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2 Answers

LNAV and LPV minimums

Asked by: 1566 views Instrument Rating

In the book of IPH(Instrument Procedure Handbook), page 4-23, there is a paragraph which says the following.

"In rare cases, the LNAV minima may have a lower HAT than minima with a glide path... obstacles exist below the MDA... glide path may be treated as a VDA..."

 

I have several questions about this.

  1. In my opinion, LNAV should be lower than LPV thinking that LNAV have no vertical guidance. So I think it doesn't make any sense to descend below LPV minimum to LNAV minimum because if I descend lower than LPV minimum, I can hit the obstacle. Actually, there exist several examples that have lower LNAV minimum than LPV minimum, but I don't get it.
  2. What kind of maneuver should be done if there is an obstacle below the MDA. Maintain level, and then steeper descent than 3 degree Glide-path?
  3. In the IPH book, "glide path may be treated as VDA"     um... I don't use VDA at all in my IAP. I just read it in briefing, but busy maintaining required altitude, lateral, and vertical standards. So, How to treat as VDA and how to use VDA in the busy IAP situation? (I know VDA is Vertical Descent Angle and usually 3 degrees.)

Thank you.

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2 Answers

  1. Best Answer


    John D Collins on Mar 28, 2020

    It all depends on where the obstacles are and how the MDA or DA is determined. LNAV and LPV use different specifications for obstacles. With LPV, obstacles must remain below a sloped plane, for example a 2 degree slope when the GP angle is 3 degrees. Only one obstacle slope is used so if an obstacle ends up penetrating the slope close to the runway, the DA is moved further from the runway so that the obstacle ends up being inside the visual segment by projecting the height of the obstacle back to a point where the obstacle slope is is the same height and placing the DA above that point. If you can see the obstacle, you can avoid it. You can’t see the obstacle when you are in the clouds, so one needs absolute protection. Also, when reaching the DA on an LPV, you are in a descent. If you need to miss the approach at the DA, physics dictates the airplane will descend below the DA while you are adding power and pitching up. By moving the DA back and up from the obstacle, you are provided with protection from hitting the obstacle, even though it is in the visual segment and because of actual conditions you can’t see it on the missed approach.

    With an LNAV, a fixed level surface is evaluated for obstacles and it provides at least 250 clearance above any obstacle at the MDA between the FAF and the MAP. You should be level at the MAP on the LNAV and not in a descent, so there isn’t any equivalent requirement to protect against sinking below the MDA. A missed approach begins with a climb. You can’t descend below the MDA until you are in visual conditions and can see the runway environment. When you are in visual day conditions, you can see any obstacles and avoid them. Night time, the obstacle has to be lit or other mitigation must be available such as a PAPI. It is a violation of the regulations if you descend below the MDA and are not in visual conditions as specified on the approach.

    An LNAV has another tool that it can use to obtain a lower MDA than the LPV. A step down fix can be used after passing an an obstacle. You can only have one slope on an LPV, so to avoid such an obstacle with the LPV, the only choices are to increase the GP angle or raise the DA. With the LNAV, utilizing a step down fix, once past the obstacle, one can descend to a lower altitude and still be 250 feet above any obstacle.

    When flying an LNAV, there are two general methods of making the descent to the MDA. One is the DnD (Dive and Drive) method where one descends relatively rapidly to the MDA and levels off. LNAV procedures are designed based on the DnD method. The other method is called Constant Descent Final Approach (CDFA) where one uses a single descent angle on the final approach and is flown like a vertically guided approach such as an ILS or LPV. The CDFA has the advantage that the aircraft is stabilized thru out the final approach, but because one may not descend below the LNAV MDA without having the runway in sight, and there is no sink thru below the MDA allowed for, pilots are guided to select a higher derived DA (DDA is typically 50 feet above the MDA) to account for any sink. Then at the DDA, if the runway is not in sight, the pilot initiates the missed approach. This works when the actual conditions are well above minimums because the point at which the miss is commenced is much further from the runway than the DnD method allows for because with DnD you can get lower and much closer to the runway before deciding a successful landing can be accomplished.

    All this said, the VDA is intended to aid the pilot in doing the CFDA method. However, when there are obstacles in the visual segment that may require the aircraft to maneuver from using the VDA to avoid hitting the obstacle, in that case a VDA is not charted on the FAA charts and a note is placed in the profile view that reads as “Visual Segment – Obstacles”. Keep in mind YOU MUST BE IN VISUAL CONDITIONS TO SEE AND AVOID THE OBSTACLES. Jeppesen charts will include the VDA on their charts when they add the profile note and most GPS systems will still provide an Advisory Vertical Glide Path indication that is to be used SOLELY for when you are above the MDA.

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  2. dgssss on Mar 28, 2020

    Detailed explanation.
    Thank you

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