Question about CYOO RNAV RWY 12 minima

[photofly]

Here's the plate:

approach.png (760.81 KiB) Viewed 2357 times

There are two sets of circling minima: one associated with the LNAV approach (1000 and 1 3/4) and one associated with the LNAV/VNAV approach (1060, 2).

At least for category A and B aircraft, why are there in fact higher minima for using the more sophisticated LNAV/VNAV equipment?

[Koalemos]

A very similar question was posted on the pilotsofamerica.com forum, a very good answer was provided by a poster there so I have just copy/pasted below. The key to your question lies in the data source used to determine altitude. LPV approaches usually are more accurate by determining the altitude from GPS positioning, while LNAV/NAV use temperature corrected altimeter readings (with the current barometer setting).

The full thread on the other forum can be found: http://www.pilotsofamerica.com/forum/sh ... hp?t=33361

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Approaches with LNAV/VNAV minimums may be flown by two types of systems, one is a TSO C145A/146A WAAS GPS and the other is a Baro VNAV system. The LNAV/VNAV minimums are primarily available for the airlines to provide a stabilized approach option to the LNAV dive and drive approach.

There are not many of the Baro VNAV systems in GA, they are mostly a creature of the airlines. They build a glide path based on sensing the altitude and comparing the altitude with a calculated altitude for the aircraft position on the glidepath. Because the calculated glidepath is based on the reference altimeter setting, it requires a local altimeter setting and has high and low temperature limits.

A WAAS GPS determines a glidepath by its vertical and Horizontal GPS position. The glidepath is independent of the altimeter setting and there are no temperature limits. The only thing that is dependent on the altimeter setting is the DA or MDA, and it is adjusted on the approach chart if necessary as a result of using an alternate altimeter setting.

With a WAAS GPS, it will only downgrade from an LPV to an LNAV minimum, even if there are LPV, LNAV/VNAV, and LNAV minimums for a given approach. It the approach does not have LNAV minimums, the WAAS GPS will cancel the approach guidance if it does not meet the criteria and revert to Terminal mode for the missed approach.

The odds of an LPV downgrading are very low. In the CONUS, a typical day has 100 percent coverage over 90+ percent of the land mass, with only 99 percent coverage over the balance of the CONUS. The worst coverage is on the West Coast. 99 percent availability means that during the day, LPV is not available for about 15 minutes sometime during the day.

With a WAAS GPS, I see no advantage to ever using an LNAV/VNAV minimums when there are LPV minimums available. In some cases, when there are LNAV/VNAV minimums without LPV minimums for a given approach, they may offer an advantage over the LNAV, but even if the minimums are lower which is not always the case the visibility requirements are often higher and an approach to LNAV minimums can offer an advantage in the probability of successfully completing the approach.
__________________
Regards,

John D. Collins
CFI, CFII, MEI

[Rookie50]

How many Ontario airports now have LPV approaches?

[Kurwa]

I'm not sure about this particular example, without knowing the approach design criteria, but it often has to do with where the missed approach segment begins. Often the case with an LNAV/VNAV is that the constant descent slope intersects with a decision altitude that is prior to the missed approach point published (usually runway threshold) as these types of approaches usually bring you down to cross the threshold at 50 feet. If you envision a straight LNAV approach, it is much like the old 'dive and drive' approaches where you could descend and level off and fly along level until the MAP. The question comes in determining obstacle clearance for the missed approach segment; sometimes the LNAV/VNAV approach will 'drop you in' prior to an obstacle that penetrates the missed segment as it takes you down to the 50' TCH. A regular LNAV only approach may have the missed approach segment commence AT the threshold at a point where a building or crane etc has already been passed laterally and thus would allow a lower descent prior to the commencement of the 'missed approach segment'.

[pelmet]

Both the straight-in and circling minimums are lower for the LNAV only approach than the straight-in LNAV/VNAV minimums.

If they made the circling minimums for the LNAV/VNAV the same as the circling minimums for the LNAV only then you would have a situation where circling minimums are lower than straight-in minimums for the LNAV/VNAV approach.

Why lower minimums for LNAV only? My answer is......and it could be wrong......but it is(drum roll)......





You have a lower recommended visibility for the LNAV only approach compared to LNAV/VNAV(1.5 vs. 2.0). This is the minimum visibility that you would expect to be able to see the approach lights at the MAP when doing the approach. Seeing as the threshold is the MAP for the LNAV only approach compared to being somewhat farther back for the LNAV/VNAV MAP(check the profile view), at the associated MAP's you can see the runway lights at a lower visibility on the LNAV only approach so you get lower minimums. Pretty stupid if my answer is correct. Have to do the less safe approach to get in during the lowest weather.

[Flying Nutcracker]

Edited for pure delayed obviousness

[costermonger]

How many Ontario airports now have LPV approaches?

11. Barrie, Dryden, Edenvale, Kincardine, Kitchener, Midland, Oshawa, Ottawa, Sioux Lookout, Thunder Bay and Timmins.

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Anyway, the difference between the LNAV and LNAV/VNAV (100 feet) is almost equal to the temperature correction you'd apply to the LNAV minimums (+106) at the temperature limit. The same relationship doesn't quite work as well (but it's close) on the RNAV 30 at CYOO - LNAV MDA is 900 (453), LNAV/VNAV DA is 980 (533) with a TLim of -33. If you were flying the LNAV minimums at -33, you'd have a correction of +96. Why this would be the case, I haven't a clue, as you'd still be temperature correcting the LNAV/VNAV DA in those conditions. Can't understand why the correction would be built in. I've never flown anything with Baro VNAV, so if I'm going totally the wrong direction with this, that's my scapegoat. I've only used WAAS based vertical guidance.

It's a good question anyway. The only thing I thought of is that it might have to do with the fact that the LNAV/VNAV minimum is a decision altitude, rather than an MDA, so the missed must be initated at that altitude (and you might go a bit below in the process of going around) as opposed to the hard limit of the MDA. Still, I can't remember seeing an approach in the US where LNAV/VNAV mins, if available, didn't take you a little lower than the basic LNAV.

[co-joe]

I'm unfamilliar with the term "dive and drive" with respect to an IAP. Is it the opposite of "chop and drop"?

[Maynard]

Dive and drive, I'm assuming, after the FAF, descending quickly to the MDA and staying there until you see or miss, (more time spent at mins), vs, maintaing a constant slope down to a DH/or MDA. (only spending a few seconds at mins to spot the runway)
Chop and drop, breaking out usually at mins, or last minute, chop the power, drop it in)

[pelmet]

Found an official word for the reason for higher visibility minimums.....

"In some cases, the visibility minimums for LNAV/VNAV might be greater than those for LNAV only. This situation occurs because the DA on the LNAV/VNAV vertical descent path is farther away from the runway threshold than the LNAV MDA missed approach point"

First paragraph on page 5-23

http://books.google.ca/books?id=zNrV6AP ... ms&f=false