Denver, CO, USA
N406CD
PIPER PA46 500TP
AIRFRAME ENGINE CONTROLS Before the engine was removed, an examination of the aircraft after the event revealed that all FCU control pressure (Py and P3) lines were intact, secured and leak-free. Additionally, the FCU control linkage and cabling from cockpit quadrant to the power lever (PLA) and the MOR lever were connected and operated smoothly and with full range of travel. The MOR linkage was further examined and no rigging errors were detected. Findings: No engine control rigging errors on the airframe were detected. ENGINE On September 16-18, 2014, the engine was examined at the P&WC facility in Montreal, Canada. Findings: Examination of the engine and accessory components revealed no anomalies that would have contributed to the reported event. GARMIN DATA A review of the data from the Garmin onboard readout device revealed that during the last taxi, the engine was allowed to decay to a sub-idle condition with gas generator speed (Ng) approximately 39% (normal idle Ng is 64%) and ITT 830° Celsius (°C) when Ng increased to approximately 47% and the ITT almost to 1300° C, corresponding to the activation of the MOR. According to the P&WC manuals, the maximum operating turbine temperature limit for takeoff is 800°C while the maximum allowable transient (limited to only 5 seconds) temperature during starting is 1000 °C. Turbine engines, at idle, require a minimum speed to operate. When operating below this speed, the compressor is operating in an inefficient manner, and cannot supply enough cooling air to the core components, causing a hot condition. When, in this already hot condition, an acceleration demand is made of the engine, excess fuel is injected into the combustor, further heating the core components, causing an overtemperature. High bleed air demands from the engine at idle can cause a decaying rpm condition. During this event, a hot day caused the pilot to increase air conditioning in the cabin which took considerable bleed air from the engine, which may have caused a decrease in RPM. To correct this decrease, the pilot must simply increase the power lever until idle speed is maintained. If the pilot does not pay attention to the idle RPM, and allows it to go to a sub-idle condition, a 'bog-down' may result and the fuel control will sense this and refuse to accelerate. The only option for the pilot is to shut the engine down and re-start. If the MOR is used at this time, an engine overtemperature and failure will likely result. Findings: The pilot did not pay attention to the engine indications and allowed the engine to 'bog down'. His subsequent use of the MOR caused the overtemperature and failure of the engine. Based on the pilots' statement, the Garmin readout data, and the lack of any anomalies in any of the engine accessories, it was concluded that the cause of the fire from the exhaust was due to the sudden introduction of fuel by the activation of the MOR, which along with the sub-idle speed condition of the engine at the time of the activation, resulted in a significant high temperature exposure of the CT blades, and their subsequent distress and failure of the engine. PIPER MERIDIAN POH MOR GUIDANCE The pilot stated that he believed that his use of the MOR was in accordance with the Piper pilot's operating handbook (POH). A review of the Piper POH Section 4 - Normal Procedures (Reference: Piper Report: VB-1689 – Revision June 4, 2013) revealed that the Piper guidance was contrary to the P&WC recommendations, which states "the emergency manual override system which is intended to be used in the event of a loss of Power Lever (PLA) control due to loss of air pressure to the Fuel Control Unit (FCU) during flight.", Piper guidance allows pilots to use the MOR on the ground, even at sub-idle RPM conditions, and further, gives the impression that reverting to the MOR is a normal procedure rather than an emergency procedure. Findings: A review of the Piper POH revealed an error in the guidance for MOR operation.
