Safe Egress Limitations for Aerospace Vehicles
(Updated 28 June 2012)


Aircraft Design: A Conceptual Approach, Third Edition, Daniel P. Raymer
Pronated Escape System (PRESS), Allen D. Disselkoen and Keith H. Heise

Types of Egress Methods and their Limitations


Maximum Safe Q: Less than 230 psf (11 kN/m2)
Mass: Several pounds
Description: Manual egress method in which the pilot either rolls the aircraft inverted, unfastens his seat belt and drops out; or where he climbs out of the cockpit and jumps. May be impossible for wounded pilots to execute, depending on the severity of the injury.

Ejection Seat

Maximum Safe Q: Less than 1,100 psf (52.66 kN/m2) (ACES II)
Mass: 151~ pounds (68.49 kg) (ACES II)
Description: Semi-automatic egress method in which the pilot triggers the ejection system and the seat takes it from there. Current “best of” technology is roughly ACES II, which is used for the statistics above. Above 1,100 psf Q with ACES II, the pilot is at severe risk of spinal/visceral injuries due to decelerative forces and flailing injuries due to windblast.
There have been proposed prone-pilot ejection seat systems that would enable ejection up to 2,000+ psf via the pilot being in a better position to survive the windblast/deceleration, along with strategically placed shields to protect from windblast.


Maximum Safe Q: Theoretically unlimited, but around 2,000+ psf (95.76 kN/m2) for practicality.
Mass: Well above 800+ pounds (F-111’s crew capsule was 3,000 lbs)
Description: Semi-automatic egress method in which the pilot triggers the ejection system and the capsule separates from the aircraft, providing complete protection against windblast and cold, along with providing shelter once on the ground. There is no theoretical limit to the protection that this system provides – it could be used for orbital ejection at any point in the re-entry envelope – it’s just a matter of how much mass the designer is willing to devote to the capsule system.
Commentary: If you look at the chart below, you’ll see that ejection from an aircraft at Mach 3 at 80,000~ feet is well within the safety limits of an ejection seat – the SR-71 demonstrated that this was possible. So why did the USAF want escape capsules for it’s “next generation” of aircraft in the 1960s (the B-70 and TFX/F-111)?
USAF regulations of the time (and still do today) required a pressure suit for operations above 50,000~ feet. The F-22A gets around this regulation by having the g-suit work as a sort of partial-pressure suit for operations at/near that altitude. Wearing a pressure suit for long periods is extremely tiring on a person, as experience with pressure suits on the B-36, B-47, B-52, and F-102 showed. A capsule allowed ‘shirt-sleeve’ comfort, or at least the wear of lighter weight flight suits while retaining safety at high altitudes.

Pre-Calculated Speed/Altitude Combinations and their Q

Sea Level (0 ft / 0 m)

100 MPH (M0.131): 25 psf
200 MPH (M0.262): 102 psf
400 MPH (M0.525): 408 psf
500 MPH (M0.656): 638 psf
600 MPH (M0.787): 919 psf
761 MPH (M1.0): 1,481 psf
951 MPH (M1.25): 2,314 psf

WWII Combat Altitudes (25,000 ft / 7,620 m)

300 MPH (M0.432): 103 psf
400 MPH (M0.577): 183 psf
500 MPH (M0.721): 286 psf
600 MPH (M0.865): 412 psf
693 MPH (M1.0): 550 psf
866 MPH (M1.25): 860 psf
1,039 MPH (M1.5): 1,238 psf
1,386 MPH (M2.0): 2,202 psf

Cold War Supersonic (50,000 ft / 15,240 m)

400 MPH (M0.605): 62 psf
500 MPH (M0.757): 96 psf
600 MPH (M0.908): 139 psf
660 MPH (M1.0): 169 psf
825 MPH (M1.25): 264 psf
990 MPH (M1.5): 380 psf
1,320 MPH (M2.0): 676 psf
1,650 MPH (M2.5): 1,056 psf
1,981 MPH (M3.0): 1,521 psf

Extreme Supersonic (80,000 ft / 24,384 m)

660 MPH (M1.0): 39 psf
990 MPH (M1.5): 89 psf
1,320 MPH (M2.0): 159 psf
1,650 MPH (M2.5): 249 psf
1,981 MPH (M3.0): 359 psf
2,641 MPH (M4.0): 639 psf
3,301 MPH (M5.0): 999 psf