Terrain Awareness and Warning System Definition Terrain Awareness and Warning System (taws) Description

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Terrain Awareness and Warning System

Definition

Terrain Awareness and Warning System (TAWS)

Description

TAWS is an airborne safety net designed to overcome the shortcomings of the original Ground Proximity Warning Systems (GPWS) by reliably and automatically providing flight crew with warning of potential collision with terrain in sufficient time for effective avoiding action to be taken. ICAO SARPs specify that it must provide "Forward Looking Terrain Avoidance".

Enhanced ground Proximity Warning System (EGPWS) is the system implemented by the pioneers of this initiative, Honeywell and as a consequence, the names TAWS and EGPWS are often used interchangebly, although since the introduction of regulatory requirements and recommendations for its carriage, the term TAWS has been universally used by Regulatory Agencies.

Whilst GPWS was based only upon radio altimeter inputs, TAWS takes account of actual aircraft position in relation to a terrain map contained in the equipment. This actual position is determined by either built-in GPS or using an input from the aircraft FMS position. TAWS is defined as Class A wih full functionality or Class B with restricted functionality.

TAWS Class A provides the following:

Forward Looking Terrain Avoidance (FLTA) by taking account of terrain both ahead of and below the aircraft to generate alerts of reduced terrain clearance and warnings of imminent terrain impact.
Premature Descent Alerting (PDA) if the aircraft descends below a normal approach path for the nearest runway.
Basic GPWS functions - Modes 1 to 6 using radio altimeter input moderated by aircraft position in relation to database terrain and obstacles (note that the Mode 6 requirement is altitude voice call out(s).
A Terrain Awareness Display which shows terrain above the aircraft current altitude and up to 2000 feet below it which can provide proactive situational awareness as well as safety net functions.

TAWS Class B equipment provides the following:

Forward Looking Terrain Avoidance (FLTA) by taking account of terrain both ahead of and below the aircraft to generate alerts of reduced terrain clearance and warnings of imminent terrain impact.
Premature Descent Alertimg (PDA) if the aircraft descends below a normal approach path for the nearest runway.
Basic GPWS Modes 1 3 and 6 only (there is no radio altimeter input).

The main differences between TAWS Class A and TAWS Class B are that TAWS Class B:

Depends entirely upon aircraft position to generate alerts/warnings including GPS altitude for the Mode 6 altitude voice callout.
Is required to have aircraft position determined by GPS.
Does not require a Terrain Awareness Display.
Cannot therefore provide proactive situational awareness or excessive terrain closure rates or alert for excessive deviation below an ILS GS.

Experience of TAWS has brought some key learning points including:

If TAWS Class A aircraft position is derived from the aircaft FMS and this does not use GPS, consequent errors in aircraft position input can reduce the terrain impact protection to no greater than that provided by basic GPWS see [AAIB - Incident Report EW/A2003/3/1 G-MEDA near CFIT Addis Ababa]

Use of a Terrain Awareness Display needs specific SOPs to guide flight crew when it should be used, espcially if it shares a screen with other displays.

The rapid spread of TAWS equipment, which has been accelerated by the introduction of ICAO SARPs which include a mandate for carriage on most commercial aircraft, has made a major contribution to the reduction in controlled flight into terrain accidents which has been observed in the past few years. Current ICAO SARPs require TAWS Class A on all aircraft with an MTAW 5700kg or authorised to carry 10 or more passengers when operated for public transport

The majority of available TAWS Class A equipment also incorporates two further situational awareness alerting functions with the core system requirements:

Mode 6: In addition to the pre-set altitude call out(s), an Voice Alert when bank angle exceeds 40 degrees at or above 150 feet agl or at increasingly smaller angles below this height

Mode 7: Windshear Detection and Annnuciation providing Alerts of Positive Windshear and Warnings of Negative Windshear.

