Medical devices are designed to function in environmentally controlled locations, such as stationary hospitals, and not within the harsh, dynamic aircraft environment. Yet, the same medical devices used to care for patients in a hospital environment are often the most capable devices for patient care during transport from one facility to another. These missions are called aeromedical evacuation (AE) missions, and they provide life-sustaining care for a vast array of patients. However, because the devices are designed for a controlled environment, concerns they may adversely affect the operation of aircraft systems must be addressed. Conversely, the aircraft may adversely affect the proper operation and efficacy of the medical equipment. Failure of medical devices during in-flight medical care may result in exposing patients and aircrew to hazardous situations.

In an Acceleration Test, a lightweight medical device was mounted in a small, ruggedized case. The item weighs 6.23 pounds, and all components of the system are con- fined inside the case. A tensile test was employed on the handle and the handle mount- ing pins to evaluate the EUT. The ELT setup for a 9g acceleration test is shown under the two worst-case loading scenarios, assuming that the equipment may potentially slide out from between the strap and litter.
All medical equipment identified for use on U.S. Air Force AE fixed-wing aircraft must undergo a safe-to-fly (STF) test process before the STF certification can be issued by the authorizing aircraft system organizations. A typical STF evaluation features three phases:

  • Baseline Assessment — The purpose of the baseline assessment is to verify that the equipment under test (EUT) operates in accordance with the manufacturer’s specifications and the operator’s manual. The EUT is evaluated for adherence to optimum human factors and basic electrical safety requirements.
  • Laboratory Tests — The purpose of the laboratory testing phase is to simulate the operational in-flight environment through testing, which is modeled after a series of worst-case event scenarios such as a rapid decompression event or other aircraft incidences or mishaps. Typical laboratory tests include vibration, electromagnetic interference (EMI), hot and cold temperature extremes for operationaluse and storage, humidity, explosive atmosphere, altitude, rapid decompression, and acceleration.
  • In-Flight Assessment (IFA) — The purpose of conducting an IFA for AE equipment is to perform functional checks onboard the aircraft during an aeromedical readiness mission. The controls, visual and audible alarms, and display screen of the AE equipment are observed and evaluated during the flight. Test personnel interact with and solicit feedback from AE crewmembers regarding the device’s form, fit, and function. These data are used to identify any remaining issue with the use of the device that may not arise during the simulated laboratory test scenarios.

The various types of analysis and test methods raise questions as to what the correct decision process is for selecting the most appropriate method for STF testing of AE equipment. The process used in this work has proven to be well suited for identifying the most appropriate test method — one that not only represents the most appropriate and effective test method, but also minimizes the use of available resources. This process includes testing both structurally simple and complex equipment, and successfully introducing the use of the Equivalent Load Testing (ELT) method, which permits the use of alternative testing approaches, such as pull testing and tensile testing.

The initial task is to evaluate the test article for any inherent safety concerns. For example, the team identified that AE devices weighing less than five pounds are usually perceived to pose no substantial risks due to acceleration; therefore, a quick assessment and description of the equipment tie-down were found satisfactory. When the test team finds product-level tests are required, the article is tested in a physical environment, namely sled tracks or centrifuges, or a model representing the product can be developed and analyzed using finite element analysis (FEA) simulation.

The component-level tests refer to the tests specific to a sub-component or a structural member of the equipment, i.e., mounting brackets, screws, beams, straps, etc. When the decision is a component-level test type, the team applies the ELT method by conducting an in-depth evaluation of the test article, identifying the critical areas within the item, and noting any potential safety concerns within the environment. The outcome of this evaluation is a list of tasks that includes a series of tests, analyses, inspections, and evaluations.

The ELT terminology refers to the constant, or approximately constant loading that is applied to the test item for a finite duration. The magnitude, point of application, and the direction of the load are equivalent to the properties of inertial loads and moments generated in an acceleration event under g-levels. The magnitude of the equivalent load is determined using the magnitude of the sustained acceleration load that would be exerted on a tie-down component, a critical part, or a joint of the equipment in a physical test. For example, 9g of acceleration introduces 90 pounds inertial load on a device with 10 pounds of weight; and the critical areas, such as tie-downs and restraints, must be tested to verify they are capable of restraining the inertial loads and moments.

This work was done by Ismail Cicek and Gary S. Beisner of the U.S. Air Force for the Defense Acquisition University. DAU-0001


This Brief includes a Technical Support Package (TSP).
Process for Testing Aeromedical Equipment

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This article first appeared in the August, 2011 issue of Defense Tech Briefs Magazine.

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