SEA98IA141

The flight was cleared for an instrument landing system (ILS) approach. The ceiling was 200 feet overcast, visibility was 1 mile (runway visual range was greater than 6,000 feet) and the first officer was the pilot flying (according to company operating procedures, the minimum ceiling for first officer flying is normally 500 feet.) Air traffic control (ATC) radar indicated that the aircraft was left of the localizer course and/or below glide path, outside of the company's stabilized approach limits, for the majority of the final approach segment below 1,000 feet above touchdown. The aircraft broke out approximately at minimums (200 feet above touchdown), left of (and diverging away from) the runway centerline. The captain called, "push it down, push it down, push it down", and asked, "you got it or you want me to get it?". The first officer replied, "I can get it", and the captain said, "OK." The aircraft pitched down and banked up to approximately 14 degrees in correcting back to the runway. A high sink rate existed in the last few seconds before touchdown; the flight data recorder registered a vertical acceleration of about 1.8 G at touchdown. The aircraft's number 1 main landing gear wheel separated from the aircraft at or shortly after touchdown. The separated wheel entered the airport ramp area and struck two parked trucks and a baggage cart in front of the main passenger terminal. Post-incident examinations of the aircraft's number 1 wheel retaining nut disclosed that the nut threads were worn approximately 0.030 inch beyond engineering drawing specifications. FAA guidance directs FAA principal operations inspectors (POIs) to ensure their operators' operations manuals contain stabilized approach criteria as well as required actions in the event of deviations ("i.e. missed approach or go/around [sic]"). The company's DC-8 operations manual states only that deviations from the stabilized approach profile "are cause for consideration to abandon the approach."

History of flight

On July 18, 1998, approximately 0700 Pacific daylight time, Emery Worldwide Airlines flight 26, a Boeing (formerly McDonnell Douglas) DC-8-63F, N921R, experienced a separation of the number 1 main landing gear wheel from the aircraft upon landing at Seattle-Tacoma International Airport, Seattle, Washington. Following the separation, the wheel entered the Horizon Air ramp area (in front of the airport main terminal building's C concourse) and struck a Horizon Air maintenance truck parked adjacent to gate C4, damaged one other vehicle and a baggage cart, and sent debris through a window of the Horizon Air maintenance office in the main terminal building. The airplane sustained minor damage in the incident and there were no injuries to the airline transport pilot-in-command, first officer, flight engineer, or two jumpseating crewmembers aboard the aircraft, nor were there any injuries to persons on the ground. Instrument meteorological conditions (IMC) existed at the time of the incident, and an instrument flight rules (IFR) flight plan had been filed for the 14 CFR 121 non-scheduled domestic cargo flight from Dayton, Ohio.

The flight was cleared by air traffic control (ATC) for the instrument landing system/distance measuring equipment (ILS/DME) approach to runway 34R. Crew statements and log entries indicated that the first officer, whom a representative of Emery reported had been with the company for approximately 9 months, was the pilot flying at the time of the incident. The first officer reported that during the approach, "I decided to try using the command bars during the descent and turned them off after intercepting final when they started giving me erroneous turns." The flight crew's statements indicated that the aircraft broke out of the weather at minimums, left of centerline. The first officer reported that at this point, the captain asked him if he thought he could continue and he replied affirmatively and aligned himself with the runway. The first officer stated that as he passed the threshold, the aircraft was on centerline and airspeed, and that as he began the landing flare the right wing dipped slightly. The first officer stated that the captain assisted on the controls to neutralize the wings. He reported that the landing was in the touchdown zone with the right main gear touching first, and that the spoilers did not automatically deploy upon landing (the flight engineer also indicated that they did not automatically deploy, that they were armed, and that he deployed them manually; however, the flight crew did not indicate any discrepancies with the spoiler system in the aircraft log after the flight.) The statements of the flight crew and jumpseating crewmembers indicated that they did not observe anything that would cause the wheel to depart the aircraft (one jumpseater reported that "The first and only indication we had of a problem was the tower report that a tire had departed from the aircraft.")

