Rolls Royce 250 Series Gas Turbine Engine Failure Case Study
The following article is based on an investigation and analysis of an aircraft accident of one Hughes 369D helicopter (Hughes 500D) equipped with one Rolls-Royce 250-C20B gas turbine engine. I was retained by the defense council as an Expert Witness in this case which resulted in a fatality. Information that was used to support these conclusions are listed at the end of this article. A physical and lab examination was performed on the accident engine components at a metallurgy forensic lab. Note that names, dates, document numbers, etc. have been changed and/or X'ed out to protect the privacy of the involved parties.
The helicopter experienced a sudden and total engine failure while stationary in an out-of-ground-effect hover at approximately 120 feet above the ground. The pilot was performing an inspection of power lines and their support towers at the time of the engine failure. This sudden failure occurred without any warning and resulted in a total loss of power. A post-accident engine investigation performed by Rolls-Royce (RR), and overseen by the FAA, concluded that the cause of the failure was due to blade separation of the 2nd stage gas producer turbine wheel. It was determined that the blade separation was caused by high cycle fatigue (HCF) and over-temperature thermal stress. The HCF is attributed to a localized over-temperature condition (hot spot) that was evident by an area of complete burn-through to a section of the 1st stage turbine nozzle assembly around the 11 o'clock position. There was also evidence of an uneven flame pattern indicated by non-uniform discoloration of the combustion liner. This uneven flame distribution, which caused the thermal damage, was the direct result of a streaking fuel nozzle. These conclusions of the turbine failure were corroborated by a very highly regarded independent Expert specializing in engine metallurgy failure analysis.
Fuel Nozzle Discussion
The fuel nozzle streaking was caused by carbon build-up near the nozzle tip which disrupted the atomized spray pattern cone. This disruption caused a stream of non-atomized raw liquid fuel to enter the combustor. This concentration of raw fuel burned in a localized area near the 11 o'clock position of the 1st stage nozzle at temperatures in excess of the safe operating limits of the combustion section (hot section). This localized over-temperature condition was undetected and was allowed to continue to the point that the internal components of the engine hot section were permanently damaged resulting in failure.
The fuel nozzle streaking had developed sometime within the last 235 flight hours and had continued to streak without detection. This condition went undetected due to the fact that the only reliable method to determine that a fuel nozzle is functioning properly is to perform the fuel nozzle maintenance per the RR maintenance manual, which had not been accomplished.
The engine manufacturer (RR) has developed and refined a maintenance program based on the production of over 30,000 RR 250 series engines with a production run spanning 50 years. The 250 series engine has accumulated an operating history of over 200 million fleet flight hours to date. This maintenance program contains a detailed checklist in the RR 250-C20 Series Operation and Maintenance Manual (OMM), chapter 72-00-00, Table 602 titled, “Scheduled Inspections.” The RR 250-C20 Series OMM states, “Scheduled inspections are made at periodic intervals in an effort to prevent engine malfunction and serve in the role of preventative maintenance for the engine.” This inspection checklist details the actions required to be performed during each inspection interval. One of the items contained within the “100 Hour Inspection” section is 21.A and states, “Remove, inspect and clean the fuel nozzle,” and, “NOTE: Operators may find it necessary to inspect and clean the fuel nozzle more often depending on past experience or operating conditions.”
This engine utilizes only one fuel nozzle and therefore it is an extremely critical component. The 100 hour inspection requirement of the fuel nozzle has developed over the years through field experience and is considered the absolute maximum number of flight hours allowable between inspections. The OMM chapter 73-10-03 states “NOTE: Due to variation in fuels and operating conditions, fuel nozzle cleaning may be necessary at more frequent intervals then stated in Table 602, 72-00-00, Engine – Inspection/Check, to maintain proper combustion flame pattern.” The critically important nature of this fuel nozzle maintenance cannot be overstated.
