More basic than the TAWS requirement are the certification requirements for transport category aircraft, specifically 14 CFR 25.729(e)(2)
14 CFR 25.729 (e) Position indicator and warning device. If a retractable landing
gear is used, there must be a landing gear position indicator easily
visible to the pilot or to the appropriate crew members (as well as
necessary devices to actuate the indicator) to indicate without
ambiguity that the retractable units and their associated doors are
secured in the extended (or retracted) position. The means must be
designed as follows:
(1) If switches are used, they must be located and coupled to the
landing gear mechanical systems in a manner that prevents an erroneous
indication of “down and locked” if the landing gear is not in a fully
extended position, or of “up and locked” if the landing gear is not in
the fully retracted position. The switches may be located where they
are operated by the actual landing gear locking latch or device.
(2) The flightcrew must be given an aural warning that functions
continuously, or is periodically repeated, if a landing is attempted
when the landing gear is not locked down.
(3) The warning must be given in sufficient time to allow the landing
gear to be locked down or a go-around to be made.
(4) There must not be a manual shut-off means readily available to the
flightcrew for the warning required by paragraph (e)(2) of this
section such that it could be operated instinctively, inadvertently,
or by habitual reflexive action.
(5) The system used to generate the aural warning must be designed to
minimize false or inappropriate alerts.
(6) Failures of systems used to inhibit the landing gear aural
warning, that would prevent the warning system from operating, must be
(7) A flightcrew alert must be provided whenever the landing gear
position is not consistent with the landing gear selector lever
For the 767, these are the requirements to trigger a landing configuration warning.
(7) Landing Configuration Warning Module Function (Fig. 8)
(a) The landing configuration warning module provides aural and visual
warnings for improper airplane configuration during landing. The
module receives a dual input of |12 volts dc and +5 volts dc from power supply modules A and B.
(b) The landing configuration warning consists of the master WARNING light, siren aural warning, CONFIG light, and level A EICAS message.
(c) The landing configuration warning logic is enabled when the module
receives the landing gear not down and locked discrete signal from the
proximity switch electronics unit.
(d) When this logic is enabled and any of the following conditions occur, the landing configuration warning is provided. The outputs for each condition are as listed below:
1) The trailing edge flaps are set to 25 or 30 degrees: a) red master WARNING lights b) amber CONFIG light c) siren aural warning d) level A EICAS message
2) Both left and right thrust levers are set to idle and the radio altitude is below 800 feet: a) red master WARNING lights b) amber CONFIG light c) siren aural warning d) level A EICAS message
3) Either left or right thrust lever is set to idle, the radio
altitude is below 800 feet, and the time delay as noted below has
expired: a) red master WARNING lights b) amber CONFIG light c) siren
aural warning d) level A EICAS message NOTE: A one time delay of 140
seconds is triggered by the edge of the landing-gear-up transition
pulse. This delay allows the airplane to reach an altitude of greater
than 800 feet and prevents a warning if only one engine is throttled
back at takeoff or go-around. If either 140 seconds have elapsed or
the airplane has exceeded 800 feet altitude, the system reverts back
to providing a warning from a single engine throttle back.
4) A radio altimeter failure exists, either thrust lever is set to the idle position, and the airplane is in a clean wing configuration (flaps and slats are fully retracted): a) amber CONFIG light b) level A EICAS message
5) A radio altimeter failure exists, either thrust lever is
set to the idle position, and either the flaps or slats are not fully
retracted (NOT CLEAN WING): a) red master WARNING b) siren aural warning