EGRESS CASUALTY ANALYSIS
By Shakila Taylor
Historically, aircraft combat survivability design metrics and evaluations have focused heavily on the conditions of the aircraft and not on casualties caused by aircraft damage or loss while in combat. Although aircrew injuries and fatalities due to in-flight escape, crash events, and post-crash egress have been documented, crew and passenger survivability evaluations have not typically been incorporated into aircraft survivability assessments. In 2007, however, the Deputy Director of Operational Test & Evaluation/Live Fire Testing stated a need for tools that will predict the probability of casualties given various crash and landing conditions/effects and failed egress. This identified need and gap in the survivability design process thus prompted the implementation of the Crew and Passenger Survivability (CAPS) project.
CH-53 INTEGRATED CAPS ANALYSIS OVERVIEW
In 2015, the Joint Aircraft Survivability Program Office (JASPO) funded the CH-53 Integrated CAPS Analysis project (M-15-07) to establish baseline CAPS metrics and determine the impact of data uncertainties on casualty metrics, as detailed in the Integrated Crew and Passenger (CAPS) Methodology Report . The CH-53E (shown in Figure 1) was selected as the target aircraft for the CAPS evaluation. The CAPS analysis involves evaluating the probability of casualties due to direct contact and indirect effects caused by various threats in flight, as well as the ability of a passenger to successfully egress a damaged aircraft once landed, before becoming a casualty. The egress casualty mechanisms are casualties that occur during the egress portion of the incident, including blocked egress and post-crash fire effects (thermal, toxicology, etc.).
The egress methodology was developed to generate casualty metrics due to a cabin fire in an in-flight scenario. For these purposes, a “casualty” is defined as an aircraft occupant who becomes incapacitated as a result of a threat interaction with the aircraft and/or with the occupant, whether directly or indirectly. Likewise, the term “incapacitated” is defined as an occupant being unable to successfully egress on his/her own. The following two casualty metrics were considered:
- Probability of a casualty given a cabin fire before landing (Pcas|ind)
- This metric considers the time period between the start of a cabin fire through the time of landing. It is the probability that an occupant will be incapacitated due to the hazards, or indirect effects, from a cabin fire before the aircraft is able to land.
- Probability of casualty given a cabin fire prior to safe egress (Pcas|egress)
- This metric considers the time period between the start of the cabin fire through the time required to egress. It is the probability that an occupant will be incapacitated due to the hazards from a cabin fire before being able to complete egress.
The Advanced Joint Effectiveness Model (AJEM) was used in these evaluations because it is the only modeling and simulation tool currently available that can handle the complex fault trees to support this analysis. That said, AJEM does have some modeling limitations. Thus, the aircraft model was divided into four zones (shown in Figure 2), which allowed for a method to approximate the expected number of casualties due to inability to egress for each zone and for the entire aircraft.
TIME ANALYSIS FOR CABIN FIRE AND CASUALTIES
A time analysis was performed, considering the landing time, incapacitation times, and egress times associated with a cabin fire. By finding the difference between the landing and incapacitation times plus time to egress, one can determine how survivable a certain engagement can be and can identify a window of time for egress, if one exists. The following relations were used to compute Pcas|ind for each occupant:
- If the time required to land exceeds the time it takes to incapacitate an occupant due to fire, then the occupant is a casualty due to fire or indirect effects.
- If Tland > Tcrit for occupant, then occupant Pcas|ind = 1.0.
- If the time required to land exceeds the time it takes to incapacitate the pilots due to fire, then all occupants are casualties due to crash, fire, or indirect effects.
- If Tland > Tcrit for pilots, then all occupants Pcas|ind = 1.0.
In these relations, Tland ≡ the time required to land aircraft from engage-ment altitude, and Tcrit ≡ the time required to incapacitate the occupant due to a hazard.
The probability of casualties that would occur prior to egressing from the aircraft was identified as Pcas|egress, which is determined by the following relation:
- If the time required to land and the time required for an occupant to egress the aircraft exceed the time required to incapacitate an occupant due to a hazard, then the occupant is a casualty prior to completing egress.
- If Tland + Tegress > Tcrit, then occupant Pcas|egress = 1.0.
In this relation, Tegress ≡ the time required to egress.
An additional assumption was that if the threat initiated a cabin fire in a zone, then all occupants in the zone would be incapacitated and Pcas|ind = 1.0 for all occupants in that zone. The values of Pcas|ind and Pcas|egress were determined for each occupant and averaged over the occupants in the zone to give a Pcas|ind and Pcas|egress value for each zone.
The landing time value (Tland) was determined by using Patuxent River CH-53E Manned Flight Simulator (PAX CH-53E MFS) data. These data were collected with the help of HM-14, HM-15, HMH-366, and HMH-461 squadron pilots, who piloted the simulators for our tests. The Marine Corps standard CH-53E landing approach was used for the flight simulations. This approach was necessary to determine the time required for a CH-53E to descend from a selected “point on the curve” (e.g., 100-ft altitude) to landing. Five straight-in approaches were executed using the flight simulator. The results of the testing (shown in Figure 3) demonstrated an average landing time of 37 s from 100-ft above ground level (AGL).
The incapacitation times were derived from the data contained in the “Crew Compartment Fire Survivability Report,” funded by Joint Live Fire Air . The report’s focus was to provide an assessment of time-to-incapacitation due to various hazards associated with onboard fuel fires. Based on the report, there were four hazards that contributed to incapacitation (Tcrit):
- Time required for second-degree burns (T2D Burns)
- Time required for inhalation of temperatures greater than 400 °F (T400F)
- Time required for loss of visibility due to smoke (Tvis)
- Time required for toxic gas levels (Ttoxic).
The actual values of Tcrit for each fire zone/occupant zone combination are the minimum values of T400F, T2d burns, Ttoxic, and Tvis. If Tcrit was less than Tland, then Pcas|ind = 1.0 for all occupants in that fire zone and occupant zone pair.
The times available to egress (Tegress) for each zone were calculated by subtracting Tland from Tcrit. To find Pcas|egress, the number of occupants that would become casualties due to incapacitation after landing, egress data were extracted from a 2016 rotary-wing egress study and documented in the Integrated Crew and Passenger (CAPS) Methodology Report . This study assessed Tegress for various equipped soldiers from a CH-47 helicopter, in realistic cabin conditions, following simulated emergency landing conditions. The data generated from the tests recorded individual egress times for each soldier.
PROBABILITY OF CASUALTIES
By having individual egress times, it was then possible to determine the probability of casualty due to incapacitation prior to egress Pcas|egress. Probability of casualties for each zone were determined by comparing the individual egress time to the available time-to-egress values. If an occupant in the zone had an egress time longer than the time available, they were considered a casualty. To obtain the value of Pcas|egress for each zone, the number of occupants considered a casualty in a zone was divided by the number of occupants in the zone.
Since the initial request for casualty prediction tools in 2007, gaps in the survivability design process have continued to be addressed with new crew and passenger survivability methodologies and roadmaps. These methodologies are making it possible to determine how survivable an aircraft is for the passengers and to include passenger survivability as part of the aircraft design process. In addition, establishing CAPS metrics for Navy aircraft is allowing future Naval aircraft programs to incorporate passenger survivability, in addition to aircraft survivability, into specification requirements, thus potentially reducing the number of casualties in future conflicts.
ABOUT THE AUTHOR
Ms. Shakila Taylor is a combat survivability analyst at the Naval Air Warfare Center Weapons Division in China Lake, CA. She has experience operating and maintaining threat systems and has supported various aircraft programs and studies, including CAPS, CH-53K, V-22, and (currently) CMV-22. Ms. Taylor has a B.S. in mechanical engineering from Tuskegee University.
- Manion, John, and Shakila Taylor. “Integrated Crew and Passenger Survivability (CAPS) Methodology.” JASPO-M-08-09-007, NAVAIR 418400D, China Lake, CA, 2012.
- Goss, Adam, Andrew Drysdale, Ryan Arthur, and Leonard Truett. “Crew Compartment Fire Survivability.” JLF-TR-13-04, 96 TG/OL-AC, Wright-Patterson AFB, OH, 2015.