Maintenance Records Investigation
The accident helicopter was operated and maintained by (name withheld, hereafter referred to as the Company). A thorough investigation of the helicopter's maintenance records revealed that the 100 hour fuel nozzle maintenance was not being performed as required by the engine manufacturer. The records indicate that the last documented fuel nozzle maintenance was performed 234.9 flight hours prior to the accident. The records also indicate that over the last 1097 flight hours, the fuel nozzle maintenance had only been performed two times (ref. Figure 1). RR requires that the fuel nozzle be removed, inspected and cleaned at intervals not to exceed 100 hours and that the operator should consider even shorter time intervals depending on their past experience or operating conditions.
There is considerable evidence contained in the Company maintenance records, wherein the information contained in one record conflicts with another record of the same inspection and date. Also, the checklists that were used to perform maintenance were inconsistent. After examining the records of 11 of the last 100 hour inspections, dating over a one year period, it was apparent that the maintenance personnel had been using incorrect/outdated checklists during most of those inspections (ref. Figure 1). Only on two occasions had the correct checklists been used. The correct checklist that should have been used is contained in the OMM chapter 72-00-00 Table 602 with a revision date of June 1, 2004 and required that the fuel nozzle be maintained each 100 hours. The use of any other prior dated checklist after the effective date of the June 1, 2004 checklist revision, would not be approved or acceptable.
Multiple maintenance records contained conflicting information. For example, on some of the Company forms titled “Approval for the Return to Service,” the maintenance personnel signed off that a specific inspection was complied with. However, on the checklist used to perform that maintenance, that specific inspection item is not shown to have been complied with. Therefore these two records are in conflict with one another.
Another issue is that it is unclear as to what type of maintenance inspection program the Company was actually using to comply with the airworthiness requirement set forth by the FARs. The records reviewed indicated that they were using the Manufacturer's Inspection Program including the checklists contained within those programs. The Company's “Approval for Return to Service” form contains a printed statement as follows, “This aircraft is in a 100 Hour / Annual / Manufacturer's Inspection Program CFR 43.15 (b), (c)(1)(3).” However, a letter addressed to the NTSB from a Company representative states, “the Company, at that time, was not performing its maintenance under that program, nor was it required to.” This was in reference to the manufacturer's program. He signed with the title, Director of Safety/Regulatory Compliance. His statement is in direct conflict with the Company's maintenance records.
The fact that the Company did not adhere to the RR maintenance program, used outdated documentation to perform maintenance, and that their own maintenance records are in conflict with each other, are all indicators that the Company's maintenance was substandard, negligent, inconsistent with industry standards, and compromised the safety of flight and rendered it unreasonably dangerous. The Company's maintenance records reflect a standard of care that breached the aviation industry's level of acceptance, manufacturer's specifications and violated applicable FARs.
100 Hour Inspection Maintenance Records
A. Total flight hours at time of inspection.
C. Documented fuel nozzle maintenance recorded on the RR checklist.
D. Documented fuel nozzle maintenance recorded on the Company's return to service form.
E. Checklist used to perform the inspection. This is contained in the RR OMM Table 602. The new Table 602 has a revision date of June 1, 2004. Columns C & D show conflicting information. Column E shows outdated checklist being used.
Fuel nozzle removal, inspection and cleaning is the only way to detect a condition that could lead to or cause fuel streaking and it is critical since there is no other way to determine a localized over-temperature condition. An early detection of a streaking fuel nozzle can preclude irreversible internal engine component damage. Had the fuel nozzle maintenance been performed in accordance with RR procedures, the malfunctioning fuel nozzle would have been discovered and the problem corrected. It is highly likely that an early discovery of such a condition would have prevented hot section damage and turbine failure. This discovery could have alerted the operator that it would be prudent to perform a simple borescope inspection of the hot section to determine if any signs of thermal stress/damage had occurred due to the streaking fuel nozzle. If any signs of thermal damage were discovered at this early stage, the procedure would be to perform a hot section inspection in accordance with RR instructions to determine the extent of the damage prior to further flight. This procedure would prevent an in-flight engine failure due to thermal damage.