HISTORY OF FLIGHT On July 7, 2014, a Piper Meridian PA-46-500T airplane, Registration Number N406CD, was holding short of the runway, ready for departure at Denver Centennial Airport, Colorado, when the engine, a Pratt & Whitney (P&WC) PT6A-42 failed to accelerate after two attempts of the pilots' power lever commands. After calling the tower and cancelling his takeoff clearance, the pilot planned to taxi away from the 'hold short line' and elected to use the fuel control unit (FCU) emergency manual over-ride (MOR) lever to increase the engine power. The sudden excess fuel in the combustor resulted in a turbine over-temperature. The pilot stated that as soon as he took the MOR lever out of detent, he heard a bang and saw a big ball of fire at the nose of the airplane and believing the airplane was on fire, he retarded all the levers, shutting the engine down and exited the airplane. He noted several small grass fires in the field which, later, were determined to have been ignited by hot fragments of the over-heated turbine blades that exited the exhaust pipe. The pilot stated that he believed that his use of the MOR was in accordance with the Piper pilot's operating handbook (POH). He further stated that he did not believe that using the MOR lever was a cause of the engine over-temperature and failure since he did not even advance it, but that in retrospect, he would not have used it if he were to do it all over again. INITIAL ENGINE EXAMINATION The airplane was transferred to Tempus Aircraft, in Englewood, Colorado and on July 10, 2014 the engine, while it was still installed on the airplane, was examined for any possibilities of control system hardware failures or rigging errors. Distress and fracture of the engine's compressor and power turbines was confirmed and fragments of the power turbine were found in the exhaust plenum and ducts. The examination further revealed that all FCU control pressure (Py and P3) lines were intact, secured and leak-free. Further, the FCU control linkage and cabling from cockpit quadrant to the power lever (PL) and the MOR lever were connected and operated smoothly and with full range of travel. The MOR linkage was further examined for any rigging errors. The lever resided in the detent properly and a positive force was required to lift the knob over the gate and out of the detent to actuate. When out of the detent, the lever appeared to have no free-play and even required positive force to advance it. REVIEW OF AIRFRAME GARMIN DATA Data from the Garmin onboard readout device was downloaded and reviewed by the NTSB recorder laboratory. During the last taxi, the engine was observed to be in a decaying sub-idle condition with gas generator speed (Ng) approximately 39% (normal idle Ng is 64%) and inter-turbine temperature (ITT) 830° Celsius (°C) when Ng increased to approximately 47% and the ITT almost to 1300° C, corresponding to the activation of the MOR. According to the P&WC manuals, the maximum operating turbine temperature limit for takeoff is 800°C while the maximum allowable transient (limited to only 5 seconds) temperature during starting is 1000 °C. ENGINE TEARDOWN AND EXAMINATION On September 16-18, 2014, the engine was examined at the P&WC facility in Montreal, Canada. Examination of the engine accessories components revealed no anomalies that would have contributed to the reported event. Based on the pilots' statement, the Garmin readout data, and the lack of any anomalies in any of the engine accessories, it was concluded that the cause of the fire from the exhaust was due to the sudden introduction of fuel by the activation of the MOR, which along with the sub-idle speed condition of the engine at the time of the activation, resulted in a significant high temperature exposure of the compressor turbine (CT) blades, and their subsequent distress and that of the downstream hot section components. REVIEW OF THE PIPER MERIDIAN POH WITH RESPECT TO MOR GUIDANCE During this investigation, it was discovered that a MOR-triggered over-temperature event on another Piper Meridian had occurred just 2 weeks previously at the Denver airport. The pilot in this event perceived an FCU problem and decided that he could take off and fly home using the MOR. This engine suffered a similar over-temperature and compressor and power turbine failure. Similarly, the pilot of this prior event stated that he believed that the Piper POH guidance allowed for the use of the MOR on the ground and for non-emergency purposes which is contrary to the P&WC installation manual guidance. The investigation was concerned that there was an error or inconsistency in the Piper POH guidance for MOR operation and therefore a review and comparison for MOR guidance between the P&WC installation manual as well as the POH other airframe companies was made. Piper POH Review With Respect To MOR Usage A review of the Piper POH Section 4 - Normal Procedures (Reference: Piper Report: VB-1689 – Revision June 4, 2013) reveals the following guidance: "CAUTION - Isolated reports of no engine response to power lever movement have occurred during low engine power (Ng idle speed below 63%) and high engine accessory load operations in hot environments. The possibility of encountering this condition (referred to as "engine roll back") may be minimized by turning air conditioning and bleed air off before final landing approach. During ground and flight operations, if an engine roll back is detected, immediately perform the FUEL CONTROL UNIT FAILURE OR POWER LEVER CONTROL LOSS (Manual Override Operation) procedure in Section 3. Pilots should review this procedure in advance and be prepared to execute if required." Clearly, contrary to the P&WC guidance, which states "the emergency manual override system which is intended to be used in the event of a loss of Power Lever (PLA) control due to loss of air pressure to the Fuel Control Unit (FCU) during flight." the Piper guidance allows the pilot to use the MOR on the ground, even at sub-idle RPM conditions, and further, gives the impression that reverting to the MOR is a normal procedure rather than an emergency procedure. A Comparison With Other Airframe POHs A review and comparison between the POHs of the Piper Meridian, Cessna 208 and Pilatus PC-12 was made and significant variations and even conflicting differences in the instructions