Tipping the Scales: The crash of Air Midwest flight 5481

Smoke billows up over Charlotte Douglas International Airport after the crash. Image source: Wikimedia commons

On the 8th of January 2003, a commuter flight leaving Charlotte, North Carolina pitched up uncontrollably on takeoff, reaching 54 degrees nose up before it stalled and fell from the sky. Despite their best efforts, the pilots were unable to recover in time, and the plane crashed into a hangar, killing all 21 people on board. Investigators discovered negligent maintenance practices at the airline that created a hidden danger in the aircraft’s flight control system, but this proved to be only part of the puzzle. It was the other half of the story that gave this small crash nationwide implications: to the Safety Board’s horror, airlines around the country were still using an average passenger weights that hadn’t been updated since 1936!

An Air Midwest Beechcraft 1900D, painted in USAir Express livery, similar to the aircraft involved in the accident. Image source: Wikimedia commons

Air Midwest was a regional airline operating short commuter flights in the central and eastern United States, usually within code sharing agreements that allowed its tickets to be sold under major brand names like Braniff, TWA, and later US Airways Express. By 2003, Air Midwest operated a fleet of over 40 19-passenger Beechcraft 1900D twin turboprop commuter planes, which spent the day hopping between small and midsized American cities. One of these routes was Air Midwest flight 5481 from Charlotte, North Carolina, to Greenville-Spartanburg, a regional airport located in Greer, South Carolina. The flight, which was marketed under US Airways Express brand, featured a full complement of 19 passengers, as well as two pilots. In command was 25-year-old Captain Katie Leslie, a rising star at Air Midwest who was known for her good judgment and extensive systems knowledge. Joining her in the cockpit was 27-year-old First Officer Jonathan Gibbs, another young pilot with a clean record and a promising future.

The basic components of a Beechcraft 1900D elevator cable. (Own work)

The plane that they would be flying had just been taken in for maintenance the previous day at a facility in Huntington, West Virginia. The facility belonged to Raytheon Aircraft Company, the manufacturer of the Beechcraft 1900 series, and it employed the mechanics and inspectors who worked on Air Midwest aircraft. On the 7th of January 2003, two Raytheon mechanics and a quality assurance inspector were tasked with carrying out routine checks on this Air Midwest Beechcraft 1900D, a process which included checking the tension of the elevator cables. Because of its small size, the 1900D does not have hydraulic flight controls; instead, pilot inputs are directly transferred to the control surfaces via cables and bellcranks. To ensure that the controls always return to the neutral position, the cables must be kept at a certain level of tension, which is measured and adjusted during the course of routine maintenance. Neither of the two mechanics assigned to this task had re-tensioned a Beechcraft 1900D elevator cable before, but the inspector had, and he planned to provide them with on-the-job training regarding the process.

How to correctly re-tension a Beechcraft 1900D elevator cable. (Own work)

Measurements of the tension of the elevator cables on this aircraft revealed that they needed adjustment. Re-tensioning the elevator cables is a complex, multi-step process. The elevator control system consists of two linked cables, designated “aircraft nose up” (ANU) and “aircraft nose down” (AND), that are connected to the pilots’ control columns and to the elevators. Which cable is in tension determines the elevators’ direction of motion. Two bellcranks — devices that transfer the motion of the cable around a corner — are located in the tail and beneath the cockpit floor. And inside the tail, both cables pass through turnbuckle assemblies, which allow mechanics to adjust the cable tension. In order to re-tension the cables, mechanics must insert “rig pins” through the bellcranks to attach them to the aircraft structure, holding the beginning and end of each cable in tension and keeping the control column and the elevators in the neutral position. Then the mechanics can use the turnbuckles to remove any remaining slack in the main section of the cable. When performed correctly, this process ensures that the elevators remain in the neutral position in the absence of any control inputs.

However, the cable re-tensioning procedure was embedded into the elevator cable rigging procedure, the set of steps that is followed when installing an entirely new elevator cable. The manufacturer’s intent was that mechanics follow the full rigging procedure when re-tensioning the cables; however, the inspector believed that they could skip the steps that he didn’t perceive as being directly related to re-tensioning. He instructed the mechanics on which steps they could skip based on his understanding of what was necessary. One of the steps on the full procedure called for the removal of several seats and part of the floor to gain access to the forward bellcrank. But the inspector instead showed the mechanics a small door that would provide access to the forward bellcrank’s associated rig pin hole, allowing them to insert the rig pin without the time consuming removal of cabin furnishings.

