Weather Radar Usage Boeing question

[BigGrumpyAlien]

Well ladies and gents, thank you for the posts......I have made some further ground and post the following. It is by no means definitive and is a hopefully brief overview with some interesting points....

I knew I had covered some of the stuff but it has been good to refresh some of those old brain cells....

It has gone some way to answering my questions.. I have produced it below and hope that it is of help to some......



The GPWS provides alerts based on radio altitude and combinations of barometric altitude, airspeed, glide slope deviation, and airplane configuration and is operative between 30 ft and 2450 ft RA. The alerts are for:

1. Excessive descent rate
2. Excessive terrain closure rate a. With Flaps NOT in landing position. b. With Flaps IN a landing position.
   
3. Altitude loss after takeoff or go-around
4. Unsafe terrain clearance when not in the landing configuration a. Landing Gear NOT down AND Flaps NOT in landing position. b. Landing Gear NOT down OR Flaps not in landing position.
5. Excessive deviation below an ILS glide slope
6. Altitude Callouts
7. Windshear Detection

The GPWS takes Input from:

• Air Data System
• IRS
• ILS
• RAD ALT

For Look Ahead Terrain Alerts additional Inputs from;

• NON Pegasus FMC
• Pegasus GPS

Notes
GPWS

• Monitors Terrain using an INTERNAL WORLDWIDE DATABASE. It does not account for manmade objects.
• Terrain display is generated using a build in ‘sweep’ on the display. It IS NOT a radar display. This display is computer generated from the database in the GPWS and correlated to GPS position.
• Altitude callouts are made based on the setting of the Captains Rad Alt.
• Does not provide an alert for flight toward vertically sheer terrain or slow descents into terrain while in the landing configuration.
• Uses WGS-84 data(be aware not all countries are WGS-84 compliant)

Terrain ahead of the airplane may exceed available climb performance. A ground proximity caution or warning does not guarantee terrain clearance.

WINDSHEAR
Can be more easily described by breaking it down into two distinct parts.

1. Windshear
2. Predictive Windshear.

Windshear
Windshear alerts are available during takeoff, approach and landing. GPWS provides a warning when the airplane is in a windshear.

• Excessive windshear at the current airplane position detected by GPWS.
• Enabled below 1,500 feet Radio Altitude.

GPWS Windshear detection begins at rotation.


Predictive Windshear

Weather radar provides “predictive windshear alerts” for excessive windshear ahead of the airplane and these are annunciated on the HSI.

The weather radar

• Uses radar imaging to detect disturbed air prior to entering a windshear.
• Provides windshear alerts for windshear events containing some level of moisture or particulate matter.
• Detects microbursts and other windshears with similar characteristics.
• Does not provide alerting for all types of windshear.

Note: The automatic function of the GPWS provides “pop-up” displays for predictive windshear and terrain. If neither pilot has weather radar on, and a predictive windshear caution or warning occurs, the weather display and predictive windshear symbol automatically show on the HSI. If neither pilot has terrain display on, and a look-ahead terrain caution or warning occurs, the terrain display will automatically show on the HSI.

Warnings(Predictive)

Windshear

• Windshear close to and directly ahead of the airplane detected by the weather radar.
• Enabled during takeoff, below 1,200 feet Radio Altitude.

Go Around Windshear Ahead

• Windshear within 1.5 miles and directly ahead of the airplane detected by the weather radar.
• Enabled during approach, below 1,200 feet Radio Altitude.

Monitor Radar Display

• Windshear within 3 miles and ahead of the airplane detected by the weather radar.
• Enabled during takeoff and approach, below 1,200 feet Radio Altitude.

The weather radar automatically begins scanning for windshear when:

• Thrust levers set for takeoff or
• In flight below 2,300 feet Radio Altitude (predictive windshear alerts are issued below 1,200 feet Radio Altitude).

Alerts are available approximately 12 seconds after the weather radar begins scanning for windshear. Predictive windshear alerts can be enabled prior to takeoff by pushing the EFIS control panel WXR switch.

If windshear is not detected, weather radar returns show only after pushing the EFIS control panel WXR switch.


During takeoff and landing,

• New predictive windshear caution alerts are inhibited between 80 knots and 400 feet Radio Altitude.
• New warning alerts between 100 knots and 50 feet Radio Altitude.

These inhibits do not remove existing predictive windshear alerts.