B737 TCAS, Terrain and Windshear Alerting

The Boeing 737 carries three systems that watch the world outside the airplane. TCAS warns of conflicting traffic. The ground proximity warning system (GPWS), enhanced with look-ahead terrain alerting, warns of terrain and, on some airplanes, of obstacles, and hosts the optional Runway Awareness and Advisory System (RAAS). Windshear is detected twice: ahead of the airplane by the weather radar (predictive) and at the airplane by the GPWS (reactive). All of them speak to the crew in voice aurals and show red or amber annunciations on the primary flight displays (PFDs) and navigation displays (NDs).

This article follows the 737 NG flight crew operations manual (FCOM) and quick reference handbook (QRH) and notes features fitted only to certain variants. The FAA Flight Standardization Board (FSB) lists no TCAS, GPWS or windshear item among the differences from the 737-800 to the 737-8, only a revised ND and weather radar range selector, and the FAA describes the MAX crew alerting system as the NG's with select alerts changed or added; on the MAX its own FCOM is the reference. The principles are in ACAS/TCAS, GPWS and TAWS and windshear and microbursts; the master caution system and the GPWS callouts are in B737 warning systems.

Alerting philosophy

The FAA describes 737 crew alerts as lights, displays, voice and tone aurals and the stick shaker, each tied to a checklist or maneuver in the QRH; normal voice callouts are indications, not alerts. Red lights mark conditions that need the crew's immediate attention, amber lights conditions that need timely attention.

Alerts are prioritized by the level of hazard and the reaction time the crew has left:

• an actual windshear warning, the airplane already in the shear, inhibits the look-ahead terrain and the radio altitude based GPWS alerts;
• those GPWS alerts and the actual windshear warning inhibit the predictive windshear alerts;
• GPWS and windshear warnings inhibit all TCAS alerts;
• RAAS callouts rank below any GPWS alert, the radio altitude callouts included, and some come back when the higher alert clears.

GPWS, TCAS, predictive windshear and altitude alert aurals reach the speakers and headsets at preset volumes that the crew can neither adjust nor switch off, although an audio control panel in degraded mode loses the GPWS, windshear and altitude alert aurals on its side. On battery power alone none of the three systems works.

TCAS traffic and resolution advisories

TCAS interrogates the transponders of other airplanes, tracks them from their replies and predicts their flight paths, independently of air traffic control. Airplanes without an operating transponder are invisible to it, and one whose transponder sends no altitude can never generate a resolution advisory.

On the transponder mode selector, TA ONLY enables the display of traffic advisories and TA/RA of traffic and resolution advisories; XPNDR and ALT RPTG OFF select the transponder with and without altitude reporting. TCAS OFF is one of the two flags expected on the flight instruments during the preflight, and the crew selects TA/RA on entering the departure runway.

A traffic advisory (TA) comes when the other airplane is about 40 seconds from the closest point of approach (CPA). "TRAFFIC, TRAFFIC" sounds once and is then reset until the next TA, TRAFFIC appears on the ND and the TA symbol is drawn at the intruder's range and relative bearing. If it keeps closing, a resolution advisory (RA) follows about 25 seconds from CPA, with a voice command and pitch guidance to maintain or increase separation. These are the FCOM's round figures; the logic varies them with own altitude (ACAS/TCAS).

• Proximate traffic is within 6 NM and 1,200 ft but not expected to cause a TA or RA; other traffic lies beyond those limits.
• An arrow beside a symbol shows a target climbing or descending at more than 500 ft/min.
• With TA ONLY or TA/RA selected, a TA or RA brings the traffic display up by itself on an ND in MAP, center MAP, VOR or APP mode, if neither pilot has selected TFC.
• OFFSCALE shows when the range selected cannot show the TA or RA; traffic without bearing becomes a text block of distance, altitude and vertical motion under TRAFFIC.
• On certain variants the altitude range switch sets the band shown: N from 2,700 ft below to 2,700 ft above, ABV from 2,700 ft below to 7,000 ft above, BLW from 7,000 ft below to 2,700 ft above.

During an RA the PFD shows red TCAS pitch command areas, the pitch regions to avoid; the airplane symbol must stay outside them. On certain variants the vertical speed tape also turns red over the rates to avoid or exit, and on most the flight path vector dims while an RA is displayed.

The RA is flown promptly, even against an ATC clearance, and reported to ATC; the crew does not maneuver on a TA alone (ACAS/TCAS). Near the ground TCAS removes the RAs that would drive the airplane toward it: INCREASE DESCENT below about 1,500 ft radio altitude, DESCEND below about 1,100 ft and all RAs below about 1,000 ft, where TA/RA becomes TA only by itself and TA ONLY shows on the ND. All TCAS voice annunciations stop below about 500 ft.