After discussing the steps to be skipped, the inspector left to perform other duties. He believed that because one of the mechanics had experience adjusting cable tension on other aircraft types, he wouldn’t need close supervision. But in his absence, the mechanics made a critical error. From the angle provided by the access door, they couldn’t see that when they inserted the rig pin through its hole in the aircraft structure, it didn’t pass through the forward bellcrank. As a result, the control column was not locked in the neutral position.

How the Raytheon mechanics performed the re-tensioning procedure. (Own work)

As the mechanics continued with the re-tensioning, this error began to compound. The aft rig pin had been inserted correctly, ensuring that the elevators stayed in the neutral position. But the control column became stuck in a nose down position, unable to rotate back to neutral because the forward bellcrank would strike the improperly inserted rig pin. So when it came time to reel in the slack, the control column had not returned to the neutral position; instead, it was applying tension to the AND cable. The mechanics tightened the turnbuckle until the cable reached the correct tension, unaware that extra tension was already being applied. When the mechanics removed the rig pins, the forward bellcrank was allowed to move freely again, reducing the tension in the cable. The result was that the elevator AND cable lacked sufficient tension. The control column has a fixed range of motion; now, when making a nose down input, some of that range of motion would be consumed by taking up the slack in the cable, causing the yoke to reach its forward stop before the elevators had reached the full nose down position.

Actual elevator positions vs. what the FDR recorded. Had the mechanics attempted to calibrate the FDR, they would have noticed the problem. Image source: the NTSB

Had the mechanics carried out the full elevator rigging procedure, they might have discovered their mistake. They skipped checks of the range of motion, believing they were not relevant, and they didn’t recalibrate the flight data recorder’s pitch sensor because they mistakenly thought that the plane had no FDR. Had they performed the calibration, they would have realized that the elevators would not move beyond 7 degrees nose down, well short of the manufacturer’s standard of 14 to 15 degrees. A visual inspection of the elevators revealed no problems, because their resting angle was difficult to detect from five meters below. A check of the control column’s range of motion also passed because this range wasn’t in fact restricted. Considering the job to be done, the inspector signed off on the work card and the plane was cleared to fly. Between the cable re-tensioning work and flight 5481, this plane completed eight flights without incident. Nose down inputs greater than 7 degrees were rarely required, so no pilots noticed that the range was restricted.

The route of AIr Midwest flight 5481. Map source: Google

The airplane’s ninth trip after the maintenance session was to be Air Midwest flight 5481 from Charlotte to Greenville-Spartanburg. As the passengers arrived at the gate, airline personnel set about calculating the weight of the airplane. On small airplanes like the Beech 1900, distributing the weight of the passengers and their baggage is a delicate affair; just a few heavier people or bags toward the back could throw the plane off balance. However, weighing each passenger individually can be prohibitively time-consuming, so airlines are also allowed to use average weights — in the case of Air Midwest, these were 77 kilograms (170lbs) for an adult, including personal items and carry-on luggage; 36 kilograms (80lbs) for a child; and 11 kilograms (25lbs) for each checked bag. Bags over 31kg (70lbs) were to be considered overweight and had to be accounted for individually. The estimated weights based on these figures were then passed on to the pilots for the final calculation. After tallying up the approximate weights of the passengers, crew, baggage, personal items, and fuel, Leslie and Gibbs determined the total weight of the aircraft to be 7,724 kilograms (17,028lbs) — just under the Beechcraft 1900D’s maximum takeoff weight of 7,765 kilograms (17,120lbs).

How center of gravity is calculated. Image source: Wikimedia commons

The pilots then used the estimated weights and their expected distribution to calculate the location of the plane’s center of gravity, or CG. The center of gravity is the theoretical point on which the plane would balance if you held it up on your finger tip. It must be within a certain range in order for the airplane to take off safely. This range is measured as a percentage of the mean aerodynamic chord (MAC), or the average width of the lifting surface (in this case, the wings). A center of gravity located 30% of the way from fore to aft along the mean aerodynamic chord would therefore be denoted as an “aft CG of 30% MAC,” as shown in the above diagram. On the Beechcraft 1900D, the center of gravity cannot exceed 40% MAC. On flight 5481, Leslie and Gibbs calculated an aft CG of 37.8% MAC, well within that limit. As far as they could tell, they were good to go.

