N622DL

HISTORY OF FLIGHT

On September 5, 2006, at 1301 Atlantic standard time, a Delta Air Lines Boeing 757-232, N622DL, experienced a tail strike upon landing at the Luis Munoz Marin International Airport, San Juan, Puerto Rico (SJU). The flight was operating under 14 CFR Part 121 as Delta flight 565. The tail strike resulted in substantial damage to the lower rear fuselage of the airplane. Neither the two pilots, five flight attendants or 109 passengers were injured. The flight originated at Atlanta Hartsfield International Airport, Atlanta, Georgia (ATL), at 0950 eastern daylight time.

The flight departed ATL after a short delay, and according to the crew, the departure and enroute phases of flight were uneventful. The crew briefed a practice autoland ILS runway 8 approach. The First Officer was the pilot flying the approach, and the Captain was the pilot monitoring.

About 1646 Universal Time Coordinated (UTC), the San Juan Approach controller advised the crew to expect the Instrument Landing System (ILS) approach to runway 8 and verified that they had received the most recent weather. The crew acknowledged. For the next few minutes, air traffic control (ATC) provided vectors and altitude clearances to establish the flight on the ILS final approach course. At 1652, ATC advised the crew to fly a heading of 110 degrees and maintain 3,200 feet until established on the approach, and cleared the flight for the ILS runway 8 approach. The crew acknowledged. One minute later, ATC advised the crew to reduce speed to 170 knots until PATTY, the final approach fix, similar to other arrivals in sequence. At 1656, ATC instructed the crew to switch to the San Juan tower frequency, the crew complied, and the tower controller issued a landing clearance. ATC advised the flight crew that an MD-11 airplane would depart ahead of them, issued a wake turbulence cautionary advisory, and instructed the crew to reduce to "slowest practical speed." The crew acknowledged. ATC then instructed the MD-11 to position and hold on runway 8, and almost immediately thereafter issued takeoff clearance. At this point, the accident airplane was approximately 3 miles from touchdown.

Flight Data Recorder (FDR) information indicated the airplane was centered on the localizer and glideslope at an indicated airspeed of approximately 140 knots. The pilots reported that the airplane began to drift to the right of the localizer course and extended centerline of the runway at about a 3 / 4 - mile final. The performance studies indicate a localizer deviation and the autopilot responded to a "fly right" command on short final. The First Officer disconnected the autopilot and hand flew the airplane back toward the runway centerline. The approach procedure chart for the ILS runway 8 was notated "unusable from 0.8 nm to threshold." (see "Aids to Navigation" section below)

The flight crew reported that, after touchdown, the airplane began pitching up higher than normal and the speedbrakes deployed. The First Officer reported that the "touchdown was smoother than normal and the aircraft nose was rising. I felt we had touched down, but the Captain said 'we're not down yet'." Both pilots reported keeping forward pressure on the yoke until landing. The airplane main gear touched down approximately 4,000 feet from the threshold of runway 8 and pitch attitude continued to increase until the rear fuselage contacted the runway surface. Thrust reversers were applied nominally. Initially, the pilots attributed the behavior of the airplane to the effects of wake turbulence from the departing MD-11. After landing, both pilots reported that they noticed the stabilizer trim indicated full nose up.

According to dispatch records, the airplane's estimated gross weight at the time of touchdown was approximately 188,300 pounds. Maximum landing weight for the airplane is 198,000 pounds. Center of gravity was calculated at 28.3 percent mean aerodynamic chord.

DAMAGE TO AIRCRAFT

The airplane exhibited multiple deep scrapes and wear damage to lower fuselage skin and stringers. The aft pressure bulkhead exhibited a 22-inch-long buckle, and wear damage. The damage required a major repair, and the airplane was ferried to Atlanta for permanent repair.

PERSONNEL INFORMATION

The Captain had been with Delta since October 1985. He reported 15,085 total flight hours, with 6,479 hours in the B757. His last B757 proficiency check was in April 2006. He held an Airline Transport Pilot certificate with type ratings in B737, B757, B767, and CE500. The accident flight was the Captain's first flight of the day, and he was acting as pilot-in-command (PIC) and pilot-monitoring.

