CEN13TA113

The flight crew's decision to fly close behind a heavy airplane, which did not ensure there was adequate distance and time in order to avoid a wake turbulence encounter with the preceding heavy airplane’s wake vortex, which resulted in a loss of airplane control during final approach.

While on the right downwind leg, the flight crew advised the air traffic control tower controller that they would make a full stop landing. The tower controller acknowledged, told them to extend their downwind, and stated that he would call their base turn. The controller then called out the landing traffic on final, which was an Airbus A300-600 heavy airplane. The flight crew replied that they had the traffic in sight, and the controller cleared the flight to land behind the Airbus, and to be cautious of wake turbulence. The flight crew observed the Airbus abeam their current position and estimated that they made their base turn about 3 miles from the runway. Before turning onto final approach, the flight crew discussed wake turbulence avoidance procedures and planned to make a steeper approach and land beyond the Airbus's touchdown point. They also added 10 to 15 knots to the Vref speed as an additional precaution against a wake turbulence encounter. The reported wind provided by the tower controller was 180 degrees at 4 knots. The flight crew observed tire smoke from the Airbus as it touched down and discussed touching down beyond that touchdown point. The tower controller advised the flight crew to be prepared for a go-around if the Airbus did not clear the runway in time, which the flight crew acknowledged. The flight crew estimated that the Airbus had turned off the runway when their airplane was about 1,000 feet from the threshold and about 200 feet above ground level (agl). The flight crew reported having a stabilized approach to their planned landing point. When the airplane was about 150 feet agl and established on the runway centerline, the airplane experienced an uncommanded left roll. The heading swung to the left and the nose dropped. The crew reported that the airplane was buffeting heavily. Immediately, they set full power, and the flying pilot used both hands on the control wheel in an attempt to roll the airplane level and recover the pitch. He managed to get the airplane nearly back to level when the right main gear struck the ground short of the threshold and left of the runway. The airplane collided with a small drainage ditch and a dirt service road, causing the right main gear and the nose gear to collapse. Videos from cameras at the airport recorded the accident sequence, and the accident airplane was about 51 seconds behind the Airbus. A wake vortex study indicated that the accident airplane encountered the Airbus's right vortex, and the airplane's direction of left roll was consistent with the counter-clockwise rotation of the right vortex.