The black line denotes the weight and CG limit for the Beechcraft 1900D. Overlayed are the plane’s calculated weight/CG and its actual weight/CG. (NTSB)

Unfortunately, Captain Leslie and First Officer Gibbs could not possibly have known that the figures they had been given were wrong. Had the airline actually weighed all the passengers and their baggage, they would have discovered that the real weight of the airplane was 8,028 kilograms (17,700lbs), well over the maximum takeoff weight. And because passengers and bags mostly sit aft of the wings, this extra weight also moved the center of gravity rearwards to 45.5% MAC — also far over the limit. In practice, the plane would still be able to fly even in such an unbalanced state, but it would encounter a large pitch up motion on takeoff that the pilots would need to actively suppress.

Once all the bags had been loaded, flight 5481 prepared to leave the gate. In the cockpit, the atmosphere was relaxed. Observing a nearby Bombardier CRJ-100 commuter jet, Gibbs remarked, “That CRJ sure is a good looking plane, isn’t it?”

Leslie chuckled. “Yeah, I sure wish I was flying it!”

Like true aviation buffs, they continued to ogle the various planes that surrounded them, excitedly pointing out everything from small Gulfstreams to huge Airbus cargo jets. They carried on their animated conversation all the way until engine start, at which point they were legally obligated to avoid off-topic discussions.

Shortly thereafter, the flight taxied to the runway and received its takeoff clearance. Then the pilots accelerated down the runway, holding the nose down to keep on the ground until reaching takeoff speed. When the plane finally lifted off, it did so vigorously and with little prompting. Within seconds, it became clear that something was wrong with the airplane’s pitch. First Officer Gibbs uttered an astonished “Whoa,” while Leslie pushed the nose down to try to return to a normal climb angle.

But no matter how hard she pushed down on the control column, the plane kept pitching up. “Help me!” she exclaimed.

Countering the pitch up motion created by the aft center of gravity required at least 9 degrees of nose down elevator, but due to the botched repair job, they only had 7 degrees available. Although they didn’t know it, there was nothing they could do to save their plane.

Flight 5481 climbed steeply and began to lose airspeed. “You got it?” Leslie asked.

“Oh shit,” Gibbs muttered. “Push down!”

Both pilots fought with all their might, but their efforts were useless. The plane reached an astonishing 54 degrees nose up, at which point a stall warning began to blare in the cockpit.

“Push the nose down!” Leslie shouted.

Moments later, the left wing stalled and the plane rapidly rolled into an inverted dive from an altitude of 1,150 feet above ground level.

“Oh my god!” Leslie exclaimed.

In the cabin, a child could be heard screaming, “Daddy!”

In frantic tones, Leslie transmitted a message to air traffic control. “We have an emergency, Air Midwest 5481!”

A simulation of flight 5481’s brief flight. Video source: Mayday (Cineflix)

As the plane plunged toward the ground, the pilots struggled to slow their acceleration and pull out of the dive. “Pull the power back!” Leslie shouted. But there wasn’t enough altitude to recover. Just as the plane began to level out, they ran out of room. In a last ditch effort to avoid an airport hangar, they rolled their aircraft sixty degrees to the right, streaking past the building’s massive rolling doors before slamming head-on into a concrete wall. The last sound captured on the cockpit voice recorder was Captain Leslie’s terrified scream.

Smoke billows up over Charlotte Douglas International Airport after the crash. Image source: Wikimedia commons

The impact with the side of the hangar completely destroyed the aircraft and instantly killed all 21 passengers and crew. An explosion ripped across the nearby parking apron, sending a plume of smoke up over Charlotte Douglas International Airport. Emergency crews rushed to the US Airways maintenance hangar where the crash occurred, but after quickly extinguishing the fire, they came to the grim conclusion that no one had survived. Save for a maintenance worker who suffered minor injuries due to flying debris, paramedics found no one to treat, and ambulances en route to the scene received a heartbreaking order to return to base.