GPWS modes and radio altitude callouts

The GPWS gives radio altitude based alerts, worked out from radio altitude combined with barometric altitude, airspeed, glideslope deviation and configuration. They are the classic GPWS modes 1 to 5 (GPWS and TAWS). Every one except GLIDESLOPE also shows PULL UP in red on both attitude indicators.

The GPWS also makes the radio altitude and minimums callouts and the BANK ANGLE alert at 35°, 40° and 45° of roll (B737 warning systems); on airplanes so equipped, AIRSPEED LOW sounds when the speed enters the amber minimum maneuver speed bar. The pilot flying acknowledges the callouts, and the pilot monitoring makes any expected one that is not heard.

The amber GPWS INOP light shows a computer malfunction, a power loss or invalid inputs from sensors such as the radio altimeter, ADIRU, ILS receiver, IRS or FMC. The SYS TEST switch, inhibited in flight, tests the lights, the PULL UP and WINDSHEAR annunciations, the aurals and a terrain test pattern.

Look-ahead terrain alerting and terrain display

The enhanced GPWS keeps an internal worldwide terrain database, detailed near major airports and coarser between them. From the airplane's position, barometric altitude, vertical flight path and groundspeed it predicts conflicts and alerts on the estimated time to impact, so it can warn of a ridge ahead in level flight.

The solid areas show in expanded MAP, center MAP, expanded VOR and expanded APP modes, and the TERR INHIBIT switch inhibits all of these alerts.

On most variants the terrain display colors terrain by its height relative to the airplane's current altitude:

The terrain elevation numbers on the ND are green for terrain more than 500 ft below the airplane, amber from 500 ft below to 2,000 ft above and red higher still. The EFIS TERR switch shows terrain in expanded MAP, center MAP, expanded VOR and expanded APP modes, arms it in PLN, center VOR and center APP, and deselects the weather radar; a second push removes it. TERR in cyan shows that the display is enabled, whether by the pilot or automatically. TCAS and the LNAV route can share the display with terrain, but weather radar cannot.

Two limitations apply: the terrain display must not be used for navigation, and the look-ahead terrain functions must not be used within 15 NM of takeoff, approach or landing at an airport or runway not in the GPWS database.

Runway Awareness and Advisory System

RAAS, fitted to certain 737s, tells the crew by voice where they are on the airport. It needs the terrain and airport runway database in the GPWS computer, an airport in that database, and GPS signals of the required accuracy.

The amber runway INOP light shows a RAAS failure, inadequate GPS accuracy, an airport not in the database, or the runway inhibit switch left at INHIBIT at 250 kt or more for over 60 seconds. RAAS must not be used for navigation or in place of NOTAM or ATIS information.

Predictive and reactive windshear

Windshear warnings come with a voice aural and a red WINDSHEAR annunciation: on the PFDs and NDs for a predictive windshear warning, on both PFDs for the reactive one. The predictive caution shows an amber WINDSHEAR on the NDs with "MONITOR RADAR DISPLAY". The ND annunciation appears in every ND mode.

The weather radar starts scanning for windshear by itself when the thrust levers are set for takeoff, even with an engine off or the IRS not aligned, and in flight below 2,300 ft radio altitude. With predictive windshear enabled, on the ground or below 2,300 ft, the sweep is limited to 120°; above 2,300 ft it returns to 180°. During takeoff and landing new predictive cautions are inhibited between 80 kt and 400 ft RA, and new predictive warnings between 100 kt and 50 ft RA; alerts already showing stay. On the ground, thrust levers not advanced, the radar TEST position tests the predictive windshear indications.

View full schematic

The automatics do not protect against shear. If windshear is met during a flight director takeoff or go-around, the flight director holds the target speed until the vertical speed falls to about +600 ft/min, then commands 15° nose-up, and if the vertical speed keeps falling it holds 15° until near stick shaker speed and then commands pitch for intermittent shaker. On an ILS approach the flight director and autopilot hold the altitude or glideslope regardless of angle of attack or stick shaker, so the speed can decay into a stall unless the pilot pushes TO/GA or disengages the autopilot. Assumed temperature reduced thrust is not used when windshear is possible.

Inhibits and crew response

The three GPWS inhibit switches are normally guarded at NORM, and no switch silences SINK RATE, TERRAIN, DON'T SINK or the reactive windshear warning. Ground proximity and windshear warnings are answered with recall maneuvers, flown from memory:

A ground proximity caution calls only for correcting the flight path or configuration. With the autopilot engaged in windshear, the pilot flying pushes TO/GA, verifies the TO/GA annunciation and go-around thrust, retracts the speedbrakes and monitors, ready to fly by hand if the shear exceeds the autopilot. On the NG the thrust levers may go full forward with the EECs in normal mode; in hard alternate mode only when terrain contact is imminent, to avoid an overboost. The MAX engine control has no overboost rating, so its own FCOM applies (B737 memory items).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.