The (failed) military missions that paved the way for new controls on threadings

It is early 1979, in Iran. The last Shah of Persia, Mohammad Reza Pahlavi, has just left the country, in the wake of a wave of protests and riots, and has gone into exile. His main opponent, Ayatollah Khomeini, can return triumphantly to Tehran. The situation, however, is still unstable and anti-Americanism is rampant, especially when it becomes known that the Shah is being opened the doors of the United States to receive medical care.

On November 4, 1979, a group of several hundred Iranian students storms the US embassy and takes 52 diplomats and American citizens hostage. The American public opinion rages against the act, seen as a violation of the principles of international law.

27 January 1980: Operation "Canadian Caper"
This is the story told by Ben Affleck's film Argo, if you haven't seen it yet. In the midst of the Islamic revolution, the CIA and the Canadian government manage to rescue six U.S. diplomats who had taken refuge in the Canadian Embassy. However, the vast majority of the hostages remain in the hands of Iranian students.

April 24, 1980: Operation "Eagle Claw"
President Carter decides to implement a complex rescue plan; the operation is called Eagle Claw.
Airplanes and helicopters are sent on a secret mission to transport an assault force and attempt a takeover: the first stopover is called Desert One, a desolate and uninhabited location 200 miles southeast of Tehran. Eight helicopters from the aircraft carrier Nimitz must land there; at least six are needed to complete the mission.

Although the plan had been meticulously worked out, the mishaps multiplied. A first helicopter reports a rotor blade failure. A second helicopter – struggling with a sudden, unexpected dust and sand storm – detects anomalies in the navigation instruments and decides to abort the mission. A third one reports hydraulic problems. Five helicopters are left: not enough. The ground commander decides to abandon the rescue mission, informs his commander in Egypt, who in turn forwards the recommendation through the chain of command to Carter. The President reluctantly approves. Noise, dust, and confusion reign at Desert One at that point. While maneuvering to move, the rotor blade of one of the helicopters strikes an aircraft. Eight men die in the resulting explosion - a disaster.
Although President Carter continues to work to free the hostages, they are held captive until January 20, 1981. First day of the Reagan presidency.
What happened?
The Iranian revolution, the hostage crisis, and the handling of the Eagle Claw mission all played a major role in sinking President Carter's popularity, and he lost re-election.
April 15, 1985
Another accident occurs to a UH-60A Blackhawk helicopter; 9 individuals lose their lives. The failure is caused by a fatigue crack that originated near the base of the first thread. Such spindles are normally replaced every 6,700 hours; this one broke after only 1,080 hours of use.
March 12, 1989
A CH-3E helicopter takes off from U.S. Libby Army Airfield in Arizona on a training mission carrying 15 people. After only 15 minutes of flight, it crashes, disappearing from air traffic control systems without a trace. A board of inquiry determined that one of the craft's rotor blades had separated from the main rotor. The spindle of the detached blade remained attached to the rotor.
Why did such perfect machines – the pinnacle of American technology at the time, designed to withstand anything – fail and crash on several occasions in seemingly inexplicable ways? In 1991, an investigation was conducted involving several government agencies, organizations, and companies, including the Johnson Gage Company, the results of which are still accessible in this document.
In many of the accidents cited, malfunctioning threaded parts caused the helicopter rotor blades to separate from the main rotor head.
But how could something like this happen?
Design error? No way, dozens of other helicopters of the same type worked perfectly.
Construction error? Impossible, every single piece was checked meticulously and with the utmost care.
Human error? Difficult, but not impossible.
Components intended for the construction of military vehicles are all individually serialized because the manufacturing process must be traceable in case of need.
Broken parts were checked and rechecked several times with different methods and tools and, eventually, the cause of the breakage was discovered.
What did they find out?
The dimensions measured with the required control scheme read the maximum material condition, i.e., they saw the part larger than it was.
Today:
When we measure a threaded part by traditional methods, by go/no-go gauges, or, for added safety, by crozes, wire thread measuring, or other methods, and the dimensions turn out to be exact, are we really sure we know what we are measuring?

Accidents in American helicopters have shown that this may not be as easy! Although all of the parts were found to meet the design specifications, they broke.
The parts had defects that the control plan provided in their production was unable to "see."


A thread is a complex geometric figure that varies three-dimensionally depending on its rotation. If we carry out a careful investigation, we realize that in this shape there can be single errors or heaps of errors that cannot be seen with common control methods. Not only that, there may be "formal errors" that have been opportunely exploited to obtain screw types with mechanical characteristics that are particularly effective and/or easier to produce.
Usually, the commonly used control methods, be they electronic (eddy currents, lasers, cameras, infrared, light beams), or traditional (crozes, wire thread measuring, thread ring gages, profile projectors, etc.) are not able to control individual dimensions over the entire thread but only the maximum material condition, i.e. they read the part larger than it is.
In other words, these systems measure the points where they are in contact with the heap of features but, during the thread development, deformed or distorted areas with less material may occur.
Somewhere, the missing material will result in the thread section being less strong.
This is what happened to US helicopters.
The investigating committee determined that in reality there are two parameters that must be considered when evaluating the accuracy of a thread:
- the pitch diameter;
- the functional size.
What is the pitch diameter of a thread? It is the diameter of the virtual cylinder that passes through the exact point where the full part of the thread and the empty part are perfectly equal. In other words, imagine filling in the gaps between the grooves with the material found in the full part. “It is defined as the diameter of the fastener where the thickness of the thread and the width of the groove between two threads are equal.” Screw resistance has been calculated based on this principle for decades.
What is the functional size? The functional size is the average diameter of the thread corrected from possible shape errors. It allows calculating and securing screws in particularly demanding applications. It can be obtained by adding to the pitch diameter – in the case of an external thread – or subtracting from the pitch diameter – in the case of an internal thread – the cumulative effects of the variations from the specified profile, including those in the lead and other characteristics over a specified length. The effects of conicity, out-of-roundness, and surface defects may be positive or negative on external or internal threads. The functional size is the one we normally detect during the measurements of our pieces with traditional methods and it is the one that always records the condition of maximum material. Thus, the average diameter of our thread is almost always smaller than we have measured.
The military fasteners investigation commission has also developed methods and inspection tools that allow measuring all parameters and errors of threads.
The difference between the functional size value (obtained by measuring the part with "traditional" tools) and the theoretical size of the pitch diameter will tell us how accurate our thread is. But we'll talk about that on another occasion.   Giovanni Stagni
Fasteners Consultant  
Ed. For comments and feedback on this article, please email us at [email protected]

Image credits: US Navy, Public domain, via Wikimedia Commons