Requirement to Carry TAWS

ICAO Annex 6 Part 1 (International Commercial Air Transport) SARPs require that TAWS Class A shall be carried in all aircraft with an MTWA greater than 5700kg or authorised to carry 10 or more passengers.TAWS Class A is Recommended for carriage in the case of aircraft authorised to carry 6-9 passengers.

ICAO Annex 6 Part 2 ( International General Aviation)SARPs require carriage of either TAWS Class A or TAWS Class B in all aircraft with an MTWA greater than 5700kg or authorised to carry 10 or more passengers

For EU and EEA States, the public transport requirement is actioned under the requirements of EU-OPS 1.665 which has the effect of mandating TAWS Class A:

"(a) An operator shall not operate a turbine powered aeroplane having a maximum certificated take-off mass in excess of 5 700 kg ... or a maximum approved passenger seating configuration of more than 9 unless it is equipped with a ground proximity warning system that includes a predictive terrain hazard warning function (Terrain Awareness and Warning System – TAWS).

(b) The ground proximity warning system must automatically provide, by means of aural signals, which may be supplemented by visual signals, timely and distinctive warning to the flight crew of sink rate, ground proximity, altitude loss after take-off or go-around, incorrect landing configuration and downward glide slope deviation.

(c) The terrain awareness and warning system must automatically provide the flight crew, by means of visual and aural signals and a Terrain Awareness Display, with sufficient alerting time to prevent controlled flight into terrain events, and provide a forward looking capability and terrain clearance floor."

Warning Provided by TAWS

Hazard awareness is provided by GPWS aural alerts or warnings and illumination of alert and warning lights in response to different situations. The table below illustrates a typical TAWS system.

Mode	Condition	Aural Alert	Aural Warning
1	Excessive descent rate	"SINKRATE"	"PULL UP"
2	Excessive terrain closure rate	"TERRAIN TERRAIN"	"PULL UP"
3	Excessive atitude loss after take off or go-around	"DON'T SINK"	(no warning)
4a	Unsafe terrain clearance while gear not locked down	"TOO LOW - GEAR"	"TOO LOW - TERRAIN"
4b	Unsafe terrain clearance while landing flap not selected	"TOO LOW - FLAP"	"TOO LOW - TERRAIN"
4c	Terrain rising faster than aircraft after take off	"TOO LOW - TERRAIN"	(no warning)
5	Excessive descent below ILS glideslope	"GLIDESLOPE"	"GLIDESLOPE"(1)
6	Advisory Callout of Radio Height	(for example) "ONE THOUSAND"	(no warning)
6	Advisory Callout of Bank Angle	"BANK ANGLE"	(no warning)
7	Windshear protection	"WINDSHEAR"	(no warning)
NOT MODE NUMBERED	Terrain Proximity	"CAUTION TERRAIN"	"TERRAIN TERRAIN PULL UP"

Aircraft Equipment

Aircraft equipment operated by the pilot comprises a horizontal situation display, usually integrated as a mode of operation of the EFIS, and a control and indicator unit. The control and indicator unit contains as a minimum two lights: a red light to indicate a hard warning (imminent danger) and an amber light to indicate an alert (soft warning or caution).

Response to a TAWS Activation

TAWS is a safety net in which a (Hard) Warning indicates that the aircraft is in a dangerous situation and immediate action is required and an Alert (or soft warning) indicates an abnormal status in reation to terrain which invites prompt review and a possible change of flight path or aircraft configuration.

Appropriate TAWS response procedures for flight crew are determined after careful study of aircraft type performance capability. They must be clearly defined by operators and, in the case of a Warning, should be followed without hesitation as soon as a triggered. Operators normally define different response procedures based upon memory drills for a Warning (sometimes called a Hard Warning) and an immediate review in the case of an Alert (sometimes called a Soft Warning)

The EU-OPS Regulatory requirements related to TAWS are:

When undue proximity to the ground is detected ... by a ground proximity warning system, the commander or the pilot to whom conduct of the flight has been delegated shall ensure that corrective action is initiated immediately to establish safe flight conditions. (EU-OPS 1.395)

CFIT