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On October 26, 2009, at approximately 1202 eastern daylight time, a Boeing 717-200, N935AT, operated by Air Tran Airways as flight 669, was not damaged when it maneuvered to avoid another airplane during cruise-descent near Pleasant Grove, North Carolina. The 2 certificated airline transport pilots, 2 flight attendants, and 116 passengers were not injured; while 1 flight attendant received serious injuries, and 1 passenger received minor injuries. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the air carrier flight conducted under 14 Code of Federal Regulations Part 121 that originated from Orlando International Airport (MCO), Orlando, Florida, destined for Westchester County Airport (HPN), White Plains, New York .
According to the operator at 1220, their operations center received an "ACARS" (Aircraft Communications Addressing and Reporting System) message from the flight crew advising them that a flight attendant had fallen and a boy had a "bump on head." They also advised that they had been descending from flight level (FL) 350 to 330 due to turbulence, when they halted their descent "for an RA" (Resolution Advisory) from the onboard traffic alert and collision avoidance system (TCAS).
According to the cabin crew, approximately one hour into the flight the captain had made a public address (PA) announcement to report turbulence, "somewhat inaudibly." Within seconds of the PA announcement, one of the flight attendants in the forward galley was "thrown" into the galley counter, and another flight attendant, in the front of the cabin "came up slightly" off her jumpseat. It appeared to both of them that the airplane had "dropped," several hundred feet.
The passengers had been seated prior to the occurrence with the exception of a 10 year old boy, who was exiting an aft lavatory. One of the flight attendants had begun to return to her jumpseat, which was located in the back of the cabin when she noticed the boy and decided to wait to be seated until the boy had made his way back to his assigned seat. It was at this time that she and the boy were "tossed to the ceiling," and then back down to the floor.
Passengers then began to repeatedly ring the flight attendant call button and the two other flight attendants ran to the back of the cabin where they discovered the flight attendant lying on the floor of the airplane, along with the boy who was crying, and whose mother was attempting to console him.
One of the flight attendants then called the captain and advised him of the situation. An eye doctor and a retired paramedic assisted the child who had a "knot" on his upper forehead and knees, and the flight attendant. They applied an ice pack to the knot on the child's head, and assisted the flight attendant "who may have lost consciousness for a brief time," and was "stiff and hurting" to a seat and gave her some "Aleve."
Paramedics met the flight when it arrived at the gate in HPN, and both the flight attendant and child were transported to a local hospital where the flight attendant was admitted and remained under a doctor's care for more than 48 hours.
According to the captain, they were cruising approximately 100 miles south southeast of Kinston, North Carolina. They were in instrument meteorological conditions, cruising at FL350 in continuous light chop with the seat belt sign "OFF".
The light chop began to "intensify" and the airplane received "a couple of moderate bumps". The light chop then became a constant steady chop with periodic larger bumps. The captain then turned "ON" the seat belt sign.
The captain then asked the first officer to query air traffic control (ATC) about ride reports for the area ahead of the airplane and ATC advised that FL290 and below was smoother. The captain then requested to descend to FL310, and ATC told them to descend and maintain FL330. The captain then selected FL330 and started to descend down to the selected altitude.
Once the airplane was stabilized in the descent, the captain handed control over to the first officer. He then made a PA announcement apologizing to the passengers for the rough ride, and assured them that they were working with ATC to get a smoother ride at a lower altitude. As he was making the announcement, the TCAS annunciated "TRAFFIC". He put down the handset and placed his hands on the control wheel. There was a "red square" illuminated on the primary flight display (PFD) almost directly under the airplane at the "1 o'clock" position and almost simultaneously the TCAS annunciated "TRAFFIC" again. The captain then disconnected the autopilot, and arrested the descent.
Another "TRAFFIC" annunciation then occurred prior to FL330 and then a "MONITOR VERTICAL SPEED" annunciation. They then came out of the cloud layer and the captain observed ether a Boeing 737 or Airbus of similar size at his "1 o'clock" position pass below him moving to his right side.
He then leveled off at FL330 and reengaged the autopilot and continued on course. He then was cleared down to FL310 and handed off to another controller. He did not recall if he had ever been given a traffic advisory by ATC regarding other aircraft in his vicinity during the descent.
