NASHVILLE, TN, USA
N245AE
Saab-Scania AB (Saab) S340B
THE ACFT WAS PARKED ON THE RAMP OVER NIGHT, UNCOVERED, IN CONTINOUS RAIN. TAKEOFF WAS MADE IN HARD RAIN. AFTER REACHING CRUISE FLIGHT (FL250), ENGINE POWER INTERRUPTIONS OCCURRED IN BOTH ENGINES. FLIGHT CONDITIONS WERE IMC, -20 DEG. OAT, AND LIGHT RIME ICING. ALSO, ALL AIRSPEED INDICATIONS DECREASED TO ZERO. A DESCENT WAS BEGUN, AND THE FLIGHT CREW DECLARED AN EMERGENCY. THE AIRPLANE WAS LANDED SAFELY AT ITS DESTINATION. THE INVESTIGATION DISCOVERED THAT ENGINE POWER INTERRUPTIONS HAVE OCCURRED IN LIGHT ICING CONDITIONS SINCE THE INTRODUCTION INTO REVENUE SERVICE OF THE ACFT. AD'S WERE ISSUED TO PROHIBIT REVENUE FLTS, THEN FLTS IN ICING CONDITIONS WITHOUT AN AUTO IGNITION. THE MANUFACTURER HAD DETERMINED THAT ICE/SLUSH COULD BUILD UP ON THE INLET PARTICLE SEPARATOR, BREAK OFF, AND ENTER THE ENGINE, RESULTING IN FLAMEOUT. THE ICE/SLUSH COULD NOT BE SEEN BY THE PILOTS, AND WAS NOT REDUCED BY ENGINE ANTI-ICE. AN AUTO IGNITION WAS ADDED THAT RESTARTED THE ENGINE WITHIN 2 SECONDS. ACFT CERTIFICATION RULES REQIRE THAT NO ACCUMULATION OF ENGINE INLET ICE OCCUR THAT WILL ADVERSELY AFFECT THE ENGINE OPERATION.
HISTORY OF FLIGHT On May 3, 1994, about 0603 central daylight time, a Saab S340B, N245AE, had inflight intermittent power interruptions of both engines, and all airspeed indications decreased to zero, while enroute to Nashville, Tennessee. The airplane was operated by Wings West Airlines, Inc., under the provisions of 14 CFR Part 135. An instrument flight rules flight plan was filed for the scheduled, domestic, passenger flight. Instrument meteorological conditions (IMC) prevailed. There were no injuries to the occupants, and the airplane was not damaged. Flight 5204 departed Baton Rouge, Louisiana, about 0512 for the flight to Nashville. The flight crew reported "power roll backs" of the left engine at Flight Level 250, while in IMC that included light rime icing. Shortly afterwards, all three airspeed indications decreased to zero. A descent was begun and the flight crew declared an emergency. The flight crew reported that during the descent, the right engine also exhibited power interruptions, The power roll backs continued in each engine, alternately, down to 11,000 feet. During the descent, the airspeed indications returned, however, the captain's indication remained about 40 knots below the first officer's airspeed indication. An uneventful landing was made, about 0705, at Nashville. The airplane had remained overnight at Baton Rouge, Louisiana. Wings West personnel reported that there were heavy thunderstorms and rain at Baton Rouge throughout the early morning. The five hourly reports from the Baton Rouge Automated Surface Observing System, prior to the departure of Flight 5204, indicated that there were rain showers, thundershowers, and gusting winds. Temperatures were not recorded. The flight crew reported that steady rain began at Baton Rouge shortly after passenger boarding, and continued throughout taxi and takeoff. Intermittent moderate rain showers were encountered during the terminal area departure and climb. According to the captain, the pitot heat was operating throughout the flight. The operator stated that protective covers for the various openings on the airplane are normally not used during idle, overnight periods. PERSONNEL INFORMATION The captain holds an airline transport pilot certificate with an airplane multi-engine rating. He is type-rated in the Saab S340. His last proficiency check, in the same make and model airplane, was conducted on April 19, 1994. He holds a first class medical certificate, with no limitations nor waivers, that was issued on January 13, 1994. The captain has 5,671 total flight hours with 1,171 flight hours in the Saab 340. According to the operator's report of the incident, the captain accrued 115.8 flight hours in the Saab 340 within the 90 days prior to the incident. He had flown 7.8 hours within the 24 hours prior to the incident flight. The first officer also holds an airline transport pilot certificate with an airplane multi-engine rating. His last proficiency check in the same make and model airplane was on September 11, 1993. His last medical certificate was issued on February 1, 1994, with no limitations nor waivers. The first officer had 4,440 total flight hours with 1,916 flight hours in the Saab 340. According to the operator's report of the incident, the first officer accrued 217 flight hours in the Saab 340 within the 90 days prior