ATL94IA092

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 boa