ACES II
The ACES II (Advanced Concept Ejection Seat II) is a third-generation ejection seat system developed for United States Air Force aircraft, enabling pilot escape from zero altitude and zero airspeed conditions up to 600 knots equivalent airspeed (KEAS) and maximum operational altitudes.[1] Manufactured primarily by McDonnell Douglas Corporation and Weber Aircraft Company, it features a lightweight aluminum alloy monocoque structure weighing approximately 127 pounds for the seat assembly plus 21 pounds for the rocket-catapult, with subsystems including a sequencer for mode selection, stabilization parachute (STAPAC), drogue gun, and an integrated survival kit.[1] The system operates in three ejection modes to optimize performance: Mode 1 for low speeds (0-250 knots) with rapid parachute deployment under 2 seconds, Mode 2 for higher speeds up to 600 KEAS with deployment under 6 seconds, and Mode 3 for high altitudes above 15,000 feet with delayed stabilization until safer conditions.[1] Development of the ACES II began in the mid-1970s as a U.S. Air Force initiative to standardize and improve ejection seat performance across its fleet, addressing limitations in prior systems during low-altitude and high-speed scenarios.[2] The initial production contract was awarded to McDonnell Douglas in November 1976, with Weber Aircraft as a secondary producer; the first flight test occurred in an A-10 Thunderbolt II in April 1978, marking the start of widespread integration.[2] By May 1997, approximately 8,000 units had been produced (about 6,500 from McDonnell Douglas and 1,500 from Weber), with over 10,000 produced by 2013 and production continuing, including a 2025 contract for 144 seats for the F-15EX.[2][3][4] The seat has since been installed in key aircraft such as the F-15 Eagle, F-16 Fighting Falcon, A-10 Warthog, B-1B Lancer, B-2 Spirit, and F-22 Raptor, with approximately 6,000 in service as of 2023.[2][3] The ACES II has demonstrated exceptional reliability, with a 94.4% success rate in operational envelopes based on 416 non-combat ejections and an 89.9% overall rate including 10 combat ejections (data as of circa 1997).[2] It incorporates safety enhancements like emergency oxygen supply, inertia reel harnesses, and redundant pressure transducers in the sequencer to ensure functionality across extreme conditions, including peak accelerations of about 12g from the CKU-5/A rocket-catapult (with environmental upgrades implemented via CKU-5 variants).[1] As the original equipment manufacturer, Collins Aerospace (a division of RTX) continues to provide maintenance, repair, and overhaul services from a 65,000-square-foot facility in Colorado Springs, Colorado, including inspections, upgrades, and OEM-certified parts to sustain the fleet's operational readiness.[5] This ongoing support underscores the seat's enduring role as the safest ejection option for Air Force pilots in emergency situations.[6]Development
Origins and requirements
The United States Air Force (USAF) expressed dissatisfaction with first- and second-generation ejection seats in the late 1960s and early 1970s, as these systems suffered from high injury rates and inadequate performance in low-altitude, low-speed scenarios. Vertebral injuries, particularly back fractures, occurred in 19-43% of cases across early seat designs, with thoracic and lumbar regions accounting for the majority of such trauma due to poor body positioning and high g-forces during ejection.[7] Limitations in zero-zero ejection capability—safe escape from ground level at zero airspeed—further compounded issues, as many seats required minimum altitudes above 300 feet for successful deployment, leading to increased fatalities in combat and training incidents.[7] Seats like those produced by Weber Aircraft exemplified these challenges, contributing to operational inconsistencies across USAF aircraft fleets and complicating maintenance and pilot training.[2] To address these shortcomings, the USAF launched the Advanced Concept Ejection Seat (ACES) program in the early 1970s, aiming to develop a standardized third-generation seat for integration into emerging fighters such as the F-15, F-16, and A-10.[8] The program emphasized rocket propulsion for reliable zero-zero performance, environmental sensing for automated sequencing (initially analog, with later digital upgrades), and weight-compensating stabilization to accommodate varying pilot sizes without compromising trajectory control.[8] This initiative sought to create a unified escape system capable of operating from zero altitude and speed up to 600 knots equivalent airspeed (KEAS) and maximum operational altitudes, thereby reducing injury risks and standardizing logistics across the fleet.[9] Following competitive evaluation of industry proposals, McDonnell Douglas was selected to lead prototype development, leveraging its extensive experience in escape systems.[9] Initial testing prioritized zero-zero capability and injury mitigation, with over 59 static ejections and 130 dynamic trials conducted to validate performance.[9] Key requirements included a minimal ejection altitude of approximately 140 feet at 150 knots indicated airspeed (KIAS) in inverted flight, peak catapult acceleration limited to 12g to minimize spinal loads, and compatibility with narrow fighter cockpits while meeting MIL-S-9479B survivability standards.[9] These specifications ensured enhanced pilot protection across the full flight envelope, marking a significant advancement over prior generations.[9]Production history
