"Richard Kaplan" wrote in message
news:b91eda6947d6388332cfdfafef004481@TeraNews...
This is a follow-up to my earlier posting.

I received a follow-up email and then spoke by phone today with an FAA
employee who has been working on GPS issues for a number of years. He
clarified this to indicate that a complete WAAS failure (horizontal plus
vertical data failure) would require the pilot to switch from LPV minimums
to LNAV minimums. There is also a very rare partial failure mode of WAAS
(apparently theoretical only but nonetheless programmed into WAAS LPV
boxes)
where one might lose LPV accuracy but retain enough accuacy for LNAV/VNAV
approaches.

That is because the minimums are more dependant on the pressure altitude and
the user's baro-correction input, than on WAAS itself.

Interestingly, the LPV and LNAV/VNAV approaches will be programmed into
the
database as separate approaches, although the waypoints will be identical.
If the approach is flown as an LPV approach, then the box will stop the
approach upon receiving a WAAS failure. However, if the same approach is
flown as a VNAV/LNAV approach, then the box will continue the approach
after
a WAAS failure since the approach can still be flown to LNAV minimums.

So the question (or I should say temptation) will arise on these
approaches
whether to program the box to fly an LPV approach and thus have no
means
to revert to the LNAV-only approach, or alternatively to fly the LNAV/VNAV
approach using LPV minimums, which would not be legal but could offer the
additional backup of continuing at LNAV minimums after a WAAS failure.

It sounds like there will be a notable learning curve to all of these
approaches.

And 30 years to create them.