to the incident, and 7.9 flight hours within the 24 hours prior to the incident flight. AIRCRAFT INFORMATION N245AE, a Saab S340B, serial number 340B-245, was registered to Flagship Airlines, Inc. It was operated by Wings West Airlines, Inc. The airplane was maintained under the provisions of a Continuous Airworthiness inspection program. The last inspection was conducted on April 26, 1994, at 46.3 hours since the previous inspection. The airframe had 6,002.54 total flight hours. This airframe was equipped with the Cox engine air intake system. The system is configured the same as the Lucas air intake system, with which the airplane was initially certificated,except that the Cox system is made of composite materials. The airplane was powered by two General Electric CT7-9B turbopropeller engines. The left engine, serial number 785475, had 3,577.7 total hours, with 46.3 hours since the last inspection, The right engine, serial number 785319, had 5,622.1 total hours, also with 46.3 hours since the last inspection. The engines were equipped with an auto-ignition system. The auto-ignition system is operated via the Digital Engine Control Unit, and will activate engine ignition, with the cockpit ignition switch in the normal position, when the rate-of-change of the engine compressor speed exceeds a preset, threshold value. The airplane is equipped with three airspeed indicators. The captain's airspeed indicator receives its signals from the left pitot/static system through the air data computer. Airspeed signals to the co-pilot's indicator is raw data via the right pitot/static system. The pilot's standby airspeed indicator receives input from the left pitot system and an independent static chamber. METEOROLOGICAL INFORMATION The flight crew reported that at the time of the occurrence, the airplane was in instrument meteorological conditions with an outside air temperature of -20 degrees, Celsius. According to the captain, light rime ice had been deposited on the wings and propeller spinners. No ice was observed on the engine inlets. FLIGHT DATA RECORDER The digital flight data recorder (DFDR) was removed from the airplane and sent to the National Transportation Safety Board laboratory for readout. According to the data, both cockpit engine anti-ice switches were placed in the "on" position, during the climb, at 10,944 feet mean sea level (msl), at a static temperature of 3.8 degrees, Celsius. Both anti-ice switches were positioned "off" at 18,864 feet msl at -10.5 degrees Celsius, then back "on" again at 20,992 feet msl at -15.8 degrees Celsius. About 22 minutes later, at DFDR elapsed time of 0051:54, and 24,976 feet msl, all parameters associated with the left engine fluctuated between one and two seconds. Gas generator speed decreased from 94.2% to 84.0% in one second, then returned to 94.2% in two seconds. Four additional fluctuations of the left engine parameters occurred, all of similar, or less duration to the first. Two torque fluctuations were found associated with the right engine, but, the other engine parameter deviations were not characteristic of engine power interruptions. However, the flight crew statements, and the statement by the flight attendant, which referenced passenger verbal statements, indicated that more than one power interruption of the right engine occurred. The captain and the flight attendant referred to one power interruption from the right engine as the most noticeable. According to the flight attendant, one passenger reported that "he saw backfiring out of engine #2 (right engine) inflight." TESTS AND RESEARCH After the airplane arrived at Nashville, it was examined by the operator's maintenance personnel. The operator reported that less than 10 cubic centimeters of water was drained from the static system drain traps. The drain traps are required to be visually inspected each 800 flight hours. They were last inspected and drained, prior to the incident, on April 8, 1994. The operator reported that minimal fluid was drained on April 8. Maintenance personnel also discovered that the air data computer (ADC), which provides signals to the captain's airspeed indicator, was inoperative. The ADC was tested at the manufacturer's facility and found to have foreign material on the A4 printed circuit board. After the foreign material was removed, the ADC functioned normally. Additionally, the right generator control unit was found to be inoperative. Upon its removal and disassembly, corrosion was found on the WB4 printed circuit