N329MX

HISTORY OF FLIGHT

On March 20, 2000, approximately 0731 mountain standard time, a Dornier Luftfahrt GmbH 328-100, N329MX, operated by Air Wisconsin, Inc., and doing business as United Express flight 7128, was substantially damaged when the right main and nose landing gears retracted during landing at Denver International Airport, Denver, Colorado. There were no injuries to the airline transport certificated captain and first officer, one flight attendant, and 21 passengers. Visual meteorological conditions prevailed, and an IFR flight plan had been filed for the scheduled domestic passenger flight being operated under Title 14 CFR Part 121. The flight originated at Bismarck, North Dakota, approximately 0534.

According to the flight crew, during the approach they confirmed the landing gear was down and locked. They broke out of the overcast some distance from the runway and had good visibility, so both pilots were looking outside. The first officer landed the airplane on runway 35L in a slight left crosswind. When the airplane touched down, the landing gear warning horn sounded. The right main and nose landing gears retracted. The airplane slid to a halt on the right side of the runway.

PERSONNEL (CREW) INFORMATION

The captain, age 37, held an airline transport pilot certificate, dated December 16, 1998, with an airplane multiengine land rating; commercial privileges, airplane single engine land, and a type rating in the Dornier DO-328. He also held a flight instructor certificate, dated July 22, 1999, with airplane single, multiengine, and instrument ratings, and a ground instructor certificate, dated November 7, 1994, with an advanced rating. His first class airman medical certificate, dated October 5, 1999, contained the restriction, "Holder shall wear corrective lenses while exercising the privileges of his airman certificate." His last biennial flight review or equivalent (proficiency check) was dated January 13, 2000, and was accomplished in the Dornier 328-100.

The first officer, age 34, held an airline transport pilot certificate, dated December 4, 2000, with an airplane multiengine land rating; commercial privileges, airplane single engine land, and type ratings in the Dornier DO-328 and Shorts SD3. He also held an expired flight instructor certificate, dated June 12, 1996, with airplane single engine land and instrument ratings, and a ground instructor certificate, dated February 2, 1996, with an advanced rating. His first class airman medical certificate, dated September 1, 1999, contained no restrictions or limitations.

According to Air Wisconsin records, the flight attendant was certified to serve on both the Dornier 328-100 and the British Aerospace BAe 146.

AIRCRAFT INFORMATION

N329MX, a Dornier Luftfahrt GmbH 328-100 (s/n 3049), was equipped with two Pratt & Whitney PW119C turboprop engines (s/n PCEAB0010, PCE116051), each rated at 2,180 horsepower. The engines had accrued 7,265:14 and 9,070:02 hours of operation, respectively, at the time of the accident. The airplane was being maintained under a continuous airworthiness inspection program, and had accrued 7,596:18 hours at the time of the accident.

METEOROLOGICAL INFORMATION

The following weather observation was recorded at Denver International Airport 1 minute after the accident:

Wind, 350 degrees at 19 knots, gusts to 28 knots; visibility, 9 s.m.; ceiling, 1400 feet overcast; temperature, 28 degrees F.; dew point, 21 degrees F.; altimeter setting, 29.74 inches of mercury.

AERODROME INFORMATION

Denver International Airport is at an elevation of 5,431 feet msl. Runway 35L is 12,000 feet long and 150 feet wide. It is concrete and grooved. It is served by an ALSF-2 approach light system, precision approach path indicator, high intensity runway lights, and runway centerline lights.

FLIGHT RECORDERS

The airplane was equipped with a Fairchild A-100A cockpit voice recorder (CVR) and a digital flight data recorder (DFDR). Data from both recorders were downloaded at NTSB headquarters on March 23, 2000.

According to the CVR transcript, the first officer requested that the landing gear be lowered at 0729:34. Two seconds later, a sound similar to the landing gear being operated was recorded. At 0730:28, the captain reported "gear down, three green," to which the first officer replied, "down, three green, I see it." At 0730:43, the captain reported, "gear down, three green...before landing checklist is complete." At 0731:53, a sound similar to a decrease in engine rpm was recorded and the captain remarked, "Nice." At the same time, a sound similar to the landing gear warning horn started. The captain said, "Oh, *," and the first officer asked, "What the #'s that?" The captain replied, "#, I don't know. At 0731:59, the sound of a thump was recorded and the captain said, "Hang on." The captain reported to the control tower that the gear had collapsed at 0732:03. At 0732:08, the sounds of impact and grinding noises started and continued to 0732:29. At 0733:00, after the passengers had been evacuated, the captain asked, "We had three green, didn't we?" The first officer replied, "We did. We called for the..." The captain then said, "Don't ask me, dude. I have no idea. I should have had you go around." The first officer said, "We touched down but..." This was the end of pertinent conversation.

