Twenty years ago this month, Captain Ted Thompson and First Officer Bill Tansky lost the fight to keep their McDonnell Douglas MD-83 in the air while making an emergency landing at Los Angeles airport. After a steep dive, the plane flipped over, and 80 seconds later Alaska Airlines Flight 261 plunged into the Pacific. All on board were killed.

Grease played a central, if at times misleading, part in the investigation into the cause of the crash after the National Transport Safety Board began the painstaking process of sorting through the debris.

Thumping noises audible on the cockpits voice recorder shortly before the crash turned out to be the horizontal stabilizer, which stops the planes nose from pitching up and down. The thread of the nut holding the stabilizers trim jackscrew had stripped off, jamming it into a nose down position. NTSB investigators initially suspected the grease on the jackscrew assembly had not lubricated the part effectively.

A year before, Alaska Airlines had swapped the Mobilgrease 28 recommended for the assembly by the planes maker, McDonnell Douglas, for Aeroshell 33, a fairly new product developed for Boeing. The two greases were not the same specification or chemistry – one had a clay thickener and the other a lithium soap. Despite this, the airline got the go-ahead from McDonnell Douglas, which said it would carry out a study to determine the acceptability of Aeroshell 33 grease for use in Douglas-built aircraft, reported the NTSB. McDonnell Douglas completed no such study.

The airline claimed the swap was an effort to standardize greases used across its fleet of Boeing 737s and MD-80s. Boeing acquired McDonnell Douglas in 1997. With so many different products being used, some operators felt the margin of error was high. In addition, inventory control was difficult, particularly where mixed fleets were operated, said Terry Dicken, chairman of the European Lubricating Grease Institute who at the time was the chairman of an ASTM committee on aviation grease. Dicken was invited by the NTSB to join the probe as an independent observer.

Many specifications and products were over 30 years old. There was an obvious driving force from the airline operators for a change in the number and way specifications for aircraft were developed and managed, he told LubesnGreases.

When closely examined, the jackscrew appeared to have traces of two greases on it, samples of which were sent to a U.S. Navy laboratory for analysis. At that point, the NTSB suspected the unauthorized mixture had not effectively lubricated the parts. Alaska tried to insist that Aeroshell allowed corrosion to occur and was to blame for the accident, making Boeing and Shell culpable. But Boeing had warned about mixing greases in a service letter to operators in 1993.

Yet the lab ruled out both corrosion and mixing. It was the lack of any lubricant at all that caused the failure after months of grinding wear. An engineer should take four hours to apply greases to an MD-83 stabilizer assembly. Although maintenance logs before the crash showed the jackscrew had been lubricated, the last application before the crash took one hour, implying that an insufficient quantity was applied or the part was skipped.

In a hangar environment, it tends to be the lower-qualified people doing lubrication. Grease nipples are often missing, so that particular point gets left, an aeronautical maintenance engineer told LubesnGreases on condition of anonymity.

After the disaster, a committee was set up comprising major oil companies, technical associations, airlines and manufacturers to train and educate operators, Dicken said.

Mechanical failure accounts for a fifth of air disasters. In this case, the absence of a humble yet crucial lubricant turned the world upside down for the families of the 88 people on board.