Safety Board wants stricter night limits at mountainous U.S. airports

 - May 16, 2008, 5:42 AM

Would a broader aeronautical definition of “night” have helped to prevent the fatal crash of a Gulfstream III last year? The NTSB has not yet released its final report, but it’s clear from the following recommendation that the Safety Board believes a “black-hole effect” may have been a factor in the accident.

The NTSB has asked the FAA to revise prohibitions and restrictions that currently refer to night operations at airports in mountainous terrain to account for the “entire period of insufficient ambient light conditions” rather than the 30 min after sunset currently used.

The recommendation emerged from the Safety Board’s continuing investigation into the crash of a Gulfstream III on March 29 last year while on short final to Sardy Field in Aspen, Colo.

The accident, which killed all 18 aboard, occurred about 30 min after official sunset. But the Safety Board determined that the sun would have set below the mountainous terrain about 25 min before official sunset. Aspen’s VOR/DME-C approach procedure was not authorized for use at night. Witnesses reported that it was “very dark” at the time of the crash. Visibility was 10 mi in light snow.

Although the pilot told ATC that he had the runway in sight, that “would not ensure that he could also have seen intervening unlighted terrain, especially given a higher-than-normal descent rate and his maneuvering to align with an upsloping runway,” said the NTSB.

The instrument approach path is from the left side of the extended runway centerline and requires a left turn for final alignment with the runway.

“Making an approach to an upsloping runway at night can aggravate a visual illusion known as the black hole,” said the Safety Board. “A pilot experiencing this illusion is not able to judge the runway’s relationship to areas of unlighted terrain in the vicinity and flies a descent that is much more rapid than it should be, leading to CFIT.”

The GIII also descended 500 ft below the published minimum altitude during the final segment of the approach, according to investigators.

Immediately ahead of the GIII, two other airplanes made missed approaches because their pilots could not adequately see and identify the airport environment. The first airplane made the miss 17 min before the GIII crashed and the second airplane took the miss just four minutes before the accident.

The Safety Board determined that the combined effects of the surrounding terrain’s high elevation and weather conditions created night visual conditions much earlier than would have occurred in non-mountainous terrain and in clear weather.

Therefore, the FAA’s definition of night “does not adequately describe the conditions under which darkness exists in mountainous terrain and, therefore, use of this term [night] may not adequately restrict potentially hazardous flight operations.”