Aircraft Accidents and Lessons Unlearned XX: LAS Registration Number HK-4246X

When Flying Tiger and Seaboard World premiered in the 1940s, air cargo carriers were considered the ‘Pinnochios’ of aviation, meaning they were never considered ‘real’ airlines by the aviation industry. Even with the dawn of Federal Express’s overnight package delivery, air cargo rarely enjoyed recognition from the passenger versions of the jet age. These are not bizarre statements; the lack of recognition extended to accident investigation, where cargo carrier safety was viewed as unimportant when compared to passenger carriers. The belief that, “It’s only three pilots and a bunch of boxes,” generated little concern and less attention. Indeed, air cargo accidents rarely made the front page.

At the turn of this century, the National Transportation Safety Board (NTSB) continued to downplay cargo aircraft accidents; victim count always fell below public notice. However, cargo aircraft accidents deserved the airline industry’s full attention; we ignored these air carriers at our own risk.

Why? Because air cargo accidents involve the same equipment used by passenger carriers, often under identical flight operating conditions. Because cargo profits were greater, many operational advances were developed in all cargo environments. Cargo airlines shined an embarrassing light on what issues made passenger operations less safe. Finally, accidents were often due to the unexpected; small anomalies that grew into larger issues. Some accidents took us in directions never imagined.

On December 18, 2003, a Lineas Aéreas Suramericanas (LAS) DC-9-15F, tail registration HK-4246X, crashed in the jungles between Bogota and Mitú, Colombia. Onboard were two pilots and a mechanic; the cargo was four 5000-pound pallets of bagged concrete. The thirty-seven year-old airliner (manufactured as a cargo aircraft) was approaching Mitú from the northeast, the aircraft suddenly plummeted from 23,000 feet after executing a right turn.

Because the crash site was in the jungle bordered by the drug cartel, rescuers were only able to remove the three bodies, the cockpit voice recorder (CVR) and the flight data recorder (FDR) – none of the aircraft was recovered. The NTSB only assigned one maintenance investigator (MI) as the sole accident investigation liaison to Colombia, to help them investigate the accident. A recorder analyst was also assigned to interpret both the CVR and the FDR. The Federal Aviation Administration (FAA) sent an experienced accident investigator, who took lead in dealing with the Colombian government in protecting the team from the drug cartel.

Since this investigation was supportive, it wasn’t accomplished like a normal launch; the three investigators first met with NTSB management and Colombian representatives. The recorder investigator presented early interpretations of the CVR and FDR findings, which showed a complete loss of all flight and engine controls. Since the accident aircraft was never recovered, the lack of physical evidence had to be compensated for by an onsite inspection of a sister aircraft in the LAS fleet.

After meeting with the Colombian officials, the NTSB Director resolved that, in order to prevent an imaginary Horizontal Trim Stabilizer Jackscrew failure ‘epidemic’, the LAS DC-9-15F’s horizontal trim stabilizer jackscrew ‘failure’ was the cause of the accident. The Director ordered the MI to include this fabrication into the NTSB’s report to support his unrealistic effort to pressure the FAA to require redundant jackscrews in all past and future jet aircraft.

NOTE: On January 31, 2000, Alaska Airlines flight 261crashed. The Alaska Air MD-83’s horizontal trim stabilizer jackscrew was serviced with an unapproved lubrication grease. This caused the MD-83’s jackscrew to seize in flight, thus contributing to the accident.

However, as the NTSB investigator, I could not corrupt the accident’s findings, even in the pursuit of another end. Instead, the FAA investigator and I went to Bogota to examine the LAS ramp and the sister DC-9-15F cargo aircraft that was there.

While observing the load and offload of the DC-9-15F, it became obvious that the DC-9-15F’s main cargo doorframe suffered damage. Normally, cargo doorframe damage is to the frame’s sides, e.g. torn seals or metal being scratched/bent by careless cargo handlers. In this case, the damage was to the main cargo doorframe’s upper edge. This would be from consistently raising the cargo loader too high during freight movement; normally not impossible, but very unlikely.

The two investigators looked under the cargo floor for anything that could have led to a complete loss of flight and engine controls. In pre-digital technology aircraft, the control cables for the flight controls and the engines ran under the floor, from the cockpit to wing box (ailerons, flaps, spoilers) to tail (elevators, rudders, horizontal trim, engines). Routing beneath the floor was the only place all the controls shared.

The underside inspection was, initially, a dead end; the floor was supported by structure. The floor’s cargo locks, which translate cargo loads into the aircraft’s structure, appeared to be in the correct positions; the locks looked to take the heavy loads properly … from what could be observed. However, when discussing the floor layout with the ramp supervisor, it was learned that the LAS DC-9-15F, leased from an American air operator: Kitty Hawk Airlines, had eight standard unit load device (ULD) positions on its upper deck cargo compartment. A standard ULD’s dimensions were: 88 inches wide by 125 inches long by 79 inches high.

The manufacturer of LAS’s leased DC-9-15Fs – McDonnell Douglas Aircraft – certified the DC-9-15F with seven ULD positions. A position had been added; this would require a review of any supplemental type certificate (STC), an alteration to the original manufacturer’s designs, to the cargo floor.

Before leaving Bogota, an inspection was made to the upper deck and why the ULDs were hitting the cargo doorframe’s upper edge. It was found that the floor was altered to allow an eighth ULD, but that a new floor was placed on top of the original floor and secured. This extra floor created the height that made the ULDs strike the upper doorframe.

The two investigators then went to Evergreen Airlines in Portland, Oregon; Evergreen owned the DC-9-15F that was manufactured next to the accident aircraft in 1966. Built as a cargo airliner, this aircraft had the original seven position floor layout. This allowed for floor measurements to be made, using the STC’s dimensions to check against the original design.

Comparing the original cargo floor plan, to the altered cargo floor plan with a DC-9 structural layout, it was clear that the accident aircraft’s cargo locks did not align with the aircraft’s structure; the stresses that were normally transmitted into the airframe, instead, were transferred to the weak floor; no structural strength had secured the cargo. Based on this STC data, the known cargo weights caused the floor to rupture. At that failure point, all engine and flight control cables were arrested. Without the accident aircraft to verify this, the only way to check this theory was to compare the CVR and FDR timelines.

In Washington, DC, a second meeting between the Colombian officials and NTSB management was held. The updated CVR and FDR data was compared. The CVR recorded a ‘bang’, a loud unnatural noise of structure tearing. The CVR technicians determined, employing digital analysis, that the loud sound was fifteen to twenty feet behind the cockpit bulkhead where the freight was. The ‘bang’ came the exact moment all control cable movement ceased. The MI and the FAA investigator tied the recorded noise and the seized cables to the STC engineering flaw that allowed the cargo locks to be unsupported by the aircraft’s structure, that one of these locations was most likely the point where the floor failed.

Since no formal NTSB report was written, NTSB management dismissed the findings since the horizontal trim stabilizer jackscrew failure was omitted as the cause. NTSB management refused to list this LAS DC-9-15F accident in the NTSB Archives. However, the FAA investigator pursued the STC engineering issue from the FAA’s side; there were sister aircraft with the error written STC being flown in the United States. They represented a safety hazard.

It was by a cargo aircraft accident that a major alteration of a common airliner was determined to be unsafe, indeed fatal. With airlines looking to fit as many passengers and freight inside the aircraft, this Lessons Unlearned proved that redesigns, when done incorrectly, can kill.