According to Federal Aviation Administration (FAA) records, the Captain held an airline transport certificate with multiple ratings including a type rating for the accident airplane. According to company records, he had accrued 9,145 total hours of flight experience. His most recent application for a FAA first-class medical certificate was dated September 17, 2009.
According to FAA records, the first officer held an airline transport certificate with multiple ratings including a type rating for the accident airplane. According to company records, he had accrued 8,360 total hours of flight experience. His most recent application for a FAA first-class medical certificate was dated March 5, 2009.
The accident aircraft was a twin engine, single aisle, Boeing 717-200 airplane. It was equipped with an electronic flight instrumentation system, a dual flight management system, a central fault display system, global positioning system, and TCAS which is designed to reduce the incidence of mid-air collisions between aircraft.
It was powered by two Rolls Royce BR-700-715C1-30 turbofan engines each rated at 21,000 pounds of thrust. It could cruise at mach .77 and had a maximum range of 2,060 nautical miles. It could carry 117 passengers in a mixed class configuration (12 passengers in business class and 105 in coach class).
According to FAA and maintenance records, the airplane was manufactured in 2000. The airplane's most recent continuous airworthiness inspection was completed on September 4, 2009. At the time of the accident the airplane had accrued 23,031 total hours of operation.
A weather observation taken at Norfolk International Airport (ORF) about 31 minutes after the accident, recorded the wind as 070 degrees at 11 knots, gusting to 17 knots, visibility 10 miles, broken clouds at 12,000 feet, temperature 16 degrees Celsius, dew point 09 degrees Celsius, and an altimeter setting of 30.23 inches of mercury.
On November 17, 2009, the Safety Board's Vehicle Recorder Division received an electronic file containing flight data recorder (FDR) data from N935AT's FDR.
The electronic recording contained approximately 27 hours and 9 minutes of data. The accident flight was the 7th flight out of 15 on the recorder.
Review of the recorded data indicated that during the cruise descent there was a TCAS resolution advisory followed 1.5 seconds later by a positive vertical acceleration of approximately 1.6 g's. One second after the positive vertical acceleration the airplane sustained a maximum negative vertical acceleration of approximately .2 g's. A second after the negative vertical acceleration the airplane sustained another positive load of 1.4 g's after which the acceleration was dampened out.
TESTS AND RESEARCH
According to the FAA, TCAS involves communication between all aircraft equipped with an appropriate transponder. Each TCAS-equipped aircraft interrogates all other aircraft in a determined range about their position, and all other aircraft reply to other interrogations. This interrogation-and-response cycle may occur several times per second.
The TCAS system builds a three dimensional map of aircraft in the airspace, incorporating their range, altitude, and bearing). Then, by extrapolating current range and altitude difference to anticipated future values, it determines if a potential collision threat exists.
TCAS will automatically negotiate a mutual avoidance maneuver between two conflicting aircraft. These avoidance maneuvers are communicated to the flight crew by a cockpit display and by synthesized voice instructions.
A protected volume of airspace surrounds each TCAS equipped aircraft. The size of the protected volume depends on the altitude, speed, and heading of the aircraft involved in the encounter.
According to the manufacturer of the TCAS unit, the flight crew should "promptly but smoothly" follow a resolution advisory (RA), and since the maneuvers are coordinated between aircraft, the crew should never maneuver in the opposite direction of the advisory. The advisories are always based on the "least amount of deviation from the flight path" while providing safe vertical separation.
Additionally, on the Boeing 717, the RA is also incorporated into the vertical speed tape. By illuminating a red or green bar next to the tape, "Fly-To" and "Fly-Away-From" commands are displayed coinciding with the required vertical rate. Some of the typical RAs shown will require a maneuver by the crew while others will warn against maneuvering. A typical maneuver only requires a crew response within 5 seconds and G-forces of ±.25G.
Review of the operator's training program and guidance for both the Boeing 717 and Boeing 737 revealed that both programs included information on testing and operation of the TCAS system however, only the training program and guidance for the operator's Boeing 737s advised how to respond to a TCAS RA.
On August 20, 2010, AirTran Airways published Revision 29 of the Boeing 717 Aircraft Operations Manual. This revision included the TCAS information guidance that was previously published in AirTran's Boeing 737 training program and guidance materials.