Does anybody have any concrete information on how these are implemented?
The one I have in my Garmin 496 appears to project your trajectory at the current GS for 2 minutes. It also does a “sink rate” (exceeding about -1500fpm) and a “five hundred” (500ft AGL, probably not connected with an approach to any runway though obviously I rarely go below 500ft when not landing!). The database is bound to be the ex Space Shuttle (SRTM) one, which (they claim) European man-made obstacle data added. The “terrain ahead” etc warnings appear to be 100% reliable and on e.g. this flight past the Italian coast to Lucca at 500-1000ft it picked up every hill. Garmin refuse to discuss any details, however.
The GNS/GTN boxes have both uncertified and “certified” GPWS options and I believe only the certified one (a few k €) gives you audio warnings – making the uncertified one pretty well useless. Honeywell also sell a €25k GPWS box which claims to take into account aircraft climb performance in its avoidance warnings.
Has any of these manufacturers ever published anything about how they work?
There are some differences:
Terrain (non certified): Displays terrain information based on your GPS position and GPS altitude. No regular database updates required, no audio callouts on Garmin panel mount equipment. Displays terrain and obstructions relative to the altitude of the aircraft. Information as advisory only as an aid to situational awereness.
TAWS-A and TAWS-B are TSO certified solutions and can be required depending on operation. Aural and visual alerts based on GPS position, GPS altitude, Radio Altimeter, flap and gear position. Alerts and warnings take in account aircraft performance, and phase of flight. Requires terrain, obstacle AND airport database to be up to date. Compliance with TAWS warnings is mandatory.
As the 496 is not certified at all, they can do what they want with that.
This is what they say in their literature, but do they say what actual criteria they use for the warnings?
Presumably they had to make some disclosures to get the TSOd models.
Yes they do.
For (uncertified) terrain mode:
Red is for terrain which is above to 100 below the current GPS altitude
Yellow is for the terrain wich is 100 to 1000 Ft below the current GPS altitude.
It will also do obstacles within 10 Nm.
For TAWS it will do more, it looks forward, anticipates on the aircraft configuration, airport location, airport level, aircraft GPS position etc.
The image above is from the GTN Pilot Guide and shows the warnings for excessive decent rate for TAWS, which meets TSO C151b
TAWS also warns for premature decent in accordance with the image above.
Then it does the same for some other configurations, suchs a negative climb rate. These are all descriped in the Pilot’s guide.
GTN-750 pilot’s guide
That is correct LeSving, though that is only for the certified TAWS.
The non certified terrain mode can partially follow the TSO, it can also deviate from that.
Even certified TAWS can deviate from the TSO or E-TSO after gaining approval for this from the FAA or EASA. These are alternative means of compliance.
