CAHOKIA, IL, USA
N508PC
Socata TB-9C
The dual student reported that during final approach to runway 30R, the airplane encountered, 'somewhat severe turbulence'. The dual student stated that, '...the turbulence was severe enough to require the assistance of his instructor...to recover from the turbulence encounter.' The dual student reported that upon completion of the landing and subsequent takeoff, '...they again encountered turbulence on takeoff and the airplane began a rolling maneuver until impact.' Reported winds taken immediately after the accident were wind from 320 degrees at 11 knots gusting to 18 knots. Prior to and during the time of the accident a United States Army Chinook helicopter was operating upwind and parallel to the active runway. A transcript of the voice communications between aircraft and Federal Aviation Administration (FAA) Air Traffic Control (ATC) did not disclose any wake turbulence cautionary advisory or hazardous condition advisory transmissions given by the local controller to any aircraft. Research revealed that rotor wake turbulence, generated by the Army helicopter, had the potential to drift across the runway and the final approach course. A 1996 FAA study, entitled 'Flight Test Investigation of Rotorcraft Wake Vortices in Forward Flight', recommended that ATC separation standards should exist between heavy helicopters and small fixed-wing aircraft to prevent rotor wake turbulence encounters.
On March 18, 1999, at 1518 central standard time (CST), a Socata TB-9C, N508PC, piloted by a dual student pilot, sustained substantial damage during impact with the terrain after a loss of control after takeoff from runway 30R (3,800 feet by 75 feet, dry/concrete) at the Cahokia/St. Louis Downtown Parks Airport (CPS), Cahokia, Illinois. The airplane was owned and operated by Parks College of St. Louis University, St. Louis, Missouri. Visual meteorological conditions prevailed at the time of the accident. The instructional flight was operating under the provisions of 14 CFR Part 91. The flight was not operating on a flight plan. The Certified Flight Instructor (CFI) suffered serious injuries and the dual student sustained minor injuries. The flight departed Cahokia/St. Louis Downtown Parks Airport at 1500 for the local flight and was conducting a touch and go maneuver at the time of the accident. A weather observation, located at CPS, reported the weather at the time of the accident as: Observation Time: 1518 cst Wind: 320-degrees at 11 knots, gusts 18 knots Visibility: 10 statute miles Sky Condition: Sky clear Temperature: 13-degrees centigrade Dew Point Temperature: -4-degrees centigrade Altimeter Pressure: 30.29 inches of mercury According to a written account of an interview of the dual student, the purpose of the flight was to conduct touch-and-go's at CPS. The dual student reported that during final approach to runway 30R, the airplane encountered, "somewhat severe turbulence". The dual student stated that, "...the turbulence was severe enough to require the assistance of his instructor...to recover from the turbulence encounter." The dual student reported that upon completion of the landing and subsequent takeoff, "...they again encountered turbulence on takeoff and the airplane began a rolling maneuver until impact." The aircraft impacted to the right-side of runway 30R and was estimated to be approximately 300-feet from the intersection of taxiway A and H. After multiple interviews, conducted up to a month after the accident, the flight instructor reported not having any recollection of the accident. According to a transcript of voice transmissions provided by the Federal Aviation Administration (FAA), a United States Army Chinook Helicopter had flown a parallel approach to runway 30R, to the right side of the runway, prior to the final approach of the accident aircraft. The transcript did not disclose any wake turbulence cautionary advisory transmissions given by the local controller to any aircraft. The full transcript is attached to this report. According to a written statement, provided by the pilot-in-command of the United States Army Chinook helicopter, the crew made their first contact with the CPS local controller approximately 10 miles from the airport. The pilot reported that the local controller issued a traffic advisory for two airplanes conducting touch-and-go maneuvers. The pilot-in-command of the helicopter reported that, "At three miles out we contacted tower as instructed and we were cleared to land on runway 30R. The winds reported were 320/09 at that time. We were established on a right base and I told my crew that I had one airplane in sight that was on a crosswind to downwind at the time. The crew could not find the second airplane at that time. As I turned final I heard ATC clear one of the airplanes to land 30R. After hearing that clearance I told my crew that this is not going to work and requested