Instrument Approach Minimums

[threeholerglory]

Can anyone hazard a guess on this one? The plate is for the GPS 3 into 1B1, Columbia County, NY. The airport has a certified LPV approach to 500' with a mile vis. LNAV only MDA is 760-1, and is the same for Circling. What gets me is the LNAV/VNAV DA of 842-2 1/4 which is obviously higher than the LNAV only/Circling minima. Anyone know why?

http://aeronav.faa.gov/d-tpp/1212/06204R3.PDF

^^^^^^ link to the plate...couldn't get a copy as an image. Also check out the circling mins of 1SM for cats A/B...doesnt really seem feasible.

This plate is valid from 15 November 2012 to 13 December 2012...just in case they change it in the next cycle.

[PhilDernerJr]

That's pretty weird. Though it doesn't make sense, I would assume it is because once you go VNAV, you're on a glide path that does not offer step-down guidance, so it is not factoring terrain, making it more limiting. I don't know if that's logical but that's my guess. Hmmm

[threeholerglory]

it's also a higher minimum than the NDB-A which is circling off a freaking NDB lol....so weird! I'm finding more of them too...CDW has an LPV to runway 22 and also LNAV/VNAV....the LNAV/VNAV minima are 50' (IIRC) HIGHER than the localizer minima. 12/21/12 is upon us. Only explanation.

[USAF Pilot 07]

It all has to do with obstacle clearance requirements and the different way each approach is TERPSed.

An LNAV approach is TERPSed laterally and carries with it a required obstacle clearance from the controlling obstacle which equates to your MDA. In the case of the LNAV approach, since there's no vertical guidance, all you care about is "I am going to clear the obstacle by X amount of feet at my MDA." (I think it's like 200-300 feet). You don't care where that obstacle lies along the approach. This is why many times you will see "step-down fixes" on LNAVs/LOCs; the final approach is divided into segments because there may be a higher obstacle farther from the runway which would preclude you from descending directly to your MDA at the FAF while still providing the required obstacle clearance.

The LNAV/VNAV has the VNAV aspect of the approach added to it. The VNAV allows for a more stabilized approach as it creates a glidepath for the aircraft to fly. While the LNAV portion of the LNAV/VNAV approach is TERPSed the same way a standalone LNAV approach is, the VNAV portion of the approach is TERPSed differently. It uses an obstacle clearance surface along the VNAV portion of the glidepath. If an obstacle penetrates that surface, the DA has to increase to satisfy the TERPS requirement.

Bottom line, when you see LNAV/VNAV mins higher than just LNAV mins, it's usually because there's a close-in obstacle. Because of TERPS requirements with each approach, that obstacle penetrates the obstacle clearance surface of the VNAV portion of the approach, and therefore to satisfy those same TERPS requirements, the pilot must be able to "see the obstacle sooner" which is why the mins and DA are higher.

In reality, if I were in this situation and the weather was marginal (i.e. below LNAV/VNAV mins but above LNAV only mins), I would request the LNAV approach, calculate a VDP for a 3* glidepath from my MDA to the runway all the while being cognizant there is more than likely a close-in obstacle. If you have the capability to do LNAV/VNAV, although you may not have the weather to legally shoot the approach, there's nothing that says you can't use the VNAV portion to "enhance your situational awareness" on the LNAV approach.


One could circle out of this approach, even with the wx at mins (not to say that it wouldn't be challenging).

Circling mins allow for 300' of clearance from the controlling obstacle within the circling radii. If the visibility was no-s**t at mins for the approach (1 mile) and we were flying CAT A, we'd just have to stay within 1 mile of the runway in order to keep the runway in sight at all times.

[megatop412]

Wow. USAF Pilot 07 I hope you're an instructor because that was an excellent explanation that a non-pilot could halfway understand. I'm bookmarking this page.

[threeholerglory]

Thanks for the clarification....I guess it boils down to the fact that they're all published based on the obstacles and that's that...just seemed counter-intuitive that LNAV/VNAV would be higher than LNAV only. I'll definitely have to study your explanation to fully comprehend it and let go of what seems logical!

[stratoduck]

There is a little more to this.

On an ILS, a radio signal is picked up by the aircraft's receiver. Based on the signal, the navigation equipment gives a reading displaying where the aircraft is in relation to the signal; on/above/below the glideslope. Even with altimetry errors, the aircraft will be on a path that crosses the runway threshold at 50 feet and intersects the runway 1000 feet later.

Non-precision approaches are highly dependent on the accuracy of the aircraft's altimeter. If there is a major error in the system, the pilot could descend into terrain or obstacles while still indicating a safe altitude.

There are three type of altimetry errors. One is user error, and that is the pilot setting an incorrect local barometric setting. Another error is a system error; altimeters must be accurate within 75 feet, but also may be off by 75 feet and still legal for instrument flight. The third error is non-standard temperature. Standard temperature is 15 degrees Celsius at sea level, 13 at 1000 feet mean sea level, 11 at 2000 MSL and so on. If the temperature is above standard, the aircraft will be higher than indicated, with the reverse for below standard temperatures. The error can translate into tens of feet at our altitudes, and hundreds of feet in mountainous elevations.

The LNAV/VNAV is a pseudo glidepath based off the aircraft's position in relation to the GPS position and altimeter reading. This glidepath with altimetry errors may be too shallow potentially taking the aircraft though obstacles or terrain. It may also be too high, leaving the aircraft in a position where it is unable to land without maneuvering (or circling) upon reaching the minimum decent altitude. These limits are considered and as such, may result in a higher LNAV/VNAV than a standard LNAV approach.