for the use of the MOR were noted. Because the Piper and Pilatus airplanes are both single engine, pressurized installations, they are good comparisons. The Piper Meridian uses a PT6A-42 engine while the Pilatus PC12 uses a PT6A-65B, different engines, however, the MOR system is very similar. The un-pressurized Cessna 208 airplane uses a PT6A-114 engine, also slightly different from the PT6A– 42, however, its FCU also features a MOR and P&WC Service Information Letter (SIL) No. PT6A-053 is applicable. See Service Information Letter Section below for more details. Significant differences were noted between the POH's with respect to MOR usage on the ground: Piper highlights the possibility of an 'engine roll back' during high bleed-air and electrical demand while taxiing or stationary and specifically recommends to use the MOR to recover from this condition. Piper does not state any minimum Ng limitations for MOR use, giving the pilot an option to use it at any speed below 63% Ng. The Piper guidance allows use of the MOR during non-emergency conditions. In comparison, Pilatus specifically prohibits any use of the MOR on ground, and additionally states that 'On the ground with no forward speed it is not possible to recover low Ng with the MOR lever'. Moreover, even while in flight, Pilatus does not allow any Ng excursions below 65% before using the MOR, a far more restrictive and safe guidance. The Cessna nomenclature for the MOR is emergency power lever (EPL), and the POH allows MOR/EPL usage only during flight and further restricts MOR usage below 65% Ng. The Cessna POH clearly states: 'Inappropriate use of the EMERGENCY POWER Lever may adversely affect engine operation and durability. Use of the EMERGENCY POWER Lever during normal operation of the power lever may result in engine surges, or exceeding the ITT, NG, and torque limits.' P&WC Installation Manual MOR Guidance Review: When an original equipment manufacturer (OEM) such as Piper installs an engine onto their airframe, it draws design, installation, performance and operation information about the engine from the installation manual (IM) document that is produced by the engine manufacturer; in this case - P&WC. Every engine type has its own specific installation manual. The IM is a technical manual intended for engineering staff rather than the end user. Guidance in the IM for the use of the FCU and the MOR are technically clearly stated, as are the caution or warning recommendations; however, the interpretation of the recommendations are left to the airframer. P&WC Service Information Letter (SIL) Guidance for Use of the MOR: An SIL is a document that contains clarifying technical or operational information for P&WC engines and is distributed to all end-users, operators and overhaul/repair centers. • SIL No. PT6A-053 Revision 3 - Emergency Power Lever (EPL)/Fuel Control Manual Override System - Issued on November 10, 2004 This is the only SIL which clarifies the usage of the MOR system; however the applicability is only for PT6A – 114/-114A engines. It states: "P&WC would like to reemphasize that the EPL system is intended 'for emergency purposes only' as outlined in the applicable Cessna pilots operating handbook POH and should be used accordingly" • SIL No. PT6A-053 Revision 4 - Emergency Power Lever (EPL)/Fuel Control Manual Override System was issued on September 23, 2014 in response to this investigation. The applicability of this SIL has been changed to 'all PT6A engines with fuel control manual override system'. Among the numerous cautions, it states: 'The EPL does not duplicate the function of the Power Lever Angle (PLA) and should not to be used as an optional means of controlling the engine during normal engine operations.' A Summary of the PT6A Customer Support Response Related to the Use of the EPL/MOR. P&WC Service Information Letter (SIL) PT6A-053 was revised on September 23, 2014 to expand the applicability of the document to include "All PT6A Engines with Fuel Control Unit Manual Override System". The key message in this revision was to highlight: "P&WC would like to re-emphasize that the Emergency Power Lever is intended "for emergency purposes only" during flight and is not intended for on-ground engine operation, as outlined in the applicable Pilot's Operating Handbook (POH) and should be used accordingly." During the Malibu Mirage Owner Pilot Association (MMOPA) Annual Convention on October 16, 2014 (approximately 70 operators of P&WC powered aircraft were in attendance), the topic was presented during a P&WC seminar presentation. Content included the revision of the SIL noted above and an overview of the relevant maintenance actions for operation of the EPL/MOR. The MOR issue was also presented during the Piper Meridian Owner`s Meeting during the Convention, which was attended by P&WC and Piper representatives. Discussion with this Operator Group indicated that there was previously a misunderstanding for the use of the EPL/MOR. There is an active bi-weekly teleconference between Piper and P&WC, where there is currently an open action item to discuss field actions in response to this issue. Following the MMOPA Convention and Meetings, current follow-on actions considered are as follows: • POH reference to ground usage of the EPL/MOR, amendments required. Specifically, the Caution under "Normal Procedures" may be interpreted that ground use not in an emergency is acceptable. • Piper release operator communication following P&WC SIL PT6A-053 revision. • Consideration regarding installation of "witness wire" to the EPL/MOR lever. • Piper has been made aware that the POH for the Meridian with respect to MOR usage is not reflective of the P&WC IM guidance, and is presently reviewing the document.
The pilots' incorrect activation of the Manual Override Lever, during ground operation, in an attempt to correct a sub-idle speed condition of the engine, resulting in an over-temperature of the CT blades, their subsequent distress and failure of the engine. Contributing to the incident was: The incorrect guidance of the Piper Meridian Pilots' Operating Handbook which, contrary to the engine manufacturer's recommendation, allowed the operation of the Manual Over ride Lever during ground operation.
Source: NTSB Aviation Accident Database
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