The FAA’s MSAW Again Fails Its Primary Raison d’etre
On August 4, 2005, at approximately 0206L, Flight N454MA an MU-2B-60 turbo-prop twin, was destroyed on impact with terrain near Parker, Colorado, while on an ILS approach to Centennial Airport (APA), near Denver, Colorado.
The pilot was killed. The flight originated at Salt Lake City International Utah at approximately 0040L. Night instrument meteorological conditions prevailed for the on-demand air cargo flight.
Recently revealed circumstances of the N45MA accident, and those of other fatal accidents, shed light, yet again, on a major weakness in the FAA’s oversight, to wit: the agency’s operational procedures for MSAW alerts.
N454MA received radar vectors to intercept the localizer for an ILS RWY 35R approach, maintaining 8,000 feet on QNH until established on the localizer. When the airplane was approximately four miles south of the final approach fix (FAF), the approach controller cleared N454MA for the approach. After his acknowledgement, the pilot was instructed to contact Centennial Tower.
At 0204:46, the pilot contacted Centennial Tower, but there was no response. He tried again at 0205:05. At 0205:19 a tower controller responded and cleared N454MA to land Runway 35R. The pilot acknowledged. This was the last radio communication received from the aircraft.
The last two minutes of radar data revealed the flight was turning toward the north at an altitude of 7,900 feet msl at a ground speed of 140 knots. As it approached the final fix, the airplane was observed on radar descending to an altitude of 7,200 feet msl, while it remained at a ground speed of 140 knots. After crossing the final fix the aircraft continued to track the localizer to the north, but continued to descend below the glide slope until the data ended at 0206:36.
Four seconds after ground contact (i.e., at 0206:40), a tower controller alerted the pilot to “…check altitude…your altitude indicates six thousand four hundred…you appear to be well below glide slope.” The pilot did not respond to several further contact attempts.
The last radar return was approximately four miles south of the landing threshold at an altitude of 6,400 feet msl, still at a groundspeed of 140 knots. The accident occurred at night in a remote area. There were no eyewitnesses. The pilot flew approximately 70 hours per month, and had accrued approximately 1,200 hours in the MU-2B.
Weather was a wind from 360 degrees at 9 knots with visibility 3 statute miles, rain, mist, scattered clouds at 800 feet agl, broken clouds at 1,600 feet, overcast ceiling at 2,500 feet, temperature 15 degrees F, dew point 14, with a barometric pressure setting of 30.37 inches of Mercury. The ceiling was reported to be varying between 600 to 1,300 feet. The tower visibility was reported as 4 statute miles.
Review of weather radar images taken between 0203 and 0208, revealed weak to moderate weather radar echoes in the vicinity of the accident site. Moderate to strong weather radar echoes were also present about 5 to 10 nautical miles south of the accident site. The tops of the echoes were reported at 30,000 feet. Conditions on the ILS would have been normal without any excessive turbulence.
The ILS RWY 35R has a decision height of 6,083 feet msl. The crossing altitude for the FAF is 7,974 feet msl. The distance between the FAF and the missed approach point at the middle marker (MM) is 5.9 nms. The MM is located 0.4 nm from the runway threshold. The airport elevation is 5,883 feet msl. The published minimums for the ILS RWY 35R approach are 200-foot ceiling and a half-mile visibility.
After the accident, the FAA observed a radar replay of the N454MA’s approach into Centennial Airport. A reconstruction of the accident sequence revealed that the pilot contacted the approach controller at Denver TRACON after entering the Denver area from the northwest. The pilot was vectored to the ILS RWY 35R final approach course and cleared for the approach about 0203. About a minute later, when the airplane was about 10 miles from the airport, the pilot was instructed to contact the tower at Centennial Airport.
