LNAV preferable over LNAV/VNAV

Take a look at the Mobile Alabama KMOB rnav Rwy 14, should be
available here

http://www.myairplane.com/databases/...s/00267R14.PDF

I notice that lnav/vnav minimums are 660 feet while lnav minimums are
620 feet. My question is, if someone has lnav/vnav authorization, why
wouldn't they prefer to do the lnav, using the glideslope as advisory,
and descend to an mda which is 40 feet below the decision altitude? I
understand the da allows a slight descent below it while initiating a
missed approach, but it still seems the lnav only approach would be
preferable.

Any gurus out there that can provide some input?

Any approaches around with an even greater disparity between lnav and
lnav/vnav minimums?

Stan

The FAA changed the obstacle clearance assumptions about VNAV about two
years ago because WAAS wouldn't provide the vertical integrity they were
hoping for. If they hadn't tinkered with the VNAV criteria, VNAV DAs
would have usually been lower than LPV DAs. That is not their political
objective. ~

The new RNAV (RNP SAAAR) criteria overcomes this political issue by using
a very different methodolgy for VNAV surfaces. But, those IAPs will be
slow in coming along and will not be available for aircraft without Baro
VNAV.

The two RNAV (RNP) SAAAR IAPs for Palm Springs, CA (KPSP) were posted on
the FAA's IAP coordination web site yesterday. They will publish on
December 22nd.

wrote:

Take a look at the Mobile Alabama KMOB rnav Rwy 14, should be
available here

http://www.myairplane.com/databases/...s/00267R14.PDF

I notice that lnav/vnav minimums are 660 feet while lnav minimums are
620 feet. My question is, if someone has lnav/vnav authorization, why
wouldn't they prefer to do the lnav, using the glideslope as advisory,
and descend to an mda which is 40 feet below the decision altitude? I
understand the da allows a slight descent below it while initiating a
missed approach, but it still seems the lnav only approach would be
preferable.

Any gurus out there that can provide some input?

Any approaches around with an even greater disparity between lnav and
lnav/vnav minimums?

Stan

Tim's right, the criteria revision caused a lot of pre-existing
LNAV/VNAV minimums to go up because of the way obstacles in the
secondary obstacle area are treated between the final area and initial
missed approach area. In the final area, there's a secondary obstacle
area where obstacle clearance is reduced based on how far the obstacle
is from the edge of the primary area. This secondary reduction is only
authorized on the final part of the LNAV/VNAV, and when you start the
missed approach, the obstacle that was given a reduction on final now
has to have full obstacle clearance applied in the initial missed
approach segment (much of the initial missed approach overlaps final).
That means if the ROC was 70 feet on final (secondary area), and the
same obstacle is in the initial missed approach secondary area, the ROC
now goes to at least 250 ft, which means the LNAV/VNAV DA must be raised
to make up the difference. The LNAV procedure can still use the
secondary reduction on the missed, so it might have a MDA lower than the
LNAV/VNAV, even though the controlling obstacle might be the same.
Compounding the problem is that raising the LNAV/VNAV DA also moves the
missed approach point further from the runway, which means the missed
approach trapezoid starts earlier and is wider than it would have been
at a given point than it was with the lower DA, which means even higher
obstacles might be picked up, which would raise the DA again, and the
computations start all over again.
Unfortunately, under current criteria (which is being reconsidered) the
Circling visibility has to be raised to match the "no-light" visibility
of the highest visibility procedure, and this is often the LNAV/VNAV due
to the high DA and consequent distance of the DA from the threshold.

I agree, it seems to make more sense to descend to the LNAV MDA using a
descent gradient comparable to the LNAV/VNAV descent angle in those
situations.

JPH

wrote:
The FAA changed the obstacle clearance assumptions about VNAV about two
years ago because WAAS wouldn't provide the vertical integrity they were
hoping for. If they hadn't tinkered with the VNAV criteria, VNAV DAs
would have usually been lower than LPV DAs. That is not their political
objective. ~


wrote:


Take a look at the Mobile Alabama KMOB rnav Rwy 14, should be
available here

http://www.myairplane.com/databases/...s/00267R14.PDF

I notice that lnav/vnav minimums are 660 feet while lnav minimums are
620 feet. My question is, if someone has lnav/vnav authorization, why
wouldn't they prefer to do the lnav, using the glideslope as advisory,
and descend to an mda which is 40 feet below the decision altitude? I
understand the da allows a slight descent below it while initiating a
missed approach, but it still seems the lnav only approach would be
preferable.

Any gurus out there that can provide some input?

Any approaches around with an even greater disparity between lnav and
lnav/vnav minimums?

Stan

Thanks Tim and JPH. Can't say I understand completely what you two
wrote, but I guess I'll continue LNAV minimums when they're lower than
LVAV/VNAV minimums.

Stan

wrote:

Thanks Tim and JPH. Can't say I understand completely what you two
wrote, but I guess I'll continue LNAV minimums when they're lower than
LVAV/VNAV minimums.

Stan

No need to understand anything other than the feds mucked around with the
VNAV obstacle clearance surfaces and revised them so VNAV DAs would be
higher than LPV (WAAS) DAs. Since that is conservative (unnecessarily so)
there is no safety issue.

A related issue is that a DA's visibility will almost always be higher
than the associated LNAV MDA's visibility when the DA's height above
touchdown is higher than standard. This is because the visibility for a
DA is always measured from the DA point to the threshold.