The fundamental precepts of debate require that terms be defined before a meaningful debate can ensue. This first step of defining terms has been largely overlooked in the ongoing debate over survivability. The terms survivability, susceptibility, and vulnerability, imply something to everyone-but they have very precise, and often misunderstood meanings in the context of survivability. In the context of aircraft combat survivability, many probably view susceptibility and vulnerability as synonyms. The basis for the definitions used is Dr. Ball's authoritative text, The Fundamentals of Aircraft Combat Survivability Analysis and Design. Additional references include MILSTD-1799, MIL-STD-2069, MIL-STD2089, and MIL-HDBKS 336-1/2/3.
Survivability is like motherhood: everyone thinks it is a good idea, but few can agree on exactly what it entails. For the purposes of this article, all use of the term survivability refers to combat survivability, which is defined as "the capability of an aircraft to avoid and/or withstand a man-made hostile environment." Several issues that are frequently raised do not fit within the scope of this definition. For example, aircraft engine reliability often is addressed in the one versus two debate, and although it is a significant issue with respect to cost and safety, engine reliability has no bearing on the "capability of an aircraft to avoid and/or withstand a man-made hostile environment." Similarly, in the one versus two crew members debate, the ability of a two-man crew to operate more safely does not directly impact the "capability of an aircraft to avoid and/or withstand a man-made hostile environment" since safety issues are primarily concerned with the dangers of a hostile natural environment.
This definition of survivability breaks down into two categories. The first is the ability to avoid the hostile man-made environment. This is the issue of susceptibility, which is defined as the probability that the aircraft is hit by a man-made, damage-causing mechanism. The mechanism may be blast or ballistic fragments from a warhead detonation, individual projectiles from a gun, or directed energy from a more exotic weapon. Susceptibility is generally referred to as the probability of getting hit (P h ). Vulnerability, defined as the ability to withstand a hit, is the probability of kill given a hit (P k/h ). Mathematically, survivability, or the probability of survival (P s ), is represented as:
P s = 1 – (P h x P k/h )
Hence, any discussion of combat survivability must give equal weight to both susceptibility and vulnerability, because neither realistically can be reduced to zero for a combat aircraft. For example, if an aircraft has a P h of 0.1 and a P k/h of 0.8, the probability of survival would be:
1 – (0.1 x 0.8) = 0.92
If a technology can be applied to reduce either the susceptibility or the vulnerability of an aircraft by 25%, the effect on the aircraft 's probability of survival would be:
1 – (0.075 x 0.8) = 0.94 (25% reduction in vulnerability)
1 – (0.1 x 0.6) = 0.94 (25% reduction in vulnerability)
or only a 2% increase in P s . Reducing both the susceptibility and vulnerability by only 10% each yields the same result:
1 – (0.09 x 0.72) = 0.94
Clearly, to optimize survivability, both vulnerability and susceptibility must be reduced.
But, back to theory. Susceptibility is most often mistaken as a synonym for survivability. Elements associated with reduced susceptibility include performance, reduced signatures, and countermeasures. These are expensive, but they get high visibility and considerable support because they are perceived as glamorous, high-tech approaches. Since susceptibility deals with the ability to avoid being hit by a threat weapon, anything that can disrupt, delay, or compress the engagement time line (detection, acquisition, identification, tracking, launch, flyout, hit) has the potential to reduce P h . In addition to the "big three" (performance, countermeasures, signatures), tactics can play a substantial role in reducing susceptibility. These may include the use of stand-off weapons, stand-off jamming support, coordinated suppression of enemy air defenses, and decoys. All of these approaches can be applied to improve survivability, and although most would agree that it is better to avoid being hit in the first place, reducing susceptibility alone may not provide an improvement in the overall survivability of the aircraft to the desired magnitude.
Vulnerability reduction probably represents the oldest application of technology to improve aircraft survivability, tracing back to World War I pilots who placed stove lids under their seats to provide protection against ground fire. The basic concepts are straightforward: protect the airframe and flight-critical functions (engine, flight controls) from catastrophic failure resulting from a hit. Protection may take the form of armor, shielding a critical component with less critical components, or incorporating a redundant system. When redundancy is used as a protective scheme, care must be taken that the redundant systems are sufficiently isolated from each other so that a single threat cannot disable both. For example, a two-engine aircraft with both engines located only a few feet apart would embody redundancy if the engines were separated by a barrier. Without such ballistic isolation, the second engine could not realistically be considered redundant from the standpoint of combat survivability, and might well increase the vulnerability of the aircraft. Peacetime safety considerations, coupled with combat experience, have resuited in modern military aircraft that have a significant degree of redundancy in structure and control systems. Although there is always room for improvement, most of the vulnerability in these aircraft resides in the fuel and propulsion systems. For a single-engine aircraft, there is very little that can be done to reduce the vulnerability of the engine other than a selective application of armor. Multi-engine aircraft offer more opportunities for protection since it is necessary only to prevent killing both engines with one threat, assuming that the aircraft can fly for a period of time on a single engine.
Historically, fires and explosions in the fuel system represent the single largest contributor to vulnerability. Prevention and suppression are the two basic methods of protecting a fuel system from fires and explosions. Prevention relies on two approaches:
- Altering the environment (fuel tanks and adjacent dry bays) with an inert gas such as halon, CO 2 , or nitrogen to prevent combustion
- Filling the environment with open-cell (reticulated) foam to prevent any flame front from propagating
If a prevention technique is not used, a fire/explosion suppression system can be incorporated to sense the presence of combustion and automatically dispense an extinguishing agent into the affected area. Both fire prevention and fire suppression systems are widely available. They are in use on some military aircraft; many military aircraft have no protection for their fuel systems, in spite of the well known danger.
Survivability is the product of vulnerability and susceptibility. The greatest improvements in survivability will be achieved through a balanced application of technology (and funding) to both of its component parts. This short discussion does not attempt to provide an exhaustive compendium of survivability enhancements, but it is intended to spur on debate using a common language.