Without even having to think about it, most (if not all) of us accept that accuracy in measurement is desirable. After all, if you’re not going to produce accurate results, why measure something in the first place? In industrial and commercial as opposed to domestic or everyday settings, the importance of the quest for accuracy is even more obvious.
Making critical instruments (like MRI machines) or structures (like bridges) involves numerous processes and parts. These components need to be manufactured (at scale, generally speaking) – which implicitly leaves little margin for error if the final product is to perform as required. And for that outcome to be achieved, components must meet rigorous standards of accuracy among other metrics.
And yet, errors in measurement occur even in the most demanding environments. In this blog, we’ll look at three famous examples to underline the general point about why investment in measurement tools is absolutely critical to manufacturing success.
Space, 1999 – a simple conversion error
We’ll start with a simple human error which, nevertheless, clearly illustrates the costly consequences of getting measurements wrong – well, not so much the measurement as such, but rather the terminology and the units adopted to provide the requisite measurements.
In 1999, due to a simple error in communication, NASA lost a $125-million spacecraft – the Mars Climate Orbiter. Why? Because engineers had failed to convert from Imperial to metric measurements when exchanging vital data before the craft was launched!
In brief detail, a navigation team at NASA’s Jet Propulsion Laboratory had used the metric system of millimeters and meters in its calculations, while Lockheed Martin Astronautics in Denver, which designed and built the spacecraft, delivered crucial acceleration data in the Imperial system (of inches, feet and pounds). The outcome: JPL engineers mistook acceleration readings measured in Imperial units of pound-seconds for a metric measure of force known as newton-seconds.
The result of the error – which wasn’t detected over the entire nine-month period it took the spacecraft to make its 461-million-mile flight to Mars- was that over the course of the journey, the miscalculations were enough to throw the spacecraft so far off track that it flew into the Martian atmosphere misaligned and was destroyed when it entered its initial orbit around Mars. $125 million of spacecraft wasn’t the only thing lost; billions of dollars of investment in research were also wasted.
Alaska Airlines flight 261 – a failure of maintenance and continual measurement
Alaska Airlines Flight 261 crashed into the Pacific Ocean in January 2000 after a catastrophic loss of pitch control, killing all 88 on board. Pitch, if you’re not familiar with the term, refers to the rotation of an aircraft around its lateral axis which runs from wingtip to wingtip and passes through the aircraft’s center of gravity. Pitch motion is controlled by the elevators on the tail of the aircraft.
In a nutshell, investigation by the NTSB determined that inadequate maintenance had led to excessive wear and the eventual failure of a critical control system during flight. What exactly happened, and why?
The flight had taken off and reached its intended cruising altitude without issue. But once cruising, the pilots identified a jammed horizontal stabilizer. This prevented the operation of the trim system, which is used to make slight adjustments to the flight control surfaces to keep the plane stable in the air (in other words, to control pitch). Neither the flight crew nor company maintenance – called for assistance – could determine the cause of the jam and repeated attempts to overcome it were unsuccessful.
At a certain point, the flight crew did manage to use one of the plane’s systems to unjam the stuck stabilizer but having done so, it quickly moved to an extreme «nose-down» position, resulting in an almost vertical nosedive that dropped the plane from about 9,600 m to between 7,000 and 7,300 m in around 80 seconds. Disaster was narrowly averted by the pilot’s actions pulling back forcefully on the control yoke.
Shortly after this, after agreeing an emergency diversion to the flight, the Cockpit Voice Recorder heard the sound of a number of «thumps», followed by an «extremely loud noise». We now know that this was an overstrained jackscrew assembly failing completely with the jackscrew separating from the nut holding it in place. As a result, the plane’s horizontal stabilizer failed completely at 5,400 m and the aircraft rapidly pitched over into an unrecoverable dive, plunging into the ocean below. It was destroyed by impact forces, and all aboard were killed.
After a recovery operation, both the horizontal stabilizer trim system jackscrew and the corresponding acme nut, through which the jackscrew turns, were found. The jackscrew was constructed from case-hardened steel and is 56 cm long and 3.8 cm in diameter. The acme nut was constructed from a softer copper alloy. As the jackscrew rotates, it moves up or down through the (fixed) acme nut, and the resulting motion moves the horizontal stabilizer for the trim system. Upon examination, the jackscrew was found to have metallic filaments wrapped around it, which were later determined to be the remains of the acme-nut thread.
Detailed analysis of the two parts estimated that 90% of the thread in the acme nut had already worn away before the flight and that it had finally stripped out completely during it. Once the thread had failed, the horizontal stabilizer assembly was subjected to aerodynamic forces that it was not designed to withstand, leading to the complete failure of the stabilizer assembly. Based on the time since the last inspection of the jackscrew assembly, the NTSB determined that the acme-nut thread had deteriorated at 0.30 mm per 1000 flight hours, much faster than the expected wear of 0.025 mm per 1000 flight‑hours. The point is, safety margins are minimal and maintenance without measurement invites disaster.
Hubble telescope – small error, potentially huge cost
Most of us know the Hubble telescope for the space images we’ve become familiar with over the years. It’s one of NASAs resounding success stories despite getting off to a near disastrous start. Why? Because the first images it sent back were fuzzy. The reason, it was discovered, was that the telescope’s main mirror was too flat. The error was small – only 2.2 microns, or about 1/50th the thickness of a human hair – but that was enough to put the entire project in jeopardy. One theory is that a speck of paint on a device used to test the mirror resulted in distorted measurements, but we’ll never know for sure.
Fortunately, the problem was fixed using an instrument called the Corrective Optics Space Telescope Axial Replacement (Costar). This cancelled out the error in the main mirror, by matching it in reverse.
The dangers of inaccurate measurement
The point of recounting these three stories is to underline the critical importance of accurate measurement and, by extension, the use of accurate measuring tools. While each example is interesting in its own right, the subtext is that inaccurate measurements and measuring tools were involved in all three and together present serious risks to both people and equipment in any engineering project. The dangers are worth spelling out.
Inaccurate measurements and measurements and measurement tools at a minimum risk:
Safety
Even a small discrepancy in size, distance, angles, or other variables can result in manufacturing components that don’t fit together correctly, in turn leading to breakages and failure of equipment under operational conditions resulting in accidents which, as we’ve seen, can sometimes be catastrophic.
Material waste
Beyond safety, there’s scarcity. Inaccurate measuring tools result in excessive and unnecessary raw material waste, a particularly undesirable outcome as raw materials become both scarcer and more expensive to acquire in the current economy.
Cost
When measurements are sub-optimal, cost overruns invariably follow (because incorrectly measured parts must either be discarded or re-manufactured) and for large-scale engineering projects, this can be a serious issue. Measurement errors can also lead to the purchase of incorrect amounts of material, sometimes delaying project progress. Additional costs can force a project to go over budget, a scenario that might have been avoided with better measurement tools.
There’s also something else. Next time you hear the words NASA, Hubble telescope, or Alaskan Airlines, you’ll probably think twice about them after reading this blog. That’s because measurement errors also lead to reputational damage when project and products fail due to inaccurate measuring tools being deployed. In a competitive economic landscape, that matters too.
About Conoptica
Conoptica is the market leader for measurement equipment in the wire & cable industry and has been providing high-tech camera-based measurement solutions for the metal working industry since the 1993. We make sure that the metal working industry has access to key quantitative data about their products and tools.