John, the 747 cabin altitude warning is an intermittent horn with a tonal frequency of 220 to 280 Hz and is identical to the unsafe takeoff warning. The warning will persist untill cancelled by the ALT HORN CUTOUT button on the pressurization control module on the the FE's panel. The altitude alert warning is a C chord output which consists of three tones of 461-563Hz, 576-704Hz and 691-845Hz. The duration of the chord is 1.5 to 2.5 seconds and cannot possibly be mistaken for loss of cabin pressure. I'm sure I don't have to point out that the Altitude Alert is a NAV function and has nought to do with cabin altitude.
Last edited by HotDog; 6th Jul 2002 at 06:50.