The First Officer had been with Delta since March 1998. He reported 6,317 total flight hours, with 3,282 in the B757. His last B757 proficiency check was in February 2006. He held an Airline Transport Pilot certificate with type ratings in L382, B757, and B767. The accident flight was the First Officer's first flight of the day and he was acting as pilot-flying.

During post-accident training evaluation, the Delta Air Lines instructor pilots noted:

"during these landings that I observed the First Officer 'trimming' in the flare and confirmed this by watching the stabilizer trim indicator. First Officer was unaware that this was occurring. He was trimming accidentally while pulling back on the control column and gripping the trim switches. I corrected this by changing the First Officer's hand placement on the control column during approach and landing."

AIRCRAFT INFORMATION

The accident airplane was a Boeing 757-232, serial number 22912, powered by two Pratt & Whitney 2037 turbofan engines. At the time of its last inspection on September 2, 2006, it had accumulated a total of 65,019 flight hours. It exhibited no maintenance deficiencies prior to or during the flight that would have affected the accident sequence of events.

The overall function of the Boeing 757-232 stabilizer trim system is to maintain the airplane in a longitudinally trimmed condition by varying the incidence of the horizontal stabilizer and to provide automatic mach and speed stability. The optimum pitch attitude is maintained during various flight conditions. The stabilizer may be trimmed manually or automatically.

The stabilizer can move from zero units to approximately 15.5 units of trim. The ballscrew actuator is controlled by two, identical, Stabilizer Trim Control Module's (STCM). These control modules respond to either mechanical or electrical inputs and provide direction and rate control to the stabilizer trim ballscrew actuator. The simultaneous operation of both STCMs will produce twice the trim rate of just one STCM. According to the 757 Aircraft Maintenance Manual (AMM) 27-41-00, the stabilizer rate varies between 0.1 and 0.25 degree/sec for one active STCM and between 0.2 and 0.5 degree/sec for two active STCMs. This rate depends on operational control mode, flap setting, and airspeed.

The STCMs are operated from two inputs, mechanical and electrical. Mechanical input is with the manual trim levers, which move control arms on both STCMs. All electrical ARM and CONTROL input signals come from the stabilizer trim and elevator asymmetry limit modules (SAM). These signals can be interrupted by column cutout switches and limit switches between each SAM and STCM. The column cutout switches interrupt electric trim commands in the event of a runaway stabilizer when both control columns are moved in a direction opposite to the commanded trim direction. The stabilizer trim limit switches provide travel limits during electric trim control and signals to the takeoff configuration warning system on whether the stabilizer surface position is within takeoff limits.

The stabilizer trim system has the following operational control modes:

(1) Manual Mechanical Mode In the manual-mechanical mode, stabilizer operation is controlled by dual mechanical STAB TRIM control levers on the control stand. The levers are connected to the STCMs by mechanical control cables. One lever activates the ARM hydraulic valve in both STCMs and the other lever activates the CONTROL hydraulic valve. Movement of the STAB TRIM control levers overrides all electric trim commands. Stabilizer motion continues until the levers are released or until the mechanical travel, limits are reached. Both STAB TRIM control levers must be moved in the same direction for stabilizer operation. In this mode, the stabilizer rate varies between 0.2 and 0.5 degree/sec due to both STCMs being activated.

The STAB TRIM control levers have priority over any other control mode by mechanically positioning two servo valves in the STCMs. When the levers are operated, the valves hydraulically interrupt any other trim signals.

(2) Manual Electrical Mode

In the manual-electrical mode, stabilizer operation is controlled by a set of stab trim control switches that are located on the outboard horn of each control wheel. Moving both switches simultaneously in the up direction issues airplane nose down ARM and CONTROL commands. Moving both switches simultaneously in the down direction issues airplane nose up ARM and CONTROL commands. When the electric trim switches are engaged, both trim channels are activated, and the stabilizer is driven at full-rate trim (between 0.2 and 0.5 degree/sec). The STCMs responds to electrical trim commands by applying hydraulic pressure to the motors on the actuator.

(3) Automatic Electrical trim Modes

The auto trim mode is operative whenever the autopilot controls the airplane trim. In this mode, the Flight Control Computers (FCCs) issue commands through the SAMs to the STCMs to trim the stabilizer to relieve sustained elevator loads. The automatic stabilizer trim only uses one hydraulic motor. Only one SAM is in command at any time as determined by the FCC engaged. The rate of stabilizer movement is 1/2 of that used for manual ...