The Company's Operations and Risk Assessment
The Company performs aerial power line inspection, maintenance and construction for the electrical utility industry utilizing helicopters. Typical day-to-day operations performed by the Company
require that the helicopter and its occupants be exposed to extreme risk for a majority of the flight profile. The helicopter spends most of its flight time operating outside of the established safe autorotation envelope contained in the helicopter's height/velocity (H/V) curve. An autorotation is a descending maneuver, or power-off glide, which allows the helicopter to safely land in the event of a loss of power. The H/V curve is established by the helicopter manufacturer and is depicted by a graphical chart which illustrates which combinations of altitude and airspeed allow a safe autorotational landing in the event of engine failure. Operations outside the safe area of the chart expose the helicopter's occupants to extreme risk in the event of an engine failure. This is due to the fact that it is unlikely that the pilot will be able to accomplish a safe autorotation under this set of conditions following an engine failure, as demonstrated in this fatal accident. This area is commonly referred to as the “dead mans curve.” At the time of the engine failure, the helicopter was in a stationary hover out-of-ground-effect at 120 feet above the ground. This flight profile put the helicopter in the “dead man's curve.”
In light of this fact, an operator should understand these risks and elect to perform it's maintenance above and beyond the minimum recommended standards. The Company was very aware of these risks, yet they have demonstrated that the standard of care they used regarding their maintenance is well below that of the industry's acceptable standard of safety.
The Company had posted a video titled “Company is Seeking Pilots and Aerial Linemen.” In this video the President, states “You're taking two very dangerous things, electricity and hovering in the height velocity curve, dead mans curve, most of your career,” and, “Ninety percent of his (pilot) time is sitting in one spot (hovering).”
Federal Aviation Regulations (FAR) Compliance Requirements
An investigation of the helicopter maintenance records shows that the Company was using the airframe and engine manufacturer's maintenance programs. These maintenance programs allow the helicopter operator to maintain the airworthiness status of the helicopter on a continuing basis, provided the programs are strictly adhered to. The RR engine maintenance program was used to maintain the engine, however, the scope and detail contained in the scheduled inspections in the OMM chapter 72-00-00 Table 602 of that program was not strictly adhered to. This table contains the Inspection Checklist for each type of inspection, IE, 100 Hour Inspection, 200 Hour Inspection, etc.
As shown in Figure 1 above, the Company was either using an incorrect engine checklist or none at all for a majority of the 100 hour inspections. On only two inspections were the correct checklists used. The correct checklist required the fuel nozzle to be inspected each 100 hours whereas the incorrect and outdated checklist did not have this requirement. It was the use of this incorrect checklist that caused the maintenance personnel to overlook this critically important inspection item. The correct 100 hour checklist contained this fuel nozzle inspection, therefore, if the Company had been using the correct checklist, maintenance personnel would have realized that the fuel nozzle inspection was a part of the required inspection items.
According to the Federal Aviation Administration (FAA) such an oversight would be considered an issue of non-compliance with the manufacturer's maintenance program, and as such would constitute a violation of the FARs and breach the aviation industry's level of acceptance. This state of non-compliance would render the helicopter unairworthy due to the fact that the maintenance program was not being adhered to as specified by the scope and detail contained within that program.
The FAA has established the FARs to regulate and guide the operator to promote flight safety. Therefore, the FARs are material to flight safety. Non-compliance with the FARs materially affects the safety of flight since such a violation would have a negative impact on that which is relevant and significant to flight safety. Compliance with the FARs is the absolute minimum standard of care that an operator must abide by.
It is not clear at this time whether the Company was required to maintain the helicopter under FAR Part 91 or Part 135. This is due to the fact that this information has been requested but has not yet been produced. If the helicopter had been maintained under Part 91, the Company was in violation of Part 91 Subpart E, 91.409 (e). If the helicopter had been maintained under Part 135, the company was in violation of Part 91 Subpart E, 91.409 (c) (2) and Part 135 Subpart J, 135.421 (a) & (b).