board. Corrosion control was performed on the circuit board, a hole under the data plate was sealed, and after re-assembly, the generator control unit functioned normally. After the incident, the flight crew entered discrepancies in the Aircraft Maintenance Log indicating that both engines had "several roll backs." Action taken by the operator's maintenance personnel was a ground run of each engine, in accordance with GE Aircraft Engines CT7 Turboprop Engine Maintenance Manual, 72-00-00, page 148 (attached). According to the maintenance log, no discrepancies with the engines were noted. A visual examination of the first stage compressor blades revealed that the left engine's first stage compressor blades were bright and shiny; the right engine's first stage compressor blades appeared gray and dirty. Service Difficulty Reports for both Saab S340A and Saab S340B aircraft were obtained from the Federal Aviation Administration (FAA), for the period January 1986 through April 5, 1994. Seventy records were presented. Four records reported a power loss associated with in-flight icing conditions in the Saab 340B, and three records reported the same circumstances in the Saab S340A. A list of CT7-9 engine in-flight power interruptions was provided from the General Electric Aircraft Engines event database for the time period from aircraft introduction into revenue service through July 1994. The event reports are limited to reports by GE field service representatives, for power interruptions experienced by United States and foreign air carriers which identify the event engine serial number. These events represent only a sample of fleet events. Forty-four reported power interruptions were associated with environmental icing conditions. Nine of the event reports specifically stated that the engine anti-ice system was "on," while the remainder of the reports made no comment regarding switch settings. Nine of the event reports indicated that engine damage was discovered during post-event maintenance inspections. Saab Aircraft AB produced a certification test report, Icing Protection, dated April 5, 1984. The certification testing about which this report was written, involved the original certification tests that were accomplished on the Saab S340A, fitted with the GE CT7-5 engine. The report contained the results of flight tests in natural icing conditions, and engine air intake anti-icing system dry air tests. Regarding the engine air inlet, the report stated "...after the lowest temperature condition test performed (test 3158) ice was found in the bird catcher, on both sides of the splitter and in the bottom....this corresponds to the reported results of the Lucas Icing Wind Tunnel Tests. As the bird catcher is meant to collect foreign objects it has been judged to be acceptable with some ice in the catcher." This airplane was certificated under 14 Code of Federal Regulations (CFR) Part 25. 14 CFR Part 25.1093 contains the requirements for Induction system icing protection. It states, in part, "(b) Turbine engines. (1) Each turbine engine must operate throughout the flight power range of the engine (including idling), without the accumulation of ice on the engine, inlet system components, or airframe components that would adversely affect engine operation or cause a serious loss of power or thrust-." The Saab S340A was issued a Type Certificate by the FAA on June 27, 1984, in accordance with the bilateral agreement between the United States and Sweden. In November 1985, Saab S340A/CT7-5A2 had engine flameouts in icing conditions, accompanied by ice FOD to stage 1 compressor airfoils. Emergency Telegraphic Airworthiness Directive (AD) T85-24-51, applicable to Saab-Fairchild model SF 340A series airplanes, was issued by the FAA on November 29, 1985. The AD related four reports of engine flame-out during cruise at 14,000 to 16,000 feet, at ambient temperatures between -5 degrees Celsius and +1 degree Celsius. The AD stated that ice ingestion could not be excluded as the cause of the flame-outs. The AD required the following: 1. When the previous flight was conducted in icing conditions, an inspection must be accomplished prior to each departure to assure that no snow, ice, or slush accumulation is present in or around the inlet and in the inlet protection device. 2. Maintain a propeller rpm of 1384 during all operation in icing conditions. 