my pilot to contact tower and request landing at the parallel taxi-way for spacing. ATC responded and approved the request as I side stepped to the right. Seconds after sliding right, ... informed me of an airplane to my immediate left and low. As I scanned the left area I saw the airplane and moved farther right to avoid his flight path. As I slowed my aircraft we watched the airplane do a touch and go without incident. I also heard ATC clear the second airplane for touch and go "number two behind company traffic." Without a visual reference of the second airplane we could not determine where it was located and with our flight path now aligned for the taxiway, I presumed there was sufficient spacing for the airplane. My approach took me to an intersection of the taxiway and a ramp where we would turn right and ground taxi to parking. I terminated my approach at a 10-foot hover, made a right pedal turn and put the aircraft on the ground." The pilot-in-command of the helicopter stated that, "The crew preformed the after landing check, I transferred the brakes to the PI [Pilot] and we proceeded forward to parking. After moving approximately 20-25 feet, the Fe [Flight Engineer] ... shouted "Crash Crash". We immediately stopped our forward motion, and asked the FE what was wrong. He stated that an airplane just crashed. I asked where and he responded to our right rear." The pilot-in-command of the helicopter stated, "The aircraft was empty and had an approximate gross weight of 28,000 lbs." Complete statements, as provided by the pilot-in-command of the Army helicopter and his crew, are attached to this report. A witness to the accident reported that, "Shortly before 3:30 PM, I was walking easterly on the apron on the south side of hanger 9 when I observed a Ch-47 Chinook helicopter on short final to the sod area adjacent to the apron in front of hanger 18. His approach path paralleled runway 30R on the north side. He terminated to an approximate 15 ft. hover, hovered forward to the taxiway, made a 90 degree pedal turn, and continued his hover forward. As he entered the taxiway, he initiated a decent to and settled onto about the midpoint of the taxiway. He then rolled to the entrance to the apron and stopped. The hover speed, decent, and taxi were at a very slow walk. I detected no change in engine sounds or rotor thump so I believe he stayed at taxi pitch. At that time, a Tampico aircraft from Parks College departed on 30R. I first observed it climbing through about 20 to 25 feet, in the proper departure direction for 30R and apparently over the center of the runway. As it got to within 35 to 55 degrees of the right rear quarter of the Chinook, it started a right roll. The rate appeared normal at first, but then increased rapidly until the wings were perpendicular to the runway. The aircraft turned toward me so that I had a clear view of the left wing and a saw no aileron displacement. I believe the roll was uncommanded. The Tampico descended rapidly as the turn continued with no apparent attempt to recover. The right wing tip contacted the ground short of the taxiway the Chinook was sitting on at about 150 ft from the center line of the runway, immediately followed by the right side of the nose. The aircraft slammed to the ground on it's belly facing opposite the departure direction and bounced a short distance further. The time between the Chinook stopping its taxi roll and my first seeing the Tampico on climb out was about 15 to 30 second. The whole roll event from its start to contact with the ground was about 2 seconds. Before the roll started, the departure looked normal in airspeed and attitude." The witness stated that, "Realizing a crash had occurred, I ran into the easterly man-door of hanger 9 and yelled 3 times "Call 911, a plane has crashed!" Seeing three men respond to my crys, I ran back outside and went to the plane. The Chinook was still running full rpm and waiting at the ramp as I approached the taxiway. Watching the Chinook out of the corner of my eye to stay clear of it, I crossed the taxiway and approached the downed aircraft." The full witness statement is appended to this report. Inspectors from the Federal Aviation Administration (FAA) St. Louis Flight Standards District Office (FSDO) represented the National Transportation Safety Board (NTSB) for the on-scene portion of the accident investigation. FAA FSDO inspectors reported no anomalies with the airplane's structures, systems, or engine. The airplane operator listed on NTSB Form 6120.1/2, that there were no mechanical malfunctions or failures with the accident airplane. NTSB Form 6120.1/2 is attached to this factual report. The Boeing Vertol Company, the current type certificate holder of the Army CH-47 Chinook helicopter, was contacted for information concerning rotor wake turbulence. Boeing Vertol provided hover downwash velocity diagrams and a company generated report