Some of the findings of the DFDR group chairman's factual report were:

"(1) At 0729:44, while descending through 7,000 feet pressure altitude, the airplane's landing gear discrete changed state from 'up' to 'down and locked' (the landing gear was extended and indicated down and locked, the source data being the position of the nose landing gear);

"(2) At 0731:17, the autopilot engage discrete changed from 'engaged' to 'not engaged' (the autopilot was disengaged);

"(3) At 0731:50, vertical acceleration was recorded at a local maximum of 1.080 g's. At 0731:51, the landing gear discrete changed state from 'down and locked' to 'up' while at an airspeed of 120.0 knots (the nose landing gear became "unlocked," or unsafe, about 2 seconds after the radio altitude reached 0 feet);

"(4) During the approach, hydraulic pressure was recorded at 3,024-3,029 pounds per square inch (psi). At 0731:51, hydraulic pressure was 3,009 psi, dropped to a local minimum of 2,683 psi at 0731:53, then 3,024 psi at 0731:55.

"(5) At 0731:51, roll attitude was recorded at a local minimum of -2.37 degrees (left wing down). Roll values increased to about zero at 0731:55 and then increased to a local maximum of 4.83 degrees (right wing down) at 0731:57, decreased to another local minimum of -0.26 degrees at 0731:59, then increased and remained at or about a maximum of 18.02 degrees at 0732:27 through the end of the recorded data (uncommanded right main landing gear retraction).

"(6) The left and right engine torque and propeller speed values approximated each other during the descent and landing. At 0732:07, however, the left and right engine torque values diverged, 7.53% and 11.59% left and right engine respectively. [According to Fairchild Dornier, flight idle is 12% which "is enough to produce considerable lift, thus reducing the load on the landing gear and WOW (weight on wheels) onset."] Over the next several seconds the left engine values continued a gradual increase whereas the right engine values increased to a local maximum of 33.12% at 0732:11 and then decreased to zero at 0732:16. The left and right engine propeller speeds were approximately equivalent until 0732:10 when the values diverged. The right propeller speed values dropped to zero at 0732:16.

WRECKAGE AND IMPACT INFORMATION

The airplane was examined in a hangar after it had been towed off the runway. Damage was confined to the right side skin and bulkheads, aft of the copilot's station and forward of the wing.

Examination of the cockpit shortly after the incident disclosed the landing gear control lever was in the DOWN position, and the landing gear status lights indicated the left main landing gear to be DOWN AND LOCKED (green) and the right main and nose landing gears to be UNSAFE (red). After the airplane was jacked up and the right main and nose landing gears were lowered and pinned in place, it was towed back to a hangar where initial tests were conducted. The airplane was placed on jacks and the landing gear was cycled 30 times without difficulty. The up and down locks were undamaged and moved freely. The proximity switches tested satisfactorily. The landing gear stopped once a WOW (weight on wheels) condition was simulated.

The landing gear control lever and lever lock solenoid were tested and found to operate normally. Attempts were made to get the handle stuck in the middle position, but the over-center lever prevented this. The handle could not be forced past the lever without going into manual override.

The landing gear selector valve was undamaged and operated normally.

The hydraulic pressure-sensing valve was unremarkable. The hydraulic system was full of fluid and none of the filters had been bypassed.

Several gear extensions were made utilizing the emergency gear extension system. No anomalies were noted.

The proximity switch electronics unit (PSEU) tested normally. A complete PSEU IBIT was performed. All systems tests were satisfactory. The PSEU nonvolatile memory (NVM) was downloaded and revealed numerous faults, none of which were relevant to the accident.

TESTS AND RESEARCH

On April 12, 2000, the PSEU (p/n 8-700-04, s/n 77) and the right main landing gear harness and proximity switches were tested at the facilities of Eldec Corporation in Lynnwood, Washington. The PSEU tested satisfactorily. It was then subjected to temperature extremes (cooled to -40 degrees C. for 2 hours, then heated to 65 degrees C. for 1.5 hours, then returned to ambient temperature). Again, it tested satisfactorily. The harness and sensors tested satisfactorily.

On May 10, 2000, the landing gear selector valv...