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#1
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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 |
#3
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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 |
#4
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LNAV preferable over LNAV/VNAV
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 |
#5
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LNAV preferable over LNAV/VNAV
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