Weather Radar and Predictive Windshear

This radar's official character is "automatic first": in Multiscan mode you touch neither tilt nor gain — it sets everything by altitude, terrain, season, and time of day, serves "the weather that matters" to the ND, and removes ground clutter and dead convection. The V2 version adds a "detective" layer (hazard prediction): on top of reflectivity, it infers hail, lightning, and cells growing rapidly beneath you. Its other identity is life-saving: predictive windshear (PWS), calling "GO AROUND WINDSHEAR AHEAD" 5 NM before you plough into a microburst. This article covers system and logic (how it works, when it auto-activates, how alerts are graded); the technique — how to scan, read, and deviate — is a companion article.

1. Multiscan V2 — a robot that scans the sky for you

Per FCOM DSC-34-20-30-10:

The aircraft is fitted with two Multiscan weather radar systems with a Predictive WindShear (PWS) function and a weather hazard prediction function .

The core of the automation is "two eyes plus a base map." The AMM mechanism. Per AMM 34-41-00:

This function determines the optimum tilt angle for the radar through monitoring of: - the aircraft altitude above the terrain (RA and ADR information) - the aircraft position (IR information)

Two antenna scans are performed, each scan is optimized for a particular region in front of the aircraft. The upper beam detects medium-range weather and the lower beam detects short and long-range weather by an automatic tilt and gain adjustment.

The FCOM adds how the image is assembled. Per FCOM DSC-34-20-30-10:

‐ The radar alternatively scans at two antenna tilt settings. The weather radar image that is displayed is the result of the stored and combined information from each beam.

So what you see is not "what the antenna illuminated this frame" but an image stitched from a database — which is why the image-refresh cadence changes with the selected range. The V2 increment. Per AMM 34-41-00:

- additional antenna scanning and new algorithm implemented in multiscan V2 function that allows to depict inferring threat area on top of detected reflectivity.

"Inferring" is the key word: reflectivity sees only water drops, but V2 dares to paint a red dot where there is no echo (the hazard prediction, §4). Two hardware facts: the two transceivers share one antenna, the waveguide switch alternating by the Radar sw 1/2 (so "changing radar" really changes transmitters); and the antenna's horizontal reference comes from the IR. Per AMM 34-41-00:

The three ADIRUs give data information to the weather radar system. These data ensure the radar antenna stabilization and the computations of the windshear function.

So moving the ATT HDG rotary re-sources even the radar's "horizontal plane"; lose all IR and the radar cannot even stabilise the antenna. Coverage envelope: ±90° azimuth, to 320 NM; turbulence detection to 40 NM; PWS detection ±60°, display ±30° / 5 NM.

2. The panel and two personalities — AUTO's radar and MAN's radar

Panel elements: Radar sw (1/OFF/2, selects transceiver), GAIN (CAL normal), display mode (WX / WX+T / TURB / WX+T+HZD / MAP), TILT (for MAN), MULTISCAN (AUTO/MAN), GCS (spring-loaded to AUTO), PWS (AUTO/OFF). In the AUTO personality, CAL gain is "smart calibration." Per FCOM DSC-34-20-30-10:

‐ When the gain selector is set to the Calibrated position (CAL), the radar automatically adjusts the gain based on various parameters (aircraft altitude, geographical area, season, time of the day) to obtain the best weather display.

It even subtracts — a "quiet and dark" philosophy that removes convection off the flight path and decayed (post-convective) cells; GCS then wipes the ground away. Per FCOM DSC-34-20-30-10:

‐ The Ground Clutter Suppression (GCS) function removes the ground returns from the ND.

In the MAN personality all that cleverness is gone: manual tilt (the TILT knob, zero = the IRS horizon), no GCS, and CAL is only a fixed calibrated value, not geographically adaptive. Hence the official spoiler. Per FCOM DSC-34-20-30-10:

As a consequence, when the flight crew switches from the Multiscan Automatic mode with CAL gain to the Manual mode with CAL gain, the weather displayed on the ND may be significantly different.

Not a fault — two personalities see the world differently (the technique article covers when it is worth switching to MAN to "hand-inspect" a cell). The recommended mode. Per FCOM DSC-34-20-30-30:

Weather, Turbulence and Hazard mode (recommended position) : The ND indicates precipitation, turbulence areas in magenta (within 40 NM) and hazard prediction risk areas.

Hazard prediction function is only available when the MULTISCAN sw is set to AUTO.

In MAN, WX+T+HZD downgrades to the equivalent of WX+T — switch to manual and you have dismissed the detective.

3. The ND's four lines of small print — the radar introduces itself

The four lines at the ND's lower right are the radar's status bar: line 1, MAN GAIN (when manual gain); line 2, the active mode (cyan = MAN, green = AUTO — the colour itself is the mode indicator); line 3, tilt. Per FCOM DSC-34-20-30-30:

The tilt angle is the angle between the horizon and the radar beam axis.

