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On November 17, 1995, approximately 0845 central standard time, a Saab 340B, N344SB, experienced a total power loss on the left engine and power fluctuations on the right engine during descent. An emergency landing was made in Corsicana, Texas. There were no injuries to the 3 crew members and 30 passengers and the airplane was not damaged. The airplane was being operated by Simmons Airlines as American Eagle Flight 3570 under Title 14 CFR Part 121. The flight departed Corpus Christi, Texas, at 0720 and the intended destination was Dallas/Fort Worth Airport, Texas. Instrument meteorological conditions prevailed en route and an IFR flight plan was filed. At the time of the power loss, the airplane had descended into visual meteorological conditions.
According to the flight crew, they had flown in light precipitation at 17,000 feet MSL for approximately 45 minutes and had accumulated "trace ice on the wing leading edges." During descent, the airplane exited the cloud layer and "one or two minutes" later the left engine "abruptly" lost power. The right engine began to "surge and a popping noise was heard." The flight attendant informed the crew that "flames" were coming out the right engine exhaust. According to the Fort Worth Air Route Traffic Control Center, at 0844:46, the pilot declared an emergency, and at 0847:42, made a request to divert to the nearest airport.
The attached cockpit voice recorder (CVR) transcript begins at 0843:16 as the airplane was descending through an altitude of "fifteen eight for one three thousand." At 0844:00, the captain stated that an engine "just failed," and at 0844:23, the flight crew initiated the engine shutdown procedure. Immediately following the captain's statement (at 0845:55) that the propeller "has feathered," the first officer remarked (at 0845:56) that the remaining engine was "starting to surge." During a transmission to air traffic control, at 0848:57, the captain reported "numerous power surges" of the right engine were occurring.
At 0851:01, approximately 5 minutes prior to touchdown, the captain started to read the single engine landing checklist. At 0855:22, the captain read the final items in the checklist including "prop sync's off." An uneventful landing was made, about 0856, in Corsicana. After the airplane was parked, the flight crew had a conversation during which the first officer remarked that he thought the left engine "ingested a bunch of ice" and both crew members stated "there was no auto ignition."
The Saab 340B is equipped with a limited authority propeller synchrophaser system. The system uses the left propeller as the master, while the right propeller is the slave. When the synchrophaser is engaged, the speed of the right propeller is adjusted to match the speed of the left propeller. The synchrophaser's authority to adjust propeller speed is limited to changes of +/- 30 rpm with corresponding torque variations of +/- 3 percent.
A review of the operator's Quick Reference Handbook for the Saab 340B revealed that instructions to turn the synchrophaser "off" were not included in the engine failure checklist. Instructions to turn the synchrophaser "off" were included in the single engine landing checklist as one of the final items before landing.
The digital flight data recorder (DFDR) and the cockpit voice recorder (CVR) were removed from the airplane and sent to the National Transportation Safety Board for readout. A CVR Group Chairman's Report and a DFDR Factual are attached to this report.
According to the DFDR data, both cockpit engine anti-ice switches were placed in the "on" position approximately 8 minutes after takeoff, at 9,952 feet MSL, with an outside air temperature (OAT) of 5 degrees Celsius. About 57 minutes later, at 15,232 feet MSL, with an OAT of -7 degrees Celsius, the left engine lost power. Approximately 3 minutes later, the right engine experienced a power interruption, during which all parameters associated with the engine fluctuated for about 2 seconds. At the time of the right engine power interruption, the airplane was at 11,200 feet MSL and the OAT was 2 degrees Celsius.
In addition to the right engine power interruption, the DFDR recorded approximately 30 fluctuations confined to right engine torque and propeller rpm parameters. These fluctuations began when the left propeller stopped rotating and continued for about 6 minutes. The fluctuations consisted of propeller speed decreases of approximately 30 rpm accompanied by torque increases of approximately 3 percent. During these fluctuations, the core engine parameters (compressor speed, compressor discharge pressure, and turbine temperature) remained stable.
TESTS AND RESEARCH
Borescope examination of both engines by the operator's maintenance personnel revealed no physical evidence of foreign object damage. Engine ground runs were conducted. The right engine performed within specifications. The left engine auto-ignition and overspeed protection systems were found to be inoperative. These systems are controlled via the Digital Electronic Control Unit (DECU.) After replacement of the left DECU, the left engine performed within specifications. According to the operator, the left DECU had accumulated 4,543 hours total time and 5,151 cycles at the time of the incident. The most recent functional check of the auto-ignition system had been accomplished 69 hours prior to the incident, during the airplane's last "A" check, on November 6, 1995. "A" checks are conducted at intervals of 400 hours.
