Saab 340B accident at New Roads, Louisiana, 1 Feb 1994

Accident narrative

On February 1, 1994, at 2140 central standard time, Simmons Airlines/American Eagle Flight 3641, a Saab 340B, sustained substantial damage during a forced landing at False River Air Park in New Roads, Louisiana. The airplane was operating a passenger flight when both engines lost power during the descent. Of the 23 passengers, two flightcrew, and one flight attendant on board, the flight attendant received a minor injury during the evacuation. There were no other injuries, and no postcrash fire occurred.

### The flight

Flight 3641 was a scheduled direct flight from Dallas/Fort Worth International Airport to Baton Rouge Airport, operating under visual meteorological conditions.

The captain was the flying pilot and the first officer was the nonflying pilot. The captain had accumulated over 20,000 total flight hours, with about 300 hours in the Saab 340. The first officer had approximately 6,500 total flying hours, including about 1,700 in the Saab 340B.

### The sequence of events

At 2114:25, while en route at flight level 210, air traffic control instructed the flight to descend at the pilot's discretion. During the descent, at 2120:09, the cockpit voice recorder captured a sound similar to the airspeed overspeed warning, which stopped about 13 seconds later. The captain remarked on the high speed, stating, "Man, we're almost the speed of heat here."

At 2125:16, the first officer checked in with Baton Rouge approach control, reporting they were descending. The controller cleared them for a visual approach to runway 13 and instructed them to descend. The captain announced to the passengers that they were about 35 miles from Baton Rouge and would land in about ten minutes.

At 2127:19, the captain noted, "A little bouncy bouncy here," and the first officer suggested they were in a cloud layer. At 2127:41, the captain said, "Yeah, we'll just...kinda slow this baby up a little bit." Five seconds later, the autopilot disconnect chime sounded.

At 2127:52, the cockpit voice recorder captured the sound of an increase in propeller and engine rpm frequency and amplitude. Four seconds later, a master caution warning chime sounded. The first officer asked, "What happened?" and the captain replied with an expletive. The first officer then stated, "Both engines flamed out...you've got an airport underneath you."

At 2128:43, the first officer broadcast a MAYDAY to Baton Rouge approach, reporting the loss of both engines and asking if there was an airport beneath them. The controller confirmed False River Air Park was below them with a 5,000-foot lighted runway.

The captain flew a circling, power-out descent to runway 18. The first officer lowered the landing gear using the hydraulic pump-override shortly before touchdown. The airplane touched down, became airborne again with about 1,600 feet of runway remaining, and touched down a second time 606 feet from the departure end. The airplane departed the end of the runway, traversed a grass overrun, a 25-foot-wide ditch, and a chain link and barbed wire fence before coming to rest upright in a sugar cane field 1,425 feet beyond the runway. The passengers and crew evacuated rapidly.

### What the investigation found

Investigators found no external physical damage to either engine and no evidence of an in-flight fire. However, internal examination revealed extensive and similar damage to the power turbine sections of both engines. The blade tip shrouds and 50 to 70 percent of the airfoil areas of the fourth stage power turbine blades were missing, and the third stage blades were also severely damaged.

Flight data recorder information showed that the power levers had remained near the flight idle stops for about one minute before the event. At 2127:43, the power levers began to move aft. By 2127:54, the levers had moved about four inches aft of the flight idle gate to positions aft of the ground idle detents, entering the beta range. The propeller rpm for both engines then rose rapidly to at or above the maximum recordable reading of 1,500 rpm. A sound spectrum analysis indicated that one propeller reached about 1,965 rpm and the other about 2,190 rpm, representing 142 percent and 158 percent of red line rpm, respectively.

The airplane flight manual prohibits moving the power levers below the flight idle stop while airborne. In flight, moving the levers into the beta range removes the governors' control of blade angle, allowing high airspeeds to windmill the propellers to high rotational overspeeds. The Board noted that the engine overspeed protection system functioned properly by cutting fuel flow, but because the propellers and power turbines were driven by air loads from the high airspeed, the overspeed condition continued until the turbine blades separated.

Examination of the engine controls revealed no mechanical failures that would have permitted inadvertent movement of the levers into the reverse range. To move the levers aft of the flight idle stops, spring-loaded triggers had to be lifted. The Board concluded that the captain manipulated the triggers and moved the power levers into the beta range. The Board believed the rate of movement was consistent with a deliberate action to increase propeller drag to slow the airplane, though the captain's actions may have been unintentional.

The Board found that the Saab 340B design met existing certification regulations, which did not require the power lever system to prevent intentional or inadvertent operation in the beta mode in flight. The Board noted that previous accidents involving in-flight beta operation on other turbopropeller airplanes had prompted safety recommendations for positive lockout systems, but the Federal Aviation Administration and the industry had been slow to act.

Additionally, the Board found that the flightcrew and flight attendant failed to prepare the passengers for the emergency landing, although they performed an effective evacuation once the airplane stopped.

### Probable cause

The National Transportation Safety Board determined that the probable causes of this accident were "the captain's movement of the power levers below flight idle in flight, the inadequate certification requirements and consequent design of the airplane's power levers that permitted them to be moved below the flight idle position into the beta range, either intentionally or inadvertently, while in flight, and the inadequate action taken to require a positive means to prevent beta operation on airplanes for which such operation is prohibited."