Survivability Discipline


The Survivability Discipline (Ref. 1)

Military Aircraft Design and Operation

"U.S. military aircraft are designed, developed, and operated to fill a need initially expressed in a DoD Mission Need Statement (MNS). They are special purpose aircraft that must operate efficiently in peacetime and effectively in wartime. As a result of this dual ‘peacetime–wartime’ environment, they have more requirements imposed upon them during the acquisition process than those imposed upon civilian aircraft. Table P.1 contains a list of some of the characteristics, capabilities, or attributes that must be considered when developing or acquiring a military aircraft. Some of them, such as ‘carries a large payload over a long distance,’ are operationally related; others, such as ‘easy to modernize,’ are not. Many of them, such as ‘stealthy’ and ‘delivers lethal weapons accurately,’ are unique to military aircraft. Of interest here is the relatively recent aircraft attribute known as survivability." (Ref. 1)

Table P.1 Some of the Important Attributes of a Military Aircraft (Ref. 1)
Easily maintained
Safe to operate & maintain
Easy to produce and repair
Good handling qualities
Flies fast and high (enough)
Secure communications
Carries a large/heavy payload over a long distance
Delivers effective weapons accurately from long range or high altitude
Good target acquisition capability
Multi–mission capable
Capable of terrain following
Short field capable
Long lived
Few logistics requirements
Maneuverable and agile
Heavy weapons bringback capability
Low number of crew members
Night–time capable
Dirty environment capable
Long endurance
Low crew workload
All weather capable
Precise navigation
Easy to modernize
Crew situational awareness
Carrier suitable
and of course, survivable
-- and easy to repair when damaged in combat


What is survivability, and how does it relate to system safety?

Aircraft combat survivability (ACS) is defined in Ref. 1 as the capability of an aircraft to avoid or withstand a man-made hostile environment. It can be measured by the probability the aircraft survives an encounter (combat) with the environment, PS. The more general term aircraft survivability refers to the capability of an aircraft to avoid or withstand hostile environments, including both man-made and naturally occuring environments, such as lightning strikes, mid-air collisions, and crashes. The more traditional discipline known as system safety attempts to minimize those conditions known as hazards that can lead to a mishap in environments that are not made hostile by man. Thus, together, the system safety and survivability disciplines attempt to maintain safe operation and maximize the survival of aircraft in all environments in both peacetime and wartime.

What is aircraft susceptibility?

Susceptibility is the inability of an aircraft to avoid (the guns, approaching missiles, exploding warheads, air interceptors, radars, and all of the other elements of an enemy's air defense that make up) the man-made hostile mission environment. The more likely an aircraft on a mission is hit by one or more damage-causing mechanisms generated by the warhead on a threat weapon (e.g. warhead fragments, blast, and incendiary particles), the more susceptible is the aircraft. Susceptibility can be measured by the probability the aircraft is hit by one or more damage mechanisms, PH. Thus,

Susceptibility = PH

What is aircraft vulnerability?

Vulnerability is the inability of an aircraft to withstand (the hits by the damage-causing mechanisms created by) the man-made hostile environment. The more likely an aircraft is killed by the hits by the damage mechanisms from the warhead on a threat weapon, the more vulnerable is the aircraft. Vulnerability can be measured by the conditional probability the aircraft is killed given that it is hit, PK|H. Thus,

Vulnerability = PK|H

What is aircraft killability?

Killability is the inability of the aircraft to both avoid and withstand the man-made hostile environment. Thus, killability is the ease with which the aircraft is killed by the enemy air defense. Killability can be measured by the probability the aircraft is killed, PK. Killability is given by the joint probability the aircraft is hit (its susceptibility) and it is killed given the hit (its vulnerability). Thus,


Killability = Susecptibility • Vulnerability

If the threat weapon contains a high explosive (HE) warhead with proximity fuzing, the subscript H for a hit is replaced with an F for warhead fuzing.

For a closer look at the words susceptibility and vulnerability, click here.

How is an aircraft's combat survivability enhanced?

The survivability of an aircraft is related to the aircraft's killability, or susceptibility and vulnerability, by the equation

PS = 1 - PK = 1 - PHPK|H

Survivability = 1 - Killability = 1 - Susceptibility • Vulnerability

Thus, an aircraft's combat survivability is enhanced when it's killability is reduced. The killability of an aircraft is reduced when the susceptibility and the vulnerability of the aircraft are reduced. Table P.2 presents a partial list of the many design and operational features that can enhance the survivability of an aircraft by reducing either the aircraft's susceptibility or its vulnerability.

Table P.2 Some Survivability Enhancement Features (Ref. 1)
Speed and altitude Maneuverability/agility Chaff and flares
Fire/explosion protection Terrain following Fighter escort
Self-repairing flight controls No fuel adjacent to air inlets Rugged structure
Redundant and separated hydraulics Self defense missiles and guns Good target acquisition capability
Night-time capability Crew situational awareness Threat warning system
More than one engine - separated Hydrodynamic ram protection Mission planning system
Low signatures Crew training & proficiency Antiradiation weapons
Tactics Nonflammable hydraulic fluid Armor
On-board electronic attack equipment Lethal launch-and-leave
or stand-off weapons
Stand-off electronic attack equipment

According to Table P.2, an aircraft's susceptibility can be reduced by reducing the aircraft signatures (stealth), installing a threat warning system, employing on-board electronic attack equipment known as electronic countermeasures (ECM), mounting expendables (chaff and flares), and using survivable tactics, such as flying outside of the enemy weapon envelopes and preemptively destroying the enemy's air defense weapons.

