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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.
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.
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.
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).
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)."
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, origin locations and propagation directions were…consistent with the stresses due to loading caused by routine operation with a supertug…" A "supertug" is a "towbarless" tractor that does not use a tow bar. Delta decided to test this theory using a supertug on another MD-90.
The nose landing gear was pulled into a cradle platform by the tug's telescoping arms. The telescoping arms use rollers to contact the aft side of the tires. There is a small gap between the arm and the rear deflector once the nose landing gear is attached to the tug. As the supertug turned the airplane, the gap between the arms and the rear deflectors was reduced. As the turn angle was increased, the arm contacted the rear deflector, fracturing both the left and right deflector assembly lugs that attach the deflector support assembly to the shock strut cylinder at the main axle). Delta now prohibits the towing of its MD-88 and MD-90 airplanes using the supertug, and requires nose gear spray deflector inspections during every "A" check.
In its May 2004 Fleet Team Conference bulletin, Boeing (which bought McDonnell Douglas) noted that deflector damage can result "from operating over a rigged military arresting cable (which may be found at joint-use military-civilian airports), taxiing over a wheel chock, striking a taxiway light or other obstruction, and operating on damaged runways." Boeing said that when the nose landing gear is retracted, "the dislocated side deflector was held horizontal by the airstream and did not follow the normal path into the forward deflector guides. This allowed the deflector to enter the wheel well, move outboard, and jam on the door frame during gear extension."
Boeing has advised operators not to use the supertug for towing MD-80, MD-90, and Boeing 717 airplanes, and has revised Flight Operations Manual and Aircraft Maintenance Manual, mandating inspection of the spray deflectors during every "A" check. The company is encouraging operators to expand their ground personnel training programs to include procedures for towing these airplanes and inspecting the spray deflectors, and plans to release a service action later this year that will incorporate a mechanical stop to prevent nose gear retraction with a damaged spray deflector. Meanwhile it has revised an existing Service Letter to raise awareness of the towbarless tug issue. The company says it is keeping operators informed by publishing pertinent articles in its Fleet TEAM Digest (FTD) and Multi-Operator Messages (MOMs).
The following two similar accidents were recorded in NTSB's accident data base:
On September 2, 2003, a McDonnell Douglas DC-9-82, N454AA, operated by American Airlines as flight 1048 from Dallas-Fort Worth International Airport (DFW), Dallas, Texas, to Newark Liberty International Airport (EWR), Newark, New Jersey, landed nose wheel-up on runway 04L at John F. Kennedy International Airport (JFK), Jamaica, New York. There were no injuries to the 133 passengers and 5 crewmembers, but the airplane was substantially damaged (NYC03FA186).
On June 20, 2006, a McDonnell Douglas DC-9-83, N961TW, operated by American Airlines as flight 1740 from Los Angeles, California, landed nose wheel-up on runway 14R at O'Hare International Airport, Chicago, Illinois. There were no injuries to the 131 passengers and 5 crewmembers, but the airplane was substantially damaged (CHI06FA158). Investigation revealed the center spray deflector had failed. According to a Boeing Materials and Processing Report (LR-16190), cracks were found "in a number of other MD80 NLG center spray deflectors." Three of those deflectors (AA-053, AA-157, and AA-198) were submitted to Boeing for metallurgical examination. It was determined that all of the cracks were due to fatigue.
Other than the Federal Aviation Administration, there were no other parties to this investigation.
The airplane was released to Delta Air Lines on October 12, 2006.