Tarver Engineering

"Richard Kaplan" <rkaplan@flyimc.com> 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
> re: 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.