BARO-VNAV Question on Instrument Written Exam

8 Answers

1. 
   

   
   

   Jon on Jan 23, 2012

   According to the Instrument Procedures Handbook (http://www.faa.gov/library/manuals/aviation/instrument_procedures_handbook/media/CH-05.pdf) Ch. 5, p 5-23:
    
   In some cases, the visibility minimums for LNAV/VNAV might be greater than those for LNAV only. This situation occurs because DA on the LNAV/VNAV vertical descent path is farther away from the runway threshold than the LNAV MDA missed approach point.

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   David Cooper on Jan 24, 2012

   Thanks. I’m still not clear on how this applies to Figure 152, but at least I have the answer.

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3. 
   

   
   

   John D. Collins on Jan 24, 2012

   David,

    

   Figure 152 shows the visibility requirement for the LNAV/VNAV DA to be 3/4 mile and the visibility for the LNAV MDA to be 1/2 mile.  The question asks why this is the case.  The missed approach point for the LNAV/VNAV is at the DA on the glidepath.  The distance to the threshold at the LNAV/VNAV DA is approximately 5,500 feet, or just over one statute mile.  With the MALSR starting at 2400 feet from the threshold, it will be just possible to see the first 4 or 5 flashers of the approach lights if the visibility is at 3/4 of a mile and this is where you must make a decision to continue the approach or start your missed approach.  If you see the approach lights, you may continue the descent below the DA, although you must spot one of the other visual cues such as the threshold lights before descending below 100 feet above the runway to complete the approach.

    

   In the case of the LNAV procedure, the missed approach point is at the threshold which is much closer, so you don’t have to see as far to complete the approach, although the descent will be steeper if you leave the MDA when you have a half a mile to go to the runway.

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   David Cooper on Jan 25, 2012

   John, thank you for the helpful explanation. If I read the chart correctly, the VDP applies only to the LNAV approach, in which case, despite the fact that I could legally fly the MDA to the runway threshhold, I should stronlgy consider going missed at the 1.5 NM waypoint if the runway isn’t in sight. Correct/comments?

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5. 
   

   
   

   Nathan Parker on Jan 25, 2012

   TERPS paragraph 330(c) points out that the MAP only affects visibility minimums when the MAP is further away from the runway than standard visibility minimums.  So if the standand visibility were 1 mi, but the MAP were 2 mi away, then the visibility minimums would be 2 mi.
    
   I don’t think that applies in this case.   TERPS table 6 gives the standard visibilities for all types of approaches based on HAT/HAA.  A value of 500 feet HAT/HAA demands a vsibility of 1 mi.  This value can be reduced to 1/2 mi with certain approach lighting systems, which the approach in question has.  So it looks like one approach has been granted the reduction, but not the other.  Why?  It might have to do with the evaluation of the visual segment and any obstacles that might lie there.  If an obstacle penetrates the 20:1 plane, then the visibility reduction for lights will not be granted.
    
   Why would the 20:1 be penetrated for one approach and not the other?  TERPS paragraph 251 says that for a precision approach, the visual area extends to DA, but for a Non-precision, it extends to the VDP.  I don’t know if the Baro VNAV is evaluated like a precision approach, since it technically isn’t one, but it might explain why one might contain an obstacle, but not the other.

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6. 
   

   
   

   John D. Collins on Jan 25, 2012

   David,

    

   Some recommend that you consider the VDP location as a point to initiate the missed approach, but for a GA piston aircraft, I would not agree.  In the case of a jet or heavy aircraft, it might be the best thing to do because of the fact that they might not be in a position to make a normal descent to the runway after the VDP.  For a piston GA aircraft, this would be very limiting and effectively require a much higher visibility than is needed.  On this approach, at the location of the VDP at the MDA, you will need 1.75 mile visibility to see the runway and about 1.4 miles to see the first approach light.  In this case the approach lights will help, but most NPA approaches are to runways without approach lights where locating the runway cues is much more difficult and it takes more time to be able to recognize where the runway is.   Treating a VDP at the MDA as if it were a DA provides only a single instant in time for the pilot to recognize the runway and at a much higher visibility requirement than is needed to safely complete the approach.  For these reasons, remaining at the safety of the MDA while continuing to fly the approach, allows the extra time to spot the runway, sort out that what you see isn’t a Walmart parking lot, and confirm that it is safe to descend using a normal descent to the runway, all while requiring lower visibility.  Regardless if you miss the approach at the VDP location or not, you still need to continue on the approach to the MAP before you can commence any missed approach procedure turns.  My Bonanza is quite happy to safely descend at more than a 6 degree slope while still carrying power in the landing configuration, so why would I abandon the approach at the VDP location that is only a 3 degree slope? All the Cessna, Pipers, and even the Mooney may be comfortably be flown in the landing configuration at steeper descent angles than the standard 3 degree slope.

    

   There are good reasons why using a CANPA (Constant Angle Non Precision Approach) procedure can increase safety, but it is not always the best method for completing an NPA approach.  The alternative which is often called, dive and drive, can get you below an overcast sooner and allow for more time at the safety of the MDA to locate the runway.

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   John D. Collins on Jan 25, 2012

   Nathan,

    

   It isn’t an obstacle issue for this approach as it is depicted on the approach chart to be clear on a 34 to 1 slope by the gray feather symbol extending to the runway threshold.  It has to do with the distance to the runway threshold from the DA for the LNAV/VNAV procedure.

    

   From change 23 of the TERPS:

    

   3.3.2 b. (1) When the NPA MAP is located at or after the LTP, proceed to Step 3. Otherwise, determine the distance from the NPA MAP (plotted position) or PA/APV DA to the LTP. When authorized by paragraph 3.1.3b, subtract the ALS length (2400 ft for FALS, 1400 ft for IALS, and 700 ft for BALS). When this distance is less than or equal to the visibility from Step 1, use the Step 1 value. When greater than the visibility from Step 1, use the next higher visibility value (RVR or SM) from the applicable table or the next higher whole SM when the distance exceeds 3 SM.

    

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   David Cooper on Jan 26, 2012

   Thanks for the thorough explanations. This makes perfect sense now.

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