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In one of the comments on this question (under the accepted answer), someone remarks that

Aircraft can legally fly without a system to alert the crew when they are landing with gear up, etc.

Are these systems installed on commercial aircraft as a legal requirement?

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All turbine-powered aircraft used in scheduled airline service under CFR Part 121 are required to have an approved TAWS installed;

§121.354 Terrain awareness and warning system.

(a) Airplanes manufactured after March 29, 2002. No person may operate a turbine- powered airplane unless that airplane is equipped with an approved terrain awareness and warning system that meets the requirements for Class A equipment in Technical Standard Order (TSO)-C151. The airplane must also include an approved terrain situational awareness display.

(b) Airplanes manufactured on or before March 29, 2002. No person may operate a turbine-powered airplane after March 29, 2005, unless that airplane is equipped with an approved terrain awareness and warning system that meets the requirements for Class A equipment in Technical Standard Order (TSO)-C151. The airplane must also include an approved terrain situational awareness display.

(c) Airplane Flight Manual. The Airplane Flight Manual shall contain appropriate procedures for—

(1) The use of the terrain awareness and warning system; and

(2) Proper flight crew reaction in response to the terrain awareness and warning system audio and visual warnings.

The TAWS system includes the functions of the older Ground Proximity Warning System (GPWS).

GPWS Mode 4, Flight Into Terrain with Less Than 500 Feet Terrain Clearance and Not in Landing Configuration, provides for alerts to the crew when entering a defined envelope below 500 feet (based on height and rate of descent) and the gear is not in landing configuration. There is also an alert envelope for Flaps not in landing Configuration.

The Mode 4 alert consists of an audible alert "TOO LOW, GEAR. TOO LOW, GEAR."

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  • $\begingroup$ Is it possible to comment on the MEL requirements about an operative TAWS? It seems possible to fly with inoperative units which de facto make the TAWS inoperative itself. E.g: In this A320 MEL, page 34-2, IR1 may be inop, in this case "The terrain function of TAWS is inoperative". There are other occurrences. It would mean, if confirmed, that it's possible to fly some days with some TAWS functions inop, maybe the gear up condition. $\endgroup$ – mins Feb 12 at 18:20
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    $\begingroup$ @mins That MEL entry and a similar for FMGC 1 apply only to the 'Terrain' function which includes the FLTA function, PDA function, and Terrain Display function. They use A/C position and inertial vector compared to the digital terrain database to predict terrain conflict. The Mode 4 alert is a GPWS function. Look at 48-01 on page 32-21 for the limitations. It may be INOP if "alternate procedures are established and used and repairs are made within 2 flight days." A failure of Radar Alt 1 will also render GPWS failed - see 42-01 on page 34-18. $\endgroup$ – Gerry Feb 12 at 20:01
  • $\begingroup$ Thanks for having digged into the MEL and added your analysis. $\endgroup$ – mins Feb 12 at 20:06
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    $\begingroup$ @Mazura - According to the black box (CVR) transcript from the accident report, "too low, gear," was sounded at 15:30:06. $\endgroup$ – ymb1 Feb 12 at 22:31
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    $\begingroup$ @Cloud its not any more inaccurate than any dramatization of a RW event. They perhaps spiced up the courtroom drama for cinematic suspense. Its not a documentary. It's good entertainment if you're into that sort of thing. $\endgroup$ – Jamiec Feb 13 at 16:36
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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 improbable.

(7) A flightcrew alert must be provided whenever the landing gear position is not consistent with the landing gear selector lever position.

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

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