The published inbound course for OKC's instrument landing system (ILS) runway 17L approach was 175 degrees magnetic, with the published straight in decision altitude of 1,486 feet msl, with a height above touchdown of 200 agl. The glide slope intercept altitude at the HANGS intersection was 2,800 feet. The distance between HANGS and the touchdown zone was 4.6 nm. The runway's threshold elevation was 1,286 feet. The OKC ILS RWY 17L approach plate is appended to the docket associated with this investigation. The airplane was equipped with an enhanced ground proximity warning system (EGPWS) that provided aural and visual alerts and warnings to prevent controlled flight into terrain and for low altitude windshear conditions. The accident airplane's model was a Honeywell MK VIII EGPWS with serial number 0967. The accident model VIII took inputs from airplane sensors and systems such as, the air data inertial reference unit, weather radar, GPS, and radio altimeter to calculate projected position and potential collision with nearby terrain. Audio alerts messages will sound over the flight deck speakers and visual warnings will appear on the electronic flight indication system to the flight crew if a potential collision is detected. An EGPWS is designed to protect against seven different scenarios categorized by modes. Mode 1 is excessive descent rate, Mode 2 is excessive closure to terrain, Mode 3 is altitude loss after takeoff, Mode 4 is unsafe terrain clearance, Mode 5 is excessive deviation below glideslope, Mode 6 is advisory callouts, and Mode 7 is windshear protection. Each mode has soft and hard limit alert criteria with the hard limit requiring more urgent attention by the crew. The accident model is designed to record events triggered by exceeding the hard and soft limits set by the device. Once a limit has been exceeded, a new event will be recorded at one sample per second that includes 20 seconds before and 10 seconds after the exceedence. The EGPWS parameters are sampled one time per second, but the actual time of occurrence can be anywhere within the second. Each record is identified by the unit's operating time, the mode that was exceeded, and a unique flight number from the device. A status log is also recorded that contains each landing and takeoff. OKC, located approximately nine miles southwest of downtown Oklahoma City, Oklahoma, was a certificated airport under 14 CFR Part 139. It was owned by The City of Oklahoma City, Oklahoma (City). The Oklahoma City Airport Trust, a public trust whose sole beneficiary is the City, leases, operates, manages, and holds title to airport property in trust for the City. OKC's field elevation was 1,295 feet above mean sea level. Its class C airspace had continuous air traffic control services. The airport supported aircraft rescue and firefighting index C requirements. OKC had 4 runways: runway 17L/35R - 9,802 feet by 150 feet, concrete/grooved; runway 17R/35L - 9,800 feet by 150 feet, concrete/grooved; runway 13/31 - 7,800 feet by 150 feet, asphalt/concrete/grooved; runway 18/36 - 3,078 feet by 75 feet, asphalt. Runway 17L was marked as a precision approach runway and had high intensity runway edge lights. It was not serviced by any visual approach path system. The runway had MALSR (medium intensity approach lighting system with runway alignment indicator lights) approach lighting and centerline lighting. The EGPWS download was performed by its manufacturer with oversight by NTSB staff. The downloaded files were sent to a vehicle recorder specialist in the NTSB Vehicle Recorder Laboratory for subsequent analysis. According to the vehicle recorder specialist's factual report, the total operating time of the device was 494:44:55. Each power cycle was tagged with a sequential flight leg number. The accident event was identified as flight number 1420 with a bank angle and terrain clearance exceedance at the recorded operating times of 494:20:21 and 494:20:28, respectively. Correlation of the EGPWS data from unit operating time to central standard time was established using ATC radar data. The radar data was overlaid against the EGPWS data in Google Earth. The location at ATC time of 11:59:03 was aligned with the similar EGPWS location at 494:20:01 EGPWS Operating Time. Accordingly, 478:20:58 was subtracted from EGPWS operating time to convert to local time. The accident flight's recorded EGPWS data was plotted. A plot contained basic airplane parameters such as airplane attitudes, airspeed, altitude, and system discrete conditions. It showed the bank angle exceedance occurs at 11:59:23, with a roll to the left of 59.1 degrees. The terrain clearance exceedance occurs at 11:59:30, with a GPS altitude of 1,326 feet. The vehicle recorder specialist's EGWPS factual report is appended to the docket associated with this investigation. A NTSB senior transportation safety specialist produced an ATC specialist's report. The report included ATC radar data and ATC audio recordings from the OKC airport traffic control tower and its terminal radar approach control. The recorded data was consistent with the accident flight crew's report. The ATC specialist's report is appended to the docket associated with this investigation. A NTSB aircraft performance national resource specialist produced an aircraft performance wake vortex study. This study included data from OKC airport surveillance radar, EPGWS data, the NASA Langley Aircraft Vortex Spacing System Predictor Algorithm (APA), meteorological information, and aerodynamic considerations. The study's calculated results indicated that the location of the Airbus' right vortex after one minute supports an encounter with the accident airplane, and it direction of roll (to the left) is consistent with the counter-clockwise rotation of the right vortex (as viewed from behind). The study is appended to the docket associated with this investigation. The instructor pilot in the T-1 reported that the ATC tower told the crew they were behind a heavy Airbus and were number two for the runway. The instructor stated that his student pilot requested a 360-degree turn to build spacing, which was granted as requested. Following the 360-degree turn, the tower cleared the flight for the visual straight in approach. The tower indicated that the flight was now number three for the runway behind the Airbus airplane and cleared the Citation behind the Airbus. According to the operator, the agency's flight training included wake turbulence training. However, following the accident, the operator increased the structure of their wake turbulence training, in part, to include: •Developing new wake turbulence procedures to be incorporated into their Office of Air and Marine's (OAM) Air Operations Handbook (AOH) •Developing new wake turbulence training procedures to be incorporated into OAM's Aircrew Standardization Manuals (ASM) •Incorporating two new slides outlining new wake turbulence procedures into our training materials, which are discussed during training •Posting a front page Quarterly Special Emphasis Item banner on wake turbulence in the Opstar web portal •Produced a Quarterly Special Emphasis Item, which is required to be completed and documented in pilots' training folders •Developed a Wake Turbulence awareness video that will be disseminated to all OAM personnel via OAM's website and OpSTAR •Delivering human factors training and crew resource management training to all instructor pilots at the National Air Training Center by a FAA contractor The toxicological samples from the PNF were collected and tested. The results were negative for the tests performed. A National Transportation Safety Board (NTSB) senior meteorologist produced a meteorology group chairman's report for the investigation. His report indicated that a surface analysis chart for 0900 on December 21, 2012, depicted a col or neutral zone centered over Ok