LNAV vs VNAV minima?

BigBadLou

BigBadLou

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Lou
Hi guys,

IR student here. Why would some GPS approaches have higher VNAV DA than the LNAV MDA? Similar for visibility.
Examples: RNAV GPS approaches into KEDC rwy 13 and rwy 31.

Coincidence? Error? Mistake? Left hand not talking to right hand?
Or is this on purpose? To allow the GS riders extra room from DA to go missed without descending below MDA?

Not sure who to ask in real life so I'm asking the smart individuals here.

Thanks in advance.
Lou
 
There are many examples of LNAV/VNAV minimums that are higher than LNAV. There are even examples of LNAV being lower than LPV or LNAV/VNAV being lower than LPV. This all has to do with the fact that each approach type has its own specification and under certain circumstances will result in lower LNAV minimums.

Here are two common reasons.

1) The missed approach is different between a vertically guided procedure and an LNAV procedure. The first allows for the decision to occur as one is passing the DA and therefore sink below that altitude before arresting the descent and starting the climb. The LNAV starts from level flight at the MDA, without any sink thru, and can start the climb immediately. So obstacles in the missed approach path can force the DA higher so that the aircraft can clear them while they have no effect on the MDA.

2) Close in obstacles to the runway may require that they be avoided visually. To put them into the visual segment, the vertically guided procedure must move the DA to a height above the obstacle. This involves moving the DA back and up along the glidepath to a sufficient amount to place the DA where the pilot can see the obstacle if they continue the approach and at the same time clear the obstacle if they execute a missed approach. If this is more than the 250 feet required obstacle clearance for an LNAV, the LNAV will be lower than the DA.

So most of the time, LPV offers the lowest DA, LNAV/VNAV the next lowest, and the LNAV the highest MDA, but not always. Often when visibility is the limiting factor, the LNAV is the winner with a lower visibility because it is closer to the runway at the last point an aircraft may be able to effect a landing.
 
BigBadLou said:
Hi guys,

IR student here. Why would some GPS approaches have higher VNAV DA than the LNAV MDA? Similar for visibility.
Examples: RNAV GPS approaches into KEDC rwy 13 and rwy 31.

Coincidence? Error? Mistake? Left hand not talking to right hand?
Or is this on purpose? To allow the GS riders extra room from DA to go missed without descending below MDA?

Not sure who to ask in real life so I'm asking the smart individuals here.

Thanks in advance.
Lou
Click to expand...

Just accept that missed approach criteria are different for vertically guided procedures than for procedures with an MDA. Trying to delve into it serves no useful purpose, especially at your stage of training.

More useful to know is for Approach Category A the required visibility on an non-precision approach (MDA) is generally 1 mile without approach lights. But, the visibility on a vertically guided approach (DA) is the geometric distance from the DA point to the runway threshold. This latter case is more restrictive than the MDA case because the criteria presume you want to be able to see the runway at the DA point. If the DA is sufficiently low then this visibility "discrepancy" you have noted won't come into play.
 
Thank you, gentlemen, great answers, as always.
And a great blog with nice explanations and illustrations, I bookmarked it.
Now if I could only find any good IMC below 5,000'. The weather has been lifting here drastically and will be CAVU soon for the next week or two. :(
 
Thanks - that's my blog and I'm glad somebody's reading it. Although my updating is sporadic at best, I do appreciate the feedback!

Russ
Click to expand...

Thanks also, Russ. That was a great explanation. I will be checking back in from time to time to see what you have posted. Very informative.
 
John Collins said:
Here are two common reasons.

1) The missed approach is different between a vertically guided procedure and an LNAV procedure. The first allows for the decision to occur as one is passing the DA and therefore sink below that altitude before arresting the descent and starting the climb. The LNAV starts from level flight at the MDA, without any sink thru, and can start the climb immediately. So obstacles in the missed approach path can force the DA higher so that the aircraft can clear them while they have no effect on the MDA.
Click to expand...

This is pretty timely. A couple days ago my flying buddy asked me the same question. After thinking for a minute (maybe a few minutes) my answer was this. I qualified my answer by saying I didn't know for sure but it sounded logical. She told me that at least I had put some thought into it, because other people she had asked didn't even try to guess.
 
Also keep in mind that if you wanted to fly the "glideslope" of the LNAV/VNAV approach without a WAAS GPS, you'd need baro-vnav capability which is found in larger aircraft such has commercial airliners. Ergo another reason the minimums might be higher for the LNAV/VNAV approach would be due to expected errors(if equipped with baro-vnav) because of temperature extremes.

Sort of useless because it's not likely that your aircraft is equiped with Baro-VNAV capability, but it's another explanation for weird minimums. As previously stated, not much need to know the whys and details, what's on the plate and your GPS annunciations are the things that matter.
 
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