Concept: Passenger Survival Systems for Commercial Aircraft:
especially cosidering Ejection Seats
Description of system: All passenger seats to be replaced with a seat capable of being jettisoned from aircraft ON PILOT or COCKPIT CREW COMMAND. The seats are to be outfitted with a recovery system (parachute ) to lower the seat occupant to the ground safely. The Crew seats would be similarly equipped.
Pro: Safe recovery of passengers in the event of a catastrophic disaster.
Con:
Discussion: Ejection seats are complex devices which when used in military service have proven to be quite successful at saving the lives of aircrew. As complex devices they require much maintainance and training to function with a high degree of success. Even though modern ejection seats are designed for fully automatic function after initiation, the occupant requires training to use the seat effectively and safely. Ejection seats are designed to cause an occupant to seperate from the aircraft at a high enough rate of speed to clear any part of the aircraft structure. This requirement necessitates a high impulse force to be used to launch the seat and its occupant. Military seats are fired with an impulse in the Z axis of between 12 and 22Gz epending on the seat design. This impulse varies with the type of seat propulsion with the rocket/catapult seat being the lower value, and the pure gun type being the higher. This value, however, is also influenced by the weight of the occupant, and the values quoted above are based on a seat occupant in the 150-200 lb weight range. With the continued influx of female pilots and crew, the weight of the average pilot is dropping. This is leading to much research to provide a propulsion system for an ejection seat that is usable over a much wider range of occupant weight.
Other considerations include the occupant's connection to the seat. Parachutes require more than just a simple lap belt. At a minimum, the harness requirements include a pair of leg straps, a pair of shoulder straps and a chest strap. These straps must be adjusted for each individual to be a snug (read uncomfortable) fit for each passenger. In most airline trips I've been on, most passengers unbelt the minute the cockpit crew turn off the lab belt indicator (this is confirmed by recent news stories about passenger injuries due to turbulance). In the case of a situation requiring a mass ejection, this would have to be delayed until ALL passengers AND crew are strapped in securely prior to depressurizing the cabin, blowing the hatches and initiating ejection.
When the cabin is depressurized and the hatches are jettisoned, the passengers would be exposed to the lower oxygen pressure in the upper atmostphere, the wind blast which would cause flail injuries and injuries by loose flying objects such as handbags, cameras, camcorders, trays, carry-on bags, and other objects.
Jettisoning a large number of hatches in the roof of an airliner will also cause significant changes in the aerodynamics of the aircraft leading to control problems for the flight crew.
The aircraft structure would require massive modification to make ejection seats feasable, including strengthing the cabin floor for the additional weight and the recoil of the seat firings. The cabin roof would have to be configured with the aforementioned hatches. The fuselage would therefore need a major increase in supports to allow it to hold its shape when the hatches were jettisoned. Overhead baggage compartments, and underseat storage would have to be eliminated to give adequite clearance above and, because of the seat depth, below. Legroom would have to be adjusted to make sure that adequite clearance was maintained on ejection to prevent leg injuries.
Mechanical ejection (spring/bungie) would provide inadequite thrust to ensure that passengers would clear the empennage. Compressed gas systems that would have enough force would provide too great of an initial force for safety. This means that pyrotechnic rocket/catapult systems be used. These systems would necessitate significantly increased training for maintainance personel, cabin and flight crews. The pyrotechnics would require maintainance on a regular basis in an explosive rated hanger with explosive rated storage.
The ejection sequence would have to be from the rear to the front of the cabin, with the flight crew being the last of all to be ejected. In an airliner with 30 rows of seats, the seats would have to be jettisoned in row sets, with seperation rockets to insure dispersal of the seats and prevent mid-air seat collisions. There would have to be a delay between rows for the same reason. This delay in military jets is in the vicinity of .4 to .5 second. This adds up to some 15-16 seconds for a full ejection of the aircraft. Seats that are unoccupied must be weighted to ensure that they seperate in a predictable path. While the seats are firing, the aircraft would be exposed to forces from the catapult charges, and the center of gravity would be changing rapidly. This would cause significant difficulty to the flight control system to maintain stability.
