DEN07FA008

When the landing gear was extended on final approach, the nose gear UNSAFE light illuminated. A low fly-by was made, and control tower personnel confirmed that the nose gear appeared to be retracted. After consulting with the company's maintenance department, unsuccessful attempts were made to lower the nose gear, and a nose gear-up landing was made. Postaccident examination revealed that the nose landing gear center spray deflector had fractured and rotated, preventing gear extension. The spray deflector is designed to deflect water and other runway material kicked up by the nose wheel away from the rear-mounted engines which could cause flameouts and engine damage. About one-third of the spray deflector was broken off. Laboratory examination of the deflector revealed the fracture was typical of overstress. The nose gear doors were scraped, and the aluminum skin just aft of the nose wheel well was scraped through, exposing 5 longerons and 6 stringers, compromising the pressure vessel.

History of flight

On October 11, 2006, at 1827 mountain daylight time, a McDonnell Douglas MD-90-30, N906DA, registered to and operated by Delta Air Lines, Inc., as Flight 1636, and piloted by an airline transport certificated pilot, was substantially damaged when the nose landing gear failed to extend and the crew was forced to make a nose gear-up landing at Denver International Airport (DEN), Denver, Colorado. Visual meteorological conditions (VMC) prevailed at the time of the accident. The scheduled domestic passenger flight was being conducted under the provisions of Title 14 Code of Federal Regulations (CFR) Part 121, and an instrument flight rules (IFR) flight plan had been filed. The captain, first officer, three flight attendants and 150 passengers were not injured. The flight originated at Salt Lake City International Airport (SLC), Salt Lake City, Utah, at 1703 and was en route to DEN.

According to the crew's written statements, the takeoff and en route portions of the flight were uneventful. When the landing gear was extended on final approach to DEN, the nose gear UNSAFE light illuminated. A low fly-by was made and control tower personnel confirmed that the nose gear appeared to be retracted. After consulting with the company's maintenance department, attempts were made to lower the nose gear but they were unsuccessful. After the flight attendants briefed the passengers, a nose gear-up landing was made on runway 16R. As the nose section skidded on the runway, a small fire erupted in the nose wheel well area and self-extinguished. No emergency evacuation was ordered. Passengers deplaned through the cabin door and were bussed to the terminal.

Damage to aircraft

After the airplane had been removed from the runway and towed to a secured area, Delta maintenance personnel found that the nose landing gear center spray deflector had fractured and rotated, preventing gear extension (the spray deflector is designed to deflect water and other runway material kicked up by the nose wheel away from the rear-mounted engines which could cause flameouts and engine damage). About one-third of the spray deflector was broken off. The nose gear doors were scraped. The aluminum skin just aft of the nose wheel well was scraped through, exposing 5 longerons and 6 stringers. Two antennae and one drain mast were also broken off. The pressure vessel was compromised.

Personnel (crew) information

The captain, age 50, held an airline transport pilot certificate with an airplane multiengine land rating, commercial privileges in airplanes single-engine land, and type ratings in the Douglas DC-9 and Lockheed L-382. He also held a flight engineer certificate, dated January 1, 1988, with a turbojet rating. His 1st-class airman medical certificate, dated September 1, 2006, contained no restrictions and limitations. He had logged 10,879 total flight hours, and 165 hours in the MD-90 in the previous 90 days. His last proficiency check was accomplished in the MD-90 on August 21, 2006.

The first officer, age 37, held an airline transport pilot certificate with an airplane multiengine land rating, commercial privileges in airplanes single-engine land, and type ratings in the Douglas DC-9 and British Aerospace BAe-3100. He also held a flight engineer certificate, dated March 18, 1999, with a turbojet rating. His 1st-class airman medical certificate, dated December 16, 2005, contained no restrictions and limitations. He had logged 3,762 total flight hours, and 191 hours in the MD-90 in the previous 90 days. His last proficiency check was accomplished in the MD-90 on July 21, 2006.