P-3 aircraft ejecting infrared flares
P-3 Orion ejecting IR flares to decoy an infrared (IR) guided missile
An aircraft is killed when one or more of its critical components is killed. According to Table P.2, an aircraft's vulnerability can be reduced by using redundant and separated critical components, such as multiple, widely spaced engines and hydraulic power components; designing critical components to contain or minimize any hit-caused damage, such as installing an On-Board Inert Gas Generator (OBIGGS) in fuel tanks to suppress internal fires and explosions and designing helicopter rotor blades to withstand a hit by a high explosive warhead; and using armor to shield critical components from the damage mechanisms.
HE Warhead Ddetonation
Detonation of an HE warhead beneath an aircraft.

What are the goals of the ACS discipline?

According to Ref. 1, the goals of the ACS discipline are:

  • The early identification and successful incorporation of those specific survivability enhancement features that increase the combat cost effectiveness of the aircraft as a weapon system.
  • In those situations where the damage will eventually lead to an aircraft kill, the survivability enhancement features should allow a graceful degradation of the system capabilities, giving the crew a chance to depart the aircraft over friendly territory.

The survivability discipline is dedicated to saving lives in combat by reducing an aircraft's susceptibility and vulnerability at an affordable cost. If you want to learn more about survivability, you should click on the link to the AIAA survivability textbook at the top of the page.


The Fiscal Year 05 National Defense Authorization Act added a new requirement for survivability in Section 141. The requirement reads as follows:



(a) Requirement for Systems Development - The Secretary of Defense shall require that the Department of Defense regulations, directives, and guidance governing the acquisition of covered systems be revised to require that --

  • (1) an assessment of warfighter survivability and of system suitability against asymmetric threats shall be performed as part of the development of system requirements for any such system; and

  • (2) requirements for key performance parameters for force protection and survivability shall be included as part of the documentation of system requirements for any such system.

(b) Covered Systems - In this section, the term 'covered system' means any of the following systems that is expected to be deployed in an asymmetric threat environment:

  • (1) Any manned system.

  • (2) Any equipment intended to enhance personnel survivability.

(c) Inapplicability of Development Requirement to Systems Already Through Development - The revisions pursuant subsection (a) to Department of Defense regulations, directives, and guidance shall not apply to a system that entered low-rate initial production before the date of the enactment of this Act.

(d) Deadline for Policy Revisions - The revisions required by subsection (a) to Department of Defense regulations, directives, and guidance shall be made not later than 120 days after the date of the enactment of this Act.

Note: In addition to this addition to the NDAA, the JCS/J8 has identified survivability as a key performance parameter (KPP) in their capability requirements process.


Susceptibility and vulnerability – what do these words really mean?

The specific use of the word susceptibility to refer to the occurrence of an undesirable event (the inability of an aircraft to avoid being hit by an air defense weapon) and vulnerability to refer to the undesirable consequence of that event (the inability of the aircraft to withstand the hit) is not widely known outside of the aircraft combat survivability discipline. The word vulnerable is generally used by those unfamiliar with the discipline to describe something that can be killed or injured in a particular situation.

For example, in an Associated Press article in the San Francisco Chronicle, July 17, 1998, page A17, that described the results of a new study of earthquakes in the Los Angeles area, the reporter wrote "Geologists have dug up evidence that at least two earthquakes of magnitude 7.2 to 7.6 struck downtown Los Angeles within the last 15,000 years, suggesting the region is vulnerable to more powerful quakes than previously known." In the survivability discipline, this would have been written as " . . . , suggesting the region is susceptible to more powerful quakes than previously known."; i.e. more powerful earthquakes are more likely to occur than previously thought. In the vernacular of the combat survivability discipline, the vulnerability of the area to a more powerful earthquake depends upon the inability of the buildings, bridges, and roads to withstand the more powerful earthquake.

In the military world, if an Air Force General asks "What is the vulnerability of the B-2 bomber?" - he or she may may want to know - "What is the killability of the B-2 bomber on a particular mission?" - or they may want to know - "What is the likelihood the B-2 bomber is killed given that it is hit by a particular weapon?" Both definitions are acceptable using the general definition of vulnerability given above. In the first interpretation, the particular situation is the mission, whereas in the second interpretation, the particular situation is the aircraft is hit. The latter interpretation is the one used by the survivability community.

This ambiguity in terminology must be kept in mind by the survivability community when communicating with those unfamiliar with the combat survivability discipline. Dividing the event of an aircraft kill by the air defense into two sequential events (hit and kill given hit) is very useful to the survivability community, but these words with their associated meanings may not be familiar to everyone else.


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The information presented herein does not represent official Naval Postgraduate School, Department of the Navy, or Department of Defense policies or positions

Ref. 1. Ball, Robert E., The Fundamentals of Aircraft Combat Survivability Analysis and Design, Second Edition, AIAA, Reston, VA, 2003 (The AIAA Textbook)