The Boeing 717 aircraft was departing Melbourne for Coolangatta
on a regular public transport service. Following a normal take-off
the pilot in command (PIC), the handling pilot, called for the
landing gear to be retracted. A short time later, he noticed an
amber warning appear on the airspeed scale on his primary flight
display (PFD) screen. The PIC immediately reduced the aircraft
pitch attitude in response to that warning. At about the same time,
he noticed that the flaps/slats lever was at the `slats retract'
position. The PIC immediately called for the flaps to be
re-positioned, but the copilot selected the landing gear up. The
PIC again called for the flaps to be re-positioned and the copilot
then returned the flap selector to the take-off position. The PIC
reduced the pitch attitude further. The airspeed then quickly
increased to 15 knots above the reference speed (Vref) as the flaps
reached the take-off position. The PIC re-established the normal
climb attitude and the flaps and slats were subsequently retracted
in accordance with the normal profile. The remainder of the flight
was uneventful. Both the PIC and the copilot believed that the
stick shaker had activated momentarily during the sequence.
Examination of the flight data recorder from the aircraft
revealed the following:
- Three seconds after the aircraft became airborne, and at about
30 ft above ground level, the flaps/slats lever was moved from the
take-off position and the flaps began to retract. - One second later, as the flaps/slats lever reached the retract
position, the stick shaker warning commenced. At that time the
aircraft pitch angle was 18.6 degrees and the computed airspeed was
157 kts. Over the next three seconds, the stick shaker warning
continued and the aircraft pitch angle reduced to 10.2 degrees. A
second later the landing gear handle was recorded in the up
position. - The flaps/slats lever began to move from the retracted position
about one second after the landing gear handle reached the up
position. The flaps reached the fully retracted position less than
two seconds later, before immediately beginning to extend again.
The slats began to retract but did not reach the fully retracted
position before moving back to the extended position. The
flaps/slats movement was accompanied by a very brief reactivation
of the stick shaker and a further reduction in aircraft pitch angle
to about 6 degrees. Computed airspeed at that time was 165 kts. The
aircraft then began to accelerate and quickly returned to a normal
climb profile. - One altitude loss of 5 feet, and lasting less than three
seconds, coincided with the reduction in pitch angle that was made
in response to the second stick shaker activation. At that time the
aircraft was more than 240 ft above ground level. There was no
altitude loss associated with the first stick shaker
activation.
Interviews with the PIC and copilot did not reveal any obvious
issue that might have led to the copilot retracting the flaps/slats
instead of the landing gear. His work schedule, recent life
history, and activities leading up to the occurrence were
unremarkable. Against that background, the actions of the copilot
appear to have been an `action slip', a type of procedural error
associated with two actions (landing gear and flaps/slats
retraction) that are sequentially linked. As was the case here, in
human behaviour there can sometimes be a `spill-over' that triggers
the associated action at an inappropriate time.
Primary flight displays are liquid crystal screens that provide
attitude, airspeed, altitude, and heading information for each
pilot. The airspeed is displayed as a vertical `tape' on the left
side of the PFD screen. The aircraft is equipped with a stall
warning system that alerts the crew of an `approach-to-stall'
condition. The warnings are generated by angle-of-attack,
horizontal stabiliser position, and flaps/slats position inputs to
the flight control computer (FCC). The FCC displays amber and red
sectors (the so-called amber foot/red foot) on the airspeed tape to
warn of an `approach-to-stall' condition. The FCC also activates
the stickshaker. If the crew does not initiate recovery action, red
STALL annunciations appear on the PFDs, a klaxon sounds, and a
synthetic voice `stall' warning is activated.
In this occurrence, by reducing the aircraft pitch angle, the
PIC responded appropriately and very rapidly to the `amber foot'
warning that appeared on the speed tape as soon as the flaps/slats
lever was moved from the take-off position. As a result, the
aircraft moved from a deteriorating and potentially unsafe flight
performance regime to a safe one.
Anecdotal evidence was received early in the investigation that
there may have been other inappropriate/inadvertent flap/slat
selections in B717 aircraft. As a result, and in cooperation with
the operator, a survey was issued to company pilots who flew the
aircraft. The survey revealed three other instances of the
flap/slats lever being moved through the flaps zero position to the
slats retract position. However, all those events occurred above
3,000 feet altitude during initial `clean-up' flap retraction after
take-off and the error was recognised and corrected on each
occasion. None was associated with landing gear selection after
take-off.
In response to the occurrence, the company amended its
procedures for flaps/slats retraction approaching the ramp after
landing to include the following CAUTION note:
`When retracting flaps/slats to UP/RET, pause at the
UP/EXT position until the flaps indicate UP on the PFD prior to
retracting the slats. Never move the flap/slat handle to UP/RET in
one motion.'
The purpose of the change was to separate the retraction of the
flaps and slats into two distinct actions, in an attempt to prevent
the retraction of the flaps and slats becoming `learned' as a
single continuous action.