Yellow tarps cover human remains at the crash site. Image source: planecrashinfo.com

Investigators from the National Transportation Safety Board (NTSB) soon arrived at the crash site and began trying to piece together the sequence of events. Looking into the maintenance carried out on the plane’s elevator cables the previous day, they discovered that mechanics had mistakenly left too much slack in the nose down cable. Further digging revealed a whole host of problems with the Huntington, West Virginia maintenance facility where the work took place. First of all, it lacked any formal classroom training for mechanics on the Beechcraft 1900D. Instead, they offered only on-the-job training, which can be beneficial if done correctly, but is often conducted by personnel with no teaching experience who can only give their trainees a haphazard and incomplete education. Furthermore, quality assurance inspectors weren’t supposed to provide on-the-job training (OJT) because maintenance and inspection tasks must be clearly separated. And yet, nobody at the facility appeared to be aware of this rule, least of all the inspector who gave OJT to the mechanics who misrigged the elevator cable. He also failed to supervise the mechanics, rendering him unable to detect their mistake. And finally, he signed off on the inspection form for the job, even though he was involved in the work — something which wasn’t explicitly prohibited, but clearly didn’t fulfill the inspector’s responsibility to act as a second set of eyes.

These failures proved symptomatic of a generally lax training environment at the facility. Many training records were incomplete, or listed completed tasks that could not possibly have been accomplished in the indicated time. Air Midwest provided no guidance on how OJT ought to be conducted. And just two months before the crash, Air Midwest had audited the facility and found it to be understaffed. It should have employed two foremen and two inspectors, but it had only one of each. Raytheon, the maintenance contractor, responded by hiring two new mechanics but had not moved to hire a new inspector or foreman at the time of the accident.

Officials observe the mangled wreckage of flight 5481. Image source: the Bureau of Aircraft Accidents Archives

According to FAA rules imposed after the 1996 crash of ValuJet flight 592, it was Air Midwest’s responsibility to ensure that its maintenance contractor was in compliance with company standards. But while Air Midwest had technically met the letter of the law, its oversight system was completely ineffective for one simple reason: the Air Midwest manager who was supposed to monitor the facility worked a day shift, while most of the maintenance took place during the night shift. There was simply no way for him to discover whether maintenance was being done correctly.

This raised another question: might other Beechcraft 1900Ds have incorrectly rigged elevator cables? To find out, the Federal Aviation Administration issued an airworthiness directive calling for immediate checks to ensure that the range of motion of Beechcraft 1900D elevators was within 1 degree of the manufacturer’s specified value. Of 296 aircraft surveyed, 40 failed the check, including 5 at Air Midwest, and 39 more failed a follow-up check after 100 flight hours. All were subsequently fixed. Although none were as badly misrigged as the accident aircraft, it was clear that action needed to be taken. As a result of these findings, Air Midwest revised its cable re-tensioning work card to explicitly state that mechanics should follow the entire cable rigging procedure, not just the steps they thought were relevant to adjusting the tension. Raytheon also fired one of the mechanics, demoted the inspector, and sent two other employees to retraining.

Firefighters thoroughly doused the wreckage to ensure that spot fires didn’t reignite. Image source: Rob Brisley

But for investigators, the search for the cause was not over. The plane had flown eight times with the misrigged elevator cable without encountering any problems. In fact, it was only when it encountered a situation that called for nose down inputs greater than 7 degrees that the misrigging became a problem. The cause of the pitch up that brought down the plane turned out to be an entirely different line of inquiry that would come to affect every airline in America.

Initial testing showed that with the weight and balance information listed on the flight manifest, flight 5481 should have had no trouble taking off, even with the faulty elevator cable. Digging deeper, investigators pieced together the actual weight of the plane using medical records of the passengers and estimates based on baggage remnants recovered at the scene. They found that the plane was likely more than 272 kilograms (600lbs) heavier than indicated on the load manifest, and that much of that extra weight was located in the back of the plane, resulting in an excessively aft center of gravity and a steep pitch up on takeoff. This raised a further question: did flight 5481 feature a set of unusually heavy passengers and luggage, or was there something wrong with the average weights in use across the country?

The average weights used by Air Midwest — 77 kilograms (170lbs) for an adult, including personal items and carry-on luggage; and 11 kilograms (25lbs) for each checked bag — came from FAA guidelines first published in 1965. These weights were specifically identified as nothing more than suggestions, and the regulation encouraged — but did not explicitly require — airlines to determine their own averages. The original source of the data for those suggested averages was even older. In fact, those average weights dated back to a survey conducted in 1936, significantly predating the FAA itself!