In AUTO it shows a green "average tilt" (the mean of the two beams); in MAN, cyan actual tilt. Losing the auto-tilt function raises a prompt. Per FCOM DSC-34-20-30-30:

When the multiscan function is lost, the tilt value is dashed and the NO AUTO TILT message appears on the ND, until the flight crew sets the MULTISCAN sw to MAN.

(It is asking you to take over — auto died, so scan manually.) Line 4 is the fault list, read on one axis. Per FCOM DSC-34-20-30-30:

If the message is in red, the ND does not display the radar image. If the message is in amber, the image is not affected.

The red family (NO WXR / WXR RT / WXR ANT / WXR CTL / WXR RNG) = image gone; the amber family (WXR DU overheat / WXR WEAK / WXR ATT / WXR STAB / PRED W/S / NO AUTO TILT / WXR TEST) = image present but a function is ailing. PRED W/S is amber — the radar image is fine but windshear prediction is dead, matching the ECAM NAV PRED W/S DET FAULT (action: consider switching to the other radar).

4. The hazard trio and two-level turbulence — painting threats beyond the echo

Core Threat Assessment. Per FCOM DSC-34-20-30-30:

The Core Threat Assessment function operates until 320 NM.

The adjustment can go up to a color level (green cells may become yellow, yellow cells may become red).

It promotes "reflectivity colour" from a pure rain gauge to a threat scale — the same green, if it has convective features, is "sentenced up." Associated Threat. Per FCOM DSC-34-20-30-30:

Associated threats are represented by red dots on the ND and can be within the cell boundary or outside.

Two kinds of red dot: (1) potential lightning/icing in precipitation near the freezing level —

This information is available only when the aircraft is below the freezing level or up to 6 000 ft above the freezing level.

and (2) the dashed red-dot rectangle at a cell top (potential icing/hail/lightning, available at all altitudes, the rectangle able to extend beyond the cell — the anvil the echo cannot see). Predictive Overflight — the cure for "am I safe flying over its top?" Per FCOM DSC-34-20-30-30:

The predictive overflight function is active above 25 000 ft and operated up to 40 NM.

an auxiliary beam with very low tilt searches for growing cells.

As it is based on actual convection measurement, the flight crew should avoid predictive overflight areas.

The icon evolves in three acts: first "a red-dot area floating alone on black" (the cell still far below your altitude) → co-displayed with the echo once the cell grows to your level → withdrawn one minute after the threat clears (or once the red core displays directly). Two-level turbulence is another V2 refinement. Per FCOM DSC-34-20-30-30:

(1) Basic plain magenta: It indicates potential severe turbulences (available in WX+T or WX+T+HZD display mode).

(2) Speckle magenta: It indicates potential moderate turbulences that could decrease passenger comfort (available only in WX+T+HZD display mode).

Solid = severe (deviate), speckle = moderate (fasten seat belts). Plus a "don't-be-fooled" indicator, PAC. Per FCOM DSC-34-20-30-30:

The PAC alert displays a yellow arc on the outermost scale of the ND, when an intervening rainfall creates an attenuated area behind a storm cell (also called a radar shadow or attenuation effect).

The PAC alert is only available when the MULTISCAN sw is in the AUTO position.

The yellow arc means: the black behind this bearing line may not be no weather, but weather the radar cannot see through (within 80 NM, CAL gain, WX-family modes). The technique article's "black behind red = active area" is the same source.

5. PWS — the radar's life-saving second job

What a microburst looks like — the AMM gives the standard picture. Per AMM 34-41-00:

A microburst is a cool shaft of air, like a cylinder, between 1000 and 3000 ft. When it encounters the ground (airflow velocity from 40 to 110 kts) the downward moving airflow is translated to a horizontal flow (from 80 to 220 kts), at the base of the air shaft.

The radar measures the Doppler speed difference of the raindrops swept up by the outflow, so it can see it before you enter. Activation is fully automatic. Per FCOM DSC-34-20-30-20:

The weather radar has a Predictive WindShear System (PWS) that operates when the PWS sw is in the AUTO position, and the aircraft radio height is below 2 300 ft, and ‐ Weather radar is ON (Radar sw on position 1 or 2), or ‐ Weather radar is OFF, and ‐ At least one engine is running, and ‐ Aircraft ground speed is greater than 30 kt, or ‐ Aircraft longitudinal acceleration is above a given threshold during at least 0.5 s

"It works even with the radar off" is the design floor: forget to switch the radar on and PWS wakes itself the moment you accelerate for take-off (the AMM qualifier chain: qualifier B = engine oil pressure proves the engine really turns; qualifier C = IR ground speed, or D = longitudinal acceleration, proves you are really taking off — three hardware signals guarding against a spurious ground activation). Per AMM 34-41-00:

Two types of qualifier inputs are required to prevent automatic activation of the windshear function (B and C or B and D).