As N454MA continued inbound on the ILS approach, it descended below the glide slope. At 0205:37, the Denver TRACON MSAW visual alert and aural alarm activated for about 5 seconds and again from 0206:00 until terrain impact about 42 seconds later. The airplane was about 7.2 and 6.3 nm from the airport, respectively, when the MSAW alerts at Denver TRACON activated. However, the approach controller had already handed the airplane off to TWR frequency.
Unfortunately, because of the MSAW software configuration at Denver TRACON, Centennial Tower was not eligible to receive aural MSAW alarms for any aircraft more than 5 nm from the airport. As a result, the controller at Centennial Tower only received non-attention-getting visual alerts when the MSAW alerts activated at Denver TRACON. When N454MA reached a point 5 nm from Centennial Airport at 0206:35, the ongoing MSAW visual alert then triggered a loud aural alarm in the tower. The tower controller immediately transmitted a low altitude alert to the pilot, but the airplane had only just impacted.
In February 2004, the FAA revised FAA Order 7110.65 Chapter 2-1-6: Safety Alert, which eliminated the obligation for an approach controller to alert a pilot of an MSAW alert once the airplane had entered a control tower’s aural alarm area. In an interview, the Denver approach controller stated that she thought the tower aural alarm area extended to 10 miles from Centennial Airport (instead of the 5 nm limit that was actually the case).
Based on that, along with the FAA’s revised policy on responsibility for issuance of safety alerts, the approach controller stated that MSAW alerts involving aircraft operating within 10 nm of an airport and on tower frequency were the tower’s responsibility. She also stated that she did not hear or see the MSAW alerts generated by N454MA. The first MSAW alert commenced when the aircraft was about 7.2 nm from Centennial Airport, 2.2 nm outside the tower’s aural alarm boundary. Even if the approach controller had heard or seen the MSAW alerts, FAA policy did not require her to respond to them.
The aural MSAW alarm received by the approach controller consisted of a tone that came from a speaker on the left side of her radar display console. The visual alert consisted of two blinking “LA” (low altitude) characters above the airplane’s data block, displayed on the controller’s radar screen. The tower controller received the same visual data on his radar display, but the Centennial Airport control tower was configured with two speakers located in the tower cab, and when tested, the tower’s MSAW aural alarm was a much louder tone than the one received by the approach controller.
ATC tower MSAW aural alarm requirements were based upon a previous safety recommendation claiming that visual-only alerts to tower controllers were ineffective. Tower controllers are not obligated to continuously monitor the radar display for MSAW visual alerts because their attention needs to be directed visually to the airport surface and surrounding airspace. The aural alarm was added to attract controller attention to the radar display when necessary, such as when an MSAW or conflict alert occurred, so that a controller would react in a timely manner.
Because the frequency change from the approach controller to Centennial Tower occurred when N454MA was about 10.7 nm from the airport, this left a 5.7 nm muted segment where both controllers could receive visual alerts, but only the approach controller could receive an aural alarm. The airplane was about 7.2 and 6.3 nm from the airport, respectively, when the MSAW alerts at Denver TRACON activated.
However, because the outer marker for the approach was 7.1 nm from the airport, and Denver TRACON procedures required aircraft to be handed off to tower frequency outside of the outer marker, the unavailability of any timely MSAW warning to the pilot was built-in to the FAA’s procedure.
The pilot’s instrumentation was recovered and checked but no reason was found for the pilot’s continued descent below the electronic glideslope. The scenario was typical of a low-level of situational awareness related to fatigue, leading to an unstabilized night approach in poor weather with a low ceiling. However, the fact remains that the accident could easily have been avoided had the Tower’s MSAW aural alert been configured correctly.
Failures of the MSAW to effectively function as originally designed continue to occur. For example, two instances relate to Guam’s Agana Airport alone: a Dec. 17, 2002 collision with powerlines on Nimitz Hill by Flt 110, a Philippines Air Lines A330, and a 747 crash of KAL Flt 801 on Nimitz Hill with heavy loss of life on Aug. 29, 1997. The FAA urgently needs to review the parameters for controller monitoring of MSAW altitude alerting radar.