3. Revise the FAA Approved Flight Manual to change the temperature stated in icing conditions from 5 degrees to 10 degrees Celsius. The FAA issued Emergency Telegraphic Airworthiness Directive (AD)T85-25-52 on December 10, 1985. This AD, also applicable to the Saab-Fairchild Model SF-340A series airplanes, indicated that a fifth engine flame-out had occurred in ambient conditions conducive to icing. Ice ingestion was considered a potential cause. The AD imposed operating restrictions to prohibit all revenue flight, and non-revenue flight in IMC or known or forecasted icing conditions. The FAA issued AD 85-26-51, Amendment 39-5376 for the Saab-Fairchild SF-340A series airplanes on July 25, 1986. It was issued "To minimize the hazards associated with engine flameout due to potential ice ingestion...." The AD required the installation of a continuous ignition switch. Flight limitations were imposed by the AD, which required the use of continuous ignition during all operations in which icing could reasonably be expected to occur. On October 30, 1987, Saab-Scania petitioned the FAA for type certification of a derivative of the Saab SF-340A, designated the Saab S340B, as a revision to the existing type certification. Configuration Definition 72THS6596 Rev B, was attached to the petition and listed the changes in the Saab S340B as compared with the Saab SF340A. The Configuration Definition stated that the stabilizer deicing boots would be extended, and that there would "...be no other changes to ice protection including the engine intake protection system." The Saab S340B auto ignition system was changed to use the digital electrical control unit to activate engine ignition, with the cockpit ignition switch in the normal position, when the rate-of-change of the engine compressor speed exceeds a preset, threshold value. Saab Aircraft AB has stated that on the S340B, the ability to start the engine is the check of the auto-ignition system. It was further stated that the flight crew would be alerted to an auto ignition software failure by numerous indications in the cockpit. The indications that the flight crew would see erratic torque and propeller speed parameters. Operating Manual procedures instruct the crew to lock out the torque motor, but do not instruct the crew to activate continuous ignition. Operation of the auto ignition can also be confirmed via maintenance checks accomplished during ground maintenance. On March 9-10, 1988, the first familiarization meeting occurred among SAAB, GE, LFV, and FAA representatives, for FAA certification of the Saab S340B. The minutes of that meeting contained, as Attachment C, the Saab 340B Configuration Definition, 72THS6596 rev F, dated March 9, 1988. As noted above, the Configuration Definition stated that there would be no changes in the engine intake ice protection system. The minutes of the meeting stated that "The SAAB 340B was described as basically an engine change for the SF340A." A description of the new engine installation was provided by SAAB engineering Manager-Propulsion Systems. The minutes did not contain any discussion regarding the aircraft's history of power interruptions while operating in icing conditions. On June 17, 1988, Amendment 39-5916 to AD 85-26-51, Amendment 39-5376, became effective. It provided for placing the ignition switch in the "norm" position after accomplishing Modification 1414 "Ignition-Introduction of Auto Ignition System." The NPRM for Amendment 39-5916 stated in part "Since issuance of that AD (85-26-51), the manufacturer has developed an automatic ignition system which activates when compressor discharge pressure (P3) drops below 70 psi and the power levers are above flight idle." The NPRM also stated "Prior to ice ingestion, this system would be activated automatically so as to prevent ingestion of ice and resultant engine flameout. It would also prevent excessive replacement of ignitors
ENGINE ICE/SLUSH INGESTION THAT RESULTED IN POWER INTERRUPTIONS AND FAILURE OF THE AIR DATA COMPUTER COUPLED WITH MOISTURE INGESTION OF THE STATIC SYSTEM. FACTORS WERE THE FAILURE OF THE MANUFACTURER TO CORRECT A PREVIOUSLY IDENTIFIED DESIGN DEFICIENCY THAT PERMITTED THE ACCUMULATION OF ENGINE INLET ICE/SLUSH, WHICH COULD NOT BE OBSERVED BY THE FLIGHT CREW, NOR PREVENTED BY ON-BOARD ENGINE ANTI-ICE EQUIPMENT, THE LACK OF A PROACTIVE STANCE BY THE FAA WHEN INVOLVED WITH AIRCRAFT CERTIFICATION UNDER BI-LATERAL AGREEMENTS, AND THE INADEQUACY OF THE FEDERAL AVIATION REGULATIONS REGARDING ENGINE INLET ICE PROTECTION.
Source: NTSB Aviation Accident Database
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