I assume you mean the speed of the aircraft, so: As slow as practical. Since most eject situations happen when the pilot no longer has full control of the airplane, I expect you take what you can get. Since different classes of jet have different operating envelopes, I would be very surprised if there is a single answer to this question. Basic rules of physics suggest that the slower you are moving when you eject, the safer you are. They also suggest that ejecting at supersonic speeds would be very dangerous, if not instantly fatal. If you mean "how fast does that rocket seat move as it leaves an out of control airplane?" I am sure it depends on the seat, which varies with the type of plane.

The Martin Baker Type 10A Ejector Seat used in all Tornado aircraft accelerates from 0-100mph in 0.4 seconds. In reaction to the response 'but at what speed are pilots ejected?', I say that the aircraft can be travelling at any speed within its capability, and the pilot is at this speed when he is ejected, although a pilot hates to lose his aircraft, so will spend as long as possible trying to regain control, during which time the aircraft is likely to be decellerating. If you want the relative speed of the pilot to the aircraft when he is ejected, this is zero. The pilot must be accelerated (as in the example given above), and starts from zero.

The only aircraft which actually used a truly "supersonic" ejection seat for the aircrew was the SR-71 Blackbird. When something goes wrong with a supersonic aircraft they tend to slow down very rapidly so ejection is around 400mph or lower but the seat in fighters are made so that theoretically at least a pilot can eject at well over 600mph. However most fighter aircraft get into serious trouble close to the ground and during landing and takeoffs. The importance of ejections seats is mainly that they can get an aircrew out of danger a very low speeds and altitudes; thus the importance today of having a "zero-zero" ejection seat. Please note that the F-111 strike aircraft utilized an escape module for the aircrew and they can eject at virtually any altitude or speed. Their escape module also doubles as a shelter or life raft as it has been designed to float.

There are two main types of ejection seats: cartridge fired and rocket fired. If I remember correctly, the old cartridge fired seats accelerated at about 20g's (196m/s^2). The current Martin Baker ACES II rocket powered seat accelerates at about 12g's (about 120m/s^2) I can find no resources to give the burn duration of the rocket in the ACES II, but I'm guessing it is anywhere between 0.5sec and 2.0 sec. This would set the peak velocity of the seat itself at anywhere between 60m/s^2 (116knots, 134mph) to 240m/s^2 (467knots, 537mph)). That's the speed the ejection seat leaves the aircraft, if that's what you were asking. ---- If you were referring at what aircraft speed the pilots can eject, it depends on the specifications of the ejection seat. The ACES II seat is rated for up to 600knots indicated air speed. It is important to understand that supersonic windblast can kill - ejecting into 1300kph winds is pretty nasty. Most ejections take place below 600knots - dogfights are rarely supersonic, and most losses of control happen either at stall or low altitude/low speed. However, having said that, I don't believe there is any device in an ejection seat to prevent it deploying at supersonic speeds. There are examples of supersonic ejection seats. The XB-70 Valkyrie had a good example - the seats had clamshell doors that closed around the seat at ejection, forming an airtight capsule. The F-111 Aardvark fighter-bomber has the entire cabin and part of the wing-roots as a capsule. This capsule has its own liferafts built into it, and even a bilge pump! As long as the cabin hasn't been damaged, it is well sealed, and the crew can survive for a while in it. I am not aware of any operational deployments of the F-111 ejection capsule, though.

Once during the cold war the crew of a B-52 had to eject at 600 MPH due to an on board fire. The pilot was fine, the co-pilot ended up with scars all over his face because the wind shattered his visor, and the navagator died because the fire weakened his parachute straps.