Examination and testing of the DECU at the engine manufacturer's facility revealed that a fuse in the unit was open. The function of this 1 amp fuse is to protect the power supply unit which provides power to the auto-ignition circuit. Without supply voltage for the auto-ignition logic, the DECU was not able to command the auto-ignition relay to energize. Further examination of the fuse disclosed evidence that the fuse element became discontinuous "due to Joule heating" and "the current level that caused final melting was relatively low." For additional details of the DECU examination see the attached Systems Group Chairman's Factual Report.
A flight test was conducted by the airframe manufacturer on January 17, 1996, in order to determine "the results of single engine operation with the synchrophaser engaged." Testing was conducted in a Saab 340B production aircraft. With the synchrophaser engaged, the left engine was shut down in flight and the propeller feathered. Intermittent drops of approximately 20 rpm in right propeller speed together with corresponding peaks in right engine torque occurred. The manufacturer's flight test report stated that "a change in the frequency of the internal noise induced by a sudden drop in the propeller speed will be apparent to the crew and could be mis-interpreted as pops."
On June 27, 1984, the Saab 340A was issued a type certificate by the FAA. In December 1985, following five occurrences of engine flameouts in icing conditions, the FAA issued Telegraphic Airworthiness Directive (AD) T85-25-52 prohibiting revenue operation of the Saab 340A in icing conditions. Research by the manufacturer determined that the flameouts were caused by the ingestion of ice/slush into the engine compressor. On August 18, 1986, the AD was amended to permit operations in icing conditions provided continuous ignition was selected, and further amended on June 17, 1988, to allow the use of a newly developed auto-ignition system as an equivalent alternative to continuous ignition.
On August 15, 1989, the Saab 340B, a derivative of the Saab 340A, was added to the existing type certificate. The major difference between the two models is the substitution of the General Electric (GE) CT7-9B engine for the CT7-5A used on the 340A. An auto-ignition system is standard equipment on the Saab 340B.
Service Difficulty Reports for both the Saab 340A and Saab 340B were obtained from the FAA for the period January 1986 through November 30, 1995. Ninety one records identified as involving engine flameout, engine power loss, or engine stoppage were presented. Three records reported a power loss associated with in-flight icing conditions in the Saab 340A, and four records reported the same circumstances in the Saab 340B.
The Safety Board has investigated one other incident in which a Saab 340B experienced multiple engine power interruptions while operating in light rime icing conditions. For details of this incident, refer to the NTSB report styled: ATL-94-I-A092, Nashville, Tennessee, May 3, 1994, Saab 340B, N245AE.
Saab reported that their records indicated the Saab 340B fleet had experienced 475 power interruptions worldwide during the period from September 1989 to October 1995. A power interruption is defined as a "1 to 3 second duration reduction in power manifested by sudden reduction and immediate recovery of torque, compressor speed, and compressor discharge pressure, occasionally accompanied by noise and/or flash from exhaust."
In late 1992, a joint Saab/GE team was formed for a "reevaluation of the power interruption phenomenon." The team's actions included flight testing in icing conditions, engine altitude cell testing, and inlet icing tunnel tests. General Electric presented the team's findings to the FAA on May 19, 1994. The tests indicated that, while the engine inlet frame and inlet guide vanes were adequately anti-iced (heated), the inlet protective device or "bird catcher" collected snow/slush "under certain mixed conditions and in snow." Ice/slush was observed to build up on the rear wall of the "bird catcher" and shed from the "splitter lip" into the engine flow path.
According to the type certificate data sheet for the Saab 340A and Saab 340B, the certification basis of the aircraft includes Title 14 CFR Part 25. Title 14 CFR Part 25.1093b contains the design criteria for air induction anti-ice systems. It states, in part:
Induction system icing protection. Turbine engines.
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-.
On December 4, 1995, the investigator-in-charge attended a briefing on the status of the Saab 340 Power Interruption Improvement Program given by Saab Aircraft to the FAA. Saab reported that a modified inlet design utilizing increased electrical heat in the splitter lip area "gave promising results" during additional inlet ice tunnel testing performed in February and March 1995. They further reported that a Controlled Service Introduction Program (CSIP) is in progress to evaluate the effectiveness of the modified inlet in reducing power interruptions. The documented plan calls for 42 modified inlets to be installed and flown on revenue service aircraft over the next two years.
On January 23, 1996, the FAA issued AD 96-01-04, applicable to both the Saab 340A and Saab 340B, "to prevent failure of the auto-ignition system to re-light the engine in the event of power interruptions due to ingestion of ice and/or slush into the engine which could result in engine flameout and subsequent shutdown; and to provide the flight crew with guidance to aid in avoidance of such occurrences." The AD requires the flight crew to perform a functional check of the auto-ignition system on the Saab 340A prior to each flight and on the Saab 340B after each engine shutdown. As stated in the preamble, the AD was "prompted by a report" of the incident which is the subject of this report.
The airplane was released to the operator on November 20, 1995. All of the retained components were returned to the operator and the final wreckage release was signed on February 22, 1996.