entitled, "Groundwash Investigation of the Boeing 234 Helicopter." According to a hover downwash velocity diagram (# 234-5255), a CH-47 helicopter weighing 33,000 lbs would produce a horizontal downwash component of 19 mph at a maximum distance of 225-feet without any meteorological wind present. The hover downwash velocity diagram, labeled as "234-5255", and Boeing Vertol report "Groundwash Investigation of the Boeing 234 Helicopter" are appended to this factual report. In FAA Order 7110.65M, Air Traffic Control, it is stated in Appendix A - Aircraft Information - Fixed Wing Aircraft that a Aerospatiale Tampico TB-9 is classified as a "Small" aircraft. In FAA Order 7110.65M, Air Traffic Control, it is stated in Appendix B - Aircraft Information - Helicopters/Rotorcrafts that a Boeing Vertol Company Chinook Model 234 is classified as a "Large" aircraft. In FAA Order 7110.65M, Air Traffic Control, it is stated in chapter 2: 2-1-19 Wake Turbulence a. Apply wake turbulence procedures to aircraft operating behind heavy jets/B757's and, where indicated, to small aircraft behind large aircraft. 2-1-20 Wake Turbulence Cautionary Advisories a. Issue wake turbulence cautionary advisories and the position, altitude if known, and direction of flight of the heavy jet or B757 to: b. Issue cautionary information to any aircraft if in your opinion wake turbulence may have an adverse effect on it. When traffic is known to be a heavy aircraft, include the word heavy in the description. NOTE - Wake turbulence may be encountered by aircraft in flight as well as when operating on the airport movement area. Because wake turbulence is unpredictable, the controller is not responsible for anticipating its existence or effect. Although not mandatory during ground operations, controllers may use the words jet blast, propwash, or rotorwash, in lieu of wake turbulence, when issuing a caution advisory. According to a Final Report, issued February 1996, of a FAA study entitled "Flight Test Investigation of Rotorcraft Wake Vortices in Forward Flight", report number DOT/FAA/CT-94/117, the purpose of the study was indicated in the study's abstract: "This report presents the results of helicopter flight test and wake vortex measurements which were designed to provide data necessary for the assessment of hazards to following aircraft. The tests described in this report were conducted using small probe airplanes and a Laser Doppler Velocimeter for wake vortex measurements during forward-flight helicopter operations. Four helicopters, having weights ranging from 7,600 to 70,000 pounds, were used in the tests as the wake vortex generating aircraft. Wake vortex strength and decay characteristics as determined from the flight test results are discussed. In the absence of encounter measurements for the case of hover flight, it is recommended that small airplanes, at the same altitude and downwind of a hovering helicopter, maintain at least 500 feet of separation." At the completion of the FAA's report, several recommendations were listed as result of the flight tests and research: "Recommendations 1. For air traffic control (ATC) separations standards, helicopters should be placed into two weight classes, with a dividing line of 25,000 pounds maximum takeoff gross weight (MGTOW). Adjective classification should be assigned by the FAA, e.g., "Helo Light" or "Helo Small" for helicopters under 25,000 lbs and "Helo Medium" or "Helo Large" for helicopters over 25,000 lbs (MGTOW). 2. ATC procedures should be established for rotorcraft making decelerating final approaches when mixed with fixed-wing traffic. 3. Characteristics of rotorcraft vortex descent should be more thoroughly investigated. 4. Future vortex probe tests should utilize fully instrumented probe aircraft. 5. Future wake vortex flight tests should include measurements of atmospheric stratification, wind shear, and turbulence at the test flight altitude. 6. The dynamic response of a rotorcraft to fixed- and rotary-wing generated wake vortices should be investigated. 7. Hazards associated with rotor wash generated by helicopters in hover or in air taxi operation should be investigated. In the absence of encounter measurements for the case of hover flight, it is recommended that small airplanes, at the same altitude and downwind of a hovering helicopter, maintain at least 500 feet of separation." As of the accident date and the writing of this factual report, none of the above recommendations had been implemented.
the in-flight loss of control by the student pilot due to wind turbulence. Additional causes were the flight instructor's inadequate supervision of the flight and the flight instructor's decision to continue the touch and go after encountering turbulence on the vfr final approach that had required the flight instructor's intervention to correct.
Source: NTSB Aviation Accident Database
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