With no immediate evidence of a technical failure on board the ill-fated UPS Airbus A300-600 freighter, US investigators have yet to explain why the twinjet was approaching Birmingham below the safe glidepath.
The straight-in localiser pattern to runway 18, for which the crew briefed, is a non-precision approach that crosses largely featureless, undulating terrain, with few visual reference points at night.
Approach charts detail a 3.28° glidepath from a 4.7nm final approach fix designated BASKN.
On this glidepath the A300 should have been at 1,380ft - equating to 736ft above the airport - as it passed a waypoint known as IMTOY while still 2nm from the runway.
This glidepath should have kept the aircraft above 1,000ft, and some 400ft above the airport, at the point where it struck a bank of trees, about 1nm from the threshold, and collided with a slope.
The IMTOY crossing altitude of 1,380ft corresponds to the minimum descent altitude. Charts indicate this lowers to 1,200ft if the waypoint is confirmed.
National Transportation Safety Board investigators have not detailed the A300's precise approach pattern nor any reasons for the its deviation from the prescribed glidepath - although the inquiry has found that the crew received sink-rate warnings just before the accident.
Charts indicate that, at the expected approach airspeed of 140kt, the aircraft's descent rate should have been around 800ft/min.
Birmingham's approach is complicated by hilly terrain, with charts showing slopes reaching 270ft above the airport elevation within the last 2nm.
NASA's Aviation Safety Reporting System contains an entry dated May 1999 from a Fokker 100 crew illustrating the challenging nature of the localiser approach at night, even in good visibility with precision-approach path indicator (PAPI) lighting.
"The [first officer] and I were both bothered by the close visual [proximity] of the ground while on the final stages of the approach," it states, adding that they had seen a car pass 80-100ft beneath the aircraft while 1nm from touchdown. This had prompted the crew to cross-check the PAPI to ensure they were still on the glidepath.
The entry does not confirm the glidepath angle or other approach references in use at the time, but highlights shallow terrain clearance north of the airport and illustrates the pilots' concerns: "The approach to runway 18 at Birmingham is marginally safe at best and is a set-up for an accident at worst."
Featureless terrain at night is characteristic of "black hole" approaches which can mislead pilots into misjudging their altitude, although the visual stimuli contributing to these erroneous estimates are not entirely understood.
Birmingham's runway 18 slopes downward and is only 7,100ft long. But downward sloping terrain and runways, and short runway lengths tend to give crews the visual impression that the aircraft is too low, rather than too high.
Landmark research by Conrad Kraft and Charles Elworth in 1969 suggested that pilots conducting black-hole approaches unwittingly flew a descent profile during which the runway subtended a constant visual angle, resulting in an arcing flightpath that dipped below the correct glideslope.
But subsequent analysis of these findings in 1991, by NASA and the San Jose State University, indicated that the data was not entirely consistent with this hypothesis. It also highlighted research into alternative visual references including the runway length-to-width ratio and the broader detail of the approach scene.
NTSB investigators looking into a "black hole" non-precision approach crash by a FedEx Boeing 727-200 freighter at Tallahassee - which, like the UPS accident, occurred about an hour before sunrise - found that the aircraft had sunk below the glideslope despite the presence of PAPI lights.
It determined that the captain had called the approach as stable at 500ft despite the 727's low engine thrust setting and excessive vertical speed. The inquiry found that the two pilots had been affected by fatigue and that colour vision deficiency in one of them might have affected his ability to identify the PAPI lighting signal.
All three crew survived the July 2002 crash. The captain of the 727 told investigators that "everything visually looked normal, based on the runway, and that's why I was somewhat shocked when I felt the thumping".
Investigators noted FedEx's training programme specifically addressing the threat of black-hole illusions, aimed at heightening awareness of misleading cues that could result in lower-than-normal approaches - such as the airport's position on the edge of a small city and the brightness of runway lighting. The training stressed cross-checking of altitude and distance, and monitoring of sink rates, to ensure that aircraft remained on the correct glidepath.