Technique: VDP’s Shaded Arrow Can Cause Confusion

AINsafety » September 10, 2012
The meaning of the shaded arrow is almost as difficult to identify as the arrow itself.
September 10, 2012, 2:40 PM

The second greatest shock to pilot Jim Huddleston and his co-captain after their Learjet 45 struck some trees during a night approach at Saratoga Springs (5B2) in July 2008 was that an almost obscure gray arrow symbol on the GPS Runway 5 approach plate apparently did not live up to expectations.

When the gray shaded arrow appears on the plate, Huddleston told AIN, “It means, if the pilot is visual at the visual descent point (VDP), he is guaranteed a 34:1 descent profile to the end of the runway. It also means there should be no obstructions in the way.” That 34:1 ratio equates more practically to a three-degree visual glideslope. If the shaded arrow is not on the approach plate, however, the pilot still flies to the minimum descent altitude (MDA) or decision altitude (DA) and continues if the runway is in sight. Without the shaded arrow on the approach plate, though, there is no guaranteed slope, and no guarantee the path is clear of obstacles.

At 5B2 that July night in 2008, trees did penetrate the guaranteed clear area, despite the shaded arrow. The Aeronautical Information Manual (chapter 5-4-18) addresses VDPs only by explaining that, “A VDP will be published on most Rnav IAPs. VDPs apply only to aircraft [using] LP or Lnav minima, not LPV or Lnav/Vnav minimums.” No mention of obstacle clearance appears. A slightly more obscure document called Terminal Procedures Publication Symbols details the 34:1 slope and clear area.

Huddleston said his near-accident taught him “to be very cautious when flying a GPS approach at night in IMC into a non-controlled airport with which the crew is unfamiliar.”

 

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