Engine Overhauler (name withheld) Work Performed
Research of the engine maintenance records shows that the Engine Overhauler had performed an overhaul of the fuel nozzle, part number 23077068, and that the Company installed this fuel nozzle on the helicopter after the overhaul. Research of the Engine Overhauler's records of work performed on this fuel nozzle indicate that they did a functional test as received. The test indicated that voids and streaks were noted at all PSIs prior to any work being performed. They completed the overhaul per the Rolls-Royce Overhaul Manual 10W3, Edition 4 Revision 1, dated August 15, 2004, and complied with AD 2004-24-09, CEB A-1394 R1, incorporated MOD PMI G0005 and installed a new filter assembly, part number 139968, per Service Bulletin 1394. After completion of the overhaul, the fuel nozzle was tested in accordance with the Overhaul Manual to overhaul limits and the results were documented on the “Fuel Nozzle Calibration Record” form. This record indicates that the fuel nozzle had a final functional test performed after the overhaul to verify that it met all the overhaul limits as specified in the Overhaul Manual. The records show that the fuel nozzle met all the required parameters and did not exhibit any hysteresis or spray pattern anomalies.
The Engine Overhauler's procedures and records regarding the fuel nozzle overhaul are well documented, consistent with the regulations, and the standard of care exceeded the industry norm. There were no discrepancies, conflicts, irregularities or errors contained in those documents. Based on these observations, the Engine Overhauler performed the overhaul in compliance with both the FARs and the manufacturer's mandatory overhaul procedures and standards, and without any wrongdoing.
I have performed a thorough investigation and review of the documentation, and inspection of the accident helicopter engine components. As an expert in aircraft maintenance and operations, within a reasonable degree of certainty, my professional opinions are:
1. The Company, as operator of the helicopter, had the duty to maintain that helicopter in accordance with the FARs and industry standards, but did not do so, failing to fulfill their duty as an operator and breaching an acceptable standard of care,
2. The Company did not maintain the helicopter's maintenance records in accordance with the FARs and industry standards,
3. The Company's substandard maintenance and record keeping, along with a disregard to follow the manufacturer's maintenance program, demonstrated a lack of standard of care and is inconsistent with aviation industry standards,
4. The Company substandard approach to maintenance led to the use of incorrect inspection checklists, which in turn led to the critical inspection of the fuel nozzle being overlooked,
5. The lack of proper fuel nozzle maintenance by The Company led to fuel nozzle streaking,
6. This streaking was allowed to go undetected due to the fact that the fuel nozzle was not being removed and inspected at each mandated 100 hour interval, thereby causing an over-temperature condition within the hot section. The over-temperature condition caused thermal stress and high cycle fatigue (HCF) of the 2nd stage turbine wheel blade which led to the sudden engine stoppage. This inspection was critical since there is no other way to determine a localized over-temperature condition,
Items 1 through 6 above caused the engine failure which led to the fatal accident.
It is always a sad day when we have to look at a fatal aircraft accident to again be reminded of the obvious: Preventative maintenance is a very valuable tool and one that should be taken seriously. This accident should have never happened. It is my opinion that had the manufacturer's maintenance instructions been carried out, to the letter, this engine failure would never had happen.
I am qualified to review, analyze and offer my expert opinion regarding this accident based on the following experience:
I have 37 years of professional experience working in the aviation industry as both a pilot and as a mechanic. I have hands-on experience with the Rolls-Royce 250 series gas turbine engine, both from the operations side as a commercial helicopter pilot and from the maintenance side as a mechanic and Chief Inspector. These operations were conducted under FAR part 91, 133, 135, and 145 Repair Station. I hold an Airline Transport Pilot (ATP) pilot certificate, Airframe and Powerplant (A&P) and Inspection Authorization (IA) Mechanics certificates. This combined experience qualifies me to render expert opinion in this case.