Any solo-stage flight student can explain the importance of a stabilized approach and decry the vices of destabilization down low. Any instrument student can discuss how IFR approach procedures divide into precision and nonprecision approaches and the implications of those classifications.
That’s a fine ground-school session. But before declaring class dismissed, describe a scenario where the potential for destabilization is intrinsic to the procedure—and where a precision approach can morph into something less on short notice.
The pilots of a Gulfstream jet approaching a snowy airport in Colorado discovered a hidden hazard of that idea the hard way.
The scenario that set them up for trouble contained two basic conditions: an approach to one of two parallel runways, and a late change in the runway-of-landing clearance, requiring a sidestep.
Sidestep approaches are published procedures. At Detroit, Mich.’s Willow Run Airport, for example, the ILS or LOC RWY 5R approach can be flown straight-in, as a circling approach, or with a separate set of minimums for a “sidestep RWY 5L.”
That’s where things may get hectic.
"The pilot is expected to commence the side-step maneuver as soon as possible after the runway or runway environment is in sight," explains the Instrument Flying Handbook. For sidestepping, parallel runways must be separated by 1,200 feet or less. Landing minimums to the adjacent runway "are based on nonprecision criteria and therefore higher than the precision minimums to the primary runway, but are normally lower than the published circling minimums."
Sometimes sidestepping can be a more impromptu solution. On Jan. 6, 2005, the Gulfstream jet had been cleared for a visual approach to Runway 17L at Denver, Colo.’s Centennial Airport, when "approximately 30 seconds prior to landing, he was advised to ‘go around or sidestep’ to runway 17R," said a National Transportation Safety Board narrative. The NTSB report arose from the incident that occurred when the Gulfstream touched down on the plowed taxiway between the bare runway 17L and the snow-covered runway 17R.
As a general aviation pilot you can fly many years to destinations without parallel runways. That points out a deficiency-of-experience risk for coping with their unique requirements—especially that business of a visual maneuver to be "executed as soon as possible after the runway or runway environment is in sight."