The detection window. Per FCOM DSC-34-20-30-20:

The system scans the airspace for windshear within a range of 5 NM ahead of the aircraft.

Three alert levels (aural + PFD + ND): approach warning = «GO AROUND WINDSHEAR AHEAD» + red W/S AHEAD + ND icon; take-off warning = «WINDSHEAR AHEAD» ×2 + red W/S AHEAD + icon; caution = «MONITOR RADAR DISPLAY» + amber W/S AHEAD + icon; advisory = ND icon only. The window stretches with phase. Per FCOM DSC-34-20-30-20:

During the takeoff roll, up to 100 kt, both warnings and cautions are available within a range of 3 NM.

During final approach, the visual and aural warning alerts are downgraded to caution alerts between 370 ft AGL and 50 ft AGL, and range between 1.5 NM and 0.5 NM.

At takeoff, alerts are inhibited above 100 kt and up to 50 ft. During landing, alerts are inhibited below 50 ft.

The inhibition philosophy: in the few seconds around V1 (> 100 kt) and the last tens of feet, any "new message" only distracts. The loudspeaker pecking order. Per FCOM DSC-34-20-30-20:

‐ Have priority over TCAS, GPWS and other FWC aural warnings ‐ Are inhibited by reactive windshear detection and aural messages of stall warnings.

Memorise the ranking: stall > reactive windshear > PWS > TCAS/GPWS — the more "already happening," the louder; prediction yields to reality. When the selected radar fails, remember the other. Per FCOM DSC-34-20-30-20:

When the selected weather radar fails, the flight crew can recover the PWS function by selecting the operative system on the Radar sw of the radar control panel.

6. Memos, borrowed data, and cross-article signposts

ECAM memo, PRED W/S OFF. Per FCOM DSC-34-20-30-30:

PRED W/S OFF : This memo appears in green during flight phases 2 and 6.

In take-off/approach phases (3/4/5/7/8/9) or after T.O CONFIG with it still off, it turns amber to scold — the PWS switch's normal position is always AUTO. One easily-missed bus. Per AMM 34-41-00:

One ARINC 429 high-speed bus (Hazard) provides radar data to the Enhanced Ground Proximity Warning System (EGPWS).

The radar and the EGPWS have a private line (display coordination: the mutual-exclusion of TERR and WX images). Cross-article signposts: how to scan/read/deviate and identify ice crystals → the technique article; the PWS/reactive windshear memory items and escape → the technique article; PRED W/S DET FAULT / the radar fault family and the MEL (dual failure flyable in daylight VMC!) → the abnormal and MEL articles.

Key numbers

Self-test

[!note]- Q1. What three inputs feed Multiscan auto-tilt, and what range band does each beam cover? RA + ADR (height above terrain), IR (position), plus the internal terrain reference table. Upper beam = medium range; lower beam = short and long range by auto tilt/gain.

[!note]- Q2. Switching AUTO+CAL to MAN+CAL, why can the picture change greatly, and which four smarts are lost? Two personalities see differently. Lost: auto-tilt, geographically adaptive gain, GCS, and the quiet-and-dark subtraction (and hazard prediction).

[!note]- Q3. HZD red dots appear where there is no echo — on what basis? The freezing-level red-dot altitude band? On inference above reflectivity. Freezing-level type: below the freezing level, or up to 6000 ft above it.

[!note]- Q4. Cruising FL380, a "red-dot area floating on black" 30 NM ahead — what is it and how to handle? Predictive overflight of a cell still below you (based on actual convection measurement) — avoid it.

[!note]- Q5. What are the PWS "qualifiers B/C/D," and what do they guard against? B = engine oil pressure (engine turning), C = IR ground speed, D = longitudinal acceleration. Two are required (B+C or B+D) to prevent spurious ground activation.

[!note]- Q6. Take-off roll at 80 kt, «WINDSHEAR AHEAD» ×2 — window? At 110 kt would the same threat sound? Up to 100 kt: warnings and cautions within 3 NM. At 110 kt (> 100 kt) alerts are inhibited up to 50 ft.

[!note]- Q7. On final both PWS and GPWS warning conditions are met — who is heard first? And if reactive windshear also triggers? PWS has priority over GPWS, so PWS first. But reactive windshear (and stall) inhibit PWS, so reactive windshear would win.

Key takeaways

References

• FCOM DSC-34-20-30-10 — Multiscan, CAL, quiet-and-dark, GCS, AUTO-MAN difference.
• FCOM DSC-34-20-30-20 — PWS activation, three alert levels, windows, inhibition, priority.
• FCOM DSC-34-20-30-30 — panel, ND four lines, fault colour, PAC, hazard trio, two-level turbulence, memo.
• AMM 34-41-00 — V2 mechanism, dual beam, coverage, microburst, qualifier signals, EGPWS bus, antenna stabilisation.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.