The three flight attendants had been trained and were certificated on the MD-90.

Aircraft information

The McDonnell Douglas Airplane Company built N906DL (s.n. 53386), a model MD-90-30, in 1995. It was powered by two International Aero V2528-D5 turbofan engines (serial number 020027, left; 020015, right), each rated at 28,000 pounds of thrust.

The airplane was maintained under a continuous airworthiness maintenance program. The following inspections were performed:

"A" Check September 22, 2006 31,605.9 hours 16,662 cycles

"B" Check February 15, 2006 30,004.4 hours 15,726 cycles

"C" Check February 15, 2006 30,004.4 hours 15,726 cycles

"D" Check July 26, 2001 18,476.1 hours 9,899 cycles

At the time of the accident, the airplane had accrued 31,746.9 hours and 16,739 cycles.

Meteorological information

Denver's ASOS (Automated Surface Observing System) recorded the following special METAR (Aviation Routine Weather Report) at 1837:

Wind, 120 degrees at 5 knots; visibility, 10 statute miles (or greater); sky condition, few clouds at 7,000 feet, 10,000 feet scattered, 13,000 feet scattered, 20,000 feet scattered; temperature, 10 degrees Celsius (C.); dew point, 3 degrees C.; altimeter, 29.88 inches of Mercury. Remarks: Altocumulus standing lenticular clouds VC southwest through northwest.

Aerodrome information

DEN is located 16 miles northeast of downtown Denver, and is situated at an elevation of 5,431 feet msl (above mean sea level). The airport has 12 runways. N906DA landed on runway 16R (16,000 feet x 200 feet, concrete, grooved).

Flight recorders

The airplane was equipped with a cockpit voice recorder (CVR) and a digital flight data recorder (DFDR). The recorders were sent to NTSB's Vehicle Recorder Division in Washington, DC, for download and readout. Summary reports are attached to this report (see EXHBITIS).

Wreckage and impact information

DEN operations officials examined runway 16R prior to it being reopened. They reported the airplane touched down approximately the 1,000-foot mark, and the nose section touched down at the 5,200-foot mark. A 15-inch x 7-inch portion of the nose gear door was found at the 5,500-foot mark. A 7-inch x 1-inch strip of metal and a 16-inch x 7-inch piece of nose gear door was found at the 5,900-foot mark. There was a piece of insulation and strap, 1.5-inch in diameter, at the 6,000-foot mark. At the 6,500-foot mark, there was a 2.5-inch x 5-inch piece of sheet metal. The airplane came to rest at the 7,200-foot mark.

Tests and research

The National Transportation Safety Board's (NTSB) Materials Laboratory Division examined the nose gear spray deflector. According to the factual report (06-101), "The center deflector assembly was fractured at two locations separating the left side deflector. The fractures were located around the two bolts attaching the left side deflector to the center deflector assembly. Magnified optical examinations of both fractures uncovered uniformly rough, gray fracture features typical of overstress separations in aluminum castings. Fracture features and fracture orientations were consistent with downward bending loads on the left deflector. No indications of preexisting cracking or porosity were observed on the fractures." The report concluded that Brinell hardness values (100 and 121 HB) of the center deflector were "consistent with the specified alloy (cast aluminum C355) and heat treatment (T61 temper)."

Additional information

On the MD-90, the spray deflector assembly -- which is attached to the nose landing gear - is a composite and consists of a center deflector, two rear deflectors, two side deflectors, and two supports. The spray deflector assembly on the MD-88 is similar to that of the assembly on the MD-90, but is aluminum and has a single piece rear/side deflector, which attaches to the same center deflector assembly.

Following this accident, Delta Air Lines inspected its fleet of 16 MD-90s and 120 MD-88s. One -- an MD-90, N903DA -- had large cracks in the spray deflector assembly, but the assembly had not failed completely. According to Delta's Engineering Report (2-458305-20), the cracking was the result "of an overload condition." Further investigation revealed that the "fracture mode