McDonnell Douglas MD-11 and Boeing 717 (inherited from McDonnell Douglas after the merger with Boeing) series aircraft in their Central Aural Warning Systems (CAWS). These systems provided a voice for most warnings, including fire, altitude, cabin altitude, stall, overspeed, autopilot disconnect, and so on. In more advanced cockpits, on newer aircraft, there may be many other voice warnings managed by an integrated indication and crew alerting system (ICAS) such as "Gear up, Gear up!" These may be warning words or phrases, or simply declarative statements that augment the pilot's situation awareness. Voice gender Early human factors research in aircraft and other domains indicated that female voices were more authoritative to male pilots and crew members and were more likely to get their attention. Much of this research was based on pilot experiences, particularly in combat situations, where the pilots were being guided by female air traffic controllers. They reported being able to most easily pick out the female voice from amid the flurry of radio chatter. In October 1996, a report by UK's Defence Research Agency on the fast jet Collision Warning System Technical Demonstrator Programme (Reference DRAMS/A VS/CR96294/1) reported: "The primary alerting signal from the CWS to the crew was an audible warning passed over the aircraft intercom system. For the first flight these warnings were given in a male voice but, on the advice of the crew, this was changed to a female voice for the second flight onwards. They said that a female voice offered greater clarity." More recent research, however, carried out since more females have been employed as pilots and air traffic controllers, indicates that the original popular hypothesis may be unreliable. Edworthy and colleagues in 2003, based at the University of Plymouth in UK, for example, found that both acoustic and non-acoustic differences between male and female speakers were negligible. Therefore, they recommended the choice of speaker should depend on the overlap of noise and speech spectra. Female voices did appear to have an advantage because they could portray a greater range of urgencies due to their generally higher pitch and pitch range. They reported an experiment showing that knowledge about the sex of a speaker has no effect on judgments of perceived urgency, with acoustic variables accounting for such differences. Arrabito in 2009, however, at Defence Research and Development Canada in Toronto, found that with simulated cockpit background radio traffic, a male voice