To Err Is Human

On April 27, 2000, a Canadian commercial helicopter pilot (who was also a helicopter flight instructor) took off with a maintenance engineer in a Bell 206 from an airport in Quebec to perform a test flight. Five minutes later, they disappeared from radar. The Transportation Safety Board of Canada determined (rather quickly) that the main rotor hub and rotor blades had departed the aircraft in flight. What makes this occurrence particularly worth discussing is that both individuals knew that the mast nut, which is what holds the rotor head in place, had been removed two days earlier. In addition, two other individuals who actually took part in the removal of the mast nut — and who were present when the aircraft took off — didn’t remember that this critical part was still sitting in the hangar.

How could such a thing happen?

The answer is human error, of course. People get distracted, complacent, they make assumptions, and they see what they expect to see during a preflight, but … the mast nut? (In the Robinson helicopters that I’m familiar with, a similar item holds the main rotor assembly on, and it goes by the colloquial name of the Jesus bolt, for obvious reasons.) Unlike airplanes, from which you would have to painstakingly remove various fairings to see the strut bolts holding the wings onto the fuselage, in helicopters it sticks out like a sore thumb for all to see. This item is always a de rigueur preflight inspection item, according to the aircraft flight manual — not to mention one’s basic instincts for self-preservation.

There are lessons to be learned here for all of us.

Late in 1999, a Canadian pilot purchased a Bell 206B JetRanger. Although he held a helicopter pilot license, he hired a commercial helicopter pilot (who was also an instructor) to oversee routine aircraft operations. He also arranged for maintenance and repair of some discrepancies. Transport Canada issued a registration in March of 2000, and an airworthiness certificate in April. Among the repairs needed were a defective transponder and minor corrosion in washers on the droop restrainers (which limit the sagging down of the main rotor blades at low rpm). A single nut atop the rotor mast secures both restrainers as well as the rotor head itself. (As with most such critical parts, this is further secured by a lock wire to prevent the nut from ever unscrewing in flight.) The installation of this one part is so critical that Canadian regulations require that two aviation maintenance engineers, or one AME and a qualified pilot sign it off. On April 25, the AME asked an apprentice AME to remove the mast nut and droop restrainers for stripping and priming. Due to an error observed by a second AME, the parts were set aside to be stripped and primed again the next day, and carefully placed on a nearby tool box and work table.

On April 27, the primary AME was doing some paperwork and the apprentice AME was working on an another aircraft right beside the 206B. The pilot arrived that morning and asked the AME to work on the JetRanger because its owner wanted to fly it the next day. The AME dropped what he was doing and complied. (It is unknown whether the pilot had been advised that the droop restrainers had been removed.) The pilot and AME then flew off to check the transponder with the Montreal control center, apparently unaware (or in the case of the AME, forgetting) that the only things holding the rotor blades on were the pitch control rods for the main rotor, when then the inevitable happened.

No entries were made in any logs, job lists, inspection sheets, or anywhere (contrary to Canadian regulations), and there were no warning notices or flags in the cockpit indicating that the aircraft was temporarily non-airworthy (which is not actually required). The pilot was a chief flight instructor, and had taken a Transport Canada decision-making course just over a year earlier that covered performance issues and how to counteract human errors. It was noted that the 15-year AME had worked 12-hour days, nearly seven days a week, for the last several months. Although it was thought likely that the pilot had not explicitly been made aware that the mast nut had been removed, both the mast nut and droop restrainers can be seen not only from the cockpit roof, but also from the ground. The pilot’s peers had considered him to be a conscientious pilot who didn’t neglect pre-flight checks. As mentioned, the other two people who had participated in removal of the critical parts were also present when the aircraft took off. One would guess that they simply didn’t remember. It is also quite likely that the pilot felt secure in the knowledge that the responsible AME was on board when he took off.

Although the apprentice AME was the one who removed the mast nut, he knew that both the AME and the AME’s partner knew that it had been removed. So surely, someone on the team would remember, right? And certainly “the boss” would take care of documenting such a critical action, right? The day of the accident, the apprentice was focusing on another aircraft, so it would be natural to assume those in charge would keep track of the loose ends. Such assumptions were stock in trade for a slapstick exchange, but when it comes to a dialogue like: “Mast nut? I didn’t check the mast nut. I though you checked the mast nut!” then it becomes about as funny as a heart attack. Lastly, the ill-fated crew was focused on the transponder — and only that the new transponder would operate properly. We take off expecting the engine won’t quit. The list could go on. This additional human characteristic, expectation (read: assumption), also rides along with us on every flight (as well as during the preflight, I hasten to add).

Diffusion Of Responsibility: There is a term in psychology that relates to situations in which a group of individuals assigned a simple task fail that task because each assumes or expects another to have taken responsibility for it.

Pilot In Command: There is a term in aviation that strictly defines the individual solely responsible for the safety of any given flight.

Yes, the AME got jerked around a bit. Yes, the chief AME was horrendously overworked, and fatigue was probably an issue. And yes, the pilot took that technician “out of his groove” by making his impromptu request. I know I absolutely despise when I’m in the middle of something, and someone (coworker or muckety-muck) yanks me away to put out someone else’s fire. I am reminded of the popular poster captioned “An emergency on your part does not constitute an emergency on my part.” Case in point: when you’re in the middle of your preflight, and someone walks up to chat, watch out. Why? Distractions are notorious for erasing critical items right out of your short-term memory. That’s the next ingredient for this kind of disaster: distraction.

One version of “uncoordinated” flight involves two pilots of equal experience inside the same cockpit, where duties and responsibilities are not clearly delineated. This situation can be at least as dangerous as crossed controls at low airspeed and high angles of attack — another form of uncoordinated flight. In retrospect, a flight like this one, where the pilot may have abbreviated or deferred a proper preflight because, after all, he had an experienced AME aboard to vouch for the aircraft’s airworthiness, was more than dangerous; it was deadly. Perhaps he had done a preflight, but just missed that one missing part. Perhaps he didn’t want to risk offending the AME with a walk-around. After all, how many times has it happened to you? You do a meticulous preflight, and then the CFI comes out to the airplane and checks the fuel and oil. (I know how that used to make me feel: “Hel-lo? Do you really think I wouldn’t check that, pal?” But egos belong in the pilots’ lounge, folks — don’t take it personally. (I’ve learned not to.) Call it a healthy mistrust of human nature (including mine). Call it anti-complacency.

THE BOTTOM LINE: We’re only human. The moral of this story, and all others that seemingly invoke the realm of ghoulish fascination for inquiring pilots who want to know, is that we have to pay equal attention to our own nature — qualitative, abstract, and often subtle though it may be. Because these things affect us all, without exception.