down the engine to inspect the chip detector. Finding only a small quantity of metal paste (fuzz) on the forward facing chip detector, the pilot cleaned and reinstalled the chip detector before departing to continue the flight. Two minutes after take-off, a complete engine stoppage occurred. The pilot entered the helicopter into an autorotation. During the forced landing on a coastal flat, the front of the skid landing gear dug into the soft surface, and the main rotor struck and cut the tailboom. There were no injuries to any of the six occupants.
The Transportation Safety Board determined that the pilot incorrectly assessed the engine as airworthy and took off to continue the flight. The engine stopped two minutes after take-off when the No. 1 bearing failed as a result of separator and/or roller wear. The reason for the failure of the No. 1 bearing could not be determined. Contributing to this occurrence was the lack of adequate training for pilots on checking chip detectors, and the absence of any guidance on inspecting and assessing chip detectors in the flight operations manual.
Damage to Aircraft
The helicopter sustained substantial damage as a result of the main rotor striking and severing the tailboom. Also, prior to being recovered from the coastal flat, the helicopter was partially submerged in salt water when the tide came in.
Wreckage and Impact Information
The helicopter touched down with about 5 to 10 knots forward speed. At touchdown, the front of the bearpaw-equipped, low-skids landing gear dug into the soft surface, bringing the helicopter to a quick stop in a slight nose-low attitude. This caused the main rotor blades to rock fore and aft with sufficient deflection to contact and completely sever the tailboom.
An examination of the engine at the accident site revealed a mechanical lockup of the N1 shafting system. The engine was removed and transported to the operator's facilities in Goose Bay, Labrador, where it was stripped down to three major sub-assemblies: compressor, gearbox, and turbine. The compressor front support was then removed and it was discovered that the No. 1 compressor bearing (part No. 23009609, serial No. MP00948) had failed.
Engine Gearbox Examination
The No. 1 bearing, the compressor front support, and the engine gearbox were shipped to the engine manufacturer's facilities for examination and testing. All work was carried out in the presence of a TSB investigator. The gearbox was fitted for a functional scavenge flow check by applying regulated oil pressure at the oil inlet port and observing flow at the oil outlet port while motoring the oil pump with the use of a 400 rpm speed gun at the oil pump drive gear. After approximately 35- 40 seconds of motoring, the oil pump gained prime and oil began to flow at a steady rate from the oil outlet port. The gearbox cover was then separated from the housing and both the N1 and N2 geartrains were visually inspected and rotated. All gear teeth and splines exhibited a normal wear pattern and rotation was noted to be free. The oil pump was then removed from the gearbox and subjected to a production unit bench test. The bench test was conducted in accordance with Assembly Inspection No. 073 and the oil pump exceeded all minimum test criteria. Only a visual examination and photographic documentation of the No. 1 bearing and the compressor front support were carried out at the manufacturer's facilities. These components were then shipped to the TSB Engineering Branch Laboratory for detailed examination.
Required Maintenance Following a Chip Light
The Allison 250-C20R Operations and Maintenance (O & M) Manual (ref. para 9.F, "Magnetic Plug Inspection," page 338) contains the following warning:
"If a magnetic plug warning light comes on during flight, land and inspect the magnetic plugs as soon as possible. This light is an indication of conditions which could cause engine failure". The O & M magnetic particles:
- Magnetic particles and debris, chips, flakes and slivers are possible indications of bearing or gear failure and/or abnormal wear within the engine.
- Chips or flakes exceeding 1/32 inch diameter or more than 4 slivers per event are not acceptable. In this case the engine is to be removed from service and sent to an approved Allison repair facility.
- Chips or flakes less than 1/32 inch diameter or less than four slivers per event are acceptable.
Fuzz falls under this last category and, as further described in the O & M manual, para 9.I, would require the following maintenance action to be performed after reinstallation of the magnetic plug:
1. Carry out a 30 minute ground run at power and observe engine operation and chip light prior to releasing the aircraft for flight. If the chip light illuminates during 30 minute ground run, remove engine from service.
2. If warning light does not illuminate during 30 minute ground run, inspect magnetic plugs for further accumulation of magnetic particles, debris, chips, flakes, and slivers. Clean and reinstall magnetic plugs.
3. If a warning light illuminates within the next eight operating hours following a 30 minute ground run and the cause is determined to be an accumulation of magnetic particles and debris (chips, flakes, or slivers) remove the engine from service.
Pilot Authority to Check Chip Plugs
In accordance with the Airworthiness Manual (AWM), chapter 575, appendix B (Note: This is 1994, pre CARs reference), pilots of commercial aircraft can be authorized to perform certain elementary maintenance tasks without a maintenance release certification. Prior to being authorized to perform any of the tasks, such persons must have performed the tasks under the direct supervision of an aircraft maintenance engineer (AME). Included in these tasks is the "checking and continuity checking of self sealing chip detectors." Accordingly, the operator's maintenance control manual (MCM) states that, coincident with the pilot's annual training, pilots will receive instruction from a company AME on the performance of these tasks.
