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IFR Fix: A threshold questionIFR Fix: A threshold question

When an accident claims an experienced pilot flying a familiar mission, other pilots ponder whether they would have been vulnerable to a similar outcome.

A fatal accident—so recent that the National Transportation Safety Board has yet to publish its preliminary report—occurred in Houlton, Maine, when a professional export pilot delivering a twin Piper Seneca from France to Florida crashed off-airport under foggy predawn conditions after a flight leg from Goose Bay, Labrador.

News reports said the pilot was a frequent visitor to Houlton. The aircraft clipped trees, striking the ground a mile short of the airport in “zero-visibility” conditions. Waiting Customs officers reported hearing changing engines noises and a crashing sound.

Without speculating on causes, what topics come to mind when you consider the challenges that can lurk in such a scenario?

Fatigue after a long flight, or across time zones? Fog—which by the wee hours has had hours to thicken around an airport with no precision approach? Fuel? There’s the recurring question of how an aircraft ends up, inadvertently or intentionally, below minimums.

Instrument flying is based on stabilized descents and respect for minimum altitudes. In training, altitude tolerances are forgiving until you confront minimum descent altitudes, where the standard is maintaining “within +100 feet, -0 feet” to the missed-approach point.

Can you think of a published yet obscure reference altitude value that is important to know for the last leg of an approach?

Here’s a quick, no-peek vocabulary quiz: The abbreviation TCH stands for: (a) Threshold clearance height; (b) Threshold crossing height; (c) Threshold capture height.

The answer is (b): TCH describes “the theoretical height above the runway threshold at which the aircraft’s glideslope antenna would be if the aircraft maintains the trajectory established by the mean ILS glideslope or MLS glidepath.”

Threshold crossing height is also published with vertical descent angles (VDA) on nonprecision approaches and is used to compute the angle (see Aeronautical Information Manual page 5-4-18). The VDA “provides the pilot with information not previously available on nonprecision approaches” for “a stabilized descent from the FAF or stepdown fix to the MDA. Stabilized descent is a key factor in the reduction of controlled flight into terrain (CFIT) incidents.”

Note that “the presence of a VDA does not guarantee obstacle protection in the visual segment and does not change any of the requirements for flying a nonprecision approach.”

Dan Namowitz

Dan Namowitz

Associate Editor Web
Associate Editor Web Dan Namowitz has been writing for AOPA in a variety of capacities since 1991. He has been a flight instructor since 1990 and is a 30-year AOPA member.
Topics: Accident, Technique, IFR

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