N939DADiamond Aircraft Ind DA20-C1 · 2006

On September 8, 2008, at approximately 1147 mountain daylight time, a single-engine Diamond DA-20-C-1 airplane, N939DA, was substantially damaged upon impact with terrain near Pueblo Memorial Airport (PUB), Pueblo, Colorado. The flight instructor and student pilot were seriously injured. The airplane was owned by Doss Aviation Incorporated and operated under contract by the United States Air Force (USAF). The airplane departed PUB for the local instructional flight. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 91 public use flight.

According to the flight instructor and student pilot, while entering the pattern for runway 8L's overhead pattern, they were informed by the tower controller to follow a C-130 and to be cautious of wake turbulence. With direction from the flight instructor, the student pilot turned and entered the downwind leg. The flight instructor recalled seeing the C-130 on a wide pattern and elected to lengthen the downwind leg and began to turn to final approximately 1 nautical mile past the runway threshold. Both pilots recalled that engine and airspeed parameters appeared "normal." Once the airplane was level on final, the student pilot was attempting to "keep it high," but noted that the glide slope indicators displayed "2 red/2 white." Furthermore, the student pilot reported that at approximately 1/2 mile from the threshold, the airplane encountered wake turbulence from the preceding C-130. The airplane began an uncommanded pitch up, rolled left and then began descending. Full throttle and opposite flight controls were applied. Despite attempts to regain level flight by both the student pilot and the flight instructor, the airplane descended until it collided with terrain.

An examination of the airframe and engine was conducted by the NTSB with technical assistance from representatives of Diamond Aircraft and Teledyne Continental Motors. Flight control continuity was established to the ailerons, elevators, and rudder. The flaps were set to the landing position. No anomalies were noted which would have precluded the safe operation of the airplane.

Engine valve train continuity, accessory gear continuity, compression and suction were established. The fuel manifold contained traces of fuel, and the fuel screen was found clear of debris. Both magnetos produced spark at all terminals. The throttle was found at the approximate full throttle position and the mixture control was found near the full rich position. The fuel inlet screen and oil filter were clear of debris. No anomalies were noted which would have precluded the production of engine power.

A review of tower radar returns revealed that the accident airplane flew 56 to 62 seconds behind and within 1/4 to 1/2 nautical miles downwind of the preceding C-130's flight path.

Federal Aviation Administration Advisory Circular 90-23F, Aircraft Wake Turbulence released February 20, 2002, "is intended to alert pilots to the hazards of aircraft wake turbulence and recommends related operational procedures." Under the heading of "6. Induced Roll" the circular stated that "...the capability of an aircraft to counteract the roll imposed by the wake vortex primarily depends on the wingspan and counter-control responsiveness of the encountering aircraft."

An 1153, an automated weather reporting facility at PUB, reported winds from 120 degrees at 7 knots, visibility 10 miles, ceiling broken at 2,500 feet, overcast at 3,900 feet, temperature 54 degrees Fahrenheit, dew point 39 degrees Fahrenheit, and a barometric pressure of 30.30 inches of Mercury.