Five witnesses (two Seattle tower controllers including the one on duty at the Local Control position, two Port of Seattle ramp controllers, and a Horizon Air ground service agent) provided statements to investigators. Of the five witnesses, three reported they observed the aircraft's wings rocking on approach, with one witness reporting that the airplane went right of the runway centerline just before touchdown and that the aircraft's left wing almost contacted the ground. One witness also reported that the airplane was "making over correct[ion]" or "trying to correct" when landing. Additionally, two witnesses reported either that the aircraft's right wing dipped just before touchdown, or that the airplane made a right bank just before touchdown. One witness reported that the airplane dropped just before touchdown, with three witnesses reporting that the airplane landed either "hard" or "rough" (the witness who described the landing as "rough" stated that the airplane did "not [make] an excessive flare.")

Two witnesses reported that the airplane landed on one side first (one of these stated it landed on the right side first.) Two witnesses reported the airplane bounced after touchdown (one of these reported it bounced about 20 feet back into the air), with one witness reporting seeing two puffs of smoke from the aircraft's tires. One witness reported that the wheel separated at touchdown, one reported she observed it separated in the touchdown area, one reported that it separated during the touchdown-bounce-settle back to the runway sequence, and one witness reported he observed the wheel separate after touchdown. Two witnesses reported that the aircraft's touchdown point was between taxiways R and Q (taxiway R intersects the runway approximately 2,000 feet past the threshold, and taxiway Q intersects the runway approximately 2,500 feet past the threshold) and one witness reported that the aircraft touched down north of taxiway P, which intersects the runway approximately 2,900 feet past the threshold.

ATC radar data on the aircraft, furnished by Seattle approach control, was sent to the NTSB's Vehicle Performance Division in Washington, D.C., which produced a study of the radar data (attached). The radar study depicted the aircraft's recorded radar positions over time in relation to the airport runways and the runway 34R localizer and glide slope. According to the radar study, the plots showed the aircraft initially lining up slightly left of the runway 34R localizer centerline, returning to the localizer centerline, then (at 0657:53) deviating generally away from, and remaining left of, the localizer centerline for 1 minute and 38 seconds, until 0659:31. The aircraft reached a maximum deviation of approximately 3/8 of full-scale displacement (about 3/4 dot deviation) at 0659:07, before beginning to track back toward the localizer centerline. The aircraft altitude at 0659:07 was recorded as 1,200 feet. The aircraft was back within approximately 1/4 scale (1/2 dot) deviation at 0659:26 (altitude 1,000 feet), and was back on the localizer centerline at 0659:31 (altitude 900 feet). It remained approximately on centerline from 0659:31 to 0659:44, then deviated slightly less than 1/4 scale (1/2 dot) right of centerline from 0659:49 to 0659:54, and subsequently crossed back through the localizer centerline from right to left and diverged away from centerline to the left. The aircraft reached a maximum deviation of between 3/4-scale (1 1/2 dots) and full-scale (2 dots) to the left of centerline at 0700:12 and 500 feet before turning and tracking back toward the landing runway.

The radar study showed the aircraft as being nearly on glide path at an altitude of approximately 2,000 feet and 5.5 nautical miles from the threshold of runway 34R. From 1,500 feet down to 800 feet, the aircraft was shown as going below glide path; it reached approximately 1/2 scale (1 dot) low at 1,200 feet and remained generally 1/2 scale or more below glide path from 1,200 feet down to 800 feet, where it was shown as being close to a full-scale low deviation. The aircraft was shown as leveling at 800 feet until recapturing the glide path from below. Below 800 feet, the aircraft was shown as fluctuating about the glide path but remaining mainly below it. The aircraft's altitude immediately prior to crossing the runway 34R threshold (0700:17) was reported as 400 feet, and immediately after crossing the threshold (0700:21) was given as 300 feet. Subsequent plots of the aircraft in the vicinity of the runway showed the aircraft's altitude as 200 feet, increasing to 300 feet. (NOTE: The actual touchdown zone elevation is 368 feet. In the radar data, aircraft altitudes were recorded to the nearest 100 feet.)

FAA Flight Standards Handbook Bulletin for Air Transportation (HBAT) 98-22, "Stabilized Approaches", dated May 26, 1998, directs FAA principal operations inspectors (POIs) with cert