The result of a passenger weight survey at 15 airlines. Image source: the NTSB

Scientific studies have shown that over the decades, Americans have been getting heavier. To determine the impact of this trend on average passenger weights, the FAA sponsored a study that weighed actual passengers and discovered that the averages used by Air Midwest and other airlines around the country had been underestimating both passenger and baggage weights for years. In 2003, the average passenger — including clothes and a carry-on bag — weighed 88.5 kilograms (195lbs), an 11-kilogram (20-lb) increase over the 1936 data. The weight of the average checked bag had also increased by 1.7 kilograms (3.8lbs). This was in line with studies conducted by the civil aviation authorities of the UK and Australia during the 1980s, which also found that average passenger and baggage weights had increased. Despite the findings overseas, the FAA had not reviewed the suggested averages included in its published guidelines at any point prior to the accident.

The crash ignited a short-lived grass fire in an adjacent field. Image source: the International Aviation Safety Association

As a result of the study, Air Midwest increased its average passenger weight, including personal items, from 77 kilograms to 91 kilograms. Many other airlines also revised their averages based on findings from their own passenger weight surveys. The FAA ultimately introduced a new rule requiring that airlines periodically sample passenger weights to update their averages, while the FAA would update its own published averages on behalf of any airlines that used them. The NTSB, although pleased with this decision, felt that even more could be done. While the use of average passenger weights is usually a reliable method of ensuring adequate weight distribution on board an aircraft, small planes like the Beechcraft 1900D are vulnerable to random fluctuations in actual passenger weights. Even a few abnormally heavy passengers or bags placed in the rear of the aircraft could put the center of gravity out of limits without any indication on the load manifest. Therefore, the NTSB recommended that the FAA work to create a system that could reliably detect the actual weight and center of gravity of an airplane and feed that information directly to the pilots. As of 2010, the NTSB was pleased to note that the FAA was indeed working to develop this technology and had published guidelines that such systems must meet.

Updates on the investigation made front page news in Charlotte in the days after the crash. Image source: the Charlotte Observer

The NTSB also issued a wide range of safety recommendations relating to aircraft maintenance, including that maintenance facilities be surveilled in order to ensure that personnel are not skipping steps in procedures; that maintenance work on a flight control system always be followed by a complete functional check; that inspectors be prohibited from signing off on inspection items for a job on which they conducted on-the-job training; that airlines have employees physically present when their contractors are conducting maintenance work; that the FAA create official guidelines for on-the-job training; and that airline maintenance programs include human factors training. These recommendations represented part of a constant battle to improve the quality of maintenance in the United States — a fight that the NTSB appears to have won, at least for now. As of 2019, Air Midwest flight 5481 was the last fatal airliner crash in the US in which negligent maintenance constituted part of the probable cause.

The crash of Air Midwest flight 5481 still serves as a useful example of the danger of latent failures. Neither the excessive weight nor the restricted elevator travel could ever have caused a crash by itself, but when both came together, disaster struck. In effect, a failure that had been lurking under the surface for decades — the incorrect weight information being used to load American airliners — encountered a particular set of conditions that allowed it to escalate into a fatal accident. It’s a grim reminder of why no lapse in safety can ever be written off as inconsequential — after all, you never know when that seemingly small matter could be the last piece in a deadly puzzle that the universe has been putting together for years.

A memorial in Charlotte lists the names of the 21 crash victims. Image source: Robert Koehler

That so many young lives and promising careers were cut short so suddenly was a terrible tragedy. Captain Katie Leslie did all she could to save the lives of her 19 passengers, and for that she should be honored, regardless of the fact that she did not succeed. In fact, her last second turn to the right might have prevented an even worse disaster, as the plane narrowly avoided crashing through the doors of the US Airways hangar where dozens of employees were hard at work. Although she could not save her own life, there are workers in that hangar who are only alive today because Captain Leslie never stopped trying to fly her plane.

In one final piece of the story, the parents of 18-year-old crash victim Christiana Grace Shepherd helped ensure that the memory of all the passengers and crew that died that day is properly respected: in a rare victory for human decency, they successfully secured a formal apology from Air Midwest for its role in the accident. Perhaps most in need of the apology were the unfortunate pilots, who had to face a situation from which recovery was impossible — something that every airline must ensure never happens again.

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Analyzer of plane crashes and author of upcoming book (soon™). Contact me via @Admiral_Cloudberg on Reddit or by email at kylanddempsey@gmail.com.