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On the surface, "Which is better?", is a misleading question to ask. At the heart of any standard is a purpose, a requirement as the military would say, and an achievable outcome. The reason it is a misleading question is that when a product meets a Mil-Spec (the use of Mil-Spec is a generic term) such as Mil-DTL-87174A (HGU-55/P) Fixed-Wing Helmet, the spec includes design specifications, test specifications, and test methodologies. The product must meet all of the Mil-Spec requirements to include the color. The only hint of a design specification in DOI/USFS Aviation Helmet Standard – Ver 4.3 is paragraph 6.2 Test Line and note that “Each helmet shall have a protective surface of continuous contour at all points on or above the test line.” Mil-DTL-87174A specifies down to the composite material used, and the thickness of the shell, as well as references to specific drawings or source documents. Additionally, a Mil-Spec is also designed to support the government contracting process. In other words, we’re comparing apples and oranges when we ask which is better.
Both helmet standards specify that the helmet must be compatible in their defined operational environment. Basically, you must be able to communicate and see while using the provided protection. DOI/USFS ensures this by requiring a specific test methodology that must be passed for the helmet to be compatible with their standard. A Mil-Spec tells you the design that must be used and then specifies the test methodology and standard that must be passed. DOI/USFS has left the design up to the manufacturer; only the outcome is defined.
So, the real question would be something like this: which impact standard is better? Now, perhaps, we can compare apples with apples.
DOI/USFS Aviation Helmet Standard, Ver. 4.3 specifies (FMVSS No. 218) the use of both hemisphere and flat anvils for impact testing. Starting with the SPH-4 in 1982, U.S. Army helicopter helmets only use flat anvil surfaces for impact testing. “All impacts for performance assessments are conducted onto flat impact anvils. The hemispherical anvil was eliminated after ALSERP [Aviation Life Support Equipment Recovery Program] investigators revealed less than 3 percent of helmet impacts resulted from hemispherically shaped objects, while flat surfaces accounted for over 60 percent.” USAARL Report No. 98-12, U.S. Army Aircrew Helmets: Head Injury Mitigation Technology (Reprint), Pg. 10
So, what was the rationale behind the U.S. Army eliminating the hemisphere from impact testing? Yes, less than 3% shows a minimal probability of risk, while quantifying that minimal. The reasoning goes more in-depth than probabilities and statistics. The reason to eliminate hemispherical threats from testing requirements is that when designing a helmet for impact protection, hemispherical surface risks, and flat surface risk design characteristics may be considered as opposed to one another. Hemispherical surface risks require a stiff shell and higher density impact liners. Flat surface risks need pliable shells and low-density impact liners.
You can never do both well. You must compromise. The U.S. Army has chosen to protect against flat surface risks, while DOI/USFS Ver. 4.3 has chosen to protect against both surface risks. Which is better?
As an aside, I want to be very clear that I believe DOI/USFS should require all “approved” military helmets to be tested against their standard just like every other manufacturer must do.
An incorrect assumption was made by DOI/USFS that helmets that were already in use by the military, whether helicopter or fixed-wing, use an equal to or better impact standard than Ver. 4.3. This assumption led DOI to approve, without testing against Ver. 4.3, military helmets.
To oppose DOI/USFS’s assumption, I offer the publicly available USAARL Report No. 96-04, Performance Assessment of the HGU-84/P Navy Helicopter Pilot Helmet. usaarl.army.mil/TechReports/96-4.pdf
In this report, the HGU-84/P is tested against the HGU-56/P helmet standard, FNS PD 96-18. Comparing the results from 96-04 on an apple for apple comparison with DOI/USFS Ver. 4.3 standards. The HGU-84/P failed the impact standard against a flat anvil with a peak acceleration greater than 300 g’s at 6 m/s, failed the chinstrap strength/elongation standard, and failed the sound attenuation standard at 125 Hz, 250 Hz, and 500 Hz.
For simplicity, I’ve included an excerpt from DOI/USFS, and I will add the test procedure and standards document for the following items:
DOI/USFS Aviation Helmet Standard – Ver 4.3
The following identifies the sequence for testing:
g. Communication Equipment Talk-Listen Testing as necessary, §6.10 MIL-DTL-27467B