Galaxy flyer, instead of disabling the GPS inputs, why not get the navdata for the datum used in that airspace and use that datum?
Quote:
Not all EGPWSs use radar altitude.
Mode 4 does, use RADALT
Class B TAWS are specifically not required to have RADALT. Mode 4 is accomplished through using takeoff elevation when RADALT is not available.
Relationship between GPWS and FLTA
FLTA produces alerts from things ahead of the airplane using a terrain database, GPS position, velocity, etc. GPWS produces alerts from things underneath the airplane and alerts from configuration, such as excessive descent rates while close to the ground, gear and flap warnings during landing, etc.
The FLTA features in Class A and Class B are identical and represent the same level of safety. The main difference is in the GPWS portion of the system. Class A requires a "fully autonomous" GPWS while Class B does not. The practical effect of this means:
- Class A requires Radar Altitude and Class B does not.
- Class A requires Airdata and Class B does not.
- Class A requires both Gear/Flap inputs and Class B does not.
- Class A requires an STC and Class B does not. (Money and time!)
- Class A requires a map display and Class B does not.
This does not mean, however, that a Class B lacks all the GPWS features. Instead, what was done in the regulation was to specify a minimum feature set in Class B which could be implemented by substituting "synthetic radar altitude" derived from the terrain database and GPS altitude to allow most of the GPWS alerting functions. Class B GPWS alerts are not autonomous, because failure of the GPS receiver will fail both FLTA and GPWS. In a Class A system failure of GPS will not fail the GPWS.