Premium on Safety - Issue 46, 2022 - 16
LESSONS LEARNED
MINIMIZING THE POSSIBILITY OF SINGLEPOINT
FAILURES IS A CORNERSTONE OF
SYSTEMATIC RISK MANAGEMENT, IN AVIATION
AS ELSEWHERE. Less consistently appreciated is
the need to stack the odds in your favor rather
than shaving margins on the assumption that
things will still work out.
On the afternoon of October 17, 2019, a
Saab 2000 on a scheduled passenger flight
from Anchorage flew the RNAV approach to
Runway 13 of the Unalaska, Alaska airport.
After the pilots visually acquired the airport,
the initial descent became unstabilized, so
they executed a go-around and lined up for a
second visual approach to the same runway.
Full brakes and full reverse
thrust were applied after the
nose gear touched down
and deceleration was initially
normal, but within five seconds
braking effectiveness dropped
to nil.
As they ran the before-landing checklist, the
first officer requested a wind check from the
airport weather observer and learned that the
direct crosswind at the beginning of their first
approach had come around to a stiff tailwind,
24 knots from 300 degrees true. Though this
was well beyond the manufacturer's published
limitation of a 15-knot tailwind component,
the captain decided to continue straight in.
The available landing distance was 3,900
feet; before the wind switched, the captain
calculated their unfactored landing distance as
3,058 feet.
The second approach was stable, and
the airplane touched down 800 feet past
the 300-foot displaced threshold at a
groundspeed of 142 knots. Full brakes and
full reverse thrust were applied after the
nose gear touched down and deceleration
was initially normal, but within five seconds
braking effectiveness dropped to nil. The
airplane continued past the departure
threshold and through the 300-foot runway
safety area, 150-foot blast pad, and four-foot
chain-link airport perimeter fence. The left
propeller struck an eight-foot road sign and
began shedding portions of its six blades
before the left main gear dropped into a ditch
and collapsed, destroying the rest of the left
propeller. The airplane finally came to rest
pitched nose-down on the riprap between the
road and Dutch Harbor with the nose gear
submerged in shallow water. Fragments of
NTSB performance studies found that even with the braking
malfunction, the Saab 2000 should have been able to
stop on the runway had it slowed its touchdown speed by
landing into the wind, or by deploying 35 degrees of flaps
rather than the 20 degrees the crew selected.
The accident was captured on a closed circuit camera on
one of the airport's buildings.
16
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