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with an MSAW capability that monitors an aircraft‘s separation from terrain and obstacles. The MSAW warnings are provided at the TRACON and at the HOU ATCT. The MSAW operates in two modes: general terrain monitor (GTM) mode, which monitors aircraft clearance above the highest terrain and obstacles in a general area, and the approach monitor (AM) mode, which monitors aircraft clearance above terrain and obstacles within a specific area on a certain runway approach (known as the Type II Airport Area). Each of these modes alerts controllers when an aircraft is currently below or predicted to go below an established minimum safe altitude. If the minimum safe altitude thresholds are exceeded, the MSAW will provide air traffic controllers with both a visual alarm (a blinking —LA“ [low altitude] on the controller‘s screen) and an aural alert.
A predicted monitor alert will activate if two out of three consecutive radar scans show that the aircraft will descend 100 feet below the minimum altitude within the next 15 seconds. A current monitor alert will activate if a single radar scan shows that the aircraft is below the minimum altitude. The MSAW recorded a single GTM predicted exceedance of the established minimum safe altitude of 400 feet at 0613:30, but neither of the two subsequent scans predicted that the airplane would descend below the minimum altitude; therefore, no alert was generated at this time. The MSAW recorded no AM predicted exceedances; therefore, no AM predicted alerts were generated. The MSAW generated an AM current alert at 0614:35. The HOU ATCT controller began issuing a warning to the flight crew about 7.5 seconds after the alert activated, which was about 3 to 4 seconds before impact with the light pole.
The Safety Board evaluated the airplane‘s flight profile and the MSAW algorithms for the HOU airport area to determine whether the MSAW provided the HOU ATCT controller the alerts expected given the design specifications of the system. The study indicated that the system performed as designed. The study also indicated that the descent profile of the accident flight was along the edge of the GTM predicted alert threshold, such that only one GTM predicted alert was recorded by the system.
During discussions with FAA staff about MSAW-related safety issues, FAA personnel noted that, in an effort to limit —nuisance alarms,“ MSAW parameters are presently set to not trigger an alarm for aircraft that routinely fly visual approaches lower than any precision instrument approach. Because the present MSAW design does not provide any way to alter system performance to treat aircraft flying visual approaches differently from aircraft flying instrument approaches, aircraft that deviate from instrument approach limits during IMC may not generate an MSAW alert until they are well below the expected instrument approach altitude.
On July 11, 2006, the Safety Board issued safety recommendations to the FAA that addressed MSAW alert effectiveness.17 In its recommendation letter, the Board noted that modifying MSAW software to apply different alerting parameters to aircraft flying visual and instrument approaches will likely decrease the number of false alarms when aircraft are in visual approach conditions and increase warning times when aircraft are instrument dependent, which will increase the credibility and overall effectiveness of MSAW warnings and likely improve controllers‘ ability to detect hazards and potentially prevent accidents.
For more information, see the Safety Board‘s Web site at <http://www.ntsb.gov/Recs/letters/2006/A06_44_47.pdf.>

FLIGHT RECORDERS

The accident airplane was equipped with a Fairchild model A-100 CVR. The exterior of the CVR showed no evidence of structural damage. The CVR was sent to the Safety Board‘s laboratory in Washington, D.C., for readout and evaluation. The interior of the CVR and the tape sustained no apparent heat or impact damage. The recording started at 0543:32 and continued uninterrupted until 0614:47. The recording consisted of two channels of —good quality“ audio information, the cockpit area microphone and the combined captain and first officer audio panel information, and two channels of low level audio that was unusable.
　
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