In designing PCBs that are used for aircraft delivered to the mil-aero market, there are some similarities to other products within the broader military market sector. Satisfying performance metrics relative to shock, temperature, corrosion, long-term product suppor,t and the ability to adhere to outdated standards and specifications can be challenging in this vertical market space. This article will address these factors and what is necessary to support the product development requirements. Note: The primary product customer that is referred to in this article is the Air Force with some crossover to the Navy, specifically for those airplanes that are used on Naval aircraft carriers.
Similar to the other vertical market segments such as satellites, modern military aircraft contain a vast array of electronic equipment ranging from guidance computers, GPS instruments, radar, multiple radios, and weapon deployment products. Thus, the signal integrity requirements are as much the same as those required for computers and Internet products. There is nothing in an airplane that is not used elsewhere.
The shock requirements for military aircraft are not as stringent as those required for ground equipment, missiles and satellites. For airplanes, the greatest shock impacts are encountered during landing. However, it should be noted that for aircraft that are carrier-based on naval aircraft carriers, the shock requirements are applied to both the take-off and landing sequences. Therefore, it is no surprise that a large portion of PCBs developed for military aircraft are housed in ruggedized containers. Further, there is no requirement for Rad-hardened ICs but the packages containing them are primarily non-organic or ceramic.
In fighter airplanes, there are G forces (the measurement of the type of force per unit mass) at work. They are not considered an element of shock but they do cause stress. Nowadays, G-forces are not an element of concern but in previous product implementations, components could come loose under G- forces.
It should also be noted that similar to satellites and missiles, there is a concern for protecting military airplanes against lightning and EMP. Both can impact the operation of electronic components within a plane even if those components are relegated only to the cockpit.
A large portion of the components that go into an aircraft are located in the cockpit. It is both pressurized and temperature controlled. Lee Ritchey, Founder and President of Speeding Edge, notes, “I don’t think that any of the electronics in a fighter airplane are close enough to the surface of the airplane to have a heat problem. But, there is a concern about making sure that the equipment can start to function when it is cold. Components, particularly capacitors, change dramatically when it is cold and that can impact the overall performance of a PCB.”
For those electronics that are not located in the cabin, they have to be cooled when there is no air to cool them. Again, heat pipes and extra layers of metal on the PCBs that conduct the heat away from critical ICs are used.
Both types of corrosion (water and salt) can compromise the overall performance of any airplane. For example, 21 years ago, there was an Aloha airlines flight of a 737-200 jet that lost its roof while flying at 24,000 feet between Hilo and Honolulu, Hawaii. Ritchey explains, ”One of the factors that contributed to the damage was corrosion. When the plane was on the ground it was filled with high humidity air. When it got up into the level of the atmosphere where the temperature was -60° below Fahrenheit that moisture would condense on the walls. Eventually, this condensation led to the rust that eroded the rivets that held the roof onto the top of the plane.”
For those airplanes that are based on naval aircraft carriers, the other issue of concern is the corrosion that comes from the salt-air environment and the resulting galvanic cells (batteries) that are formed. These issues must be addressed when designing corrosion resistant electronics.
On a side note, Ritchey states, “There was a point in time when equipment was coated in cosmoline.” Cosmoline is a brown, wax-like petroleum-based corrosion inhibitor material. It is viscous when applied but then solidifies over time and exposure to air. The good news was that it prevented corrosion. The bad news was that it had to be removed before the equipment could be used.
In the foregoing and in many other instances, the design elements of concern are limited to those elements of electronic components that are resident within the airplane cockpit. For those pieces of equipment that are exterior to an airplane, such as navigation and targeting pods that are under the wings of fighter aircraft, factors such as shock, temperature, and corrosion have to be taken into account relative to the performance of the electronic components contained within those pods (more about that below). In addition, for some product implementations, such as a stealth fighter or bomber, there are specific metrics that can be amplified. Ritchey notes, “On a stealth airplane, you can’t have any EMI anywhere. The EMI for a stealth airplane might be the strictest EMI spec there is.”
Working with Outdated Standards, Specifications and Electronic Components
As noted in previous articles on the mil-aero market space, the standards and specifications that are levied on the products designed for military airplanes are often outdated and irrelevant for those products. Ritchey explains, “Every component in both military and commercial airplanes has to be traceable to make sure that genuine parts are being used. There is a whole counterfeit industry out there. These requirements are more rigid than what you find in missiles or in satellites. If a missile or satellite crashes, you don’t kill a whole airplane full of people.”
Also, similar to other mil-aero programs, the time frame between the prototype and production phase of a military project can be radically protracted. As an example, I was assigned to the LANTIRN (Low Altitude Navigation and Targeting Infrared for Night) program when I was with Martin Marietta in 1987. This equipment is a combined navigation and targeting system that is used on the F-15 and F-16 fighter aircraft. The research and development program began in 1980. The first initial operation and test of the navigation pod was successfully completed in 1984. The first production pod was delivered to the Air Force in 1987 and, to my knowledge as well as to those of other associates assigned to the program, the first use of the system in combat occurred during the first Gulf War in 1991.
As noted in the foregoing articles, a focus of the military is to develop products that will work for a long time. Ritchey explains, “The military wants to make the same airplane for 10 or 20 years, or, in the case of the C130, for 40 years. It takes forever for an airplane to be replaced. The B-52s have yet to be replaced. They (the military), just keep putting new engines and electronics on them and keep them flying.” The preceding all leads to the dilemma facing both the military customers and the contractors— determining how many components will be required over the lifetime of a program.
In November of 2018, John Keller of Military and Aerospace Electronics magazine wrote, “The U.S. Navy is buying more than $40-million worth of obsolete FPGAs to ensure the long term operation and maintenance of the F-35 joint striker and other military aircraft for the U.S. Air Force, Navy, Marine Corps., and allied military forces.”
The press release that is contained within the article continues, “This order illustrates a dilemma that military leaders face as they find ways of keeping military electronics fielded, functioning, and upgraded over decades...”
Similar to the other targeted sectors within the overall wider mil-aero market space, there are several factors that need to be addressed when developing products for these sectors. Careful review and knowledge of these factors can lead to the successful design of the PCBs used in a variety of military Applications.
Using a software package like Altium Designer® helps you design your next military PCBs to the important performance standards on mission-critical systems. Have more questions? Call an expert at Altium.
About the AuthorMore Content by Kella Knack