Pilots employed by the company had a good understanding as to what are and what are not considered acceptable amounts of magnetic particles found on chip detectors. However, this knowledge appears to have been acquired through informal discussions with maintenance personnel. The pilot of the occurrence helicopter had not been briefed on the significance of recurring chip lights or of the requirement for 30-minute ground runs following inspection of chip detectors. The company flight operations manual (FOM), issued to all employees involved in aircraft operations, including flight crews, does not contain any guidance on checking chip detectors, nor is it required to by regulation. A search of the TSB occurrence database identified four other helicopter accidents where the incorrect assessment of airworthiness, after recurring engine chip lights, resulted in engine failure.
Flight Manual
The 206L flight manual (FM) indicates a lesser degree of urgency for response to engine chip lights than do the manuals for other models of the Bell 206 series helicopter. The 206L FM indicates that a pilot should "land as soon as practical" if an engine chip light illuminates in flight. The manual describes "land as soon as practical" to mean: "The landing site and duration of flight are at the discretion of the pilot. Extended flight beyond the nearest approved landing area is not recommended." All other models of the 206 series helicopter assign a more urgent level of response, i.e., "land as soon as possible," which the manual interprets as: "Land without delay at the nearest suitable area (i.e., open field) at which a safe approach and landing is reasonably assured."
Flight Manual - Chip Light Emergencies
There is a discrepancy between the flight manual for the 206L and the flight manuals for other Bell 206 series helicopters in that a lower level of urgency for response to chip light indications is assigned for the 206L. Since the 206L shares similar components with other Bell 206 series helicopters, and the consequences of an in-flight failure are the same, the less urgent response of landing "as soon as practical" seems inappropriate for 206L chip light indications.
Conclusions
It was found that:
- An in-flight engine stoppage occurred as a result of the failure of the compressor No. 1 bearing.
- The No. 1 bearing failed as a result of separator and/or roller wear. However, no metallurgical cause for the failure could be established because of the extensive mechanical damage.
- The pilot incorrectly assessed the engine as airworthy following a second engine chip light in less than eight operating hours, and took off to continue his flight.
- The pilot had not been briefed on the significance of recurring chip light indications or of the requirement to perform a 30-minute ground run following inspection of a chip detector.
- The company flight operations manual did not contain any guidance for pilots on checking chip detectors and making an assessment as to the engine's airworthiness.
- The pilot had not received any formal training on the checking of chip detectors as per the requirement in the company's maintenance control manual.
- The 206L flight manual indicates a lesser degree of urgency for response to chip light indications than do the manuals for other models of the Bell 206 series helicopter.
Causes
The pilot incorrectly assessed the engine as airworthy and took off to continue the flight. The engine stopped two minutes after take-off when the No. 1 bearing failed as a result of separator and/or roller wear. The reason for the failure of the No. 1 bearing could not be determined. Contributing to this occurrence was the lack of adequate training for pilots on checking chip detectors and the absence of any guidance on inspecting and assessing chip detectors in the flight operations manual.
Operator Safety Action Taken
Subsequent to the occurrence, the operator expanded its ground-training syllabus to include more detailed instructions on the checking of chip detectors. The operator has also indicated that the company flight operations manual will be amended to provide guidance and a field reference for pilots on the checking of chip detectors.
Manufacturer Safety Action Taken
The manufacturer is in the process of revising the 206L flight manual. The manual's procedural action for chip light emergencies will be amended from "land as soon as practical" to "land as soon as possible." This revision is expected to be completed and distributed to 206L operators in the summer of 1995.
Regulatory Safety Action Taken
In response to a TSB Advisory letter, Transport Canada indicated that regional inspectors have been advised to evaluate the training procedures in operators' maintenance control manuals and, during audits, to ensure that procedures are being followed. Transport Canada has also indicated that Airworthiness Manual Chapter 575 will be amended to the effect that personnel will be trained to check chip detectors and, where applicable, assess the airworthiness of the aircraft upon completion of the task.
This report concludes the Transportation Safety Board's investigation into this occurrence. Consequently, the Board authorized the release of this report on 04 April 1996.
Contributing Human Factors
The main factors contributing to this occurrence were as follows:
- Lack of Knowledge: The pilot incorrectly assessed the engine as airworthy. There was a lack of adequate training.
- Lack of Resources: Lack of adequate training for pilots on checking chip detectors, and the absence of any guidance on inspecting and assessing chip detectors in the flight operations manual.
- Lack of Communication: The pilot had not been briefed on the significance of recurring chip light indications, or of the requirement to perform a 30-minute ground run following inspection of a chip detector.