Embraer 120 accident at Eagle Lake, Texas, 11 Sept 1991

Accident narrative

On September 11, 1991, about 1003 Central Daylight Time, Continental Express Flight 2574, an Embraer 120, experienced a structural breakup in flight and crashed in a cornfield near Eagle Lake, Texas. The 2 flight crewmembers, 1 cabin crewmember, and 11 passengers aboard the airplane were fatally injured.

### The flight

The flight, operating under the call sign "Jetlink 2574," departed Laredo International Airport, Texas, about 0909 for Houston Intercontinental Airport. The captain, 29, had accumulated approximately 4,243 total flying hours, of which 2,468 were in the Embraer 120. The first officer, 43, had 11,543 total flying hours, with 1,066 in the Embraer 120.

The takeoff weight was calculated at 22,272 pounds, well below the maximum allowable weight of 25,353 pounds. The weather was daylight visual meteorological conditions, which the Board found was not a factor in the accident.

### Sequence of events

Following takeoff, the flight was assigned a cruise altitude of flight level 250, and later instructed to descend to flight level 240. At 0948:43, the flightcrew made initial radio contact with Houston Air Route Traffic Control Center radar controllers for the Eagle Lake sector. At 0954:14, Houston instructed the flight to cross 55 miles southwest of Houston Intercontinental at 9,000 feet. The crew acknowledged they were leaving flight level 240.

At 0959:51, Houston instructed the flight to fly a heading of 030 degrees and join the Humble 234 radial. The flightcrew acknowledged at 0959:57. This was the last radio transmission from the flight. The cockpit voice recorder captured the crew receiving automated terminal information service at 1000:03.

Flight data recorder information showed the airplane descending through about 11,800 feet mean sea level at 260 knots indicated airspeed when a sudden pitchover occurred. At 1003:07, the cockpit area microphone picked up sounds of objects being upset in the cockpit, followed immediately by a sound comparable to a human "grunt." Aural warnings and mechanical sounds indicating the breakup of an aircraft in flight followed. The sound of wind began at 1003:13, and the recording stopped at 1003:40.

The flight data recorder showed a sudden negative vertical acceleration of at least 3.5 negative g (the recorder's limit was 3.375 negative g), along with roll and yaw moments. Prior to the pitchover, the engines and flight controls were operating normally. At the start of the pitchover, propeller rpm initially decreased, then increased, with the No. 2 engine rpm eventually exceeding 100 percent.

During a position relief briefing, air traffic controllers noticed the loss of the airplane's radar beacon return. At 1004:53, a controller initiated the first of four unsuccessful attempts to contact the flight. Eyewitnesses on the ground observed the airplane flying normally before it was suddenly consumed by a fireball, with the wingtips and part of the tail protruding. Witnesses reported sputtering engines, a flat spin to the left, a dangling left wing, and a missing right wing.

### What the investigation found

The main wreckage came to rest upright in a harvested cornfield about 3 miles south-southwest of Eagle Lake. The horizontal stabilizer, or top of the T-type tail, had separated before impact and was found about 650 feet west-southwest of the main wreckage. The leading edge and deice boot assembly for the left side of the horizontal stabilizer was missing from the tail and was found in a corral about 3/4 mile west of the main impact site. It was the first piece of structure found along the flight's ground track.

Investigators found that none of the 47 screws that would have attached the upper surface of the left leading edge assembly to the horizontal stabilizer were present. The upper attachment holes showed no evidence of distress. The lower attachment screws remained installed, but the leading edge assembly had separated, pulling down the lower spar cap and forming a "lip." This evidence was consistent with the upper screws being missing and the leading edge ripping down and away from the lower screws.

A review of maintenance records and personnel interviews revealed that the night before the accident, the airplane was brought into the Continental Express hangar in Houston for scheduled replacement of both horizontal stabilizer deice boots. During the second shift, mechanics and an inspector removed the attaching screws from the top of both the right and left leading edge assemblies. The right assembly was removed, fitted with a new boot, and reinstalled by the third shift.

However, the left leading edge upper screws had been removed by a second shift inspector who volunteered to help. He placed the removed screws in a bag and left it on a work platform. When the shifts changed, the second shift supervisor did not solicit a verbal turnover from the mechanics, did not give a turnover to the oncoming third shift supervisor, and did not issue or complete the required maintenance work cards. The third shift personnel were unaware that the upper screws on the left side had been removed. Because of time constraints, the third shift supervisor deferred the left boot replacement to another night. The airplane was returned to service with the upper screws missing from the left horizontal stabilizer leading edge.

The airplane completed a morning flight from Houston to Laredo without incident, reaching a maximum airspeed of 216 knots. On the accident flight, the airplane was descending at 260 knots. The Board believed that the aerodynamic download at this higher airspeed deflected the partially secured leading edge downward until it tore from the airplane. The loss of the leading edge caused an aerodynamic stall of the stabilizer, creating a large nose-down pitching moment. The airframe remained intact for about 6.5 seconds before the left wing failed negatively, causing a violent right roll that created extreme airloads, separating the horizontal stabilizer and left engine. The failure of the left wing released fuel that probably led to the in-flight fire.

The Board noted that Continental Express management and quality control personnel did not consider the removal and replacement of the horizontal stabilizer leading edge deice boots to be a Required Inspection Item (RII). The Board concluded that because the leading edge had to be removed to replace the boot, the process should have been designated an RII, which would have required a concentrated inspection by a quality assurance inspector.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this accident was "the failure of Continental Express maintenance and inspection personnel to adhere to proper maintenance and quality assurance procedures for the airplane's horizontal stabilizer deice boots that led to the sudden in-flight loss of the partially secured left horizontal stabilizer leading edge and the immediate severe nose-down pitchover and breakup of the airplane."

The Board found that contributing to the cause of the accident was "the failure of the Continental Express management to ensure compliance with the approved maintenance procedures, and the failure of FAA surveillance to detect and verify compliance with approved procedures."