Gpws/egpws

[topcat450]

Just a couple of minor questions re: GPWS...I'm sure you lot can help.

If the warning mode is 'Assessed' or 'Displayed'...well whats the difference? And what is the number of 'warning cycles' ?

Also are their only 5 modes, or do manufacturers of different models have different numbers of modes - or will every aircraft you encounter have the same 5 modes & they'll all relate to the same events?

Any help you can offer would be much appreciated.

[woderick]

I have never come across the terms Assessed or Displayed related to GPWS. Modes are, in my experience, not displayed. When you are about to become another CFIT statistic who cares what mode you are in ?

Flippant switch off

Basic GPWS has Five modes which are standard. They are part of the system definition and are the same across Manufacturers and aircraft types. Decades ago, when we were first fitting the system, I think that the various envelopes were different for slow a/c as opposed to larger, faster aircraft, but I think that this difference no longer exists. My experience is only with Jet, Transport Aircraft. I would interpret a “warning cycle” as one cycle of those detailed below e.g. “WHOOP! WHOOP! PULL UP” is one cycle

So for Basic GPWS modes are:
1. Excessive Descent Rate
Excessive Barometric sink rate initially with the warning “SINK RATE”
if the condition persists changing to “WHOOP! WHOOP! PULL UP”
2. Excessive closure with respect to rising Terrain
This is split into two sub modes 2A & 2B
2A. Flaps <25deg. Terrain closure as sensed by Radio Altimeter rate vs Airspeed Warning “TERRAIN TERRAIN” if the condition persists changing to “WHOOP! WHOOP! PULL UP”
2B Flaps>25deg. Advisory mode only. Warnings occur below 789 feet R.A. down to 200- 600 feet depending on Barometric rate of descent. When closure rate exceeds the set limits warning “TERRAIN TERRAIN”
3. Barometric Altitude Height loss after takeoff.
Flaps <25deg or Landing gear raised below 200 ft during takeoff or a missed approach disabled above 700 ft (see mode 4) warning “DON’T SINK”
4. Unsafe Terrain Clearance
Again split into two modes, A + B. Usually applied to the landing phase mode a is insufficient terrain clearance, gear up, flaps < 25deg.
Mode b is gear down flaps < 25 deg Mode a Warning “TOO LOW GEAR” Mode b Warning “TOO LOW FLAP” in both cases, should the situation deteriorate then the warning becomes “TOO LOW TERRAIN”
5. Glide Slope
Excessive deviation below the Glide slope Vs RA Warning is “GLIDESLOPE GLIDESLOPE”

EGPWS

EGPWS constitutes exactly the same warnings and modes as conventional GPWS.
In an aircraft capable of supplying the system with Present Position (IRS, GPS or GPIRS derived) then it provides two additional modes TCF and TAD. Both these are calculated by comparing PPOS to an internal terrain database and then giving the appropriate warning which above conventional GPWS is augmented by a visual warning, usually on the ND. (by implication an aircraft capable of providing PPOS in the format required will have an EFIS system)
TCF Terrain Clearance Floor creates an increasing terrain clearance floor around the intended runway complementing existing mode 4. Warning is “TERRAIN TERRAIN” + visual

TAD Terrain Awareness and Display operates full time and used Aircraft position, altitude, and a Terrain data base to predict conflicts between the aircraft flight path and the terrain. Graphic displays of the terrain are available for display on the Navigation Display. Throughout all phases of flight Terrain clearance envelopes are monitored and if there is a conflict then a warning is issued. “TERRAIN TERRAIN” + Visual

This is a simplified, generic, version. It is really much more complex but there is a limit to my typing ability !
EGPWS is based on the AIRBUS application.

[Golden Rivet]

Mode 6 - radio altitude aural Calouts ( can also provide bank angle warning )

Mode 7 - windshear



GR

[Check 6]

Here is a good TAWS/GPWS (http://www.egpws.com/general_information/videos/videos.htm) web site including training videos.