The Military Spin: The Fundamentals of Surviving

By Steve Colby | May 1, 2007
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SINCE THIS ISSUE IS DEDICATED TO the future of aircraft, engines, and components, we should review the basics of a key concern ahead — combat aircraft survivability.

Prof. Robert Ball conceived and codified these concepts in 1985 in "The Fundamentals of Combat Aircraft Survivability Analysis and Design." I’ll use some of his formulas to illustrate the issue’s key ideas. Many of these quantifications are subjective. But they show that a methodical means of analyzing threats allows designers and tacticians to build aircraft that operate effectively in a combat threat environment.

Survivability’s definition is simple. The probability of aircraft survival is 1 minus the enemy weapon system’s probability of kill (PS=1-PK). The enemy’s PK, however, is based on many things characterized as aircraft susceptibility and vulnerability. Ball’s PK formula is further broken down into pieces quantifiable and clear to the reader. Its first three elements are susceptibilities; the fourth is a function of aircraft vulnerability.


Think of susceptibility as the ability to avoid a threat and vulnerability as the ability to absorb a threat’s impact or explosion. The probability of kill is expressed as PK=PAxPDITxPLGDxPK/H.

PA is the probability that a threat system is active. If you’re over enemy territory at 0300 and the enemy is asleep, it doesn’t matter what the remaining probabilities are. If PA=0, then the whole equation does, too, and survivability is 100 percent. If the enemy is active, that PA (100 percent) is multiplied by the probability that the enemy can detect, identify, and track you (PDIT). Detection and tracking could be by sight, radar, acoustics, electro-optical, or passive electronic means. PDIT is affected by the skill of the enemy’s system operator, weather, equipment reliability, and distance to the target. We can reduce his PDIT through mission planning, tactics, and susceptibility reduction through onboard systems like radar warning, intelligence imagery, signals intelligence, and data link. Stealthy designs can minimize PDIT to the point where aircraft detection is denied. Thus, if we use radar warning systems or stealth to avoid a radar-guided threat, we’ve reduced our susceptibility to that threat to zero or close to it. Systems like radar and infrared jammers, flares, and chaff can reduce PDIT and the enemy’s ability to launch, guide, and detonate a projectile or propagator (PLGD), depending whether they are employed preemptively or reactively.

PLGD is also affected by operator skill, weapon strengths and weaknesses, guidance methods, and obscurations. In an aircraft, we can reduce PLGD against, say, a surface-to-air missile through visual threat detection. We can reduce the enemy’s effectiveness reactively by using chaff and flares (expendables) to decoy radar systems or IR missiles. We can proactively reduce his effectiveness with jammers that cause guidance problems for the threat. Maneuver combined with jammers or expendables often force the threat miss distance beyond its fusing range, making PLGD=0.

Lastly, PK/H is the probability that the threat system can kill your aircraft, given a hit. Vulnerability-reduction designs improved significantly in helicopters engineered from a Vietnam War damage study. Engineers moved critical components like fuel controls and hydraulic pumps inboard of less critical ones like generators, which serve as pseudo-armor without parasitic loss of useful load. Designing in redundancy also diminishes vulnerability. Dual engines, dual flight control rods, and separate hydraulic system paths reduce the likelihood that a single hit will down the aircraft. Self-sealing fuel systems, nitrogen inerting systems that prevent fire by displacing explosive fumes and oxygen, large-diameter drive shafts, and armor round out the design features that allow helicopters to absorb hits without failing in flight.

Aircraft in a combat environment must accomplish the mission in the following manner: avoid, degrade, defeat, or destroy the threat system to survive. If you avoid the threat, you’ve beaten him at PDIT. If you degrade his ability to detect or track you, you’ve again beaten him there. If you maneuver with jammers or expendables after he’s fired, you’ve defeated the system at PLGD by exploiting its weaknesses. If the golden BB hits you, but your aircraft tolerates the damage and continues to fly, you’ve conquered him at the PK/H function and the engineers have done well with your aircraft design. If you or your supporting aircraft destroy the threat with a missile or bomb, you’ve reduced PA for that threat to zero.

The trick is to defeat the threat as far left in the PK equation as you can to avoid exposing your aircraft to a projectile in flight.

Combat helicopters must incorporate survivability elements in ways that minimize a threat’s PK while allowing usable payload. Employment of appropriate tactics, defensive systems, maneuver, and effective vulnerability-reduction design ensures that tomorrow’s warfighters are equipped and trained to fly in harm’s way and survive. We owe this to our helicopter warriors.

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