The above paragraph details one of the primary problems with airliner ejection systems. The majority of airliner disasters occur at low altitude, usually during takeoff/landing which is a section of the airliner envelope that would not be conducive to a lengthy ejection sequence. Taking two examples: TWA Flight 800, and Souix city: TWA 800's failure (irregardless of the cause) occurred too fast for the flight crew to have initiated the ejection before the structural failure of the aircraft would have destroyed the firing system. If there was a cabin crew initiated backup, the structural failure might still have been great enough to have prevented the system from functioning (presuming that the depressurization and wind blast conditions allowed the cabin crew to reach the actuation device and fire the system). Flight 800 also had an inflight fire which would have caused casualties, and flying debris from the damage to the front of the aircraft which would be impacting the ejecting passengers. The airflow pattern from such damage would also effect the flight path of the ejection seats, probably causing mid-air collisions which would cause injuries and seat malfunctions.
Souix City's crash was one where a system failure had disabled the planes flight controls, but left the engine controls active. The plane was under pilot control for some time prior to the impact. In this case, the plane could have been controlled to altitude, the passengers strapped in, briefed on procedure, and then command ejected. This assumes that the passengers would remain calm, return to their seats and strap in. The pilots would then have to attempt to maintain control of the aircraft while the seats ejected for 15 seconds behind them (if it doesn't seem like a long time, set a timer for 15 seconds, close your eyes and wait...). When it finally got to the cockpit area, the flight controls would have to be ejected with the crew as they would be unable to release the grips before they would be injured by them during ejection.
The only viable solution I have seen for this problem is the concept of a parachute recovery system for the entire aircraft, or large portions of it. I believe that a system that would jettison the wings and engines of the airliner and recover the passenger cabin could be developed which would allow for significant possibility of saving lives. Balistic Parachute systems have been developed by a company called Ballistic Recovery Systems Inc. to lower ultralight aircraft and aircraft up to 1500 pounds. With the gross weight of a fully loaded 747 being over three quarter of a million pounds, the systems would have to be considerably larger and would be heavy and expensive to add to an existing airframe. In future designs, a parachute system could be designed integral with the airframe. In order to be feasable, the system would probably have to jettison the engines and wings to lower the gross weight that could be lowered. Accordingly, If I were the designer, I would design the craft to seperate the smallest possible sections including people. In other words, the airframe should be designed with passenger cabins that are seperable by bulkhead to break the cabin into lets say three segments. Each segment would have one or more parachute system to recover the segment individually. That would still be a large segment (in some 747 aircraft there can be 510 passengers thus 1/3 would be 150 people. At an average weight of 150 lbs. that would be 22500 pounds of people. The structure weight would also have to be considered as well as carry-on baggage, and if the underfloor baggage compartment is a part of the segment, the weight of the baggage would also have to be considered.) Even though this is viable it is not likely to be built! The costs would be very large, the modifications to the airframe extensive, and the percent chance that it would or could be used to positive effect so low that it is unlikely that aircraft company would find it economically feasable.
Conclusion:
Passenger survival by seperating the passengers from the aircraft prior to impact is not an option that could be easily or cost effectivly implemented. In my opinion, crash prevention and improving survivability of the airframe would be a better use of funds. If a plane does not crash usually the survival rate is 100%!!! In the case where an aircraft does crash, if the passengers are not killed or immobilized by the impact, they have a good chance of survival if they can egress the wreckage quickly! Smoke and fire are the main killers of people after a 'survivable' impact. The trend toward smaller and fewer escape doors must not continue. The risks to the passengers are much greater with only a few more rows between the passenger and an emergency exit.
This document is based solely on my personal analysis of the concept of equipping Commercial Aircraft with ejection seats. The information it is based on is public information on ejection seats and airliners, as well as analitical discussions with other aviation enthusiasts.
For more information on Airliner safety, see either Airline Crash Research Site or the Aviation Safety Web Pages.