Tuesday, October 8, 2013
Setting Stabilized Approach Criteria
Just because the FAA doesn’t get specific doesn’t mean you don’t need criteria
A couple of weeks after the Asiana accident, the FAA's General Aviation Joint Steering Committee selected stabilized approaches as its Safety Enhancement Topic and put out a notice stating, in part, "...if something's not right, at any time, GO AROUND! There's no shame in going back up to take another shot at it."
Runway 28L at San Francisco is 11,381 feet long with a 300-foot displaced threshold and an elevation of 12.7 feet MSL. According to briefings by NTSB chairman Deborah A. P. Hersman, the flying pilot told investigators that when they were at 500 feet MSL, he saw three red lights and one white light on the precision approach path indicator (PAPI), meaning they were low on the approach path. He also said there was a flash of light from an unknown source, possibly a reflection of the sun, but it didn't interfere with his vision.
Information gathered from the cockpit voice and flight data recorders indicated there were elements in addition to the PAPI display, which should have alerted the flight crew that their approach was unstabilized and they needed to take action much sooner than they ultimately did.
The airplane's Vref speed for the approach was 137 knots. At 1,000 feet MSL, 54 seconds before impact, the airspeed was 149 knots. At 34 seconds prior to impact, when the airplane was down to 500 feet, the airspeed was already below Vref at 134 knots. At 200 feet, 16 seconds before impact, the airspeed was down to 118 knots.
At eight seconds before impact, at 125 feet, the engine throttles started moving forward, and the airspeed was 112 knots. At four seconds before impact, the stall warning stick shaker activated. Just three seconds before impact, there was a call for a go-around as the engine power rose to about 50% and the speed was down at 103 knots. At 1.5 seconds before impact, a different crewmember made a second call for a go-around. But, it was way too late.
One airline's criteria for determining when an approach is stabilized included the glide path being no more than one dot off the center of the glideslope and/or localizer, speed no more than 10 knots above the appropriate threshold speed, zero knots below the desired speed, and having a maximum descent rate of 900 feet per minute between 300 feet and 59 feet AGL.
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