The initial production contract for the ACES II ejection seat was awarded to McDonnell Douglas Corporation in November 1976, with deliveries commencing in 1977 for integration into the U.S. Air Force's A-10 Thunderbolt II, F-15 Eagle, and F-16 Fighting Falcon aircraft, marking the seat's entry into operational service that year.[2][8] Production facilities for the ACES II began at McDonnell Douglas's site in Long Beach, California, before relocating in the late 1980s to Titusville, Florida, where manufacturing continued under McDonnell Douglas following its 1997 acquisition by Boeing. In 1999, Boeing sold the ACES II product line to BFGoodrich Aerospace, which then shifted production to Colorado Springs, Colorado, integrating it into facilities previously associated with Aircraft Manufacturers Inc.[10][11] By 2025, total ACES II production had exceeded 10,000 units, reflecting sustained demand across decades of service.[12] Approximately 6,000 seats remained active as of 2023, equipping aircraft in 29 air forces worldwide.[3][13] Subcontractor involvement included Weber Aircraft, which handled partial production in Titusville, Florida, as part of a U.S. Air Force dual-sourcing initiative to ensure supply redundancy during the 1970s and 1980s. Current production is managed by Collins Aerospace, a business unit of RTX (formerly Raytheon Technologies), following BFGoodrich's 2012 acquisition by United Technologies Corporation and subsequent integration into Collins.[4] In February 2025, Collins Aerospace received a contract from Boeing to supply 144 ACES II seats for the U.S. Air Force's F-15EX Eagle II program, underscoring ongoing production for modernized platforms.[4]Design and features
Key components
The ACES II ejection seat features a lightweight monocoque structure constructed primarily from high-strength aluminum alloy, designed to provide structural integrity during high-g accelerations while minimizing overall weight. The seat includes a 16-inch-wide backrest and a 20-inch-wide bucket for occupant support, with a total seat assembly weight of approximately 127 to 131 pounds, excluding the rocket-catapult at 21 pounds and other components like the actuator adding 5 pounds. A +2.5-inch vertical adjustment mechanism, utilizing a twin-barrel actuator, accommodates varying pilot statures for optimal fit.[9][14] Propulsion is provided by the CKU-5/A/A rocket-catapult system, which combines a solid-propellant catapult charge for initial ballistic ejection—achieving peak accelerations around 12 g and velocities up to 43 feet per second—with a sustainer rocket delivering an impulse of 1,150 pound-seconds. Later variants incorporate upgrades such as the CKU-5/B or CKU-5/C for enhanced performance. Complementing this is the STAPAC (Stabilization and Pitch Attitude Control) vernier rocket motor, mounted beneath the seat, which provides a 235 pound-second impulse for pitch stabilization and attitude correction, igniting approximately 0.18 seconds after initiation to address aerodynamic and center-of-gravity variations.[9][15] The recovery systems are managed by a digital sequencer, also known as the environmental sensing unit, which uses pressure transducers to assess altitude and airspeed, thereby selecting appropriate deployment modes and sequencing events with thermal battery power. A drogue gun fires a 1.2-pound slug to deploy a small extraction parachute, which in turn extracts the 5-foot hemisflo drogue parachute for initial stabilization in higher-speed modes, followed by the 28-foot C-9 main parachute with dual reefing line cutters for controlled descent. The system includes an inertia reel harness with shoulder and lap belts, along with automatic limb and head restraints that secure the occupant to reduce injury risk during windblast and separation.[9][16] Additional integrated features enhance post-ejection survivability, including a 22 cubic-inch emergency oxygen bottle (MS 22069-3) that deploys via lanyard or manual activation to supply breathable air up to 10,000 feet for approximately 10 minutes. The seat incorporates a nonrigid survival kit housed in a soft pack under a hinged lid, containing essentials such as a life raft, rucksack, and URT-33C beacon, with automatic deployment timed to ejection mode or manual override. For analysis, the digital sequencer includes a survivable data recorder that logs switch positions, mode selection, and event timings to aid in post-incident investigations.[9][16] The design incorporates weight compensation through the STAPAC system, which adjusts for center-of-gravity offsets up to +2 inches, ensuring consistent ejection trajectories and stability for pilots ranging from 103 to 245 pounds in body weight.[8]Ejection sequence and modes
The ejection sequence of the ACES II seat is initiated by the pilot pulling the ejection handle(s), which actuates the initiator to fire the rocket-catapult and commence the escape process, often preceded by canopy jettison or shattering to clear the path.[9][17] The system's recovery sequencer automatically selects one of three operational modes based on sensed parameters such as airspeed (in knots equivalent airspeed, KEAS), altitude, and dynamic pressure to optimize safe separation and descent across the full flight envelope from zero-zero conditions to high-speed, high-altitude ejections.[9][1] Mode 1 operates for low-speed and low-altitude ejections (0-250 KEAS and below 15,000 feet), providing immediate deployment of the main recovery parachute without a drogue parachute to enable rapid stabilization in zero-zero scenarios.[18][1] Mode 2 is used for higher dynamic pressures up to the seat's maximum rated speed (above 250 KEAS but below 15,000 feet), incorporating a delayed sequence with initial drogue parachute deployment for deceleration before main parachute inflation to manage aerodynamic loads.[9][19] Mode 3 applies to high-altitude ejections (above 15,000 feet), extending stabilization by interrupting the recovery sequence until the seat descends to Mode 2 parameters, then proceeding with a timed delay (typically 0.8-1.0 seconds) for parachute deployment to account for oxygen and environmental factors.[18][1] In the typical zero-zero Mode 1 sequence, the process unfolds rapidly as follows:| Event | Time (s) |
|---|---|
| Rocket-catapult ignition | 0.00 |
| STAPAC (stabilization vernier rocket) firing | 0.18 |
| Main parachute mortar launch | 0.20 |
| Seat-man separation | 0.45 